1. Spring Semester Final Exam Review
Thermochemistry

2. THERMOCHEMISTRY
The study of heat released or required by chemical reactions
Fuel is burnt to produce energy - combustion (e.g. when fossil fuels are burnt)
CH4(g) O2(g) CO2(g) + 2H2O(l) + energy

3. What is Energy? Kinetic energy (KE) Potential energy (PE)
Energy due to motion
Stored energy

4. Total Energy = Kinetic Energy + Potential Energy
E = KE PE
Temperature measures the average
Kinetic energy & potential energy are interchangeable
Heat is the total energy of a system:
Kinetic energy + potential energy

5. Systems & Surroundings
In thermodynamics, the world is divided into a system and its surroundings
A system is the part of the world we want to study (e.g. a reaction mixture in a flask)
The surroundings consist of everything else outside the system

6. A release of heat corresponds to a decrease in enthalpy
EXOTHERMIC & ENDOTHERMIC REACTIONS
Exothermic process: a change (e.g. a chemical reaction) that releases heat to the surroundings.
A release of heat corresponds to a decrease in enthalpy
Exothermic process: H < 0 (at constant pressure)
Burning fossil fuels is an exothermic reaction

7. Exothermic Reactions

8. Endothermic process: a change (e. g
Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat from the surroundings.
An input of heat corresponds to an increase in enthalpy
Endothermic process: H > 0 (at constant pressure)
Photosynthesis is an endothermic reaction (requires energy input from sun)

9. Endothermic Reactions

10. Endothermic or Exothermic?

11. Heating Curves
Heating Curves Animation
A plot of temperature vs. time that represents the process in which energy is added at a constant rate
The magnitude of the temperature change of an object when it absorbs or loses thermal energy depends on both the amount of thermal energy transferred (q) and the heat capacity of the object.
Heat capacity (C) — amount of energy needed to raise the temperature of the object exactly 1°C, and the units of C are joules per degree Celsius (J/°C)
Change in temperature (ΔT) is: ΔT = q/C,
where q is the amount of heat (in units of J), C is the heat capacity (in units of J/°C), and ΔT is Tfinal  Tinitial (in units of °C).
 If ΔT and q are positive, heat flows from the surroundings into the object, and if ΔT and q are negative, heat flows from the object into the surroundings.
Courtesy Christy Johannesson

12. Heating Curves Gas - KE  Boiling - PE  Liquid - KE  Melting - PE 
140
Gas - KE 
120
100
Boiling - PE  80 60 40
Liquid - KE 
Temperature (oC) 20
Melting - PE 
-20
-40
Solid - KE 
-60
-80
-100
Time

13. A plot of temperature vs
A plot of temperature vs. time that represents the process in which energy is added at a constant rate

14. The standard enthalpy of reaction (DH0 ) is the enthalpy of a reaction carried out at 1 atm.
rxn
aA + bB cC + dD
DH0
rxn
dDH0 (D) f
cDH0 (C) = [ + ] -
bDH0 (B)
aDH0 (A)
DH0
rxn
nDH0 (products) f = S
mDH0 (reactants) -
6.6

15. Example Problem Calculate the heat of combustion of methane, CH4
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
H◦f CH4 (g) = kJ/mol
H◦f O2(g) = kJ/mol
H◦f CO2(g) = kJ/mol
H◦f H2O(g) = kJ/mol
pg. 316
2 mol( kJ/mol) = kJ
Step #1: multiply the H◦f H2O(g) by 2 since there are two moles of water in the products .

16. Hrxn =  Hf(products) -   Hf(reactants)
Example Problem
Calculate the heat of combustion of methane, CH4
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
H◦f CH4 (g) = kJ
H◦f O2(g) = kJ/
H◦f CO2(g) = kJ
H◦ fH2O(g) = kJ
pg. 316
H◦f = [ kJ + ( kJ)]- [ kJ + (0 kJ )]
H◦f = kJ
Step #2: sum up all the H◦ f. :
Hrxn =  Hf(products) -   Hf(reactants)

17. Calculations Involving Specific HeatOR
C = Specific Heat Capacity
q = Heat lost or gained
T = Temperature change
Tf - Ti

22. Choose all that apply... C(s) + 2 S(g) CS2(l) H = 89.3 kJ
Which of the following are true?
This reaction is exothermic
B) It could also be written
C(s) + 2 S(g) kJ CS2(l)
C) The products have higher energy than the reactants
D) It would make the water in the calorimeter colder