Balancing a symbol equation with state symbols

Quantitative chemistry

GCSE Chemistry (8462) · 4.3.1.1 & 4.2.2.2 · Foundation & Higher

ORIGINAL

Write the formulae first, balance with the big numbers only, check every atom, then add (s), (l), (g) and (aq).

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Question

Aluminium metal reacts with dilute hydrochloric acid to form aluminium chloride solution and hydrogen gas. Write a balanced symbol equation for the reaction, including state symbols. Aluminium forms

\mathrm{Al^{3+}}

ions.

Every step worked, with the reasoning.

1
Formulae: $\mathrm{Al}$ , $\mathrm{HCl}$ , $\mathrm{AlCl_3}$ and $\mathrm{H_2}$
One $\mathrm{Al^{3+}}$ ion needs three $\mathrm{Cl^-}$ ions, so aluminium chloride is AlCl₃; hydrogen gas is made of diatomic molecules.
2
Skeleton: $\mathrm{Al + HCl \rightarrow AlCl_3 + H_2}$
Now balance by changing only the numbers in front of formulae — never the small subscripts.
3
Balance Cl: $\mathrm{Al + 3HCl \rightarrow AlCl_3 + H_2}$
Three Cl on the right needs 3HCl. That brings 3 H, but hydrogen comes in pairs as H₂, so H cannot balance yet.
4
Balance H: $\mathrm{2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2}$
The lowest common multiple of 3 and 2 is 6: 6HCl gives 6 H, which is 3H₂, and 6 Cl, which is 2AlCl₃, so 2Al. Common error: changing a formula (writing AlCl or H) to force a balance — that changes the substance, and loses the mark.
5
Check: $\mathrm{Al}\ 2 = 2$ , $\mathrm{H}\ 6 = 6$ , $\mathrm{Cl}\ 6 = 6$
Count every element on both sides before moving on.
6
$\mathrm{2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)}$
State symbols: the metal is a solid, the acid and the dissolved salt are aqueous, and hydrogen is a gas.

Answer: $\mathrm{2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)}$

Question

Stuck on balancing equations?