AQA A Level Chemistry 3.1.12 - Acids and Bases Questions and Answers

AQA A Level Chemistry 3.1.12 - Acids and Bases Questions and Answers Bronsted-Lowry acid Proton donor Bronsted-Lowry base Proton acceptor pH Measure of acidity by measuring concentration of H⁺ ions. Low pH means Acidity High pH means Alkalinity Calculating pH pH = -log [H⁺] Calculating [H⁺] -pH [H⁺] = 10 What does an acid/base reaction involve? Transfer of the H⁺ from the acid to the base. Strong acid Fully dissociates in water. Weak acid Only very slightly dissociates in water. Ka weak acid dissociation constant Ka = [H⁺] [A⁻] [HA] The more a weak acid dissociates... The more H⁺ are formed. The stronger the acid. The larger the Ka value. Calculating pKa pKa = -log (Ka) Calculating Ka from pKa -pKa Ka = 10 Dissociation of water H₂O ⇌ H⁺/H₃O⁺ + OH⁻ When water boils what happens to it's pH? Water dissociating is endothermic, and the reverse reaction is exothermic. Le Chatelier's principle. Equilibrium shifts to right to lower temperature. More H⁺ so pH lower (more acidic). Oxonium ion H₃O⁺ The ionic product of water (at 298K) Kw = [H⁺] [OH⁻] = 1.0 x 10⁻¹⁴ Calculating pH of a base [H⁺] = Kw [OH⁻] Buffer Solution that maintains an almost constant pH on addition of small amounts of strong acid or strong base. What does an acidic buffer consist of? Weak acid (HA) with its salt (Na⁺A⁻) What does a basic buffer consist of? Weak base with its salt. How can an acidic buffer be made? Mix HA with Na⁺A⁻. Part-neutralise HA with NaOH. How can a basic buffer be made? Mix weak base with its salt (e.g. NH₃ with NH₄Cl). Part-neutralise weak base with HCl. When H⁺ added to acidic buffer... Unbalances equilibrium. Higher concentration of H⁺. Le Chatelier's principle. H⁺ + A⁻ → HA When OH⁻ added to acidic buffer... OH⁻ + H⁺ → H₂O Unbalances equilibrium. Lower concentration of H⁺. Le Chatelier's principle. HA → H⁺ + A⁻ When H⁺ added to basic buffer... OH⁻ + H⁺ → H₂O Unbalances equilibrium. Higher concentration of H₂O. Le Chatelier's principle. NH₃ + H₂O → NH₄⁺ + OH⁻ When OH⁻ added to basic buffer... Unbalances equilibrium. Higher concentration of OH⁻. Le Chatelier's principle. NH₄⁺ + OH⁻ → NH₃ + H₂O1. How is a pH titration carried out? 1. Calibration curve of pH meter and electrode using buffer solutions. 2. Prepare graph of pH (y-axis) against volume of alkali added (x-axis). 3. Pipette 25cm³ 0.1M acid into 250cm³ beaker. Measure pH and record as initial pH. 4. Add 0.1M alkali from burette in appropriate portions, measuring and plotting pH after each addition. 5. Repeat up to 50cm³. 6. Plot curve of best fit. Equivalence volume/endpoint Mi-point of "vertical section". At half equivalence... [A⁻] = [HA] Ka = [H⁺] pKa = pH Buffer region Around pa where pH changes very little, despite acid/base being constantly added. Acid-base indicators Weak acids. Conjugate acid and base forms have different colours. An indicator colour change must... Be rapid Occur at exactly the same time as the equivalence point. What colour changes does methyl orange show? Yellow - pH5 Orange - pH4 Red - pH3 What colour changes does phenolphthalein show? Pink - pH10 Pale pink - pH9 Colourless - pH8