Chemical formulae and equations: writing formulae from ions, balancing symbol equations, state symbols, ionic equations and half-equations, and the law of conservation of mass.

What this dot point is asking

Edexcel wants you to write chemical formulae from the charges on ions, balance symbol equations, add the four state symbols, write ionic equations and half-equations, and apply the law of conservation of mass, including explaining the apparent mass change when a gas enters or leaves an open container. These skills are used in almost every other topic.

Writing formulae from ions

The formula of an ionic compound must be neutral overall, so the total positive charge equals the total negative charge. Combine the ions in the ratio that balances the charges. You should know the common ions, including the polyatomic ones:

When a polyatomic ion is needed more than once, enclose it in brackets, as in $Ca(NO_3)_2$ or $Al_2(SO_4)_3$.

Balancing symbol equations

A balanced equation has the same number of atoms of each element on both sides, because atoms are only rearranged. Balance by adding big numbers in front of formulae; never change a subscript inside a formula, as that changes the substance.

A reliable method:

1. Count the atoms of each element on each side.
2. Add a big number to balance the element that is most out, then recount.
3. Balance hydrogen and then oxygen last for combustion reactions, since they often appear in more than one formula.
4. Check every element balances.

State symbols

State symbols are written in brackets after each formula:

• (s) solid
• (l) liquid
• (g) gas
• (aq) aqueous, meaning dissolved in water

For example, $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$.

Ionic equations and half-equations

An ionic equation shows only the ions and atoms that actually change in the reaction, leaving out the spectator ions that are unchanged on both sides. For neutralisation, the ionic equation is always $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$.

A half-equation shows the electrons gained or lost by one species, used for electrode reactions. For example, at a cathode $Cu^{2+} + 2e^- \rightarrow Cu$, and at an anode $2Cl^- \rightarrow Cl_2 + 2e^-$. The electrons must balance the change in charge.

Conservation of mass

In a closed system (such as a sealed flask) the measured mass is always constant. In an open system the mass can appear to change:

• If a gas escapes (for example carbon dioxide from a heated carbonate), the remaining solid weighs less.
• If a gas is gained from the air (for example oxygen when a metal is heated), the solid weighs more.

In both cases mass is still conserved overall; only the mass inside the container changes.

Try this

Q1. Write the formula of aluminium sulfate, given $Al^{3+}$ and $SO_4^{2-}$. [1 mark]

• Cue. Balance charges: two $Al^{3+}$ ($+6$) with three $SO_4^{2-}$ ($-6$), so $Al_2(SO_4)_3$.

Q2. Balance: $Fe + Cl_2 \rightarrow FeCl_3$. [2 marks]

• Cue. $2Fe + 3Cl_2 \rightarrow 2FeCl_3$.

Q3. Explain why the mass of a sealed flask does not change during a reaction inside it. [2 marks]

• Cue. No atoms can enter or leave the sealed flask, and no atoms are created or destroyed, so the total mass stays the same.