A plastics pilot plant is considering addition of hydrogen chloride gas to ethene gas to make chloroethane. Before running the reactor, a chemist estimates ...
Question 1 Report
A plastics pilot plant is considering addition of hydrogen chloride gas to ethene gas to make chloroethane. Before running the reactor, a chemist estimates the energy change from bond energies. Fig. 1 shows the structural change. The chlorine atom in hydrogen chloride becomes bonded to carbon in the product.
Table 1 contains mean bond energies. These values are averages, so an experimental energy change may not be identical. The reaction is carried out as gases at a controlled temperature.
| bond | bond energy / kJ mol-1 |
|---|---|
| C=C | 612 |
| H-Cl | 431 |
| C-C | 348 |
| C-H | 413 |
| C-Cl | 338 |
(a) Write the balanced chemical equation for the reaction, including state symbols. [2]
(b) Give the two types of bond broken when one mole of ethene reacts. [2]
(c) Calculate the estimated energy change for the reaction, in kJ mol-1. [3]
(d) Suggest why a value measured in the pilot plant could differ from the value calculated using Table 1. [2]
(e) State whether the reaction is exothermic or endothermic. [1]
Answer Details
(a)
\[\mathrm{C_2H_4(g)+HCl(g)\rightarrow C_2H_5Cl(g)}\]
The equation is balanced and includes the state symbols. [2]
(b) One C=C bond and one H-Cl bond are broken per mole of ethene reacting. [2]
(c) Energy is required to break bonds:
\[612+431=1043\text{ kJ mol}^{-1}\]
Energy is released when new C-C, C-H and C-Cl bonds form:
\[348+413+338=1099\text{ kJ mol}^{-1}\]
\[\Delta H=1043-1099=-56\text{ kJ mol}^{-1}\]
[3]
(d) Mean bond energies are averages, so the energy of bonds in these particular molecules may not be exactly the mean value. A measured value can also differ because of experimental heat losses. [2]
(e) The reaction is exothermic, shown by the negative energy change. [1]
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