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 answer, you should show how you determine the initial mass of ethanedioic acid to be used. [Ar: C, 12.0; H, 1.0; O, 16.0] [You are given more writing space on the following page] No. of moles of ethanedioic acid = 100/1000 x 1 = 0.1 moles Mass = 0.1 x [2(12.0) + 4(16.0) + 2 (1.0) + 2 (18.0)] = 12.6g (i) Measure the mass of an empty weighing bottle. (ii) Measure 12.60 g of the ethanedioic acid into the weighing bottle. (iii) Transfer the sample into a 100cm 3 beaker. (iv) Dissolve the solid with less than 100 cm3 of distilled water (in beaker) and transfer to a 100cm3 volumetric flask along with the rinsings. (iii) top up with distilled water to 100 cm3 mark (iv) Inversion/shaking/swirling of flask to mix the solution [4] (e) Using the volume and concentration from (c) and ∆T to represent the temperature change, derive a mathematical expression for the enthalpy change of neutralisation

Answers (Prelim 2 H2 Paper 2) PRELIM 2  INNOVA 9647/02/2011 [Turn over For Examiner’s Use of sodium hydroxide with hydrochloric acid. [4.3 J of heat energy raise the temperature of 1 cm3 of any solution by 1 °C] [1] ∆H neutralisation = – (vol of HCl + vol of NaOH) × temp rise × 4.3 moles of water formed = – 21030 3.4)3030( 3    T = – 4300 ∆T (f) Suggest what modification you could make to your experimental procedure to ensure complete neutralisation of the acid. [1] The number of moles of acid used can be made slightly lesser than the no of moles of NaOH used so that the acid is the limiting reagent. [Total: 12] 2 The use of the Data Booklet is relevant to this question. (a) The diagram below shows an experimental setup when a copper rod is dipped into a silver nitrate solution. After one hour, the copper rod is coated with a shiny substance and the solution turned slightly blue. (i) Write an ionic equation, with state symbols, for the reaction that has occurred. Cu(s) +2Ag+(aq)  Cu2+(aq) + 2Ag(s) (ii) Given that ∆Gθ = –nF Eθ, where n is the number of moles of electrons transferred in the reaction, and Eθ is the electromotive force generated, calculate ∆Gθ, in kJ mol-1, of the above reaction at standard conditions. Cu2+(aq) + 2e Cu(s) Eθ = +0.34V Ag+(aq) + e Ag(s) Eθ = +0.80V Eθ = +0.80- (+0.34) = +0.46V ∆Gθ= –nFEθ = – 2 X 96500X 0.46 = –88.8 kJ mol -1 (spontaneous) (iii) Suggest and explain if the above reaction has a positive or negative ∆Sθ. As the number of free aqueous ions is reduced, the system will be less disordered. The sign of ∆Sθ should be negative. [4]

Answers (Prelim 2 H2 Paper 2) PRELIM 2  INNOVA 9647/02/2011 [Turn over For Examiner’s Use (b) In the laboratory, there are three bottles labelled N, P and Q. Each bottle contains one of the following reagents: Cl2(aq), NaI(aq) and KBr(aq) Three tests were carried out using the reagents in the bottles. The results are summarised in the table below: Test Procedure Observations 1 Mix reagent in bottle N with reagent in bottle P No Change 2 Mix reagent in bottle N with reagent in bottle Q Mixture turn brown 3 Mix reagent in bottle P with reagent in bottle Q Mixture turn brown (i) Deduce and explain which bottle contains C l2(aq). Write chemical equations with state symbols to support your reasoning. Bottle Q contains Cl2 (aq) which oxidized / displaced Br- (aq) to Br2 (aq) and I- (aq) to I2 (aq) which accounts for the brown mixture obtained. C l2 (aq) + 2Br- (aq)  2Cl- (aq) + Br2 (aq) C l2 (aq) + 2I- (aq)  2Cl- (aq) + I2 (aq) (ii) To determine which bottle contains Na I(aq) and KBr(aq) respectively, tests 2 and 3 were repeated. Hexane was added to the resulting reaction mixture after the tests were conducted. The bottles were then shaken and allowed to stand. State the observations which will help to identify which bottle contains Na I(aq) and KBr(aq) respectively. The bottle with reddish brown organic layer contains KBr (aq) initially. The bottle with violet / purple organic layer contains NaI (aq) initially. [5] (c) Arrange the following compounds in order of decreasing pK b, giving reasons for your answer. NH2 NH2 NHCOCH3 Highest pKb Lowest p Kb

Answers (Prelim 2 H2 Paper 2) PRELIM 2  INNOVA 9647/02/2011 [Turn over For Examiner’s Use For , the lone pair of electrons on N atom is delocalized over the carbonyl group and is not available to accept a proton. Hence it is neutral and least basic among the three compounds. Phenyl