1. 1
   A student is studying the energy changes that occur during methane combustion, in which bonds in methane and oxygen molecules are broken and new bonds form in the carbon dioxide and water produced: CH_4 + 2O_2 $\rightarrow$ CO_2 + 2H_2O.

   (a)

   During a chemical reaction, bonds in the reactants must be broken before new bonds can form in the products. What is true about the energy involved in breaking bonds?

   [1 mark]

   • AEnergy must be supplied to break bonds
   • BEnergy is released when bonds are broken
   • CNo energy change occurs when bonds are broken
   • DBond breaking only occurs in endothermic reactions

   (b)

   What is true about the energy involved when new bonds form in the products?

   [1 mark]

   • AEnergy must be supplied to form new bonds
   • BEnergy is released when new bonds form
   • CNo energy change occurs when new bonds form
   • DBond forming only occurs in endothermic reactions

   (c)

   Methane combustion is known to be an exothermic reaction. Explain, in terms of bond breaking and bond making, why this reaction releases energy overall.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A textbook example describes the thermal decomposition of calcium carbonate, CaCO_3 $\rightarrow$ CaO + CO_2, in which the energy needed to break the bonds within the calcium carbonate is greater than the energy released when new bonds form in the calcium oxide and carbon dioxide produced.

   (a)

   A student is told that in a particular reaction, more energy is needed to break the bonds in the reactants than is released when new bonds form in the products. What type of reaction is this?

   [1 mark]

   • AExothermic
   • BNeutral, with no energy change
   • CEndothermic
   • DA physical change, not a chemical reaction

   (b)

   For this reaction, what would be the sign of the overall energy change, $\Delta H$?

   [1 mark]

   • ANegative
   • B$\Delta H$ cannot be assigned a sign for decomposition reactions
   • CZero
   • DPositive

   (c)

   Explain why thermal decomposition reactions like this one need a continuous supply of heat energy from a Bunsen burner in order to continue.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is given bond energy data for the reaction between hydrogen and chlorine gas, H_2(g) + Cl_2(g) $\rightarrow$ 2HCl(g), and is asked to use this data to calculate the overall energy change for the reaction.

   (a)

   In the reaction between hydrogen and chlorine, H_2 + Cl_2 $\rightarrow$ 2HCl, the bond energy needed to break one mole of H-H bonds and one mole of Cl-Cl bonds totals 680 kJ, while the bond energy released forming two moles of H-Cl bonds totals 862 kJ. Calculate the overall energy change, $\Delta H$, for this reaction, showing your working, and state whether the reaction is exothermic or endothermic.

   [3 marks]

   (b)

   Explain, in terms of the bond energies given, why this reaction releases energy overall, and describe what this tells you about the relative strength of the bonds broken compared with the bonds formed.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemistry teacher explains that bond energy calculations can be used to predict, in advance, whether an unfamiliar chemical reaction will be exothermic or endothermic, without needing to measure any temperature change experimentally.

   (a)

   Explain fully, using the ideas of bond breaking and bond making, how the overall energy change of any chemical reaction is determined, and describe how bond energy data could be used to predict whether an unfamiliar reaction will be exothermic or endothermic before it is carried out.

   [6 marks]

   (b)

   Discuss the limitations of using average bond energy values to predict the energy change of a reaction, and explain why calculated $\Delta H$ values from bond energies may differ slightly from experimentally measured values.

   [6 marks]

   Total for question 4: 12 marks

End of questions