# Thermo Chem

Topics: Enthalpy, Standard enthalpy change of formation, Thermodynamics Pages: 5 (1327 words) Published: June 7, 2012
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]
[pic]

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 carbon disulphide. [3]
[pic]

d) i) A 0.45 g sample of carbon disulphide was burned and the heat produced was used to raise the temperature of 75cm3 water from 22 °C to 30 °C. Calculate the heat produced in this reaction and hence calculate the enthalpy change, in kJ, for the combustion of one mole of carbon disulphide. The specific heat capacity of water is 4.2 JK–1 g–1. [4]

ii) Explain why the answer you calculated in part (d) (i) is very low compared to the value you were given in part (c). [1]

4. a) i) What is meant by the standard enthalpy change of formation, ΔHØf of a compound? Explain what is meant by the term standard. [2]
ii) Write an equation, with state symbols, for the ΔHØf of water.[1]
iii) Explain why the ΔHØf for water is identical to the standard enthalpy change of combustion of hydrogen. [1]
b) When calcium is placed in water, aqueous calcium hydroxide is formed and hydrogen is given off. i) Write the equation for the reaction of calcium with water. [1] ii) When 1.00 g of calcium is placed in 200 g of water, the temperature increases by 12.2ºC when the reaction is completed. The specific heat capacity of water, c, is 4.2 Jg-1K-1. Calculate the heat released in the experiment. [2]

iii) Calculate the standard enthalpy change of reaction in kJ/mol for your equation in (b)(i). [2] c) i) State Hess’s Law.[1]
ii) Use Hess’ Law and your result in (b)(iii) to calculate the ΔHØf of Ca(OH)2(aq). You also need the ΔHØf of water which is – 286 kJ/mol. [3] iii) Calculate the volume of hydrogen, measured at room temperature and pressure, liberated in the experiment described in (b)(ii). [2]

5. a) The table below contains some mean bond enthalpy data.

|Bond |H—O |O—O |O=O | |Mean bond enthalpy/kJ mol-1 |463 |146...