1. 1.4 Energetics Practical 1.6 – Finding an enthalpy change that cannot be measured directly
e. recall Hess’s Law and apply it to calculating enthalpy changes of reaction from data provided, selected from a table of data or obtained from experiments and understand why standard data is necessary to carry out calculations of this type
Connector:
State Hess’s Law
Define the standard enthalpy of formation
What are “standard conditions”?
What symbol is used to indicate “standard conditions”?
Crowe2009

2. Hess’s Law
Hess’s Law states that the enthalpy change in turning any reactants into a set of products is the same no matter what route we take. C c b a A B d D e
a = b + c = d - e
Note: arrows in same direction as “a” are added,
Those in the opposite direction are subtracted.

3. Φ
indicates standard conditions

4. Enthalpy Cycle Calculation
The enthalpy of formation of ethanol cannot be measured directly.
However, the standard enthalpy of combustion for graphite, hydrogen and ethanol are known.
Construct an energy cycle and then use the data below to calculate the enthalpy of formation of ethanol.
Hint:
Start by writing the equation for the formation of ethanol from its elements

5. 2C (graphite, s) + 3H2 (g) + ½ O2 (g) C2H5OH (l)
2CO2 (g) + 3H2O (l)
Note – the 3O2 (g) ‘s cancel each other out and so can be omitted from the cycle
ΔH1 = ΔH2 – ΔH3

6. Calculating the heat of formation of ethanol
From the previous slide:
ΔH1 = ΔH2 – ΔH3
2C + 3H2 + ½ O C2H5OH
So:
ΔH2 = 2ΔHcθ[C, graphite(s)] + 3ΔHcθ[H2,(g)]
ΔH3 = ΔHcθ[C2H5OH(l)]
ΔH1 = ΔH – ΔH3
= [2(-393.5) + 3(-285.8)] – [ ]
= [-787 – 857.4] =
= kJmol-1