Five reaction types in everyday contexts

The five patterns

• Every reaction in this standard fits one of five patterns, and knowing the pattern lets you predict the products.
• Once you have the products, write a chemical word equation, then a balanced equation, and use it to say what happens to the mass.
• In every type the atoms are only rearranged, so the total mass of everything involved stays the same.

Neutralisation: acids meet bases

• Acids contain H⁺ ions and have a pH below 7; bases such as hydroxides, carbonates and hydrogen carbonates react with them, and soluble bases (alkalis) have a pH above 7.
• The key change is H⁺ + OH⁻ → H₂O, so as base is added the H⁺ concentration falls and the pH rises towards 7, which universal indicator shows as red or orange turning to green.
• The salt is named from the metal in the base and the acid: hydrochloric acid gives chlorides, sulfuric acid sulfates, nitric acid nitrates.
• A carbonate or hydrogen carbonate also gives carbon dioxide, so you see fizzing: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂.
• Contexts: antacid tablets neutralising stomach acid, farmers spreading lime (calcium carbonate) on acidic paddocks, and saliva and toothpaste fighting acid on teeth.

Combustion: complete and incomplete

• A hydrocarbon fuel burns in oxygen; with plenty of oxygen the combustion is complete and the only products are carbon dioxide and water.
• LPG is mostly propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, a clean blue flame that gives out the most heat.
• With too little oxygen there are not enough O atoms to turn every C atom into CO₂, so some carbon leaves as carbon monoxide or as solid carbon (soot).
• 2C₃H₈ + 7O₂ → 6CO + 8H₂O makes carbon monoxide, a colourless, odourless poison; C₃H₈ + 2O₂ → 3C + 4H₂O makes soot.
• Incomplete combustion shows as a yellow or orange smoky flame, black soot on pots and walls, and less heat from the same fuel.
• Test the products: limewater turns milky with CO₂, and cobalt chloride paper turns from blue to pink with water.

Precipitation: an insoluble solid from two solutions

• When two solutions of ionic compounds are mixed, the ions swap partners; if one new pairing is insoluble it forms a solid, the precipitate, and the mixture turns cloudy.
• Use the solubility rules to predict it: FeCl₃(aq) + Na₃PO₄(aq) → FePO₄(s) + 3NaCl(aq), because iron(III) phosphate is insoluble and sodium chloride is soluble.
• Only Fe³⁺ and PO₄³⁻ actually react: Fe³⁺(aq) + PO₄³⁻(aq) → FePO₄(s); Na⁺ and Cl⁻ are spectator ions, dissolved before and after.
• A precipitate forms when the pull between two ions is stronger than the pull of the water molecules around them, so water cannot keep them apart.
• Nothing leaves the container, so the mass of the mixture equals the mass of the two solutions added together.

Phosphate in waterways

• Phosphate from fertiliser helps pasture grow, but when it washes into streams and lakes it feeds algae, causing algal blooms that cloud the water and use up oxygen when they die.
• The same ion is useful in the soil and harmful in a lake, so its effect depends on where it ends up and how much there is.
• Precipitation can lock phosphate away: adding aluminium or iron(III) ions makes insoluble solids such as AlPO₄ and FePO₄ that settle to the bottom.
• In Rotorua, alum (aluminium sulfate) is dosed into two streams that flow into Lake Rotorua; the aluminium forms a floc that holds onto dissolved phosphate and settles out, so algae cannot use it.
• Conservation of mass matters here: the phosphorus is not destroyed, only moved into the lake-bed sediment, so it could be released again if conditions in the sediment change.

Combination: two substances become one

• In a combination reaction two or more substances join to make a single product: A + B → AB.
• Iron rusts by combining with oxygen (water is needed too, and real rust also holds water): 4Fe + 3O₂ → 2Fe₂O₃, an orange-brown, flaky solid that weakens tools, gates and car bodies.
• Because oxygen from the air joins the iron, a rusty nail weighs more than it did, by exactly the mass of oxygen it took in.
• A metal combining with a non-metal makes an ionic solid, such as magnesium oxide when magnesium burns: 2Mg + O₂ → 2MgO.
• Two non-metals combining make a molecular compound, usually a gas or liquid: sulfur in fossil fuels combines with oxygen to make sulfur dioxide, S + O₂ → SO₂, which causes acid rain.

Decomposition: one substance breaks apart

• In decomposition one reactant breaks down into two or more simpler products: AB → A + B, often when heated.
• The products can only contain the atoms the reactant had: making water needs H and O, and making carbon dioxide needs C and O.
• Fizzy drinks hold carbonic acid, which breaks down when the bottle is opened: H₂CO₃ → H₂O + CO₂, so the drink fizzes, goes flat and gets lighter as the gas leaves.
• Baking soda decomposes in a hot oven, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂, and the gas makes a cake rise; more baking soda gives more gas.
• Heating limestone makes lime: CaCO₃ → CaO + CO₂, a metal carbonate giving a metal oxide and carbon dioxide; the solid loses mass because CO₂ escapes.

1. CaCO₃ → CaO + CO₂Ca 1, C 1, O 3 on each side
2. 100 g of CaCO₃ gives 56 g of CaOthe solid left in the kiln
3. 100 − 56 = 44 g of CO₂the gas that escapes
4. lime making releases CO₂ from the rock itself, not just from the fuel burned to heat it

Observations that identify products

• Linking what you see to a particular substance is how you show a reaction is happening and which type it is.
• A gas can be identified with a simple test, and a colour change can be matched to a substance using a data sheet.
• A change in measured mass is an observation too: a drop usually means a gas escaped, a rise means a gas from the air joined in.