GoalTell strong electrolytes from weak ones, write their ionisation equations with the right arrows, and read invented conductivity results.
New words
strong electrolyte · a substance that ionises completely in water, such as NaCl or HClweak electrolyte · a substance that ionises only partly and reversibly in water, such as acetic acid or ammoniaionisation · the change of molecules into ions when they dissolve in water
Explanation
An electrolyte is a substance whose water solution carries an electric current, because the solution contains ions. Ionic compounds such as NaCl are already made of ions, and in water they separate completely. Some polar covalent compounds form ions too, but only when they meet water. Hydrogen chloride ionises almost completely, so its equation has one arrow: HCl → H⁺ + Cl⁻. Acetic acid ionises very little and the change goes both ways: CH₃COOH ⇌ CH₃COO⁻ + H⁺. Ammonia is of the same kind, NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, and so is carbonic acid, H₂CO₃ ⇌ H⁺ + HCO₃⁻, written with ⇌ and not with a one-way arrow. A strong electrolyte ionises completely and a weak one only partly. The book proves the difference with lamp tests on the mains; here we do no set-up and only read invented results. A weak electrolyte in water is a mixture of molecules and ions at equilibrium. That is why the law of mass action may be used for it, as the next lessons do, but not to a strong electrolyte, which has no molecules left to balance its ions.
Worked examples
Acid
Molecules dissolved
Ionised
Share ionised
HCl
100
100
100 %
HA (invented)
100
2
2 %
A party of 100 guests. Every guest is a dissolved acid molecule. In a strong acid all 100 split into two ions each. In a weak acid, say an invented acid HA, 98 stay together as molecules and only 2 split. So 2 out of 100 are ionised, which is 2 %. Both parties are the same size, but the weak one has far fewer ions.
Substance
Type
Equation
KNO₃
strong
KNO₃ → K⁺ + NO₃⁻
HCl
strong
HCl → H⁺ + Cl⁻
CH₃COOH
weak
CH₃COOH ⇌ CH₃COO⁻ + H⁺
NH₃
weak
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
H₂CO₃
weak
H₂CO₃ ⇌ H⁺ + HCO₃⁻
Sort and write. KNO₃ is an ionic compound, so it is a strong electrolyte: KNO₃ → K⁺ + NO₃⁻. HCl is strong: HCl → H⁺ + Cl⁻. CH₃COOH is weak: CH₃COOH ⇌ CH₃COO⁻ + H⁺. NH₃ is weak: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. H₂CO₃ is weak: H₂CO₃ ⇌ H⁺ + HCO₃⁻. Look at the arrow: one arrow for complete ionisation, ⇌ for partial. Check charge: in the last equation +1 − 1 = 0 on the right, as on the left.
Read the invented results. Per 100 dissolved molecules, a strong acid HX gives 0 HX, 100 H⁺ and 100 X⁻, which is 200 ions. A weak acid HA gives 98 HA, 2 H⁺ and 2 A⁻, which is only 4 ions. At the same concentration the strong acid has 200 ÷ 4 = 50 times more ions, so a conductivity test would show it conducting much better. No lamp or mains set-up is made here: these numbers are invented only to show the idea.
Class activity
Party cards, paper only. Cut 20 small cards. Write HA on 20 of them, as if they were weak-acid molecules. For a weak acid at one concentration, turn 1 card in 20 over and write H⁺ on one side and A⁻ on the other: the pair stands for 1 ionised molecule. Count the shares: how many molecules, how many ions, which percentage. Then repeat for a strong acid, where all 20 are turned over. Draw both piles in a table and compare with the picture.
Practice
1
Write the ionisation equation of each in water: (a) KCl, (b) ammonia, (c) carbonic acid (first step).
Of 200 molecules of a weak acid that dissolve, 6 ionise. What percentage is ionised, and how many H⁺ ions are in the solution?
6 ÷ 200 × 100 = 3 %, and there are 6 H⁺ ions.
3
True or false, and correct if false: “A weak electrolyte contains no ions.”
False. It contains a few ions in equilibrium with a large number of molecules.
4
Why can the law of mass action be applied to a weak electrolyte but not to a strong one?
A weak electrolyte is an equilibrium between molecules and ions. A strong one ionises completely, so no molecules remain to be in equilibrium with the ions.
