Edexcel A-Level Chemistry: Inorganic Chemistry

The Group 2 carbonates all decompose on heating to give the metal oxide and carbon dioxide:

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

A student finds that magnesium carbonate decomposes at a much lower temperature than barium carbonate.

Describe and explain the trend in the thermal stability of the Group 2 carbonates as the group is descended, from MgCO₃ to BaCO₃. In your answer you should refer to the size and charge density of the metal cation and to its effect on the carbonate ion.

(6 marks)

A technician measured the temperature at which four Group 2 carbonates began to decompose, recording the results below.

Group 2 carbonate	Cationic radius / pm	Decomposition temperature / °C
MgCO₃	72	350
CaCO₃	100	840
SrCO₃	118	1100
BaCO₃	135	1360

(a) State and explain the relationship shown between cationic radius and decomposition temperature. (2 marks)

(b) A 5.00 g sample of impure strontium carbonate was heated strongly until its mass stopped changing. The only reaction was:

$\text{SrCO}_3(s) \rightarrow \text{SrO}(s) + \text{CO}_2(g)$SrCO3​(s)→SrO(s)+CO2​(g)

The mass of the residue was 3.90 g, so 1.10 g of carbon dioxide was lost. Any impurity present is inert and does not decompose. Calculate the percentage by mass of SrCO₃ in the original sample.

(Molar masses / g mol⁻¹: SrCO₃ = 147.6, CO₂ = 44.0) (4 marks)

A colourless, water-soluble solid Q contains one cation and one anion. A student carried out three tests and recorded the observations below.

Test	Observation
1 A small sample is held on a nichrome wire in a roaring Bunsen flame	The flame turns brick-red
2 Dilute nitric acid is added to a solution of Q, then silver nitrate solution	A cream precipitate forms
3 The precipitate from test 2 is treated first with dilute, then with concentrated, ammonia solution	It is insoluble in dilute ammonia but dissolves in concentrated ammonia

(a) Identify the cation and the anion in Q, and hence give the formula of Q. (3 marks)

(b) Write the ionic equation for the formation of the precipitate in test 2, and explain why dilute nitric acid is added before the silver nitrate. (2 marks)

A manufacturer needs to produce a steady stream of pure hydrogen iodide gas, HI, by dripping a concentrated acid onto solid sodium iodide. A junior chemist suggests using concentrated sulfuric acid, reasoning that it works well for making HCl from sodium chloride. A senior chemist rejects this and specifies concentrated phosphoric acid, H₃PO₄, instead.

(a) Explain why concentrated sulfuric acid is unsuitable for preparing pure HI, with reference to the reducing power of the iodide ion. Include one balanced equation for a reaction that contaminates the product. (3 marks)

(b) Explain why concentrated phosphoric acid is a suitable alternative, and write a balanced equation for the reaction that produces HI from sodium iodide. (2 marks)

The reaction between peroxodisulfate ions and iodide ions in aqueous solution is very slow:

$\text{S}_2\text{O}_8^{2-}(aq) + 2\text{I}^-(aq) \rightarrow 2\text{SO}_4^{2-}(aq) + \text{I}_2(aq)$S2​O82−​(aq)+2I−(aq)→2SO42−​(aq)+I2​(aq)

Adding a small amount of cobalt(II) ions, Co²⁺, dramatically speeds it up. The cobalt is a homogeneous catalyst that cycles between the +2 and +3 oxidation states. The relevant standard electrode potentials are:

Half-reaction	E° / V
$\text{S}_2\text{O}_8^{2-} + 2\text{e}^- \rightleftharpoons 2\text{SO}_4^{2-}$S2​O82−​+2e−⇌2SO42−​	+2.01
$\text{Co}^{3+} + \text{e}^- \rightleftharpoons \text{Co}^{2+}$Co3++e−⇌Co2+	+1.82
$\text{I}_2 + 2\text{e}^- \rightleftharpoons 2\text{I}^-$I2​+2e−⇌2I−	+0.54

(a) Suggest why the uncatalysed reaction is so slow, in terms of the charges on the two reacting ions. (1 mark)

(b) Write the two equations for the steps in the catalysed mechanism, and explain, using the E° data, why both steps are feasible. (3 marks)

A student is given a solution that may contain sulfate ions, SO₄²⁻, and is asked to confirm their presence using a simple test-tube reaction.

(a) State the reagents added to the solution to test for sulfate ions, and the observation that confirms a positive result. (2 marks)

(b) Write the balanced ionic equation, with state symbols, for the reaction that gives the positive result. (1 mark)