Types of chemical reactions
Thousands of reactions fall into a few recognisable patterns. Spotting the pattern lets you predict the products.
Evidence of a reaction
A chemical reaction produces new substances. Signs include a colour change, gas bubbles, a precipitate (a solid forming from solution), a temperature change, or light. A physical change, like melting ice, changes form but not identity.
Synthesis and decomposition
In synthesis (combination), substances join: \(2\text{Mg} + \text{O}_2 \to 2\text{MgO}\). In decomposition, a compound breaks apart: \(2\text{H}_2\text{O}_2 \to 2\text{H}_2\text{O} + \text{O}_2\), the reaction that makes hydrogen peroxide fizz on a cut, sped up by enzymes in blood.
Single and double replacement
In single replacement, one element replaces another in a compound: \(\text{Zn} + \text{CuSO}_4 \to \text{ZnSO}_4 + \text{Cu}\), which works because zinc is more reactive than copper (higher on the activity series). In double replacement, two compounds swap partners: \(\text{AgNO}_3 + \text{NaCl} \to \text{AgCl} + \text{NaNO}_3\), with insoluble AgCl precipitating.
Combustion
Combustion is a rapid reaction with oxygen, releasing heat and light. Hydrocarbons burn completely to carbon dioxide and water: \(\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O}\). With too little oxygen, incomplete combustion produces carbon monoxide, which is why fuel-burning appliances need ventilation.
Precipitation and solubility rules
Solubility rules predict precipitates: nitrates and group 1 compounds are soluble; most chlorides are soluble except those of silver, lead, and mercury(I); most carbonates and phosphates are insoluble. Mixing solutions that form an insoluble combination produces a precipitate.
Net ionic equations
In solution, soluble ionic compounds exist as separate ions. A net ionic equation shows only the species that actually change, removing spectator ions. For silver chloride: \(\text{Ag}^+(aq) + \text{Cl}^-(aq) \to \text{AgCl}(s)\). Sodium and nitrate ions are spectators.
\text{Ag}^+(aq) + \text{Cl}^-(aq) \to \text{AgCl}(s)Acid-base neutralisation
An acid and a base react to form water and a salt: \(\text{HCl} + \text{NaOH} \to \text{NaCl} + \text{H}_2\text{O}\). The net ionic equation is \(\text{H}^+ + \text{OH}^- \to \text{H}_2\text{O}\). Antacids neutralise excess stomach acid this way.
Worked example
Predict the products of \(\text{Pb(NO}_3)_2(aq) + 2\text{KI}(aq)\). Swap partners: \(\text{PbI}_2 + 2\text{KNO}_3\). Lead iodide is insoluble, so a bright yellow precipitate forms. Net ionic: \(\text{Pb}^{2+} + 2\text{I}^- \to \text{PbI}_2(s)\).
Key takeaways
Reactions include synthesis, decomposition, single and double replacement, combustion, and neutralisation. The activity series predicts single replacements; solubility rules predict precipitates. Net ionic equations remove spectator ions to show the real change.
Flashcard
New substances form
Check your understanding
What type of reaction is 2Na + Cl2 -> 2NaCl?