1. Enthalpy Calculations

2. Kaupapa / Learning Outcome
At the end of todays lesson I will be able to use stoichiometry to calculate heats of reactions for known masses

3. Stoichiometry review
Chemical equations give information about the chemicals involved and the amounts of those chemicals involved
Example:
2SO2(g) + O2(g)  2SO3(g)
This tells me that Sulfur Dioxide is reacting with Oxygen to form Sulfur Trioxide

4. 2SO2(g) + O2(g)  2SO3(g)
It also tell me the amount in moles that is reacting of each chemical
2 moles of SO2 reacts with 1 mole of O2 to form 2 moles of SO3

5. Question Fe2O3 + 3CO  2Fe + 3CO2
Q1. If 200g of Fe2O3 is reacted in the above equation what mass of Fe is produced? (Mr Fe2O3 = 160 gmol-1, Mr Fe = gmol-1)

6. Fe
Fe2O3
ratio 2 1 n
2.5 mol
1.25 mol m
140g
200g Mr
55.85g mol-1
160g mol-1

7. Thermochemical Equations
These are balanced equations that include a ∆H value
2SO2(g) + O2(g)  2SO3(g) ∆rH = -188 kJ mol-1
This means that when 2 moles of SO2 reacts with 1 mole of O2 188 kJ of energy is released
Because it is kJmol-1 the amount of energy is related to the number of moles in the reaction

8. Changing the amount of reactants
2SO2(g) + O2(g)  2SO3(g) ∆rH = -188 kJ.mol-1
If we halved the number of moles of SO2 then the energy released would halve as well
SO2(g) + ½ O2(g)  SO3(g) ∆rH = -94 kJ.mol-1
If we doubled the amount of reactants the energy released would double etc…

9. Enthalpy change of a reaction
Calculating ΔrH
Energy per mol
Number of moles
ΔrH = ΔE / n
Enthalpy change of a reaction
Energy Change in kJ

10. Question Ethanol burns according to the equation:
C2H5OH + 3O2  2CO2 + 3H2O ΔrH=-1370kJ mol-1
Calculate the amount of energy released when 9.2g of ethanol is burnt.

11. Step 1. Calculate the moles Mr(C2H5OH) =46g mol-1 n=m/Mr n=9
Step 1. Calculate the moles Mr(C2H5OH) =46g mol-1 n=m/Mr n=9.2g/46g mol-1 n= 0.200mol

12. ΔE= ΔrH x n ΔE= =-1370kJ * 0.200 mol ΔE= -247 kJ
Step 2: Calculate the change in energy using the formula ΔrH = ΔE / n
ΔE= ΔrH x n
ΔE= =-1370kJ * mol
ΔE= -247 kJ

13. Question
Calcium burns in oxygen to produce calcium oxide. The equation for this reaction is: 2Ca +O2  2CaO ΔrH=-1270 kJ mol-1 Calculate the mass of calcium that must react to release 116kJ of energy

14. n= ΔE / ΔrH n= -116kJ / -1270kJ mol-1 n= 0. 0913 mol m= n x Mr m= 0
n= ΔE / ΔrH n= -116kJ / -1270kJ mol-1 n= mol m= n x Mr m= mol x 40.1g mol-1 m= 3.66g