Reactions & equations

In a chemical reaction, atoms are rearranged; none are created or destroyed. Use state symbols: solid $(s)$, liquid $(l)$, gas $(g)$, aqueous $(aq)$.

Covalent elements (exist as molecules; must learn): ${\text{H}}_2,{\text{N}}_2,{\text{O}}_2,{\text{F}}_2,{\text{Cl}}_2,{\text{Br}}_2,{\text{I}}_2$ (and ${\text{C}}_{60}$).

Common covalent compound formulae: ${\text{NH}}_3$ (ammonia), ${\text{CO}}_2$ (carbon dioxide), $\text{CO}$ (carbon monoxide), ${\text{CH}}_4$ (methane), ${\text{NO}}_2,\text{NO},{\text{SO}}_2,{\text{SO}}_3,{\text{H}}_2\text{O}$.

Common acid formulae: $\text{HCl}$ (hydrochloric), ${\text{HNO}}_3$ (nitric), ${\text{H}}_2{\text{SO}}_4$ (sulfuric), ${\text{CH}}_3\text{COOH}$ (ethanoic).

Ion charges to memorise. Positive: ${\text{Li}}^{+},{\text{Na}}^{+},{\text{K}}^{+},{\text{Mg}}^{2+},{\text{Ca}}^{2+},{\text{Ba}}^{2+},{\text{Al}}^{3+},{\text{NH}}_4^{+},{\text{Cu}}^{2+},{\text{H}}^{+},{\text{Fe}}^{2+},{\text{Fe}}^{3+},{\text{Ag}}^{+},{\text{Zn}}^{2+}$; negative: ${\text{O}}^{2-},{\text{S}}^{2-},{\text{F}}^{-},{\text{Cl}}^{-},{\text{Br}}^{-},{\text{I}}^{-},{\text{CO}}_3^{2-},{\text{OH}}^{-},{\text{NO}}_3^{-},{\text{SO}}_4^{2-}$.

Balance by placing numbers in front of formulae so each element has equal atoms on both sides; never change a formula's subscripts.

Ionic equations show only the particles that change (spectator ions are omitted):

• Acid + alkali: ${\text{H}}^{+}(aq)+{\text{OH}}^{-}(aq)\to {\text{H}}_2\text{O}(l)$
• Acid + carbonate: $2{\text{H}}^{+}(aq)+{\text{CO}}_3^{2-}(aq)\to {\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$
• Acid + ammonia: ${\text{H}}^{+}(aq)+{\text{NH}}_3(aq)\to {\text{NH}}_4^{+}(aq)$
• Acid + metal: $\text{Mg}(s)+2{\text{H}}^{+}(aq)\to {\text{Mg}}^{2+}(aq)+{\text{H}}_2(g)$
• Precipitation: ${\text{Ag}}^{+}(aq)+{\text{I}}^{-}(aq)\to \text{AgI}(s)$

Half-equations show electrons explicitly: ${\text{Na}}^{+}+e^{-}\to \text{Na}$ (reduction at cathode); $2{\text{Cl}}^{-}\to {\text{Cl}}_2+2e^{-}$ (oxidation at anode).

Some reactions are reversible and reach equilibrium in a closed system, where forward and backward rates are equal. The position can be shifted by changing concentration, temperature or pressure (for gases); a change is opposed by a shift that partly cancels it.

• Balancing by changing a subscript instead of adding a coefficient.
• Forgetting state symbols when asked for them.
• Treating a reversible reaction as if it goes to completion.