1. Hess’ Law and enthalpy cycles using enthalpy of formation
Thursday, November 22, 2018

2. Hess’ Law using enthalpy of formation
Objective: to recall and use Hess’s Law for enthalpy of formation data
Success criteria:
Recall Hess’ Law
Interpret an enthalpy cycle using Hess’ Law
Construct an enthalpy cycle using Hess’ Law, for formation data
To calculate an enthalpy change from formation data using Hess’s Law
Know reasons why the enthalpy change cannot always be measured directly

3. Starter Define ΔfH What are standard conditions?
What is the value of ΔfH for any element?

4. Using ΔfHӨ As well as burning things, chemists love making things!
So we also have huge tables of Δ ΔfH Ө including very many of the most common chemicals.
In a reaction which we cannot measure directly, ΔfHӨ can also give us a link between reactants and products.

5. Actually you might see it as: A = -B + C
Δ ΔfHӨ is the energy to make something from its elements, so the arrows will be going in the opposite direction to enthalpies of combustion.
Route 1: B + A
Route 2: C
Hess’ law tells us that B + A = C
We are trying to find A:
Actually you might see it as: A = -B + C
A = C - B

6. Actually you might see it as: A = -B + C
Using ΔfH
As with ΔHc many ΔHf values have also been measured and can be looked up in tables
Actually you might see it as: A = -B + C
ΔHr
Reactants
Products
Σ ΔHf(reactants)
Σ ΔHf(products)
Elements
ΔHr = - Σ ΔHf(reactants) + Σ ΔHf(products)
Draw an enthalpy cycle – then if you follow the direction of an arrow then ADD. If opposite to the direction of the arrow then SUBTRACT.

7. Example on worksheet

8. Practice question 1 a
You are provided with the following enthalpy changes of formation
Determine the enthalpy change for the following reaction:
2NO (g) + O2(g)  2NO2(g)
Substance
NO2(g)
NH3(g)
NO(g)
H2O(l)
HNO3(l)
ΔHf / kJmol-1 33
-46 90
-286
-174

9. ΔHr = -(2 x 90) + (2 x 33) = -114 kJmol-1
Substance
NO2(g)
NH3(g)
NO(g)
H2O(l)
HNO3(l)
ΔHf / kJmol-1 33
-46 90
-286
-174
ΔHr
2NO(g) + O2 (g)
2NO2(g)
2ΔHf(NO)
2ΔHf(NO2)
Elements
ΔHr = -(2 x 90) + (2 x 33) = -114 kJmol-1

10. Practice question 1 b
You are provided with the following enthalpy changes of formation
Determine the enthalpy change for the following reaction:
4NH3 (g) + 5O2(g)  4NO(g) + 6H2O(l)
Substance
NO2(g)
NH3(g)
NO(g)
H2O(l)
HNO3(l)
ΔHf / kJmol-1 33
-46 90
-286
-174

11. ΔHr = -(4 x -46) + (4 x 90) + (6 x -286) = -1172 kJmol-1
Substance
NO2(g)
NH3(g)
NO(g)
H2O(l)
HNO3(l)
ΔHf / kJmol-1 33
-46 90
-286
-174
ΔHr
4NH3(g) + 5O2 (g)
4NO(g) + 6H2O(l)
4ΔHf(NH3)
4ΔHf(NO) + 6ΔHf(H2O)
Elements
ΔHr = -(4 x -46) + (4 x 90) + (6 x -286) = kJmol-1

12. Practice question 1 c
You are provided with the following enthalpy changes of formation
Determine the enthalpy change for the following reaction:
2H2O (l) + 4NO2(g) + O2(g)  4HNO3(l)
Substance
NO2(g)
NH3(g)
NO(g)
H2O(l)
HNO3(l)
ΔHf / kJmol-1 33
-46 90
-286
-174

13. ΔHr = -(2 x -286) - (4 x 33) + (4 x -174) = -256 kJmol-1
Substance
NO2(g)
NH3(g)
NO(g)
H2O(l)
HNO3(l)
ΔHf / kJmol-1 33
-46 90
-286
-174
ΔHr
2H2O(l) + 4NO2 (g) + O2(g)
4HNO3(l)
2ΔHf(H2O) + 4ΔHf(NO2)
4ΔHf(HNO3)
Elements
ΔHr = -(2 x -286) - (4 x 33) + (4 x -174) = -256 kJmol-1

14. Other enthalpy cycles
In your AS course you have to be able to construct and use cycles that use combustion and formation data.
The same principles can be applied to any energy cycle provided it is constructed correctly.
Following the arrows ADD, opposite to the arrows SUBTRACT and you can’t go wrong.

15. Exam questions (Edexcel) printed as booklet
Hess Cycles both and enthalpy of combustion to finish for homework

16. Question 1 from old OCR January and June 2010

21. Question 2

25. Mark schemes

26. Mark scheme Q1 – cover until you have answered

27. Mark scheme

28. Mark scheme Q2

29. Mark scheme