IJC_H2_CHEM_P2_ANS

Answers (Prelim 2 H2 Paper 2) PRELIM 2  INNOVA 9647/02/2011 [Turn over For Examiner’s Use Answer ALL questions on the spaces provided. 1 You are given a number of different acids to investigate how the enthalpy change of neutralisation, ∆Hneutralisation, varies when the acids are neutralised with aqueous sodium hydroxide, NaOH. (a) (i) Hydrochloric acid, nitric acid and sulfuric acid are all strong acids. Predict how ∆H neutralisation values for these acids would compare with each other. The three acids should have the same ∆H neut values. Since all strong acids dissociate fully in water and ∆Hneutralisation is when one mole of water is produced during acid-base reaction. (ii) Predict and explain how the expected ∆Hneutralisation for a weak acid, such as ethanoic acid or ethanedioic acid, compares with that for hydrochloric acid. [2] ∆H neut for a weak acid should be less exothermic ( ∆H is more positive or less negative) than a strong acid because some of the energy is absorbed when the weak acid dissociates into ions. (b) (i) Draw and label a diagram of the apparatus you would use to determine the temperature change, ∆T, when each of the acids reacts with 30.0 cm 3 of 2.0 mol dm-3 aqueous sodium hydroxide. Any experimental method that is normally carried out in a college laboratory may be used. beaker polystyrene cup thermometer lid reaction mixture

Answers (Prelim 2 H2 Paper 2) PRELIM 2  INNOVA 9647/02/2011 [Turn over For Examiner’s Use (ii) Identify one possible source of error in the experiment and state how you would minimise the effect. [3] S/N Source of error Improvement 1 Heat is lost to the surroundings. Use wind-shield. (If a lid is not used in the set-up above, a lid can be used to cover the cup to reduce heat loss (due to convection). (If a beaker is not used in the set-up above, a beaker can be used to contain the cup to reduce heat loss (due to convection).) 2 The thermometer used may be of low precision. Use a more precise thermometer to allow temperature to be read to a greater degree of precision. 3 There could be some loss of material if the solution overflows or sprays out. Use a larger cup/container. (If a lid is not used in the set-up above, a lid can be used to minimise spray.) (c) In each experiment, 30.0 cm3 of 2.0 mol dm-3 aqueous NaOH is to be used. Suggest appropriate volumes for each of the acids to be used in the individual experiments. [1] Ans Acid Volume/ cm 3 Concentration/ mol dm -3 Hydrochloric HCl 30 2.0 Ethanedioic (CO 2H)2 15 1.0 (d) Ethanedioic acid is a hydrated crystalline solid, (CO2H)2.2H2O. Outline how you would prepare 100 cm 3 of 1 mol dm-3 solution of ethanedioic acid. In your