Part 1: Reactivity, hydroxides and neutralisation

Connect the ns² configuration of Mg–Ba to reactivity, practical observations, hydroxide solubility and neutralisation calculations.

Lose two outer-shell electrons

Mg, Ca, Sr and Ba have an outer ns² configuration and commonly form M²⁺. Down the group, added shells increase radius and shielding. Despite increased nuclear charge, outer electrons are less strongly attracted; first and second ionisation energies fall and oxidation to M²⁺ becomes easier. Reactivity generally increases from Mg to Ba.

Oxygen, water and dilute acids

Magnesium burns with a bright white flame to form white MgO. The simple Group 2 oxide equation is 2M + O₂ → 2MO; use supplied information if an unfamiliar product is specified. Mg reacts very slowly with cold water but reacts readily with steam to form MgO and H₂. Calcium, strontium and barium react increasingly vigorously with cold water to form hydroxides and hydrogen.

For calcium with water, observe bubbles, disappearance of metal and possibly a white suspension of sparingly soluble Ca(OH)₂. The mixture becomes alkaline. With dilute HCl, these metals form chloride salts and H₂. Clean Mg ribbon removes its surface oxide for a fair rate comparison; MgO consumes acid without generating H₂.

Basic oxides and increasing hydroxide solubility

Group 2 oxides react with water to form hydroxides. Hydroxide solubility generally increases down Mg–Ba, so saturated solutions provide higher OH⁻ concentrations and become more alkaline. Mg(OH)₂ is sparingly soluble; its suspension contains undissolved solid as well as a small amount of dissolved ions. Do not call low solubility “weak ionisation”.

MgO in water gives an alkaline mixture often described at AS as roughly pH 9–10; calcium hydroxide solution is roughly pH 12. These are approximate, condition-dependent values, not constants independent of temperature or concentration.

Neutralise acid with a suitable base

Calcium hydroxide can neutralise acidic soils; excess addition would make the soil too alkaline. Magnesium hydroxide and calcium carbonate are used as antacid ingredients because they react with excess stomach acid. These are chemical examples, not dosing advice.

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O does not evolve CO₂. CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O does. Both consume two moles of HCl per mole of base.

Follow both elements in a metal–water reaction

In Ca + 2H₂O → Ca(OH)₂ + H₂, calcium changes from 0 to +2: it loses two electrons and is oxidised. Hydrogen changes from +1 in water to 0 in H₂: it is reduced. Oxygen stays at −2. The fact that oxygen appears in a product does not mean it has been reduced.

Each Ca atom supplies the two electrons needed to form one H₂ molecule. This explains the 1:1 calcium-to-hydrogen mole ratio. With dilute hydrochloric acid the chloride ions are spectators, and the net change is M + 2H⁺ → M²⁺ + H₂.

Separate a rate comparison from a product test

Fizzing is an observation; production of hydrogen is an inference that can be checked with the appropriate gas test. White cloudiness when calcium reacts with water is compatible with suspended calcium hydroxide. A metal disappearing does not mean it has simply dissolved unchanged: its atoms have been oxidised to ions.

To compare rates using hydrogen volume, keep temperature, acid concentration and volume, exposed surface area and apparatus consistent, and state how the metal amount is controlled. Equal masses of different metals contain different numbers of atoms. Compare initial slopes where appropriate; the final gas volume depends on the limiting amount as well as the reaction stoichiometry.

Magnesium’s oxide coating delays access to the metal. Cleaning a consistent surface removes this confounding factor. Do not generalise every “dilute acid” to the hydrogen-producing HCl model: oxidising acids can behave differently.

Three reactions that must not be mixed up

Metal plus water produces hydrogen because a redox change takes place. Oxide plus water forms hydroxide without producing hydrogen. Hydroxide plus acid forms a salt and water by neutralisation. Write the correct starting species before deciding the products.

Increasing hydroxide solubility down the group means more OH⁻ can enter solution at saturation. It does not mean that Ba(OH)₂ releases more than two OH⁻ per formula unit or that Mg(OH)₂ should be described as partially ionised in the same way as a weak molecular base. Approximate pH comparisons refer to the specified mixtures, not every possible dilution.

Worked example: capacity of an antacid ingredient

A laboratory sample contains 0.350 g of pure Mg(OH)₂. Use M = 58.3 g mol⁻¹. Its amount is 0.350 ÷ 58.3 = 0.006003… mol. Each mole consumes two moles of HCl, so it neutralises 0.01201… mol HCl.

For 0.200 mol dm⁻³ HCl, V = n/c = 0.01201…/0.200 = 0.06003… dm³ = 60.0 cm³ to three significant figures. The factor two comes from two hydroxide groups, not from the charge of HCl. For a tablet containing inert ingredients, use the mass of active compound rather than the total tablet mass.

The same mole-ratio method works for CaCO₃, but its molar mass and gaseous product differ. Equal masses of the two bases do not necessarily neutralise equal amounts of acid.

Quick checks

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

Sources