OCR GCSE Chemistry: Chemical Reactions

Copper(II) sulfate is a soluble salt. A student is given copper(II) oxide (a black, insoluble base) and dilute sulfuric acid. The equation for the reaction is:

$\text{CuO}_{(s)} + \text{H}_2\text{SO}_{4(aq)} \rightarrow \text{CuSO}_{4(aq)} + \text{H}_2\text{O}_{(l)}$CuO(s)​+H2​SO4(aq)​→CuSO4(aq)​+H2​O(l)​

Describe a method the student could use to prepare pure, dry crystals of copper(II) sulfate from these two substances. (6 marks)

Magnesium burns in oxygen to form magnesium oxide:

$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$2Mg+O2​→2MgO

A student burns 12 g of magnesium in excess oxygen.

(Relative atomic masses: Mg = 24; O = 16.)

(a) Calculate the number of moles of magnesium in 12 g. (2 marks)

(b) Calculate the maximum mass of magnesium oxide that could be formed. Show your working. (2 marks)

Hydrogen reacts with chlorine to form hydrogen chloride:

$\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$H2​+Cl2​→2HCl

Use the bond energies in the table to calculate the overall energy change for this reaction, and state whether it is exothermic or endothermic.

Bond	Bond energy / kJ per mol
H–H	436
Cl–Cl	242
H–Cl	431

(3 marks)

A student heats a small piece of magnesium ribbon in an open crucible. After heating, the white powder left behind has a greater mass than the magnesium at the start.

Explain why the mass increases, and explain why this does not break the law of conservation of mass. (3 marks)

When magnesium burns in oxygen, the reaction can be written as:

$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$2Mg+O2​→2MgO

This is a redox reaction, which can be described in terms of oxygen.

(a) State what is meant by oxidation in terms of oxygen. (1 mark)

(b) State which substance, magnesium or oxygen, has been oxidised in this reaction, and justify your choice. (1 mark)

The pH scale is used to measure how acidic or alkaline a solution is.

Name the ion that is present in all acidic solutions and is responsible for their acidity. (1 mark)