O-Level Patterns in the Periodic Table: worked solution

Question

Certain car parts are made from an aluminium and silicon alloy. Silicon is obtained from the silicon dioxide in sand by heating the sand with carbon at 2000 degrees C in a furnace, as shown by the equation: SiO2 + 2C -> Si + 2CO Sand also holds a little aluminium oxide, but this oxide stays behind in the silicon after extraction and has to be removed by additional steps. By considering reduction, work out the relative reactivities of silicon and aluminium, and explain your reasoning. [2]

Carbon (the reducing agent) reduces silicon dioxide to silicon: SiO2 + 2C -> Si + 2CO. So carbon is more reactive than silicon. However, carbon does NOT reduce the aluminium oxide, which is why it stays behind in the silicon. This means aluminium holds on to its oxygen more strongly than silicon does, so aluminium is harder to reduce. Therefore aluminium is more reactive than silicon.