1. Representing Enthalpy Changes
There are a number of different ways to express the change in the quantity of energy stored in a chemical system.
Enthalpy changes, DH, can be expressed using thermochemical equations.
For instance the complete combustion of 1.0 mol of methanol releases 726 kJ of heat energy.
The balanced thermochemical equation is:
CH3OH + 3/2 O2 ---> CO H2O kJ

2. CH3OH + 3/2 O2 ---> CO2 + 2H2O + 726 kJ
DHcombustion = -726 kJ/mol of CH3OH
Notice the enthalpy change is negative since the quantity of energy stored in CO2 and H2O is less than the quantity of energy stored in CH3OH and O2.

3. For the thermochemical equation
2A + B kJ ---> 3C + 4D
What is the DH / mol of A?
120 kJ / 2 mol A = 60 kJ / mol A
What is the DH / mol of B?
120 kJ / 1 mol B = 120 kJ / mol B
What is the DH / mol of C?
120 kJ / 3 mol C = 40 kJ / mol C
What is the DH / mol of D?
120 kJ / 4 mol D = 30 kJ / mol D

4. For the thermochemical equation
A + 3B ---> 4C + 2D kJ
What is the DH / mol of A?
-240 kJ / 1 mol A = -240 kJ / mol A
What is the DH / mol of B?
-240 kJ / 3 mol B = - 80 kJ / mol B
What is the DH / mol of C?
-240 kJ / 4 mol C = -60 kJ / mol C
What is the DH / mol of D?
-240 kJ / 2 mol D = -120 kJ / mol D

5. For the thermochemical equation
HCl(aq) + NaOH(aq) ----> H2O(l) + NaCl(aq) + 45 kJ
enthalpy change can also be expressed as the molar enthalpy outside the equation
DHneutralization = - 45 kJ/mol of HCl or NaOH or H2O or NaCl
enthalpy change can also be shown by a graph

6. For the thermochemical equation
HCl(aq) + NaOH(aq) ----> H2O(l) + NaCl(aq) + 45 kJ
Potential Energy Diagram for the Neutralization of HCl by NaOH
HCl(aq) + NaOH(aq DH Ep
Reaction Progress
H2O(l) + NaCl(aq)

7. If the dissolving of 8. 0 g of NH4NO3(s) requires 4
If the dissolving of 8.0 g of NH4NO3(s) requires 4.0 kJ of heat write the thermochemical equation and draw a Potential energy curve.
The molar mass of ammonium nitrate is 80 g.
the n NH4NO3(s) is
0.10 mol
molar enthalpy change is
4.0 kJ / 0.10 mol = 40 kJ / mol of NH4NO3(s)
thermochemical equation is
NH4NO3(s) + 40 kJ ---> NH4NO3(aq)

8. NH4NO3(s) + 40 kJ ---> NH4NO3(aq)
Potential Energy Diagram for the dissolution of Ammonium Nitrate
NH4NO3 (aq) DH Ep
Reaction Progress
NH4NO3 (s)

9. When sulfur dioxide reacts with oxygen sulfur trioxide forms
When sulfur dioxide reacts with oxygen sulfur trioxide forms. If DHreaction is -198 kJ/mol of oxygen what quantity of heat is released when 256 g of SO2 is burned?
Write the balanced equation.
SO2(g) + O2 (g) ----> SO3 (g) 2
+ 198 kJ
Calculate the heat released per mole of SO2.
198 kJ / 2 mol SO2 = - 99 kJ / mol SO2
Multiply the kJ /mol SO2 by nSO2
n SO2 = 256 g / 64 g/mol = 4.0 mol
(- 99 kJ / mol ) x (4.0 mol) = kJ

10. C4H9OH(l) + O2(g) ----> CO2(g) + H2O(l) 6 4 5
If the molar enthalpy of combustion of butanol is kJ, what quantity of heat energy is released when 7.4 g of butanol is burned?
First write the balanced thermochemical equation for the combustion of butanol.
C4H9OH(l) + O2(g) ----> CO2(g) + H2O(l) 6 4 5 kJ
n C4H9OH(l) is 7.4 g / 74 g/mol = 0.10 mol
kJ / mol x 0.10 mol =
- 64 kJ

11. Potential Energy Diagram for the combustion of Glucose
Draw a Potential energy Diagram for the combustion of glucose if the molar enthalpy is -2.8 MJ.
Potential Energy Diagram for the combustion of Glucose
C6H12O6(s) + 6O2(g)
DH = -2.8 MJ Ep
Reaction Progress
6CO2(g) + 6H2O(l)

12. Try questions
pg. 319 # 1-5,
pg. 320 # 1-4