1. Can a weak acid be concentrated?
- No
- Yes
- Only if heated
Check the answer and reasoning
Yes
Weak describes partial ionisation; concentration measures amount per volume.
CHEM / AS level
Equilibria, acids and redox
Equilibrium constants express a composition relationship at fixed temperature. Le Chatelier’s principle predicts shifts qualitatively, while oxidation numbers track redox changes.
Learning objectives
- Explain equilibria, acids and redox using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
Equilibrium constants express a composition relationship at fixed temperature. Le Chatelier’s principle predicts shifts qualitatively, while oxidation numbers track redox changes.
Strong and weak acids differ in the extent of ionisation, not simply concentration. Acid–base and redox titrations use balanced equations to connect accurately measured quantities.
Equations and relationships
Kc = [products]^coefficients / [reactants]^coefficients
Use the equilibrium expression for the specific reaction, omitting pure solids and liquids.
Worked example
For H₂ + I₂ ⇌ 2HI, write Kc.
Follow the solution
Each concentration is raised to its stoichiometric coefficient.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Related interactive model. Compare its assumptions with the explanation; it is not a complete syllabus lesson.
Check your understanding
2. For H₂ + I₂ ⇌ 2HI, write Kc.
- [HI]/([H₂]+[I₂])
- [H₂][I₂]/[HI]²
- [HI]²/([H₂][I₂])
Check the answer and reasoning
[HI]²/([H₂][I₂])
Each concentration is raised to its stoichiometric coefficient.
3. Does increasing pressure shift every gas equilibrium right?
- No
- Yes
- Only with a catalyst
Check the answer and reasoning
No
Pressure favours the side with fewer gas molecules, if the two sides differ.