1. Grade 12 Chemistry SCH4U0 Mr. Krstovic
Thermochemistry
Grade 12 Chemistry SCH4U0
Mr. Krstovic

2. What did we learn so far?
Heat is energy transferred from one place to another
Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations.
Calorimetry is the science of measuring the heat of chemical reactions or physical changes.

3. What did we learn so far? Q = mcT
A calorimeter is a device used for calorimetry.
Q = mcT
Specific heat capacity (c) has units of J/gC
Difference in heat capacity of oil and water (recall the demo in class)

4. Remember these three processes?

6. Exothermic and Endothermic Processes
If heat is transferred from surrounding to system. Process is endothermic. Surroundings feel COOLER!
If heat is transferred from system to surroundings. Process is exothermic. Surroundings feel HOTER!

7. Enthalpy ΔHsystem = - Qsurrounding
Chemists give a special symbol, ΔH(delta H) to the heat change in a reaction
This heat change is called ENTHAPLY
ΔH (enthalpy) = energy absorbed or released to the surroundings when a system changes from reactants to products.
ΔHsystem = - Qsurrounding

8. Potential Energy Diagrams

9. Molar Enthalpy
ΔHx: the enthalpy change for a physical, chemical or nuclear change involving 1 mol of a substance
[Units: kJ/mol]
Types of Molar Enthalpies:
ΔHvap ΔHsol ΔHrx
ΔHfr ΔHcomb (See Table1 p.306)

10. Types of Molar Enthalpies
Example: ΔHcomb
CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(l)
ΔHcomb = kJ/mol

11. Enthalpies of Reaction
What is DH for the combustion of 11.0 g of CH4 in excess oxygen?
11.0 g CH4
= -550 kJ

12. Example Question
In a calorimetry experiment, 7.46 g of KCl is dissolved in mL of water at an initial T of 24.1 C. The final T of the solution is 20C. What is the molar enthalpy of solution of KCl?

13. Methods of Communicating Enthalpy Changes
Method 1: Energy value as part of the reaction
H2O (l) kJ  H2 + 1/2O2
Mg (s) + 1/2O2 (g)  MgO(s) kJ

14. Method 2 Thermochemical Equations with ΔH values next to the reaction
H2O (l)  H2 + 1/2O ΔH = kJ
Mg (s) + 1/2O2 (g)  MgO(s) ΔH = kJ

15. Method 3
Standard Molar Enthalpy of Reaction
measured at SATP conditions (100 kPa; 25 C)
ΔHorx - this symbol distinguishes standard molar enthalpies from molar enthalpies
ΔHocomb = -726 kJ/mol CH3OH

16. Method 4
Potential Energy Diagrams