This question is about silicon and compounds of silicon - AQA - GCSE Chemistry - Question 7 - 2022 - Paper 1

Aluminium is more expensive than carbon primarily because more energy is required to extract aluminium from its ores through electrolysis, whereas carbon is obtained more cheaply through heating coal.

Both products, silicon and magnesium oxide, are solid, which makes them difficult to separate by conventional methods.

First, we calculate the molar mass of silicon dioxide (SiO$_2$): $M_r(SiO_2) = 28 + 2 \times 16 = 60 \ g/mol$

Next, we find the moles of SiO$_2$ in 1.2 kg: $\frac{1200 \ g}{60 \ g/mol} = 20 \ moles \ of \ SiO_2$

According to the reaction: $2Mg + SiO_2 \rightarrow Si + 2MgO$

2 moles of magnesium are required for every mole of silicon dioxide, thus: $20 \ moles \ of \ SiO_2 \times 2 = 40 \ moles \ of \ Mg$

Calculating the mass of magnesium needed: $M_r(Mg) = 24 \ g/mol$ $40 \ moles \ of \ Mg \times 24 \ g/mol = 960 \ g$

In SiH$_4$, silicon shares its four outer shell electrons with four hydrogen atoms. Each hydrogen atom contributes one electron, resulting in four shared pairs that create a stable octet for silicon.

For 30 cm$^3$ of SiH$_4$:

determine the volume of oxygen needed: $30 \ cm^3 \ SiH_4 \times 7 \text{ (from the equation)} = 105 \ cm^3 \ O_2$

Since we have 150 cm$^3$ of $O_2$, the reaction leaves a surplus of: $150 \ cm^3 - 105 \ cm^3 = 45 \ cm^3 \ (excess)$

Water vapour formed: $3 \times 30 \ cm^3 = 90 \ cm^3$

Total volume of gases present: $45 \ cm^3 (excess \ O_2) + 90 \ cm^3 (H_2O) = 135 \ cm^3$