Coaching Centre
Reactions of acids
Neutralisation and naming salts, acids reacting with metals and with carbonates, testing for hydrogen and carbon dioxide, and titration to find exactly how much base neutralises an acid, with pH curves and indicator choice.
After you eat something sugary, bacteria on your teeth turn the sugar into acid. Within minutes the pH at the surface of your teeth can fall below about 5.5, and the enamel starts to dissolve, because tooth enamel is a base. Saliva slowly neutralises the acid and the enamel recovers, as long as it gets the chance. Tooth decay is an acid reaction, and so is the cure.
Acid + base → salt + water
When an acid reacts with a base, the H⁺ ions from the acid and the OH⁻ ions (or oxide ions) from the base combine to make water. This is neutralisation. The other ions are left behind as a salt, an ionic compound made of a metal (or ammonium) ion from the base and a negative ion from the acid.
acid + base → salt + water
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
The ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)
The ionic equation is the same for every acid–alkali reaction, which is why they all release about the same amount of heat: neutralisation is exothermic. The name of the salt comes from the acid:
| Acid | Formula | Salts are called | Example |
|---|---|---|---|
| hydrochloric acid | HCl | chlorides | HCl + KOH → KCl + H₂O |
| sulfuric acid | H₂SO₄ | sulfates | H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O |
| nitric acid | HNO₃ | nitrates | 2HNO₃ + CuO → Cu(NO₃)₂ + H₂O |
| ethanoic acid | CH₃COOH | ethanoates | CH₃COOH + NaOH → CH₃COONa + H₂O |
| phosphoric acid | H₃PO₄ | phosphates | H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O |
Neutralisation is used every day. Farmers spread crushed limestone (agricultural lime) on acidic soils, a problem across large areas of Australian farmland, so crops can take up nutrients. Antacid tablets neutralise excess stomach acid (3I), and toothpaste is usually slightly basic.
Acid + metal → salt + hydrogen
Many metals react with dilute acids, fizzing as they release hydrogen gas and forming a salt:
metal + acid → salt + hydrogen
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
Reactive metals such as magnesium fizz quickly, zinc and iron more slowly, while copper, silver and gold do not react with dilute acids at all (you will see why in 3F). Hydrogen burns with a squeaky "pop" when a lit splint is held at the mouth of the test tube: this is the pop test. The pop is a tiny explosion of hydrogen reacting with oxygen to make water.
Acid + carbonate → salt + water + carbon dioxide
Carbonates and hydrogen carbonates react with acids to give a salt, water and carbon dioxide:
acid + carbonate → salt + water + carbon dioxide
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
Carbon dioxide turns limewater (calcium hydroxide solution) milky, because insoluble calcium carbonate forms: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). This reaction explains why marble statues and limestone buildings are damaged by acid rain, why sherbet fizzes on your tongue (citric acid and bicarbonate soda meet in your saliva), and how baking powder makes cakes rise.
Titration: exactly how much?
To neutralise an acid exactly, you need to know precisely how much base to add. Chemists find this by titration. A measured volume of acid and a few drops of indicator go in a conical flask. A base of known concentration is added slowly from a burette until the indicator just changes colour. That point is the end point. The volume added tells you how much base reacted with the acid.
A pH probe shows what happens during a titration. The pH changes slowly at first, because most of the added OH⁻ is used up by the acid. Near the equivalence point, where the acid and base exactly match, a single drop can change the pH by several units. An indicator works if its colour change falls inside that steep jump. For a strong acid with a strong base the equivalence point is at pH 7, but for a weak acid such as ethanoic acid it is above 7, so phenolphthalein (which changes at about pH 8–10) is the better choice.
- With hydrochloric acid and universal indicator, add 1 mL at a time and watch the pH. At what volume does the indicator turn green?
- Reset and drip all the way to 50 mL. Describe the shape of the curve. Why is it steepest at 25 mL?
- Read the temperature at 25 mL. How much did the solution warm up, and what does this tell you about neutralisation?
- Switch to ethanoic acid. What is the pH at the equivalence point? Which of the three indicators would give the most accurate end point?
25.0 mL of hydrochloric acid of unknown concentration is titrated with 0.100 mol/L sodium hydroxide. The end point is reached after 20.0 mL of base.
HCl + NaOH → NaCl + H₂O, so one HCl reacts with one NaOH (a 1 : 1 ratio).
Equal amounts of acid and base reacted. The acid needed 20.0 mL of 0.100 mol/L base for every 25.0 mL of acid, so the acid is a little less concentrated than the base.
concentration of acid = 0.100 × 20.0 ÷ 25.0 = 0.080 mol/L
Test yourself
Zinc reacts with dilute sulfuric acid. The products are
Metal + acid → salt + hydrogen.
Which salt forms when potassium hydroxide neutralises nitric acid?
Nitric acid makes nitrates: KOH + HNO₃ → KNO₃ + H₂O.
A gas from a reaction turns limewater milky. The reaction was most likely between an acid and
Carbonates release CO₂.
During a titration of a strong acid with a strong base, the pH rises most steeply
Near equivalence, one drop changes the pH by several units.
Which is the balanced equation for calcium carbonate reacting with hydrochloric acid?
Two Cl and two H are needed on the left.
Label the titration apparatus.
What does each reaction produce?
The reaction between an acid and a base is called . Salts made from sulfuric acid are called . Hydrogen gas burns with a squeaky . Carbon dioxide turns milky. In a titration the base is added from a .
A student is given three unlabelled test tubes containing magnesium ribbon, marble chips and copper(II) oxide powder. They have dilute hydrochloric acid, limewater and a lit splint. Describe how the student could identify each solid, and write a balanced equation for each reaction.
Add a little dilute hydrochloric acid to each test tube. The tube that fizzes and gives a gas that burns with a squeaky pop when a lit splint is held at its mouth contains magnesium, which produces hydrogen: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). The tube that fizzes and gives a gas that turns limewater milky when bubbled through it contains marble chips, which produce carbon dioxide: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). The copper(II) oxide dissolves without fizzing, because it is a base that is neutralised to a salt and water, and the solution turns blue-green: CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l).