1. How do we find the enthalpy of…
CaCO3 (s) –heat CaO (s) + CO2 (g)
The calcium carbonate has to be heated continuously in a Bunsen flame for at least 5 minute.
Most of the energy is lost to the surroundings.
How would you calculate the enthalpy change for this reaction?

2. Hess Cycles What is the law of conservation of energy?
Hess’s Law: the enthalpy change for a reaction is the same regardless of the route taken
Equivalent to the gain in g.p.e. if you take the stairs or the lift!
We can’t measure the enthalpy change for the thermal decomposition of calcium carbonate – but we can react calcium carbonate and calcium oxide with dilute hydrochloric acid and measure the temperature change.

3. A Hess Cycle CaCO3 (s) –heat CaO (s) + CO2 (g) ΔHreaction
ΔH HCl (aq) ΔH2
CaCl2 (aq) + H2O (l) + CO2 (g)
ΔHreaction = ΔH1 - ΔH2

4. Hess Cycles and enthalpy of combustion
Standard enthalpy of combustion
The enthalpy change when one mole of a substance is burned with excess oxygen under standard conditions (1 atm) and a given temperature (e.g. 298K)
C2H5OH (l) + 3 O2 (g)  2 CO2 (g) + 3 H2O (l)
ΔHӨc(ethanol) = kJmol-1

5. Hess Cycles and enthalpy of combustion
2 C (graphite) + 3 H2 (g) + ½ O2 (g)  C2H5OH (l)
ΔHreaction
ΔH O2 ΔH2
2 CO2 (g) + 3 H2O (l)
ΔHreaction = ΔH1 - ΔH2

6. Hess Cycles and enthalpy of combustion
ΔHӨc (H2) = kJmol-1
ΔHӨc (C) = kJmol-1
ΔHӨc(C2H5OH) = kJmol-1
ΔH1 = 2 x x = kJmol-1
ΔH2 = kJmol-1
ΔHreaction = ΔH1 - ΔH2 = ( )
= kJmol-1

7. Hess Cycles and enthalpy of formation
Standard enthalpy of formation
The enthalpy change when one mole of a substance is formed from its elements, in their standard states, under standard conditions (1 atm) and a given temperature (e.g. 298K)
2 C (graphite) + 3 H2 (g) + ½ O2 (g)  C2H5OH (l)
ΔHӨf(ethanol) = kJmol-1
ΔHӨf of an element = 0

8. Hess Cycles and enthalpy of formation
CaCO3 (s) –heat CaO (s) + CO2 (g)
ΔHreaction
ΔH ΔH2
Ca (s) + 1½ O2 (g) + C (graphite)
ΔHreaction = - ΔH1 + ΔH2

9. Hess Cycles and enthalpy of formation
ΔHӨf (CaCO3) = kJmol-1
ΔHӨf (CaO) = kJmol-1
ΔHӨf (CO2) = kJmol-1
ΔH1 = kJmol-1
ΔH2 = (-635.5)+(-393.5) = kJmol-1
ΔHreaction = - ΔH1 + ΔH2 = -( ) + ( )
= kJmol-1

10. 2 KHCO3(s)  K2CO3 (s) + H2O (l) + CO2 (g)
A reaction…
2 KHCO3(s)  K2CO3 (s) + H2O (l) + CO2 (g)
How can I find ΔHreaction for this process?
Why can’t we measure directly?
What do carbonates react with?
Design an experiment – with calculations to find ΔHreaction.

11. 2 KHCO3(s)  K2CO3 (s) + H2O (l) + CO2 (g)
A reaction…
2 KHCO3(s)  K2CO3 (s) + H2O (l) + CO2 (g)
ΔH HCl (aq) ΔH2
2 KCl (aq) + 2H2O (l) + 2 CO2 (g)
ΔHreaction = ΔH1 - ΔH2

12. A method
50 cm3 HCl (2 moldm-3) pipetted into polystyrene beaker in glass beaker
Record temperature to nearest 0.5oC every 30 seconds for 2 ½ minutes
At three minutes add an accurately weighed sample of between 5 and 6g potassium hydrogen carbonate
Stir
At 3 ½ minutes continue to record temperature every 30 seconds
Repeat the process using between 3 and 4g potassium carbonate

13. Calculation Plot temperature data
Extrapolate the before and after mixing lines
Find ΔT at 3 minutes for both reaction
Calculate enthalpy change of system for both reactions = - (50 x 4.2 x ΔT)
Calculate enthalpy change per mole of each carbonate
Use ΔHreaction = ΔH1 - ΔH2 to find the enthalpy change for the reaction
N.B. ΔH1 is twice the ΔHreaction KHCO3 + HCl