Unit 2 – Kinetics
Unit 2 - Jan 2009
Q1.
This question is about boron and nitrogen compounds.
(a)  Draw and name the shape of a boron trifluoride, BF3, molecule.  Suggest the FBF bond angle.
(3)

Name of shape
 ............................................................................................................................................. 
FBF bond angle
 ............................................................................................................................................. 
(b)  Ammonia has the formula NH3.  Its HNH bond angle is less than the FBF bond angle in boron trifluoride.
(i)   Estimate the HNH bond angle in NH3.
(1)
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(ii)   Explain why the HNH bond angle is less than that for FBF.
(1)
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 (iii)   Name the strongest intermolecular force between BF3 molecules.
(1)
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(iv)   Name the strongest intermolecular force between NH3 molecules.
(1)
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(c)  Ammonia will react with oxygen in the presence of a platinum catalyst at 750°C
       forming water and nitrogen(II) oxide, NO.
(i)   What is the oxidation number of nitrogen in ammonia, NH3?
(1)
 ............................................................................................................................................. 
(ii)   The diagram below shows the distribution of molecular energies in the reaction at 750°C.  On the same diagram, draw a curve to show the distribution at 500°C and explain what effect this change in temperature would have on the rate of the reaction.
(3)

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*(d)  Explain how a catalyst speeds up the rate of a reaction.
(3)
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(Total for question = 14 marks)

Q2.
In the reaction between Ag+(aq) ions and Fe2+(aq) ions, what would be the effect of increasing the concentration of Ag+(aq) ions?

   A     Rate of reaction increases, yield of Fe3+(aq) stays the same.
   B     Rate of reaction increases, yield of Fe3+(aq) decreases.
   C     Rate of reaction decreases, yield of Fe3+(aq) stays the same.
   D     Rate of reaction increases, yield of Fe3+(aq) increases.
(Total for question = 1 mark)

Unit 2 - Jun 2009
Q3.
In the equilibrium below, what effect would the changes described have on the system?

(a)  Increase in temperature
(1)
   A     increase rate, decrease yield
   B     increase rate, increase yield
   C     decrease rate, decrease yield
   D     decrease rate, increase yield

(b)  Decrease in pressure
(1)
   A     increase rate, decrease yield
   B     increase rate, increase yield
   C     decrease rate, decrease yield
   D     decrease rate, increase yield
(Total for question = 2 marks)

Q4.
In the reaction profile below, which energy change would alter if a catalyst was added to the reaction?

   A
   B
   C
   D
(Total for question = 1 mark)

Unit 2 - Jan 2010
Q5.
Magnesium nitrate, Mg(NO3)2, decomposes when it is heated. One product is the brown gas, nitrogen dioxide.
(a)  (i)  Write an equation for this reaction. State symbols are not required.
(2)

(ii)  Calcium nitrate decomposes in a similar way to magnesium nitrate, but at a
       higher temperature.
Explain why the two nitrates have different stability to heat.
(2)
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(b)  Sodium nitrate decomposes to give different products to magnesium nitrate. Write
       an equation for the decomposition of sodium nitrate. State symbols are not required.
(1)

(c)  A student suggested that the structure of the nitrate ion, NO3−, is

Scientists have found that the bonds between nitrogen and oxygen in the nitrate ion are all the same length. Is the student's suggestion supported by this evidence? Explain your answer.
(1)
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 (d)  Nitrogen dioxide gas can dimerize to dinitrogen tetroxide, N2O4, a very pale yellow gas, as shown in the equation below.

(i)  What would you see when an equilibrium mixture of these gases is warmed
       gently?  Explain your answer.
(2)
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 (ii)  Explain why an equilibrium mixture of these gases eventually becomes paler in colour when the pressure on it is increased.
(2)
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 (e)  Two Maxwell-Boltzmann distributions showing the energy of particles in a gas at different temperatures, T1 and T2, are shown below. The activation energy for the reaction is labelled EA.

Use the distributions to explain why gases react faster when the temperature is
 increased.
(2)
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 (Total for question = 12 marks)

Unit 2 - Jun 2010
Q6.
(a)  In the catalytic converter of a car engine's exhaust system, the following reaction occurs.

The temperature in a catalytic converter is high.
(i) State the effect, if any, on the position of equilibrium if the temperature is lowered. Give a reason for your answer.
(2)
Effect
 ............................................................................................................................................. 
Reason
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(ii) The gases from the engine are not cooled before entering the converter.
      Explain why this is so.
(2)
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(iii) State the effect, if any, on the position of equilibrium if the pressure on the reacting gases is increased. Give a reason for your answer.
(2)
Effect
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Reason
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(b)  Nitrogen monoxide, NO, is formed when nitrate ions, , in acidic solution are reduced by silver metal.
(i)  Calculate the oxidation number of nitrogen in NO and in .
(2)
In NO
 ............................................................................................................................................. 
In 
 ............................................................................................................................................. 
(ii)  Balance the half-equation for the reduction of nitrate ions, , in acidic solution.
(1)

(iii)  Write the half-equation for the oxidation of silver metal, Ag, to silver ions, Ag+.
(1)

(iii)  (iv) Hence deduce the full ionic equation for the reaction between silver metal and nitrate ions in acidic solution. State symbols are not required.
(2)

(Total for question = 12 marks)

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)
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(ii)  Suggest the types of intermolecular force present between molecules of liquid halothane.
(2)
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 (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)
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 (iv)  Suggest why ethanol was used in this experiment.
(1)
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(v)  What would be seen in the test tube as the reaction progressed?
(1)
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 (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)
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(ii)  Suggest ONE use for CFCs other than as a refrigerant.
(1)
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*(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)
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 (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)
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*(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)
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 (Total for question = 22 marks)

Unit 2 - Jan 2011
Q8.
This question is about some reactions which can be used in the manufacture of hydrogen.

       Reaction 1 uses two naturally occurring chemicals, water and natural gas. Steam is reacted with methane to form carbon monoxide and hydrogen in an equilibrium reaction.

       Reaction 1    CH4(g) + H2O(g)  CO(g) + 3H2(g)       ΔH = +210 kJ mol−1

       In reaction 2, carbon monoxide and steam are passed over copper at high temperature. This forms carbon dioxide and hydrogen.

       Reaction 2    CO(g) + H2O(g)  CO(g) + H2(g)

       The carbon dioxide formed is removed by passing it through potassium carbonate solution in reaction 3.

       Reaction 3    K2CO3(aq) + CO2(g) + H2O(l) → 2KHCO3(aq)

       The potassium carbonate is regenerated by heating the potassium hydrogencarbonate solution in reaction 4. The carbon dioxide gas produced is released into the atmosphere.

       Reaction 4    2KHCO3(aq) → K2CO3(aq) + CO2(g) + H2O(l)

(a) For each of the first three reactions, state the initial and final oxidation numbers of any elements that change their oxidation numbers. Hence decide which are redox reactions.
(5)
Reaction 1
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Reaction 2
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Reaction 3
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*(b) (i) Discuss, with reasons, the conditions of temperature and pressure that would favour the production of hydrogen in reaction 1. You should consider the effect of the conditions on both yield and rate.
(7)
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 (ii) Excess steam is used in reaction 1. State why an excess of a reagent is used and suggest why steam, rather than methane, is chosen.
(2)
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(c) Copper is a catalyst in reaction 2. Explain how a catalyst increases the rate of a reaction.
(2)
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 (d) (i) State one economic advantage of reaction 4.
(1)
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*(ii) Reaction 4 contributes to global warming. Identify the substance formed in this reaction which is likely to be responsible and explain the processes that lead to an increase in global temperatures.

      Suggest two effects an increase in global temperatures might have on the environment.
(4)
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(Total for question = 21 marks)

Q9.
The Maxwell-Boltzmann distribution of molecular energies is useful for explaining why increasing temperature affects the rate of a chemical reaction.
(a) Which of the following statements describes how the shape of the Maxwell-Boltzmann distribution curve changes as temperature increases?
(1)
   A   The peak decreases in height and moves to the left.
   B   The peak increases in height and moves to the left.
   C   The peak decreases in height and moves to the right.
   D   The peak increases in height and moves to the right.

(b) The main reason that reaction rates increase with temperature is that
(1)
   A   all the molecules move faster.
   B   all the molecules collide more frequently.
   C   more molecules collide with the correct orientation.
   D   a larger proportion of molecules have high energies.
(Total for question = 2 marks)

Unit 2 - Jun 2011
Q10.
This question is about nitrogen monoxide, NO, which can be described both as a friend and a foe.

(a) (i)   What is meant by the term free radical?
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(a) (ii)  Suggest a dot and cross diagram for nitrogen monoxide, showing outer shell electrons only, remembering that it is a free radical.
(2)

(b) (i)   Part of the unbalanced equation for the preparation of nitrogen monoxide from nitric acid is shown below.

Identify the elements which are oxidized and reduced and give their oxidation numbers.
(3)
Element oxidized .........................

Oxidation number initial .........................    final .........................

Element reduced .........................

Oxidation number initial .........................    final .........................

     (ii)  Complete and balance the equation for the reaction between silver and nitric acid.
(2)

(c) The reaction between nitrogen and oxygen to form nitrogen monoxide reaches equilibrium.

     (i)   Explain why the yield of nitrogen monoxide is increased when the temperature is increased.
(1)
      ..............................................................................................................................................

  ∗(ii)  State and explain the effect, if any, on the yield of nitrogen monoxide when the pressure is increased.
(2)
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      ..............................................................................................................................................

   (iii)  State and explain how the rate of the reaction is affected by an increase in pressure.
(2)
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∗ (d) (i)  Explain why a jet aeroplane in flight causes much more damage to the ozone layer than cars carrying the same number of passengers at sea level. You should assume that the nitrogen monoxide outputs for both methods of conveying the passengers are the same.
(2)
      ..............................................................................................................................................

      ..............................................................................................................................................

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   (ii)  The reactions of chlorine free radicals with ozone may be represented by the following equations.

Write corresponding equations for the reactions of the free radical nitrogen monoxide with ozone. Combine your two equations to show the overall reaction.
Use these equations to explain why a small quantity of nitrogen monoxide can have a continuing effect on the ozone layer.
(5)
Equations

Explanation
 ............................................................................................................................................. 
      ..............................................................................................................................................
(Total for Question = 20 marks)

Unit 2 - Jan 2011
Q11.

(a) Write the equation for the formation of nitrogen monoxide from nitrogen and oxygen. State symbols are not required.
(1)

(b) The electronic structure of nitrogen monoxide is unusual in that it has an unpaired electron.
(i) What name is given to a chemical species such as nitrogen monoxide that has an unpaired electron?
(1)
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(ii) Chemical species with unpaired electrons occur as intermediates in some chemical reactions. What type of bond breaking produces these species?
(1)
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(c)(i) Suggest the most likely source of the hydrocarbons that react with nitrogen monoxide to form toxic compounds.
(1)
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(ii) Suggest the type of reaction that is involved when a hydrocarbon is converted into an aldehyde.
(1)
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(iii)  Draw the skeletal formula of the aldehyde with three carbon atoms.
(1)

(iv) By considering the equation

explain the effect of the reaction of hydrocarbons with nitrogen monoxide on the breakdown of ozone.
(1)
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(d) Suggest why the proportion of nitrogen dioxide might be reduced at high altitudes.
(2)
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 (e) Explain why it is important to maintain the concentration of ozone in the upper atmosphere.
(2)
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 (f)(i) Write an equation for the reaction on a catalytic converter described in the passage. State symbols are not required.
(1)

(ii) Draw an energy profile for the exothermic reaction in (f)(i). Label the axes, the reactants and products, the enthalpy change and the activation energy.
(3)

*(iii) By referring to your energy profile, explain how a catalyst speeds up a chemical reaction.
(3)
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 (g) Jet aircraft are considered a greater threat to the ozone layer than road vehicles. Suggest an explanation for this.
(2)
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(Total for question = 20 marks)

Unit 2 - Jun 2012
Q12.
The diagram below shows the Maxwell-Boltzmann distribution of molecular energies for a gaseous system at two temperatures.

      (a) The energy plotted on the horizontal axis is mainly
(1)
   A      activation.
   B      kinetic.
   C      rotation.
   D      vibration.

      (b) The rate of a chemical reaction increases with temperature mainly because
(1)
   A      the activation energy increases.
   B      the activation energy decreases.
   C      more collisions occur with energy greater than the activation energy.
   D      the molecules collide more frequently.

      (c) The total area under each curve
(1)
   A      decreases with increasing temperature.
   B      increases with increasing temperature.
   C      increases or decreases with increasing temperature, depending on the size of the molecules.
   D      does not change with temperature.
(Total for question = 3 marks)

Unit 2 - Jun 2013
Q13.
(a)  Sea water is a source of chemicals. The most abundant chemical dissolved in sea water is sodium chloride. Compounds of magnesium and bromine are also present. Magnesium occurs at 1300 parts per million (ppm) and bromine at 60 ppm by mass.

The solution left after crystallizing sodium chloride from sea water is even richer in bromine, and contains around 2.2 g dm−3 of bromine.

Bromine is extracted from this solution by passing in chlorine gas. The mixture is acidified to prevent hydrolysis of bromine by the reaction
Br2(aq) + H2O(l)  2H+(aq) + Br−(aq) + BrO−(aq)

The bromine can be separated by heating the solution to collect bromine vapour which is then condensed, or by blowing air through the solution.

(i)  Show by calculation that a solution containing 2.2 g dm−3 of bromine is richer in bromine than one containing 60 ppm.

[Assume that the mass of 1 dm3 of the bromine solution is 1000 g]
(1)

(ii)  Write an ionic equation, including state symbols, for the reaction in which chlorine gas reacts with bromide ions in solution to produce bromine.
(2)

(iii)  What would be observed when the reaction in (ii) occurs?
(1)
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(iv)  Explain why the addition of an acid, such as hydrochloric acid, prevents hydrolysis of bromine.
(2)
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(v)  Assuming the hydrolysis of bromine is endothermic, explain how an increase in temperature would affect the equilibrium position for the hydrolysis of bromine.
(2)
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(vi)  Use your knowledge of activation energy to explain why an increase in temperature increases the rate of hydrolysis of bromine.
(1)
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(vii) Use the equation for the hydrolysis of bromine to show that it is a disproportionation reaction.
Br2(aq) + H2O(l)  2H+(aq) + Br−(aq) + BrO−(aq)
(2)
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(b) At the surface of the sea, there is a dynamic equilibrium between carbon dioxide gas in air and dissolved carbon dioxide in the surface sea water.
CO2(g) CO2(aq)

(i)  State two features of a system which has reached dynamic equilibrium.
(2)
1.  .................................................................................................................................................... 
    ....................................................................................................................................................
2.  .................................................................................................................................................... 
    ....................................................................................................................................................

*(ii)  Carbon dioxide dissolves more easily in seawater than in pure water because seawater contains carbonate ions, CO32−(aq), and the following reaction occurs.
CO2(aq) + H2O(l) + CO32−(aq)  2HCO3 −(aq)

Explain how an increase in concentration of carbonate ions in sea water affects the amount of carbon dioxide gas in the atmosphere.
(2)
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(iii)  Carbon dioxide and water vapour both contain polar bonds.
What effect does infrared radiation have on the bonds in these molecules?
(1)
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*(iv)  Outline the mechanism by which molecules such as carbon dioxide and water cause global warming.
(2)
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*(v) Without water vapour in the atmosphere, the earth would be many degrees colder than it is at present. Why are many climate change scientists more concerned about warming due to carbon dioxide in the atmosphere, than warming due to the presence of water vapour? Refer to the difference between anthropogenic climate change and natural climate change in your answer.
(4)
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(Total for Question = 22 marks)

Q14.
In a chemical reaction, which of the following factors increases the proportion of particles that have sufficient energy to react?
   A    A decrease in concentration
   B    An increase in concentration
   C    A decrease in temperature
   D    An increase in temperature
(Total for Question = 1 mark)

Q15.
In the industrial process involving gas phase reactions to produce ammonia, many collisions between molecules are unsuccessful because

   A    gas phase reactions are reversible.
   B    the collisions are not energetic enough to break the bonds in the molecules.
   C    gas phase reactions can only occur when a catalyst is present.
   D    gas phase reactions can only occur when UV light is present.
(Total for Question = 1 mark)

Q16.

The energy marked X in the Maxwell-Boltzmann distribution shows
   A    the most common energy of the molecules.
   B    the activation energy of the reaction.
   C    the activation energy of a catalysed reaction.
   D    the number of molecules with energy greater than the activation energy.

(Total for Question = 1 mark)

Q17.
Nanorockets have generated a lot of excitement due to their potential uses in the medicinal field, such as in the delivery of drugs around the body.
Some bacteria have the ability to move at speeds of 100 times their body length per second. Scientists in one study made nanorockets that reached speeds of up to 200 times their length per second. These scientists made their rockets on a nano scale (10−9) from nanotubes with platinum coated on the inside as a catalyst. The fuel used to power these tiny rockets was hydrogen peroxide, H2O2, which forms water and oxygen gas when undergoing decomposition.
Other forms of nanotechnology are already being used. For example, some sun creams use nanoparticles of titanium(IV) oxide which form an invisible protective layer against UV radiation.
(a)  Write an equation for the catalytic decomposition of hydrogen peroxide. State symbols are not required.
(1)

(b)  Draw a dot and cross diagram to show the electronic configuration of the oxygen gas produced in the breakdown of the hydrogen peroxide (only outer electrons should be shown).
(1)

(c)  Suggest a dot and cross diagram for the hydrogen peroxide molecule in which each oxygen atom is covalently bonded to one hydrogen atom (only outer electrons should be shown).
(2)

*(d)  The bond angles in hydrogen peroxide are similar to those in a water molecule. Suggest a bond angle for hydrogen peroxide and reasons for your value.
(3)
Bond Angle
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Reasons
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(e)  In the future, the aim is to develop a nanorocket that can use a fuel such as glucose rather than hydrogen peroxide. Suggest an advantage of using glucose and a disadvantage of using hydrogen peroxide.																									(2)
Glucose advantage
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Hydrogen peroxide disadvantage
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(f)  The boiling temperature of hydrogen peroxide is relatively high, about 150°C, for such a small molecule. Explain fully why this is the case.
(2)
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*(g)  It is suggested that hydrogen peroxide is a polar liquid.
Describe how you might carry out an experiment to test whether a liquid is polar.
(3)
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*(h)  The speed of the nanorockets is controlled by the rate of decomposition of hydrogen peroxide. The speed at the body temperature of 37 °C is faster than under normal laboratory conditions. Draw Maxwell-Boltzmann distribution curves on the axes below. Label your diagram and use it to explain why the increase in the speed of the rockets occurred.
(4)

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(i)  The scientists used platinum in their nanorockets. Explain the catalytic role of the platinum in the reaction.
(2)
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(j)  Nanotubes can be made from carbon. These carbon nanotubes can be good electrical conductors in a similar way to graphite.
Explain why they are able to conduct electricity.
(2)
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(k)  Some scientists are concerned that the use of nanoparticles in cosmetic products, such as sun cream, could pose a health hazard. Suggest why this might be the case.
(1)
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(Total for Question = 23 marks)

Unit 2 - Jun 2014
Q18.
Which of the arrows, A, B, C, D, indicates the activation energy for a catalysed reaction on the reaction profile shown?

   A    
   B    
   C    
   D    
(Total for question = 1 mark)

Q19.
Which of the following will not affect the rate of the reaction below?
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
   A    Surface area
   B    Concentration
   C    Pressure
   D    Temperature
(Total for question = 1 mark)

Q20.
This is a question about Group 2 elements and their compounds.
*(a) Explain why the first ionization energy of calcium (590 kJ mol−1) is greater than that of strontium (550 kJ mol−1). 
(2)
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(b)  (i)  Describe how you would carry out a flame test on a sample of a Group 2 metal salt.
(2)
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(ii)  What result of the flame test would confirm the presence of a barium salt?
(1)
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*(iii)  Explain, in terms of electronic transitions, how the result of the flame test arises.
(3)
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(c)  Barium reacts with water to form a clear, colourless solution.

(i)  Give the name or formula of the barium compound formed.
(1)
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(ii)  State another observation that would be made when barium reacts with water.
(1)
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(d)  Barium reacts with chlorine gas to form barium chloride as shown in the equation below.
Ba(s) + Cl2(g) → BaCl2(s)
(i)  Use the changes in oxidation numbers to show that this is a redox reaction.
(2)
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(ii)  Write the ionic equation for the reaction between barium chloride solution and dilute sulfuric acid. Include state symbols in the equation.
(2)

(iii)  The reaction in (d)(ii) is used to test for sulfate ions.
Why is dilute hydrochloric acid added with the barium chloride solution in this test?
(1)
 ............................................................................................................................................. 

(e)  Magnesium carbonate, MgCO3, readily reacts with hydrochloric acid.
(i)  Write the equation for this reaction. State symbols are not required.
(1)

*(ii)  The rate of the reaction between powdered magnesium carbonate and dilute hydrochloric acid was monitored using the experimental apparatus shown below.

State two factors that would decrease the rate of this reaction, other than by changing the reaction temperature.
Explain how these two factors decrease the reaction rate.
(4)
Factor 1
 ............................................................................................................................................. 

Explanation 1
 ............................................................................................................................................. 
 ............................................................................................................................................. 

Factor 2
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Explanation 2
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(f)  Suggest why pressure has little or no effect on the rate of the reaction of magnesium oxide and hydrochloric acid, the equation for which is given below.
MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 21 marks)

Q21.
The diagram below shows the Maxwell-Boltzmann distribution of molecular energies for a catalysed reaction.
?
(a)  If the temperature were lowered, what would be the effect on the shape of the curve?
(1)
   A    The peak would shift to the left and be higher.
   B    The peak would shift to the left and be lower.
   C    The peak would shift to the right and be higher.
   D    The peak would shift to the right and be lower.

(b)  Which of the following would shift the activation energy line to the right?
(1)
   A    An increase in reactant concentration.
   B    The removal of the product.
   C    The removal of the catalyst.
   D    The use of smaller particles with a larger surface area.
(Total for question = 2 marks)

Q22.
When steam is passed over heated magnesium, which of the following occurs?
   A    Mg + H2O → MgO + H2
   B    Mg + H2O → MgOH + ½H2
   C    Mg + 2H2O → Mg(OH)2 + H2
   D    There is no reaction with the magnesium.
(Total for question = 1 mark)

Q23.
The reaction profile for an endothermic reaction is shown below.

(a)  Which arrow represents the activation energy for the forward reaction?
(1)
   A    
   B    
   C    
   D    

(b)  Which arrow represents the activation energy for the backward reaction?
(1)
   A    
   B    
   C    
   D    
(Total for question = 2 marks)