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Thermo Chem

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  • June 7, 2012
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Enthalpy changes can be calculated using average bond enthalpy data. i) The enthalpy change to convert methane into gaseous atoms is shown below. [pic]
Calculate the average bond enthalpy of a C—H bond in methane. [1] ii)Use the data in the table below and your answer to (a)(i) to calculate the enthalpy change for
[pic] [3]

b) The standard enthalpy of formation of 1,2-dibromoethane, CH2BrCH2Br, is – 37.8 kJmol-1. Suggest the main reason for the difference between this value and your calculated value in (a)(ii).[1]

2. . a) Define the term standard enthalpy of combustion. [3] b) Write an equation for the complete combustion of ethanol, C2H5OH [1] c) The following table gives some standard enthalpies of formation. [pic]

Use these data to calculate a value for the enthalpy of combustion,∆Hc , ofpropan-1-ol, C3H7OH
C3H7OH(l) + 4 O2(g) → 3CO2(g) + 4H2O(l) [3]
d) State how you would expect the value obtained in part (c) to differ if gaseous water, rather than liquid water, is formed. [1]
e) In an experiment 0.92 g of propan-1-ol, C3H7OH, was burned and the heat given off used to raise the temperature of 250 g of water. The temperature rise was 16 °C. The specific heat capacity of water is 4.2 JK–1 g–1. Calculate a value for the enthalpy of combustion of one mole of propan-1-ol. [4] f) Suggest why the experimental value of the enthalpy of combustion obtained in part (e) is less reliable than the value obtained in part (c). [1]

3. a) Explain the meaning of the term enthalpy change of a reaction. [2] b) Write an equation, including state symbols, for the reaction that represents the standard enthalpy of formation, ∆Hf , of liquid carbon disulphide (CS2). [2] c) Carbon disulphide is flammable and burns in air according to the following equation. [pic]

Using this equation and the data below, calculate a value for the standard enthalpy of formation of...