1. Looking at your calorimetry results…
How do your calculated enthalpies of combustion compare to the correct standard enthalpy change of combustion?
Explain why the experimental values you obtained for the heat energy evolved in the experiments might be less than the correct value.
Enthalpy changes always occur at constant pressure. Complete the definition of enthalpy change- _____ ______ ______ at constant pressure.
Standard enthalpy of combustion
Methanol
-715
Ethanol
-1370
Propan-1-ol
-2021
Butan-1-ol
-2670

2. Today we are looking at standard enthalpy changes (∆HӨ).
Hess’s Law
LO- Carry out calculations using Hess’s law.
Today we are looking at standard enthalpy changes (∆HӨ).
What are standard enthalpy changes?
This means the rxn happens under certain conditions.
These are 298K & 100kPa.
Standard conditions
The sign is ∆HӨ
We look at 2 types of enthalpy change : standard enthalpy of formation and standard enthalpy of combustion.

3. What is standard enthalpy of formation (∆HfӨ)?1.
What is standard enthalpy of formation (∆HfӨ)?
By definition:
‘The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, with all reactants and products in their standard states.’
What does this mean?
Example- methane gas, CH4.
∆HfӨ for methane is -74.9KJ mol-1. Write an eqn to show the formation of one mole of methane from its constituent elements.
ALWAYS include state symbols when writing these eqns.

4. Example 3- potassium perchlorate, KClO4
∆HfӨ for potassium perchlorate is KJ mol-1. Write an eqn to represent ∆HfӨ for potassium perchlorate.
Write an eqn for the formation of the following compds:
C2H4(g)
CH3CH2OH(l)
NH4NO3(s)
H2SO4(l)
TiCl4(s)

5. What is standard enthalpy of combustion (∆HcӨ)?2.
What is standard enthalpy of combustion (∆HcӨ)?
By definition:
‘The enthalpy change when one mole of a substance undergoes complete combustion in excess O2 under standard conditions with all reactants and products in their standard states.’
What does this mean?
Example- methane gas, CH4.
∆HcӨ for methane is -890KJ mol-1 Write an eqn to show the complete combustion of one mole of methane.

6. Write an eqn for the complete combustion of the following cpds:
Propane, C3H8
Methanol, CH3OH
Ethanethiol, C2H5SH

7. HESS’S LAW What is Hess’s law?
Using values for known standard enthalpy changes, sometimes you will be asked to work out an unknown standard enthalpy change.
This involves Hess’s law.
What is Hess’s law?
Hess’ law states that the enthalpy change in a chemical rxn is independent of the route taken.
It is related to the 1st law of thermodynamics:
“energy cannot be created or destroyed, it can only be changed from one form to another.”
Learn this!

8. ROUTE 1 ROUTE 2 A C B Hess’s law In this situation: ∆H1 = ∆H2 + ∆H3
There are two possible routes in this cycle… A
∆H1 C
∆H3
∆H2 B
In this situation:
ROUTE 2
∆H1 = ∆H2 + ∆H3

9. Hess’s law calculations
Pay close attention to the direction of the arrows. Use the diagrams to work out the unknown enthalpy change in the made-up rxns.
Example 1
Example 2
∆H1 = ?
∆H1 = ? D E G H
ROUTE 1
ROUTE 2
∆H2 = KJ mol-1
∆H2 = KJ mol-1
∆H3 = -10KJ mol-1
∆H3 = KJ mol-1 F I

10. R S U V ∆H1 ∆H1 T W Example 3 Example 4 ∆H2 ∆H3 = ? ∆H3 ∆H2 = ?
Reaction
∆H / KJ mol-1
∆H1
-200
∆H2
+40
Reaction
∆H / KJ mol-1
∆H1
-348
∆H3
-235

11. M L Q P K O N J Example 5 Example 6 ∆H2 ∆H2 = ? ∆H3 = ? ∆H3 ∆H4 ∆H4
Reaction
∆H / KJ mol-1
Q --> P
-240
Q --> N
-490
N --> O
+145
Reaction
∆H / KJ mol-1
J --> K
-20
K --> L
-50
J --> M
-80

12. Further application of Hess’s law
Example 1
Methane is formed in the following eqn:
C(gr) + 2H2(g) --> CH4(g)
Use the data below to calculate ΔHf of methane.
Pay careful attention to the info. given as this affects the direction of the arrows.
Substance
∆Hc (kJ mol-1)
C(gr)
394
H2 (g)
-286
CH4 (g)
890

13. Example 1- Hess’s law cycle
∆Hf = ?
C(gr) H2(g) CH4(g)
CO2(g) + 2H2O(l)
∆H2
∆H3
ROUTE 1
ROUTE 2
(-286)
-890

14. Example 1- Calculation
red route = green route
∆Hf + ∆H = ∆H2
From here we want to make ∆Hf the subject of the eqn:
∆Hf = ∆H ∆H3
∆Hf = [ (-286)] – -890
∆Hf = - 76 kJ mol-1

15. Answer = -86 kJ mol-1 Example 2 Ethane is formed in the following eqn:
2C(gr) + 3H2(g) --> C2H6(g)
Use the data below to calculate ΔHf of ethane.
Substance
∆HcӨ (kJ mol-1)
C(gr)
394
H2 (g)
-286
C2H6 (g)
1560
Answer = -86 kJ mol-1

16. Lesson 2- Hess’ law Butane is formed in the following eqn:
4C(gr) + 5H2(g) --> C4H10(g)
Use the data below to calculate the enthalpy change for reaction to form butane.
Substance
∆HcӨ (kJ mol-1)
C(gr)
394
H2 (g)
-286
C4H10 (g)
2878

17. In general, if you are WORKING OUT ∆Hf …
If you are working out ∆Hf use the eqn:
∆Hf Ө = ∑ ∆H Ө reactants - ∑ ∆H Ө products
Or put simply:
∆HfӨ = r - p
BUT, you need to be able to draw cycles too to fully understand the process.

18. ∆HfӨ (kJ mol-1) Example 3 MgO reacts with HCl in the following eqn:
MgO + 2HCl → MgCl2 + H2O
Use the data below to calculate the enthalpy change for this rxn.
Pay careful attention to the info. given as this affects the direction of the arrows.
Substance
∆HfӨ (kJ mol-1)
MgO
602
HCl
-92
MgCl2
-642
H2O
-286

19. In general, if you are GIVEN ∆Hf info…
If you are GIVEN ∆Hf info. in the question use the eqn:
∆HӨ = ∑∆Hproducts - ∑∆Hreactants
Or put simply:
∆HӨ =p - r
BUT, you need to be able to draw cycles too to fully understand the process.

20. Nitric acid can be made in the following eqn.
Example 4
Nitric acid can be made in the following eqn.
H2O(l) + 2NO2(g) + O2(g) → 2HNO3(l) ΔH = –128 kJ mol–1
Some ΔHf Ө are given in the table.
A) Use the data to calculate a value for ΔHf Ө of nitric acid.
B) Why is ∆HfӨ for O2 zero?

21. TRY SOME MORE….
1. Some standard enthalpies of formation are given in the table below.
Use these data to calculate the enthalpy change for the following reaction.
4NH3(g) + 3F2(g) → NF3(g) + 3NH4F(s)

22. 2. Use the data in the table to calculate the standard enthalpy of formation of liquid methylbenzene, C7H8
7C(s) + 4H2(g) → C7H8(l)

23. 3A) Use the data below to calculate the standard enthalpy change for the following rxn.
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
B) Why is no ∆HfӨ data given for O2?

24. 4. Use the standard enthalpy of combustion data given below to calculate a value for the standard enthalpy change for the following reaction.
4C(s)   +   2H2(g)   +   2O2(g)   →   C4H4O4(s)
C4H4O4(s)
C(s)
H2(g)
ΔHc / kJ mol–1
–1356
–393.5
–285.8

25. 5. The table below contains some standard enthalpy of formation data.
Use ΔHf data from the table to calculate a value for the enthalpy change for the following reaction.
NH4NO3(s) + ½C(s) → N2(g) + 2H2O(g) + ½CO2(g)

26. 2C(s) + 3H2(g) + S(s) → C2H5SH(l)
6. Calculate a value for the standard enthalpy of formation for liquid ethanethiol,C2H5SH. Use the eqn given below and enthalpy of combustion data from the following table.
Substance
C2H5SH(l)
C(s)
H2(g)
S(s)
ΔHc  / kJ mol–1
–1170
–394
–286
–297
2C(s) + 3H2(g) + S(s) → C2H5SH(l)

27. 7. Carbon disulphide is flammable and burns in air according to the following eqn.
CS2(l) + 3O2(g) → CO2(g) + 2SO2(g) ΔH = –1076 kJ mol–1
Using this eqn and the data below, calculate a value for the standard enthalpy of formation of carbon disulphide.

28. 8. The following table gives some standard enthalpies of formation
8. The following table gives some standard enthalpies of formation. Use these data to calculate a value for the enthalpy of combustion, ΔHc , of propan-1-ol, C3H7OH
C3H7OH(l) + 4O2(g) → 3CO2(g) + 4H2O(l)

29. 9. Hess’s Law is used to calculate the enthalpy change in reactions for which it is difficult to determine a value experimentally.
Consider the following table of data and the scheme of reactions.
Use the data in the table, the scheme of reactions and Hess’s Law to calculate a value for ΔHr

30. 10. Barium can be extracted from barium oxide (BaO) in a process using aluminium. A mixture of powdered barium oxide and powdered aluminium is heated strongly.
The eqn for this extraction process is shown below.
3BaO(s) + 2Al(s) →3Ba(s) + Al2O3(s)
Some standard enthalpies of formation are given in the table below.
Use the data to calculate the standard enthalpy change for the rxn shown by the eqn above.