Acids & Bases (A-Level) — A-Level Chemistry Revision
Revise Acids & Bases (A-Level) for A-Level Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel and OCR.
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Go to Electrode PotentialsWhat is Acids & Bases (A-Level)?
A-Level Chemistry introduces the Brønsted-Lowry theory of acids and bases, where acids are proton donors and bases are proton acceptors. The concept of conjugate acid-base pairs is central. The topic delves into pH calculations for strong and weak acids and bases, using the acid dissociation constant, Ka, and the ionic product of water, Kw. Buffer solutions, which resist changes in pH, are also a key area of study, vital for biological systems and chemical applications.
Board notes: All boards require proficiency in pH, Ka, and buffer calculations. AQA often includes questions involving titration curves, asking students to identify buffer regions and choose appropriate indicators. Edexcel may focus on the application of buffers in biological systems, such as the carbonic acid-hydrogencarbonate buffer in blood. OCR frequently tests the calculation of pH for a variety of acidic and basic solutions, including diprotic acids.
Step-by-step explanationWorked example
Calculate the pH of a 0.100 mol dm-3 solution of ethanoic acid (CH3COOH), given that its Ka = 1.74 x 10^-5 mol dm-3. Step 1: Write the Ka expression: Ka = [H+][CH3COO-]/[CH3COOH]. Step 2: Make the approximation that [H+] = [CH3COO-] and that the equilibrium concentration of [CH3COOH] is approximately the initial concentration (0.100 M). Step 3: Rearrange to find [H+]: [H+] = sqrt(Ka * [CH3COOH]) = sqrt(1.74e-5 * 0.100) = 1.32 x 10^-3 mol dm-3. Step 4: Calculate pH: pH = -log10([H+]) = -log10(1.32e-3) = 2.88.
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Common mistakes
- 1Confusing strong/weak with concentrated/dilute. A strong acid fully dissociates in water, whereas a weak acid only partially dissociates. Concentration refers to the amount of acid dissolved in a given volume of water.
- 2Incorrectly setting up Ka expressions. The expression for a weak acid HA is Ka = [H+][A-]/[HA], with concentrations being the equilibrium values, not the initial ones.
- 3Assuming the pH of a neutral solution is always 7. The pH of a neutral solution is only 7 at 298K (25°C). Since the autoionisation of water is endothermic, Kw increases with temperature, and the pH of a neutral solution decreases (e.g., to 6.77 at 313K).
Acids & Bases (A-Level) exam questions
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Acids & Bases (A-Level)
Core concept
A-Level Chemistry introduces the Brønsted-Lowry theory of acids and bases, where acids are proton donors and bases are proton acceptors. The concept of conjugate acid-base pairs is central. The topic …
Frequently asked questions
What is a buffer solution?
A buffer solution is a mixture that resists changes in pH when small amounts of acid or alkali are added. It is typically made from a weak acid and its conjugate base (e.g., ethanoic acid and sodium ethanoate), or a weak base and its conjugate acid.
How does a buffer solution work?
It contains a reservoir of the weak acid (to react with added alkali) and its conjugate base (to react with added acid). These reactions remove the added H+ or OH- ions, thus keeping the pH of the solution relatively stable.