OCR GCSE Combined Science: Chemistry: Chemical Reactions

A student adds an excess of solid magnesium to dilute hydrochloric acid in an open conical flask standing on a balance. The reaction produces hydrogen gas and a salt. As the reaction proceeds, the reading on the balance decreases.

$\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$Mg(s)+2HCl(aq)→MgCl2​(aq)+H2​(g)

Explain why the mass recorded on the balance decreases during this reaction, and explain how this observation is still consistent with the law of conservation of mass. In your answer refer to the idea of a closed versus an open system.

(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 4.8 g of magnesium completely.

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

(a) Calculate the number of moles of magnesium in 4.8 g. (2 marks)

(b) Using the balanced equation, calculate the mass of magnesium oxide produced. Show your working. (2 marks)

A student neutralises dilute sulfuric acid with copper(II) oxide (an insoluble base) to make the soluble salt copper(II) sulfate. Water is also formed.

(a) Complete and balance the symbol equation for this reaction:

$\text{CuO} + \text{H}_2\text{SO}_4 \rightarrow \text{………} + \text{………}$CuO+H2​SO4​→………+………

(2 marks)

(b) State how the student could tell, during the reaction, that all the acid had been used up. (1 mark)

Molten lead(II) bromide, $\text{PbBr}_2$PbBr2​, can be broken down by electrolysis into its elements. During the process, lead forms at one electrode and bromine forms at the other.

(a) State at which electrode (positive or negative) the lead is formed, and explain your choice. (2 marks)

(b) State the name given to the reaction that happens at the negative electrode during electrolysis (gain or loss of electrons). (1 mark)

The reaction profile below represents a chemical reaction.

• The energy of the reactants is higher than the energy of the products.

(a) State whether this reaction is exothermic or endothermic. (1 mark)

(b) Explain your answer to part (a) in terms of the energy of the reactants and products. (1 mark)

A student measures the pH of a solution and finds it to be pH 2.

State whether this solution is acidic, neutral or alkaline. (1 mark)