Unit 2 – Group 7 Periodic Table
Unit 2 - Jan 2009
Q1.
Which of these statements about fluorine is not correct?
   A     It is a gaseous element at room temperature and pressure.
   B     It can react with chloride ions to form chlorine.
   C     It forms salts with Group 1 metals.
   D     It is less electronegative than chlorine.
(Total for question = 1 mark)

Q2.
10.0 cm3 of a solution containing Cu2+(aq) ions was added to excess potassium iodide solution and the following reaction occurred.

(a)  What happens to the Cu2+(aq) during this reaction?  Justify your answer.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (b)  All of the mixture containing iodine was titrated using sodium thiosulfate solution of concentration 0.200 mol dm−3.  The volume of sodium thiosulfate solution added at the end-point was 12.75 cm3.
       The equation for the reaction is

(i)   The end-point is shown most effectively using an indicator.  State a suitable indicator and the colour change you would expect to see at the end-point.
(2)
Indicator
 ............................................................................................................................................. 
Colour change at end-point
 ............................................................................................................................................. 
(ii)   Calculate the number of moles of iodine in the solution.
(2)

*(iii)   Use your answer from (ii), and the equation for the reaction between Cu2+(aq) and I−(aq), to calculate the concentration of the Cu2+(aq) in the original sample of solution.
         Give your answer to three significant figures and justify why this is an appropriate level of accuracy.
(3)
............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(iv)   The whole of the solution containing iodine was used in one titration.  Explain how this affects the reliability of your answer to (iii).
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (Total for question = 10 marks)

Unit 2 - Jun 2009
Q3.
When 2-chloro-2-methylpropane is heated in a mixture of water and ethanol at 65 °C, two types of reaction occur.  A mixture of two organic products, 2-methylpropan-2-ol and 2-methylpropene, is formed.

(a)  (i)  Name the two reaction types that are taking place.
(2)
Reaction 1
 ............................................................................................................................................. 
Reaction 2
 ............................................................................................................................................. 
*(ii)  Explain how the two products form, by describing the role of the water in each case.
(4)
Reaction 1
 ............................................................................................................................................. 
 ............................................................................................................................................. 
Reaction 2
 ............................................................................................................................................. 
 ............................................................................................................................................. 
............................................................................................................................................. 
(b)  A proposed mechanism for Reaction 1 involved the formation of the carbocation,

(i)  What type of bond breaking must have occurred during the carbocation
       formation?
(1)
 ............................................................................................................................................. 
(ii)  Suggest why 1-chlorobutane reacts with water via a different mechanism.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (c)  Another halogenoalkane, 2-chlorobutane, behaves in a similar way to 2-chloro-2-methylpropane but in Reaction 2 can form three different alkenes. Suggest how three different alkenes can form and give their displayed formulae.
(4)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(d)  Suggest why 2-fluoro-2-methylpropane would react more slowly than 2-chloro-2-methylpropane in Reaction 1.

        What reagent could you use instead of water to increase the rate of this reaction involving 2-fluoro-2-methylpropane? Explain why the reagent would have this effect.
(3)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 16 marks)

Q4.
Starch is often used as an indicator in titrations between sodium thiosulfate and iodine solutions.  What colour change would you see at the end-point as sodium thiosulfate is added to iodine solution in the presence of starch?
   A     Yellow to colourless
   B     Colourless to yellow
   C     Blue-black to colourless
   D     Colourless to blue-black
(Total for question = 1 mark)

Unit 2 - Jan 2010
Q5.
The concentration of iodine in solution can be measured by titration with sodium thiosulfate solution.

(a)  Name a suitable indicator which could be used for this titration.
(1)
 ............................................................................................................................................. 
(b)  The amount of sulfur dioxide in the atmosphere can be measured by passing a known volume of air through iodine solution. Sulfur dioxide converts iodine to iodide ions.

In an experiment, 100 m3 of air were passed through 100 cm3 of iodine, concentration 0.0100 mol dm−3. The remaining iodine was titrated with sodium thiosulfate solution and reacted with 12.60 cm3 of sodium thiosulfate, concentration 0.100 mol dm−3.
(i)  How many moles of iodine were present in the solution of the iodine at the start of the experiment?
(1)

(ii)  How many moles of iodine remained in the solution at the end of the experiment?
(2)

(iii)  Calculate the number of moles of iodine which reacted with the sulfur dioxide, and hence the number of moles of sulfur dioxide in 100 m3 of air.
(2)

(iv)  The European Commission recommend exposure to sulfur dioxide in air should be less than 350 micrograms (350 × 10−6 g) per cubic metre.
Calculate whether the sulfur dioxide in this sample of air was within this limit.
 One mole of sulfur dioxide has mass 64.1 g.
(2)

(c)  Explain whether the changes below would or would not improve the experimental procedure for measuring the concentration of sulfur dioxide in air used in (b).
(i)  The 100 cm3 of iodine was divided into 25 cm3 samples before titration.
(1)
 ............................................................................................................................................. 
 (ii)  The concentration of sodium thiosulfate used to titrate the iodine was changed from 0.100 mol dm−3 to 0.050 mol dm−3.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (iii)  150 m3 of air was passed through the iodine. The solutions used were of the same concentrations as in the original experiment.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (Total for question = 13 marks)

Unit 2 - Jun 2010
Q6.
Which of the following statements about the elements in Group 7 is incorrect?
   A     They all show variable oxidation states in their compounds.
   B     They all form acidic hydrides.
   C     Electronegativity decreases as the group is descended.
   D     They all exist as diatomic molecules.
(Total for question = 1 mark)

Q7.
This question is about the chemistry of some halogenoalkanes.

Halothane is a colourless and sweet-smelling liquid. It has a boiling temperature of 50°C. Halothane vapour was used as a general anaesthetic in hospitals during the mid to late 20th Century. Patients inhaled the halothane vapour under medical supervision. However, halothane was found to have some adverse side-effects and was therefore replaced by other halogenoalkane anaesthetics.
Halothane has the structure

In an experiment, halothane was heated in a test tube with aqueous silver nitrate and ethanol, using a water bath. Compound X and bromide ions were formed. The structure of compound X is shown below.

(a)  (i)  Give the systematic name of halothane.
(1)
 ............................................................................................................................................. 
(ii)  Suggest the types of intermolecular force present between molecules of liquid halothane.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (iii)  In the above experiment, suggest ONE reason why a water bath was used rather than heating the test tube containing the reaction mixture directly over a Bunsen flame.
(1)
 ............................................................................................................................................. 
 (iv)  Suggest why ethanol was used in this experiment.
(1)
 ............................................................................................................................................. 
 (v)  What would be seen in the test tube as the reaction progressed?
(1)
 ............................................................................................................................................. 
 (vi)  Write an ionic equation to show the reaction between aqueous silver ions and aqueous bromide ions. Include state symbols in your equation.
(1)

(b)  Chloroethane, C2H5Cl, can also be used as an anaesthetic. In an experiment, chloroethane was hydrolysed by aqueous sodium hydroxide, NaOH.
(i)  Name, and give the structural formula of, the organic product of the hydrolysis of chloroethane.
(2)
Name
 ............................................................................................................................................. 
Structural formula
 ............................................................................................................................................. 
(ii)  The hydrolysis of chloroethane is an exothermic reaction which takes place in a single step.
On the diagram below, draw the energy profile for the reaction. Label clearly the activation energy for the reaction.
(3)

(c)  In the early 1900s, the CFC with formula CCl2F2, was identified as a refrigerant which was both non-flammable and non-toxic.
(i)  What does the term CFC stand for?
(1)
 ............................................................................................................................................. 
(ii)  Suggest ONE use for CFCs other than as a refrigerant.
(1)
 ............................................................................................................................................. 

*(iii)  In the stratosphere, CFCs are broken down by absorption of UV radiation to form chlorine free radicals.
The following two reactions occur.

Combine these two equations to give the overall equation for the reaction of ozone in the stratosphere. State the role played by the chlorine free radical in the overall reaction. Hence explain why many scientists consider the effect of CFCs on ozone to be harmful.
(5)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (d)  The compound of formula CH2F2 has replaced several CFCs for commercial use. If molecules of CH2F2 reach the stratosphere, they do not break down to produce fluorine free radicals.
(i)  Suggest why C-F bonds are not broken in the stratosphere.
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
*(ii)  The compound CH2F2 acts as a greenhouse gas when it absorbs a particular type of radiation.
Name the type of radiation and explain why a molecule of CH2F2 is able to absorb this radiation.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 22 marks)

Q8.
When solutions of iodine are titrated with aqueous sodium thiosulfate solution, Na2S2O3(aq), the thiosulfate ions are oxidized to
   A     S2O42−
   B     S2O62−
   C     S2O82−
   D     S4O62−
(Total for question = 1 mark)

Unit 2 - Jan 2011
Q9.
This question is about iodine and its compounds.
(a) (i) The element iodine can be obtained from seaweed. One step in the procedure is to extract the iodine from aqueous solution by shaking with a hydrocarbon solvent in a separating funnel.

           Draw a diagram of a separating funnel containing the separated layers. Label the hydrocarbon layer, and state its colour.
           [Density of hydrocarbon layer 0.660 g cm−3]
(3)
Diagram

Colour of hydrocarbon layer
 ............................................................................................................................................. 
     (ii) Iodine is also formed when an aqueous solution containing iodide ions reacts with an aqueous solution of iron(III) ions.

           Write the ionic equation for this reaction. State symbols are not required.
(1)

(b) Hydrogen iodide gas is usually prepared by adding phosphoric(V) acid to solid potassium iodide.
(i) Suggest why phosphoric(V) acid is used in this preparation rather than concentrated sulfuric acid.
(1)
 ............................................................................................................................................. 
 (ii) Describe what you would see if a test tube of hydrogen iodide gas was inverted in a beaker of water.
(1)
 ............................................................................................................................................. 
 (iii) When hydrogen iodide gas reacts with ammonia, dense white fumes form.

                    Write the equation for this reaction, including state symbols.
(2)

(c) 1-iodobutane can be made by reacting butan-1-ol with phosphorus(III) iodide, PI3, formed by reacting moist red phosphorus with iodine.
(i) Complete the following equation for the formation of 1-iodobutane.
(1)
PI3 + .................... C4H9OH →

(ii) Identify the intermolecular forces present between molecules of 1-iodobutane.
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (iii) 1-iodobutane reacts with hot aqueous silver nitrate solution. Describe what you would see when this reaction takes place.
(1)
 ............................................................................................................................................. 
(iv) Give the structural formula for the organic product of the reaction between 1-iodobutane and ammonia.
(1)

(Total for question = 12 marks)

Unit 2 - Jun 2011
Q10.
This question is about the element chlorine and its compounds.
(a) When chlorine is bubbled through water, a solution of chlorine water forms. What is the colour of chlorine water?
(1)
 ............................................................................................................................................. 
(b) Chlorine water is added to potassium iodide solution.
      (i)   State the colour of the solution produced.
(1)
 ............................................................................................................................................. 
      (ii)  Write the ionic equation for the reaction, including state symbols.
(2)

(c) The concentration of chlorine water was found by taking 10.0 cm3 of solution, adding an excess of potassium iodide solution, and titrating with 0.0100 mol dm−3 of sodium thiosulfate solution. The experiment was repeated.
      The following results were obtained.

      (i)   Name a suitable indicator for the titration. State the colour change you would expect to see at the end point.
(2)
Indicator ...........................................

Colour change from ........................................... to ...........................................
      (ii)  Calculate the mean titre and use this value to calculate the number of moles of sodium thiosulfate used in the titration.
(1)
Mean titre = .................................... cm3
Moles of sodium thiosulfate

      (iii)  Complete the ionic equation for the reaction between iodine and thiosulfate ions.
(2)
               
      (iv)  Calculate the number of moles of iodine which reacted with the sodium thiosulfate solution.
(1)

      (v)   Hence state the number of moles of chlorine present in 10.0 cm3 of the chlorine water.
(1)

      (vi)  Calculate the concentration of the chlorine water, in mol dm−3.
(1)

(d) Potassium burns in chlorine to form potassium chloride.
      (i)   Give the colour of the flame when potassium burns in chlorine.
(1)
 ............................................................................................................................................. 
      (ii)  Write the equation for the reaction between potassium and chlorine. State symbols are not required.
(1)

(e) Concentrated sulfuric acid is added to potassium chloride in a test tube. Steamy fumes are given off which react with ammonia to give dense white smoke.
      (i)   Name the gas given off in this reaction.
(1)
 ............................................................................................................................................. 
      (ii)  Steamy fumes are observed at the mouth of the test tube. Explain how these fumes are formed.

(1)
 ............................................................................................................................................. 
      (iii)  The steamy fumes react with ammonia to give a dense white smoke. Identify the white smoke by name or formula.
(1)
 ............................................................................................................................................. 
(f) 2-chlorobutane can be made from butan-2-ol.
     (i)   Name the chemical you would add to butan-2-ol in the laboratory to make 2-chlorobutane.
(1)
 ............................................................................................................................................. 
     (ii)  2-chlorobutane reacts with alcoholic potassium hydroxide at a high temperature to form a mixture of gaseous alkenes.
            Draw a fully labelled diagram of the apparatus you would use to prepare and collect this mixture.
(3)

(Total for Question = 21 marks)

Q11.
Which silver halide is a cream coloured solid which darkens in sunlight and dissolves in concentrated ammonia solution?
   A    AgF
   B    AgCl
   C    AgBr
   D    AgI
(Total for Question = 1 mark)

Unit 2 - Jan 2012
Q12.
Chlorine disinfectants are essentially solutions containing chlorine molecules and chlorate(I) ions in an equilibrium summarised by the equation

The chlorine content of a disinfectant was determined using the following procedure.
1. 10.0 cm3 of the disinfectant was transferred to a 250 cm3 volumetric flask.
2. Approximately 20 cm3 of nitric acid and 20 cm3 potassium iodide solution (both in excess) were added to the volumetric flask.
3. The solution in the volumetric flask was made up to the mark with distilled water and then mixed thoroughly.
4. 10.0 cm3 portions of the solution in the volumetric flask were titrated against a solution of sodium thiosulfate, concentration 0.109 mol dm-3. Starch solution was added near the end-point of the titration and the mean (average) titre was 27.35 cm3.
The equations for the reactions involved in this procedure are

(a)  (i) Calculate the number of moles of sodium thiosulfate used in the titration.
(2)

(ii) Calculate the number of moles of iodine, I2, that reacted in the titration (step 4).
(1)

(iii) Hence state the number of moles of chlorine, Cl2, in 10.0 cm3 of the solution in the volumetric flask.
(1)

(iv) Calculate the concentration of chlorine, in mol dm-3, in the original disinfectant.
(2)

(b)  Equation 1 is an example of a disproportionation reaction. Define the term 'disproportionation' and explain, by considering the relevant oxidation numbers, why this reaction is a disproportionation.
(3)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (c)  State the colours of the titration solution just before the starch solution is added, after the starch solution is added and the colour change at the end-point of the reaction.
(2)
Colour just before adding the starch
 ............................................................................................................................................. 
Colour after adding the starch
 ............................................................................................................................................. 
Colour at the end-point
 ............................................................................................................................................. 
(Total for question = 11 marks)

Q13.
Which of the following properties of the elements chlorine, bromine and iodine increases with increasing atomic number?
   A  Boiling temperature
   B  Bond enthalpy
   C  Electronegativity
   D  First ionization energy
(Total for question = 1 mark)

Unit 2 - Jun 2012
Q14.
(a)  Ozone, O3, is formed when oxygen is exposed to ultraviolet (UV) radiation or to an electric discharge. Ozone is a blue gas whereas oxygen is colourless. When the two gases are mixed, an equilibrium is established as shown in the following equation.
3O2(g)    2O3(g)      ΔH = +143 kJ mol−1
(i) When the temperature of the pale blue equilibrium mixture is increased at constant volume, the colour darkens. Explain this observation in terms of the changes to the equilibrium.
(2)
 ............................................................................................................................................. 
 (ii) State and explain what you would see if the pressure of the system at equilibrium were increased.
(2)
 ............................................................................................................................................. 
(iii) A small amount of oxygen gas containing the isotope 18O is added to the equilibrium mixture. After a few hours, ozone containing 18O is detected. Given that the equilibrium position is not affected, explain this observation.
(1)
 ............................................................................................................................................. 
 (b) The concentration of ozone in the atmosphere may be determined by bubbling air through a solution of acidified potassium iodide. Iodine is formed in solution, the concentration of which may be determined by titration with a solution of sodium thiosulfate of known concentration. The equations for the reactions are
O3 + 2I− + 2H+  →  O2 + H2O + I2     Equation 1
 I2+ 2S2O32−   →  2I− + S4O62−       Equation 2
In an experiment to determine the concentration of ozone in a sample of air, 100 m3 of air was bubbled through 100 cm3 of a solution containing an excess of acidified potassium iodide. The resulting solution was titrated against a solution of sodium thiosulfate of concentration 0.0155 mol dm−3. The volume of sodium thiosulfate solution required for complete reaction was 25.50 cm3.
(i) Calculate the number of moles of sodium thiosulfate that react.
(1)

(ii) Calculate the number of moles of iodine that reacted with the sodium thiosulfate.
(2)

(iii) Use equation 1 to deduce the number of moles of ozone that reacted with the acidified potassium iodide.
(1)

(iv) Calculate the volume of ozone, measured in m3, present in the original sample of air. Assume that all gas volumes were measured at room temperature and pressure and that the molar volume of any gas under these conditions is 0.024 m3 mol−1.
(1)

(v) Calculate the concentration of ozone in the sample of air in units of parts per million (ppm) by volume.
(1)

(vi) A student suggested that the 100 cm3 of acidified potassium iodide should be divided into four portions before the titration. Explain how this change increases the reliability and decreases the accuracy of the experiment.
(3)
Increases reliability
 ............................................................................................................................................. 
 ............................................................................................................................................. 
Decreases accuracy
 ............................................................................................................................................. 
 (c) Give the oxidation numbers of oxygen in equation 1, shown below. Hence state the role of ozone in this reaction.
(3)
O3 + 2I− + 2H+ → O2 + H2O + I2
Oxidation number of O
 ............................................................................................................................................. 
Role of ozone
 ............................................................................................................................................. 
(d) Ozone is used as an alternative to chlorine to disinfect flood damaged buildings, to remove residual smoke odours from fires and in the treatment of drinking water. Suggest one advantage of using ozone rather than chlorine, given that chlorine and ozone are both toxic.
(1)
 ............................................................................................................................................. 
 (Total for question = 18 marks)

Unit 2 - Jun 2013
Q15.
The percentage by mass of tin in a piece of rock containing tin(IV) oxide, SnO2, was determined as described in the procedure below.
Step 1	A sample of rock, with mass 10.25 g, was crushed and dissolved in sulfuric acid. 	
Step 2	The solution was treated with a reducing agent to convert the Sn4+ to Sn2+ ions. 
	
Step 3	50 cm3 of aqueous iodine solution with concentration 0.250 mol dm−3 was added to the solution of Sn2+ ions. The following reaction occurred: 
	
  	Sn2+(aq) + I2(aq) → Sn4+(aq) + 2I−(aq) 	
Step 4	The excess iodine was titrated with sodium thiosulfate solution with concentration 0.100 mol dm−3. The volume of sodium thiosulfate solution required was 11.60 cm3. 	
 
(a)  Thiosulfate ions react with iodine as shown below.
2S2O32−(aq) + I2(aq) → S4O62−(aq) + 2I−(aq)
(i)  Calculate the number of moles of sodium thiosulfate which were used in Step 4.
(1)

(ii)  Calculate the number of moles of iodine which reacted with this amount of sodium thiosulfate.
(1)

(iii)  Calculate the number of moles of iodine added to the solution of Sn2+ ions in Step 3.
(1)

(iv)  Use your results from (ii) and (iii) to calculate the number of moles of iodine which reacted with the Sn2+ ions from the rock.
(1)

(v)  Hence calculate the percentage by mass of tin in the rock.
(2)

(b)  (i)  What change could be made in Step 4 to improve the reliability of the result?
(1)
 ............................................................................................................................................. 

(ii)  The error each time the burette was read was ± 0.05 cm3. Calculate the percentage error in the titre value of 11.60 cm3.
(1)

(iii)  How could the percentage error in the titre value be reduced without using a different burette?
(1)
 ............................................................................................................................................. 

(c)  The titration can be carried out with or without an indicator. What colour change would be seen at the end-point if an indicator was not used? The tin ions are colourless.
(1)
 ............................................................................................................................................. 
(Total for Question = 10 marks)

Q16.
Ethane-1,2-diol, CH2OHCH2OH, is commonly used in antifreeze for cars to lower the freezing temperature of the water in the car radiator. It reacts in a similar way to ethanol but both of the alcohol groups can react.

(a)  Write an equation for the complete reaction between sodium and ethane-1,2-diol. State symbols are not required.
(2)

(b)  Ethane-1,2-diol is very quickly oxidized to ethanedioic acid, (COOH)2, even under the conditions shown below.

However, ethanol requires stronger oxidizing conditions to be converted into ethanoic acid.
Explain how you would change the above apparatus to achieve this oxidation of ethanol.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(c)  Draw the skeletal formula of ethanedioic acid.
(1)

(d)  Explain why phosphorus(V) chloride, PCl5, would not be a suitable reagent to be used to distinguish between ethane-1,2-diol and ethanedioic acid.
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(e)  (i)  Depending on the reaction conditions, ethanol can be oxidized to either an aldehyde or to carboxylic acid. Infrared spectroscopy is a suitable technique for determining whether the oxidation product obtained is an aldehyde or a carboxylic acid.
Draw, on the spectrum below, any peak(s) that you would expect to see between 4000 and 1500 cm−1 if the product was an aldehyde and not a carboxylic acid.
(2)
DATA
The IR absorption ranges associated with some organic functional groups are given below:
O—H stretching in alcohols (variable, broad) at 	3750 − 3200 cm−1 	
O—H stretching in carboxylic acids (weak) at 	3300 − 2500 cm−1 	
CO stretching in aldehydes (strong) at 	1740 − 1720 cm−1 	
CO stretching in ketones (strong) at 	1700 − 1680 cm−1 	
CO stretching in carboxylic acids, alkyl (strong) at 	1725 − 1700 cm−1 	
C—H stretching in aldehydes (weak) at 	2900 − 2820 cm−1 	
                                       and (weak) at 	2775 − 2700 cm−1 	

(ii)  The infrared spectrum of the distillate from the reaction in (e)(i) included a peak at 3750–3200 cm−1.

What substance is likely to have caused this?
(1)
 ............................................................................................................................................. 

(iii)  Mass spectrometry can be used to identify the products of the oxidation of ethanol. Suggest the formula of a fragment that would show when ethanoic acid is produced and would not be present in either ethanol or ethanal.
(1)
 ............................................................................................................................................. 

(f)  Treatment of 2-bromoethanol, CH2BrCH2OH, with aqueous sodium hydroxide would be one way to produce ethane-1,2-diol.
(i)  Complete a possible mechanism for this reaction in the space below.
(3)

(ii)  Classify the mechanism and type of reaction in (f)(i):
(2)
Mechanism
 ............................................................................................................................................. 
Type
 ............................................................................................................................................. 

(g)  Aqueous silver nitrate can be used to test for the presence of bromide ions. Write an ionic equation for the reaction. Include state symbols in your answer.
Ionic Equation

*(h)  It can be difficult to distinguish between the colours of the silver halides. The use of solutions of ammonia can be very helpful.

A silver halide dissolved in concentrated ammonia to form a colourless solution.

Explain why this result does not prove conclusively that the silver halide was silver bromide and give a further test to confirm that the silver halide is silver bromide.

(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for Question = 19 marks)

Q17.
The concentration of a solution of potassium iodate(V) can be determined by the liberation of iodine, followed by titration with sodium thiosulfate.
A suitable indicator is

   A    methyl orange.
   B    phenolphthalein.
   C    starch.
   D    universal indicator.
(Total for Question = 1 mark)

Unit 2 - Jun 2014
Q18.
Potassium dichromate(VI), K2Cr2O7, can be used to accurately determine the concentration of other chemicals, such as sodium thiosulfate, Na2S2O3.
(a) The dichromate(VI) ion has two chromium atoms sharing one oxygen to give two tetrahedral units. Each chromium atom uses six electrons in bonding and expands its outer shell to accommodate a total of 12 electrons.
Complete the dot and cross diagram for this ion below. Only show outer shell electrons.
Use x for chromium electrons and • for oxygen electrons. Use the symbol * to represent the extra electrons which give the ion its charge.
(3)

(b)  Four chemistry students were given a solution of sodium thiosulfate with a concentration of approximately 0.1 mol dm−3 and asked to determine its exact concentration using potassium dichromate(VI) solution. They were each given separate tasks to carry out as described below.
(i)  The first student was given the task of making up a potassium dichromate(VI) solution. A mass of 14.71 g of K2Cr2O7 was weighed out, dissolved in deionized water, the volume made up to 250 cm3 in a volumetric flask and the mixture shaken.
Calculate the concentration of this potassium dichromate(VI) solution, in mol dm−3.
Use the Periodic Table as a source of data.
(2)

(ii)  The second student was asked to calculate the mass of potassium iodide that would be required to add to 0.00250 mol of potassium dichromate(VI) to ensure complete reaction. The equation for the reaction is
Cr2O72−(aq) + 14H+(aq) + 6I−(aq) → 2Cr3+(aq) + 3I2(aq) + 7H2O(l)
Calculate the minimum mass of potassium iodide, KI, required and hence suggest a suitable mass to use if the potassium iodide is to be in excess.
You must show your working and your mass should be reasonable.
(2)

Minimum mass required ........................................................... g
Suitable mass to use ........................................................... g

(iii)  The third student was given the following equation.
I2(aq) + 2S2O32−(aq) → 2I−(aq) + S4O62−(aq)
This student was asked to estimate the titration reading at the end-point if a solution that contained 0.00260 mol of iodine was in the conical flask and the concentration of sodium thiosulfate was about 0.16 mol dm−3.
Calculate the volume of sodium thiosulfate solution, in cm3, that would have been added at the end-point of the titration.
(2)

*(iv)  The fourth student was given the following experimental information.
0.1 g of potassium dichromate(VI) was dissolved in a total volume of 25.00 cm3. An excess of potassium iodide and acid was added and then used in a titration with the sodium thiosulfate solution of concentration approximately 0.1 mol dm−3. The titre was 25.15 cm3.
The student suggested that the greatest uncertainty in the result arose from the mass that was measured and that the procedure was unreliable.
Explain why these views were justified. No calculation is required.
(2)
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(c)  (i)  Acidified sodium dichromate(VI) solution is commonly used for the oxidation of alcohols. It is important not to use hydrochloric acid in this reagent mixture because the chloride ions are oxidized to chlorine.
Write the ionic half-equation for the oxidation of chloride ions. State symbols are not required.
(1)

(ii)  Fumes of hydrogen chloride gas can be identified by bringing the fumes into contact with another gas, X. Identify gas X and state the observation you would make.
(2)
Gas X
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Observation
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(d)  Potassium bromide can be distinguished from potassium chloride by its reaction with silver nitrate solution, followed by the addition of aqueous ammonia solution.
State what you would see on the addition of silver nitrate solution to potassium bromide solution.
How could both dilute and concentrated ammonia be used to confirm that silver bromide is formed, rather than silver chloride?
(3)
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(Total for question = 17 marks)

Q19.
This is a question about halogenoalkanes.
(a)  Halogenoalkanes can react with hydroxide ions in different ways depending on the conditions used. Using 1-chloro-1-fluoroethane, CH3CHClF, as an example of a halogenoalkane, the following reaction could occur in aqueous solution.
CH3CHClF + OH− → CH3CHOHF + Cl−
(i)  Suggest why it is unlikely that the fluorine atom in CH3CHClF would be substituted by the hydroxide ion.
(1)
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*(ii)  A student attempted to draw the reaction mechanism for the reaction in (a)(i), but made a total of three errors.

Identify these errors and state how they should be corrected.
(3)
First error
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Second error
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Third error
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(iii)  In hot alcoholic solution, a different reaction may occur between halogenoalkanes and hydroxide ions.
Write the equation for the reaction between 1-chloro-1-fluoroethane, CH3CHClF, and hydroxide ions in alcoholic solution.
(2)

(b)  Chloroethane can be produced in various ways as shown below. Identify, by name or formula, the reagent needed for each of these reactions.
(3)

(c)  (i)  Chloroethane will react with alcoholic ammonia initially to produce ethylamine. Complete the equation for this reaction.
(1)
.............................. + NH3 → .............................. + ..............................

(ii)  Name the type and mechanism of the reaction occurring in (c)(i).
(2)
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(iii)  What feature of the ammonia molecule enables the reaction in (c)(i) to take place?
(1)
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(iv)  If aqueous ammonia was used in (c)(i), instead of alcoholic ammonia, suggest the identity of the organic product that would be formed.
(1)
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(d)  Dichlorodifluoromethane, CCl2F2, is also known as Freon 12 and its manufacture was banned in 1994 under the terms of the Montreal Protocol.
(i)  Complete the equation for the initiation stage and suggest equations for two of the propagation stages and a termination stage for the mechanism of the reaction that this molecule might undergo with ozone.
(4)
Initiation           CCl2F2 →

Propagation 1

Propagation 2

Termination

*(ii)  Explain why the effect of Freon 12 molecules on the ozone layer was such a serious issue that scientists recommended its use to be discontinued.
(2)
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(iii)  Freon 12, CCl2F2, could also be described as a "greenhouse gas". Explain what the term 'greenhouse gas' means.
(2)
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(iv)  Freon 12, and other similar molecules, are not normally viewed as contributors to the greenhouse effect. Suggest why this is so.
(1)
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(Total for question = 23 marks)