1. 1
   A power station burns methane (natural gas) to generate electricity: CH4 + 2O2 -> CO2 + 2H2O. Bond energies (kJ per mole of bonds): C-H = 413, O=O = 498, C=O = 805, O-H = 464. One mole of methane contains 4 C-H bonds and reacts with 2 O=O bonds. The products contain 2 C=O bonds and 4 O-H bonds.

   (a)

   What is the total energy needed to break all the bonds in the reactants?

   [1 mark]

   • A2648 kJ
   • B1652 kJ
   • C3466 kJ
   • D996 kJ

   (b)

   Using the same bond energies, the products of the reaction contain 2 C=O bonds and 4 O-H bonds. What is the total energy released when these product bonds are formed?

   [1 mark]

   • A1610 kJ
   • B1856 kJ
   • C2648 kJ
   • D3466 kJ

   (c)

   Using your answers above (2648 kJ to break reactant bonds and 3466 kJ released forming product bonds), calculate the overall energy change for the combustion of one mole of methane and state whether the reaction is exothermic or endothermic.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A manufacturer of industrial disinfectants reacts hydrogen gas with chlorine gas to produce hydrogen chloride gas: H2 + Cl2 -> 2HCl. Bond energies (kJ per mole of bonds): H-H = 436, Cl-Cl = 242, H-Cl = 431.

   (a)

   What is the total energy needed to break the bonds in one mole of hydrogen and one mole of chlorine?

   [1 mark]

   • A431 kJ
   • B678 kJ
   • C862 kJ
   • D194 kJ

   (b)

   The product of the reaction, hydrogen chloride, contains 2 H-Cl bonds per mole of reaction. What is the total energy released when these bonds form?

   [1 mark]

   • A431 kJ
   • B678 kJ
   • C862 kJ
   • D1293 kJ

   (c)

   Calculate the overall energy change for this reaction using the values 678 kJ (bonds broken) and 862 kJ (bonds formed), and state whether the process is exothermic or endothermic.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A school science technician repeats a classic nineteenth-century experiment, heating hydrogen gas and iodine vapour together in a sealed glass vessel: H2 + I2 -> 2HI. Bond energies (kJ per mole of bonds): H-H = 436, I-I = 151, H-I = 299.

   (a)

   Calculate the total energy needed to break the bonds in the reactants, showing your working.

   [3 marks]

   (b)

   The product, hydrogen iodide, contains 2 H-I bonds per mole of reaction. Calculate the total energy released forming these bonds, then calculate the overall energy change for the reaction and state whether it is exothermic or endothermic.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fire safety engineer is comparing two products for outdoor rescue teams: a hand warmer that releases heat by oxidising iron powder, and a cold pack that absorbs heat when ammonium nitrate dissolves in water.

   (a)

   Explain, in terms of bond breaking and bond making, why the hand warmer releases energy to the surroundings while the cold pack absorbs energy from the surroundings.

   [6 marks]

   (b)

   Describe how a reaction profile (energy level diagram) for the hand warmer reaction would differ from one for the cold pack reaction, referring to the relative energy levels of reactants and products and to activation energy.

   [6 marks]

   Total for question 4: 12 marks

End of questions