Equilibria, Acids, Bases and Buffers
Dynamic equilibrium, Le Chatelier reasoning, Kc, Kp, Brønsted-Lowry acids and bases, pH, Ka, Kw, buffers, titration curves and indicators.
How to study A-level Chemistry
Represent the species and process correctly, conserve atoms, charge and energy, calculate with units, then test the result against observations and chemical evidence.
Core concepts
Concept 1
At dynamic equilibrium forward and reverse rates are equal while concentrations remain constant in a closed system.
Exam cue: Write the equilibrium expression before substituting equilibrium quantities.
Concept 2
Equilibrium constants quantify composition at a stated temperature and omit species according to the defined expression.
Exam cue: Distinguish a change in equilibrium position from a change in the value of K.
Concept 3
Acid-base calculations connect concentration, pH, Ka and Kw; buffer action follows from coupled equilibria and component availability.
Exam cue: State the conjugate pair and reaction that removes added acid or base in a buffer.
Risk pitfalls and guardrails
Saying a catalyst changes the value of K.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Using initial instead of equilibrium concentrations without justification.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Assuming a strong acid must be concentrated.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Memory anchors
Dynamic Equilibrium
Forward and reverse reactions continue at equal rates in a closed system.
Equilibrium Constant
An equilibrium constant is a ratio of equilibrium activities represented by the specified expression.
Brønsted-Lowry Acid
A Brønsted-Lowry acid donates a proton.
pH
pH equals minus log base ten of hydrogen-ion concentration.
Buffer
A buffer resists pH change because a weak acid-base pair consumes added acid or base.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
What characterises dynamic equilibrium in a closed system?
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), what is the effect of increasing pressure at constant temperature?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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