This question is about displacement reactions - AQA - GCSE Chemistry - Question 6 - 2020 - Paper 1

To determine the limiting reactant, we first calculate the moles of each reactant.

1. Calculate moles of iron oxide (Fe2O3):

   Using the relative atomic mass, the molar mass of Fe2O3 is:

   $ext{M(Fe2O3) = 2(56) + 3(16) = 160 ext{ g/mol}}$

   The moles of Fe2O3 in 3.00 kg is:

   $ext{Moles of Fe2O3 =} \frac{3000 ext{ g}}{160 ext{ g/mol}} = 18.75 ext{ mol}$

2. Calculate moles of aluminium (Al):

   The moles of Al in 1.00 kg is:

   $ext{Moles of Al =} \frac{1000 ext{ g}}{27 ext{ g/mol}} = 37.037 ext{ mol}$

3. Determine the moles required for the reaction:

   The balanced equation shows that 2 moles of Al are needed for every mole of Fe2O3:

   • Moles of Al needed = 2 x 18.75 = 37.5 mol

4. Compare moles available and required:

   Since we have 37.037 mol of Al and need 37.5 mol, aluminium is indeed the limiting reactant.

The ionic equation for the reaction between magnesium and zinc sulfate is:

$\text{Mg(s) + Zn}^{2+}(aq) \rightarrow \text{Mg}^{2+}(aq) + Zn(s)$

In this reaction, magnesium is oxidized as it loses two electrons, going from a neutral atom to a magnesium ion (Mg²⁺). This loss of electrons constitutes oxidation. Meanwhile, the zinc ions (Zn²⁺) gain those electrons to become neutral zinc (Zn), which is the reduction process. Therefore, the reaction is a redox (reduction-oxidation) reaction, where oxidation and reduction occur simultaneously.