1. Which statement is correct for a crystal of iron(ii) sulfate in a state of equilibrium with a saturated solution of iron(ii) sulfate?

Transcription

1 1. Which statement is correct for a crystal of iron(ii) sulfate in a state of equilibrium with a saturated solution of iron(ii) sulfate? A. The colour of the solution darkens as the crystal continues to dissolve. B. The concentration of the iron(ii) sulfate solution increases as the water evaporates. C. The shape of the iron(ii) sulfate crystal does not change. D. The colour of the solution does not change but the shape of the crystal may change. (Total 1 mark) 2. The reaction below represents the Haber process for the industrial production of ammonia. N 2 (g) + 3H 2 (g) 2NH 3 (g) H O = 92 kj The optimum conditions of temperature and pressure are chosen as a compromise between those that favour a high yield of ammonia and those that favour a fast rate of production. Economic considerations are also important. Which statement is correct? A. A higher temperature would ensure higher yield and a faster rate. B. A lower pressure would ensure a higher yield at a lower cost. C. A lower temperature would ensure a higher yield and a faster rate. D. A higher pressure would ensure a higher yield at a higher cost. (Total 1 mark) IB Questionbank Chemistry 1

2 3. Consider the endothermic reaction below. 5CO(g) + I 2 O 5 (g) 5CO 2 (g) + I 2 (g) According to Le Chatelier s principle, which change would result in an increase in the amount of CO 2? A. Increasing the temperature B. Decreasing the temperature C. Increasing the pressure D. Decreasing the pressure (Total 1 mark) 4. For the reaction below: H 2 (g) + I 2 (g) 2HI(g) at a certain temperature, the equilibrium concentrations, in mol dm 3, are [H 2 (g)] = 0.30, [I 2 (g)] = 0.30, [HI(g)] = 3.0 What is the value of K c? A B. 10 C. 33 D (Total 1 mark) IB Questionbank Chemistry 2

3 5. An example of a homogeneous reversible reaction is the reaction between hydrogen and iodine. H 2 (g) + I 2 (g) 2HI(g) (i) Outline the characteristics of a homogeneous chemical system that is in a state of equilibrium. (2) (ii) Deduce the expression for the equilibrium constant, K c. (1) (iii) Predict what would happen to the position of equilibrium and the value of K c if the pressure is increased from 1 atm to 2 atm. (2) (iv) The value of K c at 500 K is 160 and the value of K c at 700 K is 54. Deduce what this information tells us about the enthalpy change of the forward reaction. (1) IB Questionbank Chemistry 3

4 (v) The reaction can be catalysed by adding platinum metal. State and explain what effect the addition of platinum would have on the value of the equilibrium constant. (2) (Total 8 marks) 6. In carbonated drinks containing dissolved carbon dioxide under high pressure, the following dynamic equilibrium exists. CO 2 (aq) CO 2 (g) Describe the effect of opening a carbonated drink container and outline how this equilibrium is affected. (Total 2 marks) 7. When a mixture of mol NO, mol H 2 and mol H 2 O were placed in a 1.0 dm 3 flask at 300 K, the following equilibrium was established. 2NO(g) + 2H 2 (g) N 2 (g) + H 2 O(g) At equilibrium, the concentration of NO was found to be mol dm 3. Determine the equilibrium constant, K c, of the reaction at this temperature. (Total 4 marks) IB Questionbank Chemistry 4

5 8. The ph of a solution changes from ph = 2 to ph = 5. What happens to the concentration of the hydrogen ions during this ph change? A. It decreases by a factor of 1000 B. It increases by a factor of 1000 C. It decreases by a factor of 100 D. It increases by a factor of 100 (Total 1 mark) 9. Which compounds can be mixed together as solutions of equal volume and concentration to form a buffer solution? A. Nitric acid and potassium hydroxide B. Nitric acid and potassium nitrate C. Propanoic acid and potassium hydroxide D. Propanoic acid and potassium propanoate (Total 1 mark) IB Questionbank Chemistry 5

6 10. The graph below shows the titration curve of 25 cm 3 of mol dm 3 of hydrochloric acid with sodium hydroxide, of mol dm 3 concentration. The indicator methyl orange was used to determine the equivalence point. Methyl orange has a ph range of If the hydrochloric acid was replaced by ethanoic acid of the same volume and concentration, which property of the titration would remain the same? A. The initial ph B. The ph at the equivalence point C. The volume of strong base, NaOH, needed to reach the equivalence point D. The colour of the titration mixture just before the equivalence point is reached (Total 1 mark) IB Questionbank Chemistry 6

7 11. Consider the equilibrium below. CH 3 CH 2 COOH(aq) + H 2 O(l) CH 3 CH 2 COO (aq) + H 3 O + (aq) Which species represent a conjugate acid-base pair? A. CH 3 CH 2 COOH and H 2 O B. H 2 O and CH 3 CH 2 COO C. H 3 O + and H 2 O D. CH 3 CH 2 COO and H 3 O + (Total 1 mark) 12. The K b value for a base is mol dm 3 at 298 K. What is the K a value for its conjugate acid at this temperature? A B C D (Total 1 mark) 13. (i) Define a Lewis acid and state an example that is not a Brønsted-Lowry acid. (2) (ii) Draw structural formulas to represent the reaction between the Lewis acid named in (i) and a Lewis base and identify the nature of the bond formed in the product. (4) (Total 6 marks) IB Questionbank Chemistry 7

8 14. In an experiment conducted at 25.0 C, the initial concentration of propanoic acid and methanol were 1.6 mol dm 3 and 2.0 mol dm 3 respectively. Once equilibrium was established, a sample of the mixture was removed and analysed. It was found to contain 0.80 mol dm 3 of compound X. (i) Calculate the concentrations of the other three species present at equilibrium. (3) (ii) State the equilibrium constant expression, K c, and calculate the equilibrium constant for this reaction at 25.0 C. (2) (Total 5 marks) IB Questionbank Chemistry 8

9 15. Determine the ph of the solution resulting when 100 cm 3 of 0.50 mol dm 3 HCl(aq) is mixed with 200 cm 3 of 0.10 mol dm 3 NaOH(aq) (Total 5 marks) 16. Some of the most important processes in chemistry involve acid-base reactions. (i) Calculate the K a value of benzoic acid, C 6 H 5 COOH, using Table 15 in the Data Booklet. (1) (ii) Based on its K a value, state and explain whether benzoic acid is a strong or weak acid. (2) IB Questionbank Chemistry 9

10 (iii) Determine the hydrogen ion concentration and the ph of a mol dm 3 benzoic acid solution. State one assumption made in your calculation. (4) (Total 7 marks) 17. Define an acid in terms of the Lewis theory. Deduce, giving a reason, whether NF 3 is able to function as a Lewis acid or as a Lewis base (Total 2 marks) 18. An experiment was carried out to determine the concentration of aqueous ammonia by titrating it with a mol dm 3 sulfuric acid solution. It was found that 25.0 cm 3 of the aqueous ammonia required 20.1 cm 3 of the sulfuric acid solution for neutralization. (a) Write the equation for the reaction and calculate the concentration, in mol dm 3, of the aqueous ammonia. (4) (b) Several acid-base indicators are listed in Table 16 of the Data Booklet. Identify one indicator that could be used for this experiment. Explain your answer. (3) IB Questionbank Chemistry 10

11 (c) (i) Determine the poh of mol dm 3 aqueous ammonia (pk b = 4.75). (4) (ii) State what is meant by the term buffer solution, and describe the composition of an acid buffer solution in general terms. (3) (iii) Calculate the ph of a mixture of 50.0 cm 3 of mol dm 3 aqueous ammonia and 50.0 cm 3 of mol dm 3 hydrochloric acid solution. (4) (Total 18 marks) IB Questionbank Chemistry 11

12 1. D 2. D 3. A 4. D 5. (i) reactants and products in same phase/state; rate of forward reaction = rate of reverse reaction; concentrations of reactants and products remain constant / macroscopic properties remain constant; Do not accept concentrations are equal. 2 max (ii) (K c ) = [HI 2 ] ; 1 [H ][I ] 2 2 (iii) no change to position of equilibrium; no change to value of K c ; 2 (iv) the reaction is exothermic/heat is given out/ H is negative; 1 (v) no effect (on the value of the equilibrium constant); as it speeds up forward and reverse reaction / concentrations of reactants and products do not change / position of equilibrium does not change / no change in yield; 2 [8] 6. CO 2 (g) /gas escapes / (gas) pressure / [CO 2 ] (above liquid) decreases / bubbles (of CO 2 gas) form in the liquid; equilibrium shifts to the right (to replace the lost CO 2 gas); 2 [2] 7. 2NO(g) + 2H 2 (g) N 2 (g) + 2H 2 O(g) IB Questionbank Chemistry 1

13 NO(g) H 2 (g) N 2 (g) H 2 O(g) Initial/mol dm Change/mol dm Equilibrium/mol dm 3 [H 2 ] at equilibrium = (mol dm 3 ); [N 2 ] at equilibrium = (mol dm 3 ); [H 2 O] at equilibrium = (mol dm 3 ); K c = [N 2 ][H 2 O] 2 /[NO] 2 [H 2 ] 2 = (0.019)(0.138) 2 / (0.062) 2 (0.013) 2 = ; Award [4] for final correct answer. Accept any value also in range Do not penalize significant figures. 4 [4] 8. A 9. D 10. C 11. C 12. D 13. (i) (Lewis acid) electron pair acceptor; appropriate example (such as AlCl 3, BF 3 etc.); 2 IB Questionbank Chemistry 2

14 (ii) structural formula of Lewis acid (e.g. BF 3, AlCl 3, Transition element etc.); structural formula of Lewis base (e.g. NH 3, H 2 O etc.); structural formula of product (e.g. F 3 BNH 3 etc.); dative covalent (bond)/coordinate (bond); Penalize missing structural formulas once. 4 [6] 14. (i) [CH 3 CH 2 COOH]: ( =) 0.8 (mol dm 3 ); [CH 3 OH]: ( =) 1.2 (mol dm 3 ); [H 2 O]: 0.80 (mol dm 3 ); 3 (ii) [CH 3CH 2COOCH 3 ][H 2O] (K c =) ; [CH 3CH 2COOH][CH 3OH] 2 [(0.80) ] (K c = [( )] =) 0.7; 2 Allow Award [1 max] for [5] 15. n(hcl) = ( ) = (mol); n(naoh) = ( ) = (mol); n(hcl) remaining = ( ) = (mol); [HCl] = = 0.10 (mol dm 3 ); 0.30 ph = 1.0; 5 Award [2 max] for just ph = 1.0 without working. [5] 16. (i) K a = / ; Accept (ii) weak (acid); IB Questionbank Chemistry 3

15 K a << 1/ small K a ; 2 (iii) [H 3 O + ]/[H + ] = K a ; [H 3 O + ]/[H + ] = (mol dm 3 ); ph = 3.10/3.1/3.12; Award [3] for correct final answer of ph. assume x << (mol dm 3 )/ ionization of water is insignificant / [C 6 H 5 COOH] initial = [C 6 H 5 COOH] aq / temperature 25 C/298 K; 4 [7] 17. a Lewis acid can accept a pair of electrons; it is a Lewis base as it can donate the lone/non-bonding pair of electrons (on the N atom); 2 Do not award second mark for simply stating it is a Lewis base with no reason given. [2] 18. (a) 2NH 3 (aq) + H 2 SO 4 (aq) (NH 4 ) 2 SO 4 (aq); 4 Accept correct equation with NH 4 OH instead of NH 3 n(h 2 SO 4 ) = (mol); n(nh 3 ) = (mol); [NH 3 ] = (mol dm 3 ); Award [3] for the correct final answer for the concentration calculation. (b) bromocresol green; reaction of weak base and strong acid; ph range of bromocresol green is 3.8 to 5.4/occurs at ph < 7; 3 (c) (i) K b = = ; + [NH 4 ][OH ] K b = /[OH ] = K b[nh 3 ]; [NH ] 3 [OH ] = ; poh = 2.83; 4 Award [4] for the correct final answer. Allow ECF, for example any correct conversion of [OH ] to poh. IB Questionbank Chemistry 4

16 (ii) a solution which resists change in ph / changes ph very slightly; when small amounts of acid or base are added; weak acid and its salt / weak acid and its conjugate base; 3 (iii) n(nh 3 ) = (mol) and n(hcl) = (mol); + [ NH 4 ] = [NH 3 ]; [OH ] = K b = ; (poh = 4.75 so) ph = 9.25 (allow 9.2 to 9.3); 4 Award [4] for correct final answer. Accept other valid methods. [18] IB Questionbank Chemistry 5