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Questions
Q1.
This question is about the thermal decomposition of ammonia. This reaction is catalysed by platinum and is represented by the equation: 2NH 3 (g) → N 2 (g) + 3H 2 (g) (^) = + 92 kJ mol−
The table shows the bond enthalpies for the N≡N and H–H bonds.
Use this data, together with the standard enthalpy change of reaction, (^) , for the decomposition of ammonia, to calculate a value, in kJ mol−1, for the mean bond enthalpy of the N–H bond in ammonia. (3)
Q2.
Propane is a saturated hydrocarbon with molecular formula C 3 H 8. *(a) Explain the meaning of the terms saturated and hydrocarbon. (2) Saturated ............................................................................................................................................. ............................................................................................................................................. Hydrocarbon ............................................................................................................................................. ............................................................................................................................................. (b) Propane is sold in small cylinders for use as a fuel in camping stoves. The enthalpy change of combustion of propane can be measured by experiment using one of these cylinders.
A known mass of propane is burned to heat a container of water, and the temperature rise of the water is measured. The results of the experiment are shown below.
(i) How would the mass of propane which was burned be measured? (1) ............................................................................................................................................. ............................................................................................................................................. (ii)following expression. Calculate the energy transferred in the experiment, using the results above and the
Energy transferred (J) = mass × specific heat capacity × temperature change The specific heat capacity of water is 4.18 J g−1^ °C−1. (1) (iii) Calculate the enthalpy change of combustion of propane, Δ H c, in kJ mol−1. Give your answer to three significant figures and include a sign. (3) (iv) The results of this experiment are inaccurate due to heat loss. Suggest one other source of error, other than measurement errors and limitations of the equipment. (1) ............................................................................................................................................. ............................................................................................................................................. (c) Another way of calculating the enthalpy change of combustion for propane is to use mean bond enthalpy data. (i) Complete the equations in the Hess cycle below. The enthalpy change of +6490 kJ mol−1^ is the total energy required to break the bonds in propane and in oxygen. (1)
be detected. When the fuel is burned, the gaseous ethanethiol undergoes complete combustion to form theoxides CO 2 (g), SO 2 (g) and H 2 O(g). (a) Table 1 shows some mean bond enthalpy data.
(i) Write the equation for the complete combustion of ethanethiol. State symbols are not required. (1) (ii) Use your equation and data from Table 1 to calculate the enthalpy change for the combustion of ethanethiol. (3) (b) Table 2 shows some enthalpy changes of formation.
(i) Why is the value for Δf H of O 2 (g) zero? (1) A O 2 (g) is an element B O 2 (g) is a gas C O 2 (g) is a molecule D O 2 (g) is a non-metal (ii) Use Hess's Law, and data from Table 2 , to calculate another value for the enthalpy change
for the combustion of ethanethiol. (2) (c) Give a reason why the values for the enthalpy change of combustion of ethanethiol in (a)(ii) and (b)(ii) are different. (1) ............................................................................................................................................. .............................................................................................................................................
(Total for question = 8 marks)
Q4. Given the following information CH 4 (g) → C(g) + 4H(g) Δ H = + Q kJ mol− the mean bond enthalpy for the C—H bond in methane is A + Q B + Q / C − Q D − Q / (Total for Question = 1 mark)
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