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Chemistry · Solutions and electrolytes · Grade 7

Acids and metals

Which metal displaces hydrogen from an acid, and why does copper stay silent?

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The situation

In the kitchen there are lemon and vinegar, in the garage a car battery — all contain an acid. Acids change the colour of indicators and react with many metals, releasing hydrogen (never taste them!). Why does a strip of magnesium dissolve “boiling” in hydrochloric acid, an iron nail bubble slowly, and a copper wire not change at all? What is the composition of acids, how are they named, and how are they made from acidic oxides?

Topic

Acids and their action on metals. Lemon, vinegar and battery acid: magnesium, zinc, iron and copper in hydrochloric acid — who displaces hydrogen (the reactivity series)

Key ideas

  • Composition of acids
  • Reactivity series of metals
  • Indicators

Formulas

Mg+2HCl→MgCl2+H2↑

A metal that stands before hydrogen displaces the hydrogen from the acid — a salt forms.

Cu+HCl→no reaction

Copper stands after hydrogen in the reactivity series: it does not react with dilute HCl.

n(H2)=mAr,V=nVm

For a divalent metal: as many moles of H₂ as moles of metal dissolved; at STP Vm = 22.4 L/mol.

P2O5+3H2O→2H3PO4

Acidic oxide + water → acid (practical).

H2+Cl2→2HCl

An oxygen-free acid: hydrogen chloride is dissolved in water to give hydrochloric acid.

Theory

Composition and classification of acids

An acid is a compound whose composition includes hydrogen atoms that can be replaced by metal atoms, and an acid residue: H₂SO₄ = 2 H + SO₄ (the sulfate residue). The valency of the acid residue equals the number of hydrogen atoms. By composition: oxygen-containing (H₂SO₄, HNO₃, H₃PO₄, H₂CO₃) and oxygen-free (HCl, H₂S, HF) acids. Basicity is the number of replaceable H atoms: monobasic (HCl, HNO₃, CH₃COOH — only one of its four H), dibasic (H₂SO₄, H₂CO₃, H₂S), tribasic (H₃PO₄). Names: hydrochloric, nitric, sulfuric, phosphoric, carbonic, hydrosulfuric acid.

Acids in everyday life: strong and weak

Lemon juice contains ≈ 5 % citric acid (≈ 48 g/L), table vinegar is a 6–9 % solution of ethanoic acid, carbonated water contains unstable carbonic acid, yoghurt contains lactic acid, and gastric juice contains hydrochloric acid. The electrolyte in a car battery is a solution of sulfuric acid: when fully charged its density is ≈ 1.27–1.28 g/cm³ (≈ 37 %). Strong acids (HCl, HNO₃, H₂SO₄) dissociate almost completely into ions in water, weak ones (CH₃COOH, H₂CO₃, H₂S) only partially, so at the same concentration their pH is higher.

Indicators

In an acid solution litmus is red, methyl orange is red (pH < 3.1), and phenolphthalein is colourless. But take care with weak acids: in saturated carbonated water pH ≈ 3.9 — methyl orange does not turn red, only orange, while litmus turns red. More about indicators — in the “Acids, bases and indicators” experiment.

Practical: preparing acids

Oxygen-containing acids are made by the reaction acidic oxide + water: SO3+H2O→H2SO4, P2O5+3H2O→2H3PO4, SO2+H2O⇌H2SO3, CO2+H2O⇌H2CO3 (carbonic acid is unstable, and CO₂ is only slightly soluble: ≈ 0.039 mol/L at 20 °C and 1 atm). Oxygen-free acids are made by dissolving a compound of hydrogen with a non-metal in water: H2+Cl2→2HCl — hydrogen chloride gas dissolves readily in water. In the laboratory HCl is also made by the action of concentrated sulfuric acid on table salt. Safety: when diluting, pour acid into water, never the reverse; P₂O₅ reacts violently with water; SO₂ is poisonous — work in a fume cupboard; a mixture of chlorine and hydrogen explodes in light — this synthesis is carried out only in industry.

Practical: with metals, and the reactivity series

Metals that stand before hydrogen in the reactivity series displace hydrogen from an acid: Mg+2HCl→MgCl2+H2↑, Zn+2HCl→ZnCl2+H2↑, Fe+2HCl→FeCl2+H2↑ (the solution is pale green). The further left in the series, the more vigorous the reaction: magnesium “boils”, zinc reacts steadily, iron bubbles slowly. Copper stands after hydrogen — it does not react with HCl or dilute H₂SO₄. The series: Li K Ba Ca Na Mg Al Zn Fe Ni Sn Pb (H) Cu Hg Ag Pt Au. Calculation: 0.1 g Mg = 0.1/24.3 = 4.1 mmol → the same amount of H₂: 92 mL (STP). For the same mass, magnesium gives more hydrogen — its atomic mass is smaller. Nitric acid and concentrated sulfuric acid react with metals differently (no hydrogen is released).

Safety and the model

Safety: safety goggles and gloves; if acid gets on the skin or in the eyes, wash at once with plenty of water; never taste an acid; do not light a flame near a test tube from which hydrogen is coming off. Model: the relative rates of the metals Mg : Zn : Fe = 1 : 0.12 : 0.03 are schematic — in reality they depend on the metal surface, its purity and the temperature; the rate is proportional to the acid concentration, and ≈ 12 times slower in ethanoic acid; the volume of hydrogen is reduced to STP (the gas in the balloons is at 20 °C); pH is calculated for ideal solutions with constants at 25 °C; excess CO₂ escapes from the water (1 atm).

Did you know?

  • Every litre of lemon juice contains on average about 48 g of citric acid — roughly a 5 % solution. That is why a lemon is much more sour than an orange.
  • The electrolyte of a car battery is a solution of sulfuric acid. When fully charged its density is ≈ 1.27–1.28 g/cm³; as the charge runs out the acid is used up and the density falls — this is how mechanics check the charge with a hydrometer.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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