AS Chemistry
Unit 1 Practice Exam Questions – Test 4

Name………………………………………………………..

1.	A student had a stomach-ache and needed to take something to neutralise excess stomach acid. He decided to take some Milk of Magnesia, which is an aqueous suspension of magnesium hydroxide, Mg(OH)2.
(a)	The main acid in the stomach is hydrochloric acid, HCl(aq), and the unbalanced equation for the reaction that takes place with Milk of Magnesia is shown below.
.....Mg(OH)2(s) + .....HCl(aq) ® .....MgCl2(aq) + .....H2O(l)
	Balance the equation by adding numbers where necessary in the unbalanced equation above.
[1]
 
(b)	The student’s stomach contained 500 cm3 of stomach fluid with an acid concentration of 0.108 mol dm–3. The student swallowed some Milk of Magnesia containing 2.42 g Mg(OH)2. He wondered whether this dose was sufficient to neutralise the stomach acid.
	Assume that all the acid in the stomach fluid was 0.108 mol dm–3 hydrochloric acid.
(i)	How many moles of HCl were in the 500 cm3 of stomach fluid?
[1]
 
(ii)	Calculate the mass of Mg(OH)2 necessary to neutralise this stomach fluid.
[3]
(iii)	Determine whether the student swallowed too much, too little, or just the right amount of Milk of Magnesia to neutralise the stomach acid.
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[1]
[Total 6 marks]

2.	Chewing chalk has been used for many years to combat excess stomach acid and indigestion tablets often contain calcium carbonate, CaCO3. Suggest, with the aid of an equation, how these tablets work.
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[Total 2 marks]

3.	Chlorine is used in the preparation of many commercially important materials such as bleach and iodine.
Bleach is a solution of sodium chlorate(l), NaOCl, made by dissolving chlorine in aqueous sodium hydroxide.
Cl2(g) + 2NaOH(aq) ® NaOCl(aq) + NaCl(aq) + H2O(l)
Determine the changes in oxidation number of chlorine during the preparation of bleach and comment on your results.
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[Total 3 marks]

4.	Iodine is extracted commercially from seawater with chlorine gas. Seawater contains very small quantities of dissolved iodide ions, which are oxidised to iodine by the chlorine gas.
(i)	Write an ionic equation for the reaction that has taken place.
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[2]

(ii)	Use your understanding of electronic structure to explain why chlorine is a stronger oxidising agent than iodine.
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[2]
[Total 4 marks]

5.	In this question, one mark is available for the quality of use and organisation of scientific terms.
Nitrogen and oxygen are elements in Period 2 of the Periodic Table. The hydrogen compounds of oxygen and nitrogen, H2O and NH3, both form hydrogen bonds.
(i)	Draw a diagram containing two H2O molecules to show what is meant by hydrogen bonding. On your diagram, show any lone pairs present and relevant dipoles.
[3]
 
(ii)	State and explain two anomalous properties of water resulting from hydrogen bonding.
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[4]
[Total 7 marks]

6.	The ‘dot-and-cross’ diagram of an ammonia molecule is shown below.
	
Predict, with reasons, the bond angle in an ammonia molecule.
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[Total 4 marks]

7.	The atomic radii of nitrogen and oxygen are shown below.
element	nitrogen	oxygen	
atomic radius/nm	0.075	0.073	
Explain why a nitrogen atom is larger than an oxygen atom.
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[Total 4 marks]

8.	Ammonia reacts with sulphuric acid, as shown in the equation below.
2NH3(g) + H2SO4(aq) ® (NH4)2SO4(aq)
(i)	Complete the statement below to describe how ammonia is behaving in this reaction.
Ammonia is behaving as a ..................... because
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[2]

(ii)	State one important use for the compound (NH4)2SO4.
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[1]
(iii)	Apart from the manufacture of (NH4)2SO4, state one other large-scale use of ammonia.
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[1]
[Total 4 marks]

9.	The Group 7 element bromine was discovered in 1826. Bromine gets its name from the Greek brōmos meaning stench because of its strong smell.
	Bromine consists of a mixture of two isotopes, 79Br and 81Br.
(i)	What is the difference between the atomic structures of 79Br and 81Br?
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[2]
 
(ii)	State two similarities between the atomic structures of 79Br and 81Br.
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[2]
[Total 4 marks]

10.	The electronic configuration of a bromine atom can be written in terms of sub-shells.
(i)	Complete the electronic configuration of a bromine atom.
1s22s22p63s23p6 ................................................................................................
[2]

(ii)	Why is bromine classified as a p-block element?
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[1]
[Total 3 marks]

11.	Bromine forms three compounds with phosphorus. The compounds have the molecular formulae PBr3, PBr5 and P2Br4.
(i)	Explain what is meant by the term molecular formula.
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[1]
 
(ii)	PBr3 can be prepared by heating bromine with phosphorus, P4.
Write a balanced equation for this reaction.
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[1]
 
(iii)	Compound A is one of the three bromides of phosphorus above. It has the following percentage composition by mass: P, 16.2%; Br, 83.8%.
	Use this percentage composition to calculate the empirical formula and to determine the identity of compound A.
empirical formula ......................................
identity of compound A ......................................
[3]
[Total 5 marks]

12.	Water and carbon dioxide both consist of covalent molecules.
State what is meant by a covalent bond.
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[Total 2 marks]

13.	Draw ‘dot-and-cross’ diagrams for a molecule of water and a molecule of carbon dioxide. Show outer electron shells only.

		
water	carbon dioxide	

[Total 3 marks]

14.	The shape of a water molecule is different from the shape of a carbon dioxide molecule.
(i)	Draw the shapes of these molecules and state the bond angles.
water

	carbon dioxide	
bond angle in water = ..............	bond angle in carbon dioxide = ..............	
[4]
 
(ii)	Explain why a water molecule has a different shape from a carbon dioxide molecule.
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[2]
[Total 6 marks]

15.	An understanding of electronegativity helps to explain why some covalent bonds are polar.
(i)	Define the term electronegativity.
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[2]
 
(ii)	Water and carbon dioxide both have polar bonds. Explain why water has polar molecules but carbon dioxide has non-polar molecules.
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[2]
[Total 4 marks]

16.	The first ionisation energies of the elements H to K are shown below. Use this diagram to help with your answers to this question.

(a)	Define the term first ionisation energy.
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[3]

(b)	Explain why the first ionisation energies show a general increase across Period 2 (Li to Ne).
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[2]
[Total 5 marks]

17.	State and explain the trend in first ionisation energies shown by the elements with the atomic numbers 2, 10 and 18.
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[Total 4 marks]