1. Lesson 6 – Hess’ law and enthalpy cycles using enthalpy of combustion
Wednesday, April 26, 2017

2. Objectives Recall Hess’ Law
Interpret an enthalpy cycle using Hess’ Law
Construct an enthalpy cycle and use it to calculate an enthalpy change of reaction from combustion data

3. Specification link
2.3.1 Enthalpy Changes, Candidates should be able to:
(a) explain that some chemical reactions are accompanied by enthalpy changes, exothermic (ΔH, negative) or endothermic ΔH, positive);
(b) describe the importance of oxidation as an exothermic process in the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration;
(c) describe that endothermic processes require an input of heat energy, i.e., the thermal decomposition of calcium carbonate;
(d) construct a simple enthalpy profile diagram for a reaction to show the difference in the enthalpy of the reactants compared with that of the products;
(e) explain qualitatively, using enthalpy profile diagrams, the term activation energy;
(f) define and use the terms:
(i) standard conditions,
(ii) enthalpy change of reaction,
(iii) enthalpy change of formation,
(iv) enthalpy change of combustion;
(g) calculate enthalpy changes from appropriate experimental results directly, including use of the relationship: energy change = mcΔT;
Bond Enthalpies
(h) explain exothermic and endothermic reactions in terms of enthalpy changes associated with the breaking and making of chemical bonds;
(i) define and use the term average bond enthalpy (ΔH positive; bond breaking of one mole of bonds);
(j) calculate an enthalpy change of reaction from average bond enthalpies;
Hess’ Law and Enthalpy Cycles
(k) use Hess’ law to construct enthalpy cycles and carry out calculations to determine:
(i) an enthalpy change of reaction from enthalpy changes of combustion,
(ii) an enthalpy change of reaction from enthalpy changes of formation,
(iii) an enthalpy change of reaction from an unfamiliar enthalpy cycle;

4. Starter - Test What are standard conditions? Define ΔHc Define ΔHf
Define ΔHr
Describe the following equations as ΔHc, ΔHr or ΔHf
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 ΔHc for H2
CH3OH C

5. Starter - 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.

6. Starter - 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 2mols 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)

7. Measuring enthalpy changes
Using a calorimeter for a combustion reaction or or other reaction can often give the ΔHr directly (think about the spirit burners and copper sulphate experiments you have done).
Bond enthalpies can also be used to estimate ΔHr
However it may not always be possible to measure the enthalpy change of a reaction directly.

8. Problems There may be: a high activation energy a slow reaction rate
more than one reaction taking place
Take for example 3C(s) + 4H2(g)  C3H8(g)
This is virtually impossible to measure directly - think of the number of compounds of hydrogen and carbon that could form!

9. Solution Measuring enthalpy of reaction indirectly
We can use Hess’ law to measure the energy change of reactions indirectly.
We use enthalpy changes that we can measure to calculate ones that we can’t.
This means we need to work out a “cycle” of reactions.

10. Using ΔHC Ө Many things burn and chemists love burning things!
So we have huge tables of ΔHC Ө including very many of the most common chemicals.
In a reaction which we cannot measure directly, ΔHC Ө gives us a link between reactants and products.

11. Hess’ Law ΔH(Route A) = ΔH(Route B) – ΔH(Route C) A B C
Definition (on sheet) 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)
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.

12. Using combustion data ΔHr = Σ ΔHc(reactants) – Σ ΔHc(products)
The combustion of reactants and combustion of products is a useful way to complete the cycle in a way that can be experimentally determined.
ΔHr = Σ ΔHc(reactants) – Σ ΔHc(products)
Reactants
Products
Combustion products
ΔHr
Σ ΔHc(reactants)
Σ ΔHc(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.

13. Example on worksheet

14. Practice question 1a
You are provided with the following enthalpy changes of combustion
Determine the enthalpy change for the following reaction:
4C(s) + 5H2(g)  C4H10(g)
Substance
C(s)
H2(g)
C4H10(g)
C2H5OH(l)
ΔHc / kJmol-1
-394
-286
-2877
-1367

15. ΔHf = (4 x -394) + (5 x -286) – (-2877) = -129 kJmol-1
Substance
C(s)
H2(g)
C4H10(g)
C2H5OH(l)
ΔHc / kJmol-1
-394
-286
-2877
-1367
4C(s) + 5H2 (g)
C4H10(g)
Combustion products
ΔHf
4ΔHc(C)+ 5ΔHc(H2)
ΔHc(C4H10)
ΔHf = (4 x -394) + (5 x -286) – (-2877) = -129 kJmol-1

16. Practice question 1b
You are provided with the following enthalpy changes of combustion
Determine the enthalpy change for the following reaction:
2C(s) + 3H2(g) + ½O2(g)  C2H5OH(l)
Substance
C(s)
H2(g)
C4H10(g)
C2H5OH(l)
ΔHc / kJmol-1
-394
-286
-2877
-1367

17. ΔHr = (2 x -394) + (3 x -286) – (-1367) = -279 kJmol-1
Substance
C(s)
H2(g)
C4H10(g)
C2H5OH(l)
ΔHc / kJmol-1
-394
-286
-2877
-1367
ΔHr
2C(s) + 3H2 (g) + ½ O2(g)
C2H5OH(l)
ΔHc(C2H5OH)
2ΔHc(C)+ 3ΔHc(H2)
Combustion products
ΔHr = (2 x -394) + (3 x -286) – (-1367) = -279 kJmol-1

18. Examination question

20. Mark scheme

21. Mark scheme