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

At the negative electrode, zinc ions gain electrons through a reduction process. The half-equation representing this reaction can be expressed as:

The half equation at the positive electrode is:

The presence of magnesium as a gas in the product side indicates that the reaction occurs at a high temperature, as magnesium typically exists as a solid at room temperature.

An endothermic reaction is one that absorbs energy from the surroundings. This typically occurs in reactions where more energy is needed to break bonds than is released during bond formation.

To find the mass of magnesium oxide (MgO) needed:

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

2. The molar mass of Mg is 24 g/mol, so for 1.2 tonnes (1200 kg or 1200000 g):

   $ext{Moles of Mg} = \frac{1200000 ext{ g}}{24 ext{ g/mol}} = 50000 ext{ moles}$

Since the molar ratio of Mg to MgO is 1:1, moles of MgO required = 50000 moles.

3. The molar mass of MgO is (24 + 16) = 40 g/mol.

4. Therefore, the mass of magnesium oxide needed is:

   $\text{Mass of MgO} = 50000 ext{ moles} \times 40 ext{ g/mol} = 2000000 ext{ g} ext{ (or 2 tonnes)}$

To calculate the percentage yield, use 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:

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

One reason could be that the reaction is reversible, which means not all reactants convert to products. Additionally, some magnesium may be lost during separation, or there may be impurities in reactants.