1. 1
   A student places some blue hydrated copper(II) sulfate crystals in a test tube and heats them gently. The crystals turn into a white powder and colourless liquid droplets collect on the cooler part of the test tube. When water is added to the white powder, blue crystals reform and the mixture becomes noticeably warmer.

   (a)

   Which symbol is used in a chemical equation to show that a reaction is reversible?

   [1 mark]

   • A⇌
   • B→
   • C=
   • D+

   (b)

   What does the formation of blue crystals and the release of heat when water is added to the white powder show about this reaction?

   [1 mark]

   • AThe reaction is not reversible because a colour change has occurred
   • BThe reverse reaction is exothermic, so the forward (heating) reaction must be endothermic
   • CThe white powder was a different substance from the original blue crystals with no chemical link
   • DWater added to any solid will always release heat

   (c)

   Explain, in terms of reversibility, why the white powder can be described as anhydrous copper(II) sulfate and the blue crystals as hydrated copper(II) sulfate.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemical company operates a reactor in which nitrogen gas and hydrogen gas are pumped in continuously. Inside the reactor, held at a temperature of 450 °C and a pressure of 200 atmospheres over an iron catalyst, the gases react to form ammonia. Not all of the nitrogen and hydrogen is converted to ammonia; unreacted gases are separated from the ammonia product and recycled back into the reactor.

   (a)

   From which raw materials are the nitrogen and hydrogen used in this reactor obtained?

   [1 mark]

   • ABoth nitrogen and hydrogen from limestone
   • BNitrogen from natural gas; hydrogen from the air
   • CBoth nitrogen and hydrogen from crude oil
   • DNitrogen from the air; hydrogen from natural gas

   (b)

   The reaction inside the reactor reaches a dynamic equilibrium. What does this mean?

   [1 mark]

   • AAll the nitrogen and hydrogen have been converted into ammonia
   • BThe forward and reverse reactions have both stopped completely
   • CThe forward and reverse reactions are still happening at equal rates, so the concentrations of reactants and products remain constant
   • DThe rate of the forward reaction is much greater than the rate of the reverse reaction

   (c)

   State two of the conditions used in this reactor for the industrial production of ammonia.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A sample of colourless dinitrogen tetroxide gas (N2O4) is sealed inside a rigid glass gas syringe with no way for gas to escape. Some of the N2O4 decomposes to form brown nitrogen dioxide gas (NO2), according to the equation: N2O4(g) ⇌ 2NO2(g). The forward reaction (decomposition of N2O4 into NO2) is endothermic. The mixture is left until it reaches dynamic equilibrium, and the intensity of the brown colour is used to monitor the relative amount of NO2 present.

   (a)

   Predict and explain what would happen to the position of this equilibrium, and to the colour of the gas mixture, if the temperature of the sealed syringe were increased.

   [3 marks]

   (b)

   Predict and explain what would happen to the position of this equilibrium if the pressure inside the sealed syringe were increased by pushing in the plunger, without changing the temperature.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A production manager at an ammonia plant is reviewing the conditions used for the Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g), forward reaction exothermic. The plant currently uses a temperature of 450 °C and a pressure of 200 atmospheres with an iron catalyst. The manager notes that a lower temperature would give a higher equilibrium percentage yield of ammonia, and that a higher pressure than 200 atmospheres would also increase the equilibrium yield, yet the plant does not use these conditions.

   (a)

   Using ideas about reversible reactions and equilibrium, explain why the plant uses a compromise temperature of 450 °C rather than a much lower temperature, even though a lower temperature would give a higher equilibrium yield of ammonia.

   [6 marks]

   (b)

   Using ideas about dynamic equilibrium, explain why increasing pressure increases the percentage yield of ammonia, and evaluate why the plant uses 200 atmospheres rather than a much higher pressure.

   [6 marks]

   Total for question 4: 12 marks

End of questions