This question is about zinc and magnesium - AQA - GCSE Chemistry - Question 4 - 2014 - Paper 1

At the negative electrode (cathode), zinc ions gain electrons in a reduction reaction. The half equation for this process can be expressed as:

$\text{Zn}^{2+} + 2 e^{-} \rightarrow \text{Zn}$

This results in the formation of solid zinc at the negative electrode.

The half equation at the positive electrode (anode) is:

$2 \text{Cl}^{-} \rightarrow \text{Cl}_2 + 2 e^{-}$

The presence of magnesium in a gaseous state (Mg(g)) in the equation suggests that the reaction occurs at high temperatures. This indicates that magnesium has enough energy to overcome its melting point and exists in the gas phase.

An endothermic reaction is one that absorbs energy from the surroundings, typically in the form of heat. This energy is required to break bonds, leading to an overall intake of energy during the reaction.

To find the mass of magnesium oxide (MgO) needed, we can use the equation:

1. The molar mass of magnesium (Mg) is 24 g/mol and that of magnesium oxide (MgO) is 40 g/mol (24 + 16).

2. From the equation, 2 moles of Mg are produced from 2 moles of MgO.

3. Therefore, if 1.2 tonnes (1200 kg) of magnesium is produced:

   • Convert 1.2 tonnes to grams: 1200 kg = 1,200,000 g

   • Moles of magnesium produced = ( \frac{1,200,000 \text{ g}}{24 \text{ g/mol}} = 50,000 \text{ moles} )

   • Since 2 moles of MgO yield 2 moles of Mg, the moles of MgO required are also 50,000 moles.

   • Now, calculate the mass of MgO:

   $\text{Mass of MgO} = 50,000 \text{ moles} \times 40 \text{ g/mol} = 2,000,000 \text{ g} = 2.0 \text{ tonnes}$

   Thus, 2.0 tonnes of magnesium oxide is needed to produce 1.2 tonnes of magnesium.

The percentage yield can be calculated using the formula:

$\text{Percentage yield} = \left( \frac{\text{actual yield}}{\text{theoretical yield}} \right) \times 100$

Here, the actual yield is 0.9 tonnes and the theoretical yield is 1.2 tonnes. Substituting the values:

$\text{Percentage yield} = \left( \frac{0.9 \text{ tonnes}}{1.2 \text{ tonnes}} \right) \times 100 = 75\%$

One reason for not obtaining the calculated yield of magnesium could be that some of the reactants were lost or escaped during the reaction process. Other factors could include incomplete reactions or presence of impurities.