Reactions of acids

Chemistry · Chemical changes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Acids produce hydrogen ions, H⁺, in water. Alkalis produce hydroxide ions, OH⁻. When they meet, the ions join to make water. This is neutralisation, and the other ions left behind form a salt.

The name of the salt comes from the metal and the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates and nitric acid gives nitrates.

The pH scale runs from 0 to 14. Below 7 is acidic, 7 is neutral and above 7 is alkaline. A strong acid ionises completely in water. A weak acid ionises only partly, so it has fewer H⁺ ions and a higher pH at the same concentration. Strong and concentrated do not mean the same thing.

Rules to remember

acid + metal → salt + hydrogen
acid + alkali or base (metal oxide or hydroxide) → salt + water
acid + metal carbonate → salt + water + carbon dioxide
Neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l)
Soluble salt: add excess insoluble base to warm acid, filter, heat the filtrate gently, leave to crystallise
Titration: measure the alkali with a pipette, add acid from a burette until the indicator changes colour
When pH falls by 1, the H⁺ concentration becomes 10 times greater

Common mistake

Students think a concentrated acid is always a strong acid. Strong or weak is about how fully the acid ionises. Concentrated or dilute is about how much acid is dissolved in the water.

Worked example

Magnesium carbonate is added to dilute hydrochloric acid. Name the products and write the balanced equation.

Acid + metal carbonate gives a salt, water and carbon dioxide.
The metal is magnesium and hydrochloric acid gives chlorides, so the salt is magnesium chloride, MgCl₂.
Two HCl are needed to give the two Cl in MgCl₂: MgCO₃ + 2HCl → MgCl₂ + H₂O + CO₂.
Check: Mg 1 and 1, C 1 and 1, O 3 and 3, H 2 and 2, Cl 2 and 2.

Answer: Magnesium chloride, water and carbon dioxide. MgCO₃ + 2HCl → MgCl₂ + H₂O + CO₂

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