Hess’ cycles C2.1 Thermochemsitry 21 September 2018

Starter Define the following key terms: Standard conditions Define ΔcH Define ΔfH Define ΔrH Describe the following equations as ΔcH, ΔrH or ΔfH 3C(s) + 4H2(g) C3H8(g) C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) C2H4(g) + H2(g)  C2H6(g) 2C2H6(l) + 7O2(g)  4CO2(g) + 6H2O(l) Write the equation for ΔcH for H2 CH3OH C

Answers Standard conditions are 298K, 100 kPa and 1M for solutions. All substances should be in their standard states The standard enthalpy change of combustion ΔHc is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states. The standard enthalpy change of formation ΔHf is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. The standard enthalpy change of reaction ΔHr is the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.

answers Describe the following equations as ΔHc, ΔHr or ΔHf 3C(s) + 4H2(g) C3H8(g) ΔHf C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) ΔHc C2H4(g) + H2(g)  C2H6(g) ΔHr 2C2H6(l) + 7O2(g)  4CO2(g) + 6H2O(l) ΔHr (because 2 moles are shown to be burnt) Write the equation for ΔHc for H2(g) + ½ O2(g)  H2O(l) CH3OH(l) + O2(g)  CO2(g) + H2O(l) C(s) + O2(g)  CO2(g)

Hess’ Law Sometimes it is too difficult to measure an enthalpy change directly. This may be because of: a high activation energy a slow reaction rate more than one reaction taking place

Hess’ Law ΔH(Route A) = ΔH(Route B) – ΔH(Route C) A B C Hess’ law states that, if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same. Reactants Products Intermediate A B C ΔH(Route A) = ΔH(Route B) – ΔH(Route C)

Worked Example one (Formation Data) Calculate the standard enthalpy change of reaction for the reaction shown below. Fe2O3 (s) + 3Ca (s)  2Fe (s) + 3CaO (s) The standard enthalpy changes of formation of the reactants and products are listed in table 1. Step 1: Construct a Hess’ cycle Step 2: Use the arrows to determine the equation for ΔrH Step 3: Input the ΔfH values into the equation. Substance Fe2O3 (s) Ca (s) 2Fe (s) 3CaO (s) ΔfH / kJ mol-1 -824 -635

Worked Example two (Formation Data) Calculate the standard enthalpy change of the following reaction: I2O5 (s) + 5CO (g)  I2 (s) + 5CO2 (g) The standard enthalpy changes of formation of the reactants and products are listed in table 1. Step 1: Construct a Hess’ cycle Step 2: Use the arrows to determine the equation for ΔrH Step 3: Input the ΔfH values into the equation. Substance I2O5 (s) CO (g) CO2 (g) ΔfH / kJ mol-1 -158 -110 -394

Knowledge Check Complete the questions on the worksheet.

Worked Example one (Combustion Data) The equation for the formation of butane, C4H10 (g) is shown below. 4C (s) + 5H2 (g)  C4H10 (g) It would be impossible to measure the enthalpy change of this reaction directly. Construct a Hess’ cycle and use the data below to indirectly determine the enthalpy change. Step 1: Construct a Hess’ cycle Step 2: Use the arrows to determine the equation for ΔrH Step 3: Input the ΔcH values into the equation. Substance C (s) H2 (g) C4H10 (g) ΔcH / kJ mol-1 -394 -286 -2877

Worked Example two (Combustion Data) The equation for the formation of hexane, C6H14 (g) is shown below. 6C (s) + 7H2 (g)  C6H14 (g) Use the data below to calculate the enthalpy change of 2 moles of hexane formed from it’s constituent elements. Step 1: Construct a Hess’ cycle Step 2: Use the arrows to determine the equation for ΔfH Step 3: Input the ΔcH values into the equation. Substance C (s) H2 (g) C6H14 (g) ΔcH / kJ mol-1 -394 -286 -4163

Knowledge Check You are provided with the following data. Calculate the enthalpy change of formation of: C6H14 C2H5OH 2.3 moles of ethanol 15 moles of solid carbon Substance C (s) H2 (g) C6H14 (g) C2H5OH ΔcH / kJ mol-1 -394 -286 -4163 -1367