Chemistry past paper questions (January 2009)

(1) Carbon occurs in a wide range of compounds and is essential to living systems.
(a) Two isotopes of carbon are 12C and 13C.
(i) State what is meant by the term isotopes.
1 mark

(ii) Isotopes of carbon have the same chemical properties.
Explain why.
1 mark

(iii) The 12C isotope is used as the standard measurement of relative masses.
Define the term relative isotopic mass.
2 marks

(b) One form of naturally occurring carbon is graphite. The table below lists some properties of graphite.

Electrical conductivity: good conductor
Hardness: soft
Melting point: very high

• Describe the bonding and structure in graphite.
• Explain, in terms of bonding and structure, the properties of graphite shown above.
5 marks

(c) In the sixteenth century, a large deposit of graphite was discovered in the Lake District. People at the time thought that the graphite was a form of lead.
Nowadays, graphite is used in pencils but it is still referred to as ‘pencil lead’.
A student decided to investigate the number of carbon atoms in a ‘pencil lead’. He found that the mass of the ‘pencil lead’ was 0.321 g.
(i) Calculate the amount, in mol, of carbon atoms in the student’s pencil lead.
Assume that the ‘pencil lead’ is pure graphite.
1 mark

(ii) Using the Avogadro constant, NA, calculate the number of carbon atoms in the student’s
‘pencil lead
1 mark

(2) Chemists have developed models for bonding and structure which are used to explain different properties.
(a) Ammonia, NH3, is a covalent compound.
(i) Explain what is meant by a covalent bond.
1 mark

(ii) Draw a ‘dot-and-cross’ diagram to show the bonding in NH3.
Show outer electrons only.
1 mark

(iii) Name the shape of the ammonia molecule.
Explain, using your ‘dot-and-cross’ diagram, why ammonia has this shape and has a
bond angle of 107°.
3 marks

(b) Ammonia reacts with hydrogen chloride, HCl, to form ammonium chloride, NH4Cl.
NH4Cl is an ionic compound containing NH4 + and Cl – ions.
(i) Complete the electron configuration of the Cl – ion.
1s2 .................................
1 mark

(ii) Draw a ‘dot-and-cross’ diagram to show the bonding in NH4
+.
Show outer electrons only.
1 mark

(iii) State the shape of, and bond angle in, an NH4 + ion.
2 marks

(iv) A student investigated the conductivity of ammonium chloride.
She noticed that when the ammonium chloride was solid it did not conduct electricity.
However, when ammonium chloride was dissolved in water, the resulting solution did conduct electricity.
Explain these observations.
2 marks

(c) Ammonium compounds such as ammonium sulfate, (NH4 )2SO4, can be used as fertilisers.
(i) Write a balanced equation to show how ammonium sulfate could be formed by the
reaction between aqueous ammonia and sulfuric acid.
1 mark

(ii) Ammonium sulfate is an example of a salt formed when an acid is neutralised by a base.
Explain what is meant by the term salt.
1 mark

(iii) Why is ammonia acting as a base in this neutralisation?
1 mark

(iv) What is the relative formula mass of (NH4 )2SO4?
Give your answer to one decimal place.
1 mark

(3) A student used the internet to research chlorine and some of its compounds.
(a) He discovered that sea water contains chloride ions. The student added aqueous silver nitrate to a sample of sea water.
(i) What would the student see?
1 mark

(ii) Write an ionic equation, including state symbols, for the reaction that would occur.
2 marks

(iii) After carrying out the test in (i), the student added dilute aqueous ammonia to the
mixture.
What would the student see?
1 mark

(b) The student also discovered that chlorine, Cl 2, is used in the large-scale treatment of water.
(i) State one benefit of adding chlorine to water.
1 mark

(ii) Not everyone agrees that chlorine should be added to drinking water.
Suggest one possible hazard of adding chlorine to drinking water.
1 mark

(c) The equation for the reaction of chlorine with water is shown below.
Cl 2(g) + H2O(l) HCl(aq) + HCl O(aq)
(i) State the oxidation number of chlorine in:
Cl2 ……… HCl ……… HClO ………
1 mark

(ii) The reaction of chlorine with water is a disproportionation reaction.
Use the oxidation numbers in (i) to explain
why.
2 mark

(iii) Chlorine reacts with sodium hydroxide to form bleach in another disproportionation
reaction.
Write an equation for this reaction.
1 mark

(d) Two other chlorine compounds of chlorine are chlorine dioxide and chloric(V) acid.
(i) Chlorine dioxide, ClO2, is used as a bleaching agent in both the paper and the flour industry. When dry, ClO2 decomposes explosively to form oxygen and chlorine.
Construct an equation for the decomposition of ClO2.
1 mark

(ii) Chloric(V) acid has the following percentage composition by mass:
H, 1.20%; Cl, 42.0%; O, 56.8%.
Using this information, calculate the empirical formula of chloric(V) acid.
Show all of your working.
2 marks

(iii) What does (V) represent in chloric(V) acid?
1 mark

(ii) Sulfur exists as S8 molecules and chlorine as Cl 2 molecules. Use this information to explain the difference in their melting points.
2 marks

(b) Explain the decrease in the atomic radii across the period from Na to Cl.
In your answer, you should use appropriate technical terms, spelt correctly.
3 marks

(5) The Group 2 element barium, Ba, is silvery white when pure but blackens when exposed to air.
The blackening is due to the formation of both barium oxide and barium nitride. The nitride ion is N3–.
(a) Predict the formula of:
barium oxide …… barium nitride ……

(b) A 0.11 g sample of pure barium was added to 100 cm3 of water.
Ba(s) + 2H2O(l) Ba(OH)2(aq) + H2(g)
(i) Show that 8.0 × 10−4 mol of Ba were added to the water.
1 mark

(ii) Calculate the volume of hydrogen, in cm3, produced at room temperature and pressure
1 mark

(iii) Calculate the concentration, in mol dm−3, of the Ba(OH)2(aq) solution formed.
1 mark

(iv) State the approximate pH of the Ba(OH)2(aq) solution.
1 mark

(c) A student repeated the experiment in (b) using a 0.11 g sample of barium that had blackened following exposure to the air.
Suggest why the volume of hydrogen produced would be slightly less than the volume collected using pure barium.
1 mark

(d) Describe and explain the trend, down the group, in the reactivity of the Group 2 elements with water.
5 marks

End of Test. Well done!
Now check how you did using the grade boundaries below!
A = 46
B = 40
C = 34
D = 28
E = 23
U = 0
Turn over for a quick recap on bonding.