AQA A Level Chemistry 3.1.12 - Acids and Bases Graded A+

AQA A Level Chemistry 3.1.12 - Acids and Bases Graded A+ Bronsted-Lowry acid Bronsted-Lowry base Proton donor Proton acceptor pH Measure of acidity by measuring concentration of H⁺ ions. Low pH means Acidity High pH means Alkalinity Calculating pH Calculating [H⁺] [H⁺] = 10 pH = -log [H⁺] -pH What does an acid/base reaction involve? Strong acid Fully dissociates in water. Transfer of the H⁺ from the acid to the base. Weak acid Only very slightly dissociates in water. Ka weak acid dissociation constant [HA] The more a weak acid dissociates... The stronger the acid. The larger the Ka value. Calculating pKa Ka = [H⁺] [A⁻] The more H⁺ are formed. pKa = -log (Ka) Calculating Ka from pKa Ka = 10 Dissociation of water -pKa H₂O ⇌ H⁺/H₃O⁺ + OH⁻ When water boils what happens to it's pH? 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) Water dissociating is endothermic, and the reverse Kw = [H⁺] [OH⁻] = 1.0 x 10⁻¹⁴ Calculating pH of a base [OH⁻] [H⁺] = Kw 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? What does a basic buffer consist of? How can an acidic buffer be made? Part-neutralise HA with NaOH. How can a basic buffer be made? Weak acid (HA) with its salt (Na⁺A⁻) Weak base with its salt. Mix HA with Na⁺A⁻. Mix weak base with its salt (e.g. NH₃ with NH₄Cl). Part-neutralise weak base with HCl. When H⁺ added to acidic buffer... Higher concentration of H⁺. Le Chatelier's principle. H⁺ + A⁻ → HA When OH⁻ added to acidic buffer... Unbalances equilibrium. Lower concentration of H⁺. Le Chatelier's principle. HA → H⁺ + A⁻ When H⁺ added to basic buffer... Unbalances equilibrium. Higher concentration of H₂O. Le Chatelier's principle. NH₃ + H₂O → NH₄⁺ + OH⁻ When OH⁻ added to basic buffer... Higher concentration of OH⁻. Le Chatelier's principle. NH₄⁺ + OH⁻ → NH₃ + H₂O1. Unbalances equilibrium. OH⁻ + H⁺ → H₂O OH⁻ + H⁺ → H₂O Unbalances equilibrium. 1. Calibration curve of pH meter and electrode using buffer How is a pH titration carried out? 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 At half equivalence... Mi-point of "vertical section". [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? Orange - pH4 Red - pH3 What colour changes does phenolphthalein show? Pale pink - pH9 Colourless - pH8 Yellow - pH5 Pink - pH10 When can you NOT use phenolphthalein? With a weak base. When can you NOT use methyl orange? With a weak acid.