Part 2: Equations, observations and reacting quantities

Translate observations into balanced equations, then use coefficients as mole ratios to calculate how much can react or form.

Conserve atoms and charge

A formula identifies a substance; a coefficient states how many formula units or moles take part. Balance by changing coefficients, never subscripts. Mg + 2HCl → MgCl₂ + H₂ conserves each element. Replacing MgCl₂ with MgCl would change the substance instead of balancing the equation. Add state symbols: (s), (l), (g) and (aq), where aqueous means dissolved in water, not a pure liquid.

An ionic equation removes spectator ions that remain chemically unchanged. Split soluble strong electrolytes into their aqueous ions; keep solids, gases, liquids and weak acids together. Cancel identical species on both sides, then check total charge as well as atoms. Aqueous does not automatically mean a substance should be fully split into ions: ethanoic acid is weak and remains mainly molecular.

An observation is evidence, not an equation

Effervescence means bubbles of gas appear; it does not by itself identify that gas. A white precipitate is an insoluble solid forming within a solution, not merely a solution becoming colourless. Connect an observed change to the species in your equation, and use a separate confirming test where identification is required.

The equation is a bridge between amounts

For 2Al + 3Cl₂ → 2AlCl₃, the ratio 2 : 3 : 2 is a ratio of amounts in mol, not of masses. Convert the known quantity into moles, apply the coefficient ratio, then convert the target moles to the requested quantity. Label each amount with the substance so that a correct number is not later assigned to the wrong species.

Example: 2.70 g Al reacts with excess chlorine. n(Al) = 2.70/27.0 = 0.100 mol. n(Cl₂) needed = 0.100 × 3/2 = 0.150 mol, so m(Cl₂) = 0.150 × 71.0 = 10.65 g, or 10.7 g to 3 s.f. n(AlCl₃) = 0.100 mol and M(AlCl₃) = 133.5 g mol⁻¹, giving 13.35 g product, or 13.4 g. The unrounded reactant masses sum to 13.35 g, checking conservation of mass.

Find the limiting reactant before predicting product

When both reactant amounts are supplied, either may run out first. Divide each amount by its coefficient; the smaller result gives the amount of reaction possible. Comparing raw masses, or even raw mole amounts without coefficients, can choose the wrong limiting reactant.

Suppose 4.86 g Mg reacts with 100.0 cm³ of 1.50 mol dm⁻³ HCl. n(Mg) = 4.86/24.3 = 0.200 mol; n(HCl) = 1.50 × 0.1000 = 0.150 mol. The reaction Mg + 2HCl → MgCl₂ + H₂ requires two HCl per Mg. Available reaction amounts are 0.200/1 and 0.150/2 = 0.0750 mol, so acid limits the reaction. Only 0.0750 mol Mg reacts and 0.0750 mol H₂ forms.

Mass of Mg consumed = 0.0750 × 24.3 = 1.8225 g; mass remaining = 4.86 − 1.8225 = 3.04 g. A gas-volume calculation must use the 0.0750 mol actually formed, not all the initial magnesium. A large piece of unreacted metal is therefore compatible with a reaction that has stopped because acid ran out.

Use reacting data to infer a ratio

If 0.0100 mol of a metal produces 0.0150 mol H₂ with excess dilute acid, the metal:H₂ ratio is 2:3. For a metal forming M³⁺, 2M(s) + 6H⁺(aq) → 2M³⁺(aq) + 3H₂(g) balances atoms and charge. The observation is consistent with a +3 ion, assuming the gas is hydrogen and there is no competing reaction. Experimental ratios constrain an equation but cannot identify every substance without additional chemical information.

To prepare an insoluble salt, mix suitable soluble salts, filter the precipitate, wash it with a little deionised water to remove soluble ions and dry it. To prepare a soluble salt from acid and an insoluble base, add excess base, filter off the excess, then concentrate and crystallise the filtrate. The first method collects the residue; the second keeps the filtrate. This difference follows from which material is soluble.

Make risk controls specific to the procedure

A hazard is a capacity to cause harm, such as corrosivity, flammability or a hot surface. Risk depends on the chance and severity of harm in the actual procedure. Use the hazard information for the concentration supplied; a dilute acid solution and concentrated acid do not present identical risks.

Wear eye protection during acid reactions, use a pipette filler, handle hot crucibles with tongs and keep hydrogen-producing experiments away from ignition sources. For precipitation work, minimise quantities and collect hazardous metal-ion waste as directed. A control should interrupt a stated route of harm: ventilation reduces inhalation exposure, while goggles protect against splashes. These notes support supervised practical work; observed competence is assessed through practical activity, not reading alone.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources