OCR A-Level Chemistry: Periodicity, Group 2 & Halogens

The first ionisation energies of the Period 3 elements do not increase smoothly from sodium to argon. Selected values are shown below.

Element	Na	Mg	Al	Si	P	S	Cl	Ar
First ionisation energy / kJ mol⁻¹	496	738	578	789	1012	1000	1251	1521

Describe and explain the trend in first ionisation energy across Period 3. Your answer should account for the general increase from sodium to argon, and explain why aluminium is lower than magnesium and why sulfur is lower than phosphorus.

(6 marks)

A student investigated the reactivity of the Group 2 metals by reacting a sample of a metal M with an excess of dilute hydrochloric acid. The metal reacted completely according to the equation:

$\text{M}(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{MCl}_2(\text{aq}) + \text{H}_2(\text{g})$M(s)+2HCl(aq)→MCl2​(aq)+H2​(g)

The following data were recorded.

Quantity	Value
Mass of metal M	0.486 g
Volume of hydrogen gas collected	480 cm³
Molar gas volume at the conditions used	24.0 dm³ mol⁻¹

(a) Calculate the amount, in moles, of hydrogen gas produced. (1 mark)

(b) Use your answer to (a) and the equation to calculate the relative atomic mass of M, and hence identify the metal. (4 marks)

(c) State one reason why the volume of hydrogen actually collected might be slightly smaller than the true value. (1 mark)

Four Group 2 carbonates were each heated strongly in turn. The temperature at which each one just began to decompose to its oxide and carbon dioxide was recorded. The carbonates are labelled W, X, Y and Z (not in order of the group).

Carbonate	W	X	Y	Z
Decomposition temperature / °C	540	900	1290	350

The general decomposition can be written as:

$\text{MCO}_3(\text{s}) \rightarrow \text{MO}(\text{s}) + \text{CO}_2(\text{g})$MCO3​(s)→MO(s)+CO2​(g)

The four metals involved are magnesium, calcium, strontium and barium.

(a) Place the four carbonates W, X, Y and Z in order of increasing thermal stability and assign each to its metal. (2 marks)

(b) Explain, in terms of the metal ions, why thermal stability changes in this way down Group 2. (3 marks)

A small water-treatment works adds chlorine gas to drinking water to kill bacteria. When chlorine dissolves in the water it undergoes a disproportionation reaction that produces chloric(I) acid, HClO, which is the active disinfectant.

(a) Write a balanced equation for the reaction of chlorine with water, and use oxidation numbers of chlorine to explain why it is described as a disproportionation. (3 marks)

(b) A regulator notes that adding chlorine to water is, on balance, beneficial despite a hazard. Suggest one benefit and one hazard of chlorinating drinking water in this way. (2 marks)

A technician has lost the labels from three colourless aqueous solutions, A, B and C, which are known to be (in some order) potassium chloride, potassium bromide and potassium iodide. To identify them she adds a few drops of bromine water to a fresh portion of each and shakes. Her results are:

Solution	Observation on adding bromine water
A	No reaction; solution stays orange
B	Solution turns a darker brown colour
C	No reaction; solution stays orange

(a) Use the results to identify solution B, and write the ionic equation for the reaction that occurs in test B. (3 marks)

(b) Explain why this single test cannot distinguish between solutions A and C. (1 mark)

This question is about the chemistry of Group 2 and the halogens.

(a) Chlorine reacts with cold, dilute sodium hydroxide solution. Write the balanced equation for this reaction. (1 mark)

(b) State the trend in reactivity of the Group 2 elements down the group (from magnesium to barium). (1 mark)

(c) A few drops of dilute nitric acid and then silver nitrate solution are added to a solution of sodium bromide. State the observation (including the colour of any precipitate). (1 mark)