1. Hess’ cycles
C2.1 Thermochemsitry
21 September 2018

2. 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

3. 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.

4. 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)

5. 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

6. 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)

7. 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

8. 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

9. Knowledge Check
Complete the questions on the worksheet.

10. 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

11. 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

12. 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