1. 1
   A camping shop sells instant hand warmers that contain solid sodium ethanoate dissolved in water; clicking a metal disc inside the pouch triggers crystallisation and the pouch becomes warm to the touch.

   (a)

   What type of reaction is occurring inside the pouch?

   [1 mark]

   • AAn exothermic reaction, transferring thermal energy to the surroundings
   • BAn endothermic reaction, taking in thermal energy from the surroundings
   • CA reaction with no overall energy change
   • DA combustion reaction, in which a fuel burns in oxygen

   (b)

   What is the sign of the enthalpy change, ΔH, for the reaction taking place in the hand warmer?

   [1 mark]

   • APositive
   • BNegative
   • CZero
   • DΔH cannot have a sign for this type of reaction

   (c)

   Describe, in terms of energy transfer, why the hand warmer pouch feels warm after the disc is clicked.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A sports physiotherapist uses an instant cold pack for treating injuries; squeezing the pack breaks an inner pouch of water, which mixes with solid ammonium nitrate, and the pack becomes noticeably cold.

   (a)

   What type of reaction is occurring inside the pack?

   [1 mark]

   • AAn exothermic reaction, transferring thermal energy to the surroundings
   • BA reaction with a negative enthalpy change
   • CA combustion reaction
   • DAn endothermic reaction, taking in thermal energy from the surroundings

   (b)

   What is the sign of the enthalpy change, ΔH, for the reaction taking place in the cold pack?

   [1 mark]

   • ANegative
   • BZero
   • CPositive
   • DUndefined

   (c)

   Describe, in terms of energy transfer, why the injured area of skin feels cold when the pack is applied.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student burns a small sample of fuel in a school laboratory to heat a beaker of water, and notices the temperature of the water rises steadily.

   (a)

   Explain, in terms of bond breaking and bond making, why this combustion reaction releases energy overall.

   [3 marks]

   (b)

   Explain what is meant by the term activation energy, and describe how the student could tell from the experiment that the fuel and oxygen molecules had sufficient activation energy to react once ignited.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An engineer is designing a portable heater that burns methane gas, CH4, in oxygen according to the equation CH4 + 2O2 → CO2 + 2H2O. The bond energies (in kJ/mol) are: C–H = 413, O=O = 498, C=O = 805, O–H = 464.

   (a)

   Calculate the overall enthalpy change for this reaction, showing your working, and state whether the reaction is exothermic or endothermic.

   [6 marks]

   (b)

   The engineer wants to represent the energy changes in this reaction for a technical report. Describe, without drawing, what a reaction pathway diagram for this reaction would show, including the position of the reactants, products, the overall enthalpy change and the activation energy.

   [6 marks]

   Total for question 4: 12 marks

End of questions