Acids, Bases and Salts: Question 7

Worked solution

Part (a): Magnesium and dilute hydrochloric acid

Magnesium is a reactive metal, so it reacts with a dilute acid to form a salt and hydrogen gas. Start with the word equation to identify the products, then balance the formulae:

$\text{Mg(s)} + \text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$

Counting atoms, the right-hand side has 2 chlorine atoms and 2 hydrogen atoms, but the left-hand side has only 1 of each. Placing a 2 in front of $\text{HCl}$ balances both:

$\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$

Check: 1 Mg, 2 H, 2 Cl on each side, balanced.

Part (b): Sodium carbonate and dilute nitric acid

A carbonate reacting with an acid forms a salt, water, and carbon dioxide:

$\text{Na}_2\text{CO}_3\text{(s)} + \text{HNO}_3\text{(aq)} \rightarrow \text{NaNO}_3\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

The left-hand side has 2 sodium atoms (from $\text{Na}_2\text{CO}_3$), but the right-hand side has only 1 (from $\text{NaNO}_3$). Placing a 2 in front of both $\text{HNO}_3$ and $\text{NaNO}_3$ balances the sodium, nitrogen and hydrogen together:

$\text{Na}_2\text{CO}_3\text{(s)} + 2\text{HNO}_3\text{(aq)} \rightarrow 2\text{NaNO}_3\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

Check: 2 Na, 1 C, 9 O, 2 H, 2 N on each side, balanced.

Part (c): Aluminium and dilute hydrochloric acid

Aluminium is also a reactive metal, so it forms a salt (aluminium chloride) and hydrogen gas:

$\text{Al(s)} + \text{HCl(aq)} \rightarrow \text{AlCl}_3\text{(aq)} + \text{H}_2\text{(g)}$

$\text{AlCl}_3$ needs 3 chlorine atoms, so start by placing a 3 in front of $\text{HCl}$:

$\text{Al(s)} + 3\text{HCl(aq)} \rightarrow \text{AlCl}_3\text{(aq)} + \text{H}_2\text{(g)}$

This now gives 3 hydrogen atoms on the left, but $\text{H}_2$ only supplies an even number on the right, so the numbers of aluminium and chlorine must also be doubled to allow the hydrogens to balance exactly:

$2\text{Al(s)} + 6\text{HCl(aq)} \rightarrow 2\text{AlCl}_3\text{(aq)} + 3\text{H}_2\text{(g)}$

Check: 2 Al, 6 H, 6 Cl on each side, balanced.

Final answers

• (a) $\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$
• (b) $\text{Na}_2\text{CO}_3\text{(s)} + 2\text{HNO}_3\text{(aq)} \rightarrow 2\text{NaNO}_3\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$
• (c) $2\text{Al(s)} + 6\text{HCl(aq)} \rightarrow 2\text{AlCl}_3\text{(aq)} + 3\text{H}_2\text{(g)}$