Unit 1 – Covalent Bonding
Unit 1 - Jan 2009
Q1.
(a)  Define the term covalent bond.
(2)
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(b)  Nitrogen forms an oxide called nitrous oxide, N2O. The bonding in nitrous oxide can be represented as:

Complete the diagram below for the N2O molecule using dots or crosses to represent electrons. Just show all of the outer shell electrons.
(3)

(Total for question = 5 marks)

Unit 1 - Jun 2009
Q2.
Which of the following covalent bonds is the shortest?
   A     H—F
   B     H—Cl
   C     H—Br
   D     H—I
(Total for question = 1 mark)

Unit 1 - Jan 2010
Q3.
The bonding in magnesium oxide, MgO, is
   A      ionic.
   B      metallic and ionic.
   C      ionic and covalent.
   D      metallic and covalent.
(Total for question = 1 mark)

Q4.
 Chloroethane, C2H5Cl, can be made from either ethane or ethene.
(a) (i) What reagent and condition would be used to make chloroethane from ethane?
(2)
Reagent
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Condition
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(ii) State the type of reaction and mechanism by which this reaction occurs.
(2)
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(b) (i) What reagent would be used to make chloroethane from ethene?
(1)
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(ii) Show, in full, the mechanism for this reaction in which ethene is converted to chloroethane.
(3)

(c) Which method of making chloroethane has
(3)
•   a higher atom economy?
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•   a higher percentage yield?
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 Explain your answers.
Higher atom economy
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Higher percentage yield
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(d) The compound chloroethene, CH2=CHCl, forms an addition polymer.
(i) Draw a diagram, using dots or crosses, to show the arrangement of electrons in chloroethene. Only the outer shell electrons need be shown.
(2)

(ii) Chloroethene can form an addition polymer. Write the displayed formula of poly(chloroethene) showing two repeat units.
(1)

*(iii) Poly(chloroethene) is commonly known as PVC. Almost a quarter of the PVC which is manufactured is used to make water pipes, which were formerly made of metal.

                     Give TWO factors which have to be considered when deciding which material, PVC or metal, contributes to more sustainable uses of resources in the long term.
(2)
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(Total for question = 16 marks)

Unit 1 - Jun 2010
Q5.
 This question is about methanol and the energy changes that accompany some of its reactions.
(a) Complete the diagram (using dots and crosses) to show the bonding in methanol, CH3OH. You should show outer electrons only.
(2)

(b) The Hess cycle below can be used to calculate the standard enthalpy change of combustion of methanol, using standard enthalpy changes of formation.

(i) Complete the cycle by filling in the empty box.
(2)
*(ii) Define the term standard enthalpy change of formation of a compound,
       making clear the meaning of standard in this context.
(3)
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(iii) Use your cycle and the data below to calculate the standard enthalpy change of
       combustion of methanol, ΔHc.

(2)

(c) An experiment was carried out, using the apparatus below, to estimate the standard
       enthalpy change of combustion of methanol.

After burning the methanol for a few minutes, the temperature of water in the beaker had risen by 20.7 °C and the mass of methanol burnt was 0.848 g.

 (i) Calculate the amount of energy transferred to the water.
 Energy transferred (J) = mass of water × 4.18 × temperature change
(1)

(ii) Calculate the number of moles of methanol, CH3OH, burnt during the experiment.
(1)

(iii) Use your answers to (c)(i) and (ii) to calculate the experimental value for the standard enthalpy change of combustion. Include a sign and units in your answer, which should be given to three significant figures.
(1)

(iv) Compare your answers to (b)(iii) and (c)(iii) and give TWO reasons to explain
       any differences.
(2)
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(Total for question = 14 marks)

Unit 1 - Jan 2011
Q6.
This question is about the element chlorine (atomic number = 17).
(a) Complete the electronic structure of chlorine.
(1)
1s2 2s2
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(b) Chlorine forms compounds with magnesium and with carbon.
(i) Draw a dot and cross diagram to show the electronic structure of the compound magnesium chloride (only the outer electrons need be shown). Include the charges present.
(2)

(ii) Draw a dot and cross diagram to show the electronic structure of the compound tetrachloromethane (only the outer electrons need be shown).
(2)

*(iii) Suggest why the melting temperature of magnesium oxide is higher than that of magnesium chloride, even though both are almost 100% ionic.
(3)
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(c) Magnesium chloride may be prepared from magnesium by reaction with chlorine or with hydrochloric acid. Compare these two preparations in terms of the atom economies of the reactions. No calculation is required.
(2)
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(Total for question = 10 marks)

Unit 1 - Jun 2011
Q7.
Covalent bonding results from the strong electrostatic attractions between
   A    instantaneous dipoles.
   B    electron clouds.
   C    electrons in the bonding pair.
   D    bonding pairs of electrons and nuclei.
(Total for Question = 1 mark)

Unit 1 - Jun 2012
Q8.

(a) Explain how the atoms are held together by the covalent bond in a molecule of
     hydrogen.
(1)
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(b) Draw the dot and cross diagrams for
     (i) methane, CH4
(1)

     (ii) ethene, CH2CH2
(1)

     (iii) nitrogen, N2
(1)

     (iv) the ammonium ion, NH4+
(1)

(c) Silicon exists in a giant covalent lattice.
     (i)  The electrical conductivity of pure silicon is very low. Explain why this is so in terms of the bonding.
(2)
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     (ii)   Explain the high melting temperature of silicon in terms of the bonding.
(2)
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(Total for question = 9 marks)

Unit 1 - Jun 2013
Q9.
Crude oil is a complex mixture of hydrocarbons. Initial separation is achieved by fractional distillation of the crude oil. The separate fractions are further refined to produce hydrocarbons such as decane, C10H22.

(a)  Give the general formula of alkanes.
(1)

(b)  Carbon monoxide, CO, is formed during the incomplete combustion of decane.

(i)  Write an equation for the incomplete combustion of decane, forming carbon monoxide and water only.
(1)

(ii)  Explain why incomplete combustion can occur.
(1)
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(c)  'Low-sulfur fuel' is now supplied to petrol stations. The removal of sulfur from diesel and petrol reduces the emission of toxic oxides of sulfur from vehicle exhausts. One such oxide is sulfur dioxide, SO2.

The bonding in sulfur dioxide may be represented as shown below.
O = S → O
Complete the dot and cross diagram below for the SO2 molecule, showing only outer shell electrons. Use dots to represent the oxygen electrons and crosses to represent the sulfur electrons.
(3)

(d)  Another alkane produced from crude oil is heptane, C7H16. The reforming of heptane produces methylcyclohexane and only one other product. A methylcyclohexane molecule is made from a ring of six carbon atoms bonded to a methyl group.

(i)  Use the information given above to give the skeletal formula of methylcyclohexane.
(1)

(ii)  Write a balanced equation, using molecular formulae, for the reforming of heptane into methylcyclohexane and one other product. State symbols are not required.
(1)

(iii)  Suggest a reason why oil companies reform alkanes such as heptane.
(1)
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(e)  Five branched-chain isomers of heptane are shown in the boxes below.

(i)  Give the systematic name of isomer A.
(1)
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(ii)  In the empty boxes above, draw skeletal formulae for two other branched-chain isomers of C7H16, with no side-chain having more than one carbon atom.
(2)

(f)  Butane, C4H10, reacts with chlorine, Cl2, at room temperature and pressure.
C4H10 + Cl2 → C4H9Cl + HCl

(i)  What other condition is essential for this reaction?
(1)
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(ii)  Write an equation for the initiation step of the mechanism for the above reaction. Curly arrows are not required.
(1)

(iii)  State the type of bond fission involved in the initiation step.
(1)
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(iv)  Write equations for the two propagation steps of this mechanism. Curly arrows are not required.
(2)
First propagation step:

Second propagation step:

(v)  Write one equation for a reaction that would terminate this mechanism.
(1)
(Total for Question = 18 marks)

Unit 1 - Jun 2014
Q10.
In which of the following electronic configurations are only two of the electrons unpaired?
   A    1s2 2s2
   B    1s2 2s2 2p3
   C    1s2 2s2 2p4
   D    1s2 2s2 2p5
(Total for question = 1 mark)

Q11.
In an experiment to make crystals of hydrated copper(II) nitrate, a sample of 5.60 g of copper(II) oxide was added to 50 cm3 of 2.50 mol dm−3 nitric acid. The following reaction occurred.
CuO(s) + 2HNO3(aq) → Cu(NO3)2(aq) + H2O(l)
(a)  Calculate the number of moles of each reactant present, and use this to show that the copper(II) oxide was in excess.
The molar mass of copper(II) oxide, CuO, is 79.5 g mol−1
(3)
Moles of copper(II) oxide added

Moles of nitric acid used

The copper(II) oxide is in excess because
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(b)  The copper(II) nitrate solution was heated gently to concentrate it, and then left to crystallize. The mass of hydrated copper(II) nitrate crystals, Cu(NO3)2.6H2O, obtained was 12.52 g.
Calculate the percentage yield.
The molar mass of Cu(NO3)2.6H2O is 295.6 g mol−1.
(3)

(c)  Give one reason why the percentage yield is less than 100%, even though the nitric acid was completely reacted.
(1)
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*(d)  (i)  The nitrate ion, NO3−, contains both covalent and dative covalent bonds.
What is the difference between these types of bond?
(2)
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(ii)  Complete the dot and cross diagram to show the bonding in the nitrate ion. Only the outer electron shells for each atom need to be shown.
Represent the nitrogen electrons with crosses (×), and oxygen electrons with dots, (•). The symbol * on the diagram represents the extra electron giving the ion its charge.
(3)

  
(Total for question = 12 marks)