1. Enthalpy = heat energy (H)
Learning objective:
To measure enthalpy of neutralisation
To explain the trend in terms of free H+
27/05/2018
Enthalpy = heat energy (H)
The enthalpy of a substance can’t be measured…
…but the change can be.
system
reactants and products
surroundings
the rest of the universe!

2. Standard enthalpy of neutralisation ΔHnӨ
The standard enthalpy of neutralisation is the enthalpy change when one mole of water is formed by neutralisation under standard thermodynamic conditions
1 moldm-3, 1 atm and 298K

3. Experimental procedure
Total volume solution: 50 cm3
Measure volumes with 25.0 cm3 pipette – you will need to measure the second reactant into a beaker for rapid addition ©M. Hatten
Measure base line temperature: every 30 seconds for 2 ½ minutes
At 3 minutes add second reactant
Record temperature at 3 ½ minutes and every 30 seconds for another 2 minutes

4. Calculation
Enthalpy change (ΔH) of system (J) = - (mass water (g) x specific heat capacity x ΔT)
Enthalpy per mole:
calculate amount water produced (amount (mol) = conc (moldm-3) x vol (dm3))
HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)
H2SO4 (aq) + 2 NaOH (aq)  Na2SO4 (aq) + 2 H2O (l)
HNO3 (aq) + NaOH (aq)  NaNO3 (aq) + H2O (l)

5. What’s happened?
What is the key reaction? What happens to the substances in solution H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq)  H2O (l) + Na+ (aq) + Cl- (aq) H+ (aq) + OH- (aq)  H2O (l)
Why is it exothermic? H+ (aq) + O-H- (aq)  H-O-H (l)
Why do we get less enthalpy with ethanoic acid? CH3COOH (aq) ⇋ CH3COO- (aq) + H+ (aq) Only partially dissociated when it starts – about 1% So why does it produce almost as much enthalpy?

6. A problem for the new year…
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?

7. 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.

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