9701 · 25.1
Acids and bases flashcards
Revision flashcards for Cambridge 9701 Acids and bases (syllabus 25.1). Flip, recall, then mark a real past-paper question.
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What is a Brønsted–Lowry acid?
A proton (H⁺) donor.
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What is a Brønsted–Lowry base?
A proton (H⁺) acceptor.
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What is a conjugate acid-base pair?
A pair of species that differ by a single proton (H⁺). For example, HA and A⁻.
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Define pH.
The negative logarithm to the base 10 of the hydrogen ion concentration. $pH = -log_{10}[H^+(aq)]$. A lower pH indicates higher acidity.
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What is the acid dissociation constant, Ka?
An equilibrium constant for the dissociation of a weak acid in water. For $HA \rightleftharpoons H^+ + A^-$, $K_a = \frac{[H^+][A^-]}{[HA]}$. A larger Ka indicates a stronger weak acid.
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Define pKa.
The negative logarithm to the base 10 of the acid dissociation constant, Ka. $pK_a = -log_{10}(K_a)$. A smaller pKa indicates a stronger acid.
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What is the ionic product of water, Kw?
The equilibrium constant for the auto-ionisation of water. $K_w = [H^+][OH^-]$. At 298 K, its value is $1.0 \times 10^{-14} \ mol^2 \ dm^{-6}$.
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What is a buffer solution?
A solution that resists changes in pH upon the addition of small amounts of acid or alkali. It consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.
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What is the Henderson-Hasselbalch equation?
An equation used to calculate the pH of a buffer solution: $pH = pK_a + log_{10}(\frac{[A^-]}{[HA]})$. It is not explicitly required in the syllabus but is a very useful tool.
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What is the equivalence point in a titration?
The point at which the amount of titrant added is just enough to completely react with the analyte. For an acid-base titration, this is where moles of H⁺ equal moles of OH⁻.
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Common Trap: What is the difference between a strong acid and a concentrated acid?
Strength refers to the degree of dissociation (e.g., HCl is a strong acid as it fully dissociates). Concentration refers to the moles of acid per unit volume. You can have a dilute solution of a strong acid.
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Why is water, $[H_2O]$, omitted from the Ka expression?
In dilute aqueous solutions, the concentration of water is very large and considered constant. It is incorporated into the value of Ka.