1. 1
   A student dissolves solid ammonium nitrate in water in an insulated cup, and uses a thermometer to record the temperature of the solution every 30 seconds; the temperature of the solution falls from 20°C to 8°C over the course of the experiment before slowly rising back towards room temperature.

   (a)

   Is dissolving ammonium nitrate in this experiment an exothermic or endothermic process?

   [1 mark]

   • AExothermic, because the temperature fell
   • BEndothermic, because the temperature fell
   • CExothermic, because the temperature rose
   • DEndothermic, because the temperature rose

   (b)

   Which term describes the direction of overall energy transfer between the solution and the process of dissolving while the temperature is falling?

   [1 mark]

   • AEnergy is transferred from the solution/surroundings into the dissolving process
   • BEnergy is transferred from the dissolving process to the solution/surroundings
   • CNo energy transfer takes place
   • DEnergy is transferred equally in both directions, cancelling out

   (c)

   Describe, in terms of relative energy levels, how the energy of the products (the dissolved ions in solution) compares with the energy of the reactants (the solid ammonium nitrate and water) in this endothermic process, and state whether energy is absorbed or released overall.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student investigates the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid by placing a conical flask containing the two solutions on top of a printed black cross on paper, and timing how long it takes for the cross to become no longer visible from above as a yellow precipitate of sulfur forms and makes the solution cloudy. The experiment is repeated using solutions of the same total volume but different concentrations of sodium thiosulfate, keeping the volume and concentration of hydrochloric acid the same.

   (a)

   As the concentration of sodium thiosulfate solution used is increased, what happens to the time taken for the cross to disappear?

   [1 mark]

   • AThe time increases
   • BThe time stays the same
   • CThe time decreases
   • DThe cross no longer disappears

   (b)

   Using collision theory, why does increasing the concentration of sodium thiosulfate decrease the time taken for the cross to disappear?

   [1 mark]

   • AThere are more thiosulfate particles in a given volume, so collisions between reacting particles happen more frequently
   • BThe particles move faster at a higher concentration, so each collision has more energy
   • CA higher concentration lowers the activation energy of the reaction
   • DA higher concentration means fewer collisions are needed to react

   (c)

   Suggest one reason why this experiment gives only a subjective (judged by eye) rather than fully accurate measure of the time taken for the reaction to reach the same point each time, and suggest one way the method could be improved to make the measurement more objective.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student adds 2.5 g of solid sodium hydroxide to 100 cm3 of water in an insulated polystyrene cup and stirs until fully dissolved, recording a temperature rise from 20.0°C to 26.8°C. (Specific heat capacity of the solution, c = 4.2 J/g°C. Assume the density of the solution is 1 g/cm3, and Mr of NaOH = 40.)

   (a)

   Calculate the heat energy released, Q, during the dissolving of the sodium hydroxide, using Q = mcΔT, where m is the mass of the solution.

   [3 marks]

   (b)

   Calculate the number of moles of sodium hydroxide used, and hence calculate the molar enthalpy change, ΔH, for the dissolving of sodium hydroxide, in kJ/mol, giving your answer to an appropriate number of significant figures.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist studies the reaction between hydrogen and iodine to form hydrogen iodide: H2(g) + I2(g) → 2HI(g). Bond energies (in kJ/mol) are: H–H = 436, I–I = 151, H–I = 299. The chemist also investigates how adding a platinum catalyst changes the rate of a different, separate, uncatalysed exothermic reaction.

   (a)

   Use the bond energies given to calculate the overall enthalpy change, ΔH, for the reaction H2(g) + I2(g) → 2HI(g), and explain, in terms of bond breaking and bond making, whether the reaction is exothermic or endothermic.

   [6 marks]

   (b)

   The chemist finds that adding a platinum catalyst greatly increases the rate of the separate, uncatalysed exothermic reaction. Explain, in terms of activation energy and reaction pathway, how a catalyst increases the rate of a reaction, and explain why the same overall enthalpy change, ΔH, is obtained for the reaction whether or not the catalyst is used.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A student heats pink, hydrated cobalt(II) chloride paper strongly, and it turns blue as water is driven off; the paper is then left in a room with damp air overnight, and by the next morning it has turned back to a pink colour, without being heated further.

   (a)

   Which symbol in a chemical equation would be used to show that this change from pink to blue and back to pink is reversible?

   [1 mark]

   • A→
   • B=
   • C+
   • D⇌

   (b)

   What does it mean to say that this reaction reaches dynamic equilibrium if it were carried out in a sealed container?

   [1 mark]

   • AThe forward reaction stops completely and only the reverse reaction continues
   • BThe forward and reverse reactions occur at the same rate, and the concentrations of reactants and products remain constant
   • CAll of the reactants have been completely converted into products
   • DThe reaction stops completely and no further change occurs

   (c)

   Explain why the paper turning pink again overnight, without any heating, does not happen if the paper is instead sealed inside a dry, airtight bag.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A reversible reaction taking place in a sealed container is represented by the equation: 2SO2(g) + O2(g) ⇌ 2SO3(g), which is exothermic in the forward direction. A student investigates how changing the temperature and pressure of the sealed container affects the position of equilibrium.

   (a)

   If the pressure inside the sealed container is increased, in which direction does the position of equilibrium shift?

   [1 mark]

   • AIt shifts towards the reactants, since there are more moles of gas on that side
   • BThe position of equilibrium is not affected by pressure
   • CIt shifts towards the products, since there are fewer moles of gas on that side
   • DIt shifts towards the reactants, since there are fewer moles of gas on that side

   (b)

   If the temperature of the sealed container is increased, in which direction does the position of equilibrium shift, given that the forward reaction is exothermic?

   [1 mark]

   • ATowards the products, since the forward reaction is exothermic
   • BThe position of equilibrium is not affected by temperature
   • CTowards the products, since increasing temperature favours the endothermic (reverse) reaction
   • DTowards the reactants, since increasing temperature favours the endothermic (reverse) reaction

   (c)

   Explain why adding a catalyst to this reaction would increase the rate at which equilibrium is reached, but would not change the position of equilibrium (the proportions of reactants and products present at equilibrium).

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A laboratory technician investigates the rate of reaction between excess marble chips and dilute hydrochloric acid by measuring the total mass of carbon dioxide gas lost from the reaction flask, placed on a balance, every 30 seconds. The results show that 0.44 g of gas had been lost after 60 seconds, and that no further mass was lost after 240 seconds, when a total of 1.10 g of gas had been lost.

   (a)

   Calculate the average rate of reaction, in g/s, over the first 60 seconds of the experiment, and calculate the average rate of reaction, in g/s, over the whole reaction, from 0 to 240 seconds.

   [3 marks]

   (b)

   Explain, in terms of collision theory, why the rate of reaction would have been greatest at the very start of the experiment, and explain why no further mass is lost after 240 seconds, even though excess marble chips remain in the flask.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A reversible reaction is represented by the equation 2NO2(g) ⇌ N2O4(g), where the forward reaction (formation of colourless N2O4) is exothermic. A sealed gas syringe containing a mixture of brown nitrogen dioxide (NO2) and colourless dinitrogen tetroxide (N2O4) gas is placed first in a beaker of hot water, and the gas mixture becomes noticeably more brown; it is then placed in a beaker of icy water, and the gas mixture becomes noticeably less brown (paler).

   (a)

   Explain, in terms of the position of equilibrium, why the gas mixture becomes more brown when the sealed syringe is placed in hot water, and why it becomes paler when placed in icy water.

   [6 marks]

   (b)

   Using the idea of bond breaking and bond making, explain why the forward reaction (2NO2 → N2O4) is exothermic, and explain how the energy released in this reaction compares with the energy required for the reverse reaction (N2O4 → 2NO2).

   [6 marks]

   Total for question 8: 12 marks

End of questions