1. Unit 5: Thermochemistry
Reaction Energy Diagrams: slides 2-7
Specific Heat: slides 8-13
Enthalpy and Hess’s Law: slides

2. Thermochemistry
thermochemistry: studies the relationships between chemical reactions and energy changes involving heat

3. Thermochemistry Heat and temperature are NOT the same thing!
Heat: transfer/flow of energy due to a temperature difference (from high to low)
Temperature: a measure of the random motions of molecules (average kinetic energy)

4. Reaction Energy Diagram
Reaction energy diagrams show the energy changes throughout the reaction
Activated complex
(Also called the transition state)
Activation Energy
Products
Energy
Energy change for reaction
Reactants
Reaction proceeds

5. energy EXITS the system
Exothermic Reactions
Exothermic:
energy EXITS the system
E<0 (neg.)
System
Energy
Surroundings

6. Endothermic Reactions
energy ENTERS the system
E>0 (pos.)
System
Energy
Surroundings

7. Catalysts create a lower energy reaction pathway
Activated complex
(Also called the transition state)
Activation Energy
Products
Energy
Energy change for reaction
Reactants
Reaction proceeds

8. Specific Heat
specific heat: willingness of an object to change temperature, with the symbol Cp (the p means "under constant pressure")
specific heat: the amount of energy required to change the temperature of one gram of a substance by 1°C

9. Specific Heat Units
1 calorie = energy required to heat 1 gram of water by 1°C
1 Calorie (food labels) = 1 kilocalorie
1 calorie = joules

10. Specific Heat
Duluth, next to Lake Superior, stays cool in the summer and relatively warm in the winter. Why?
Substance
Specific Heat (J/g°C)
copper
0.3845
granite
0.7953
lead
0.1276
ice
2.06
water
4.184

11. Calculating Heat (Energy Transfer)
q = Cp x m x T
change in temperature
(units: °C)
specific heat (units: J/g°C)
mass
(units: g)

12. Calculating Heat Example #1
q = Cp x m x T
3.05 kg of aluminum is heated from 22.1C to 67.5C. Calculate the heat absorbed in both J and kJ by the metal. The specific heat of aluminum is J/gC.

13. Calculating Heat Example #2
q = Cp x m x T
A 200-g block of copper at temperature of 90 °C is dropped into 400 g of water at 27 °C. The What is the final temperature of the mixture? The specific heat of copper is J/gC.

14. H = ΣHproducts – ΣHreactants
Enthalpy
Enthalpy is the heat energy absorbed or lost in a reaction (H or H if talking about change)
H = ΣHproducts – ΣHreactants

15. CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)
Enthalpy Example
Calculate ΔH for the following reaction:
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)
Substance
ΔH (kJ/mol)
CH4
-74.8 O2
CO2
-393.5
H2O

16. Hess’s Law
Hess’s Law: overall enthalpy changes are the sum of all steps of the reaction process
Add enthalpies (H) from all reaction steps
Note:
If a reaction is reversed, the sign of H is also reversed
The magnitude of H depends on the quantities of reactants and products
Multiply H by the coefficient for that substance

17. Hess’s Law Example
Calculate the heat of reaction, H, for the overall reaction:
N2(g) + 2O2(g)  2NO2(g)
Given:
2NO(g)  N2(g) + O2(g) H=-180 kJ 2NO(g) + O2(g)  2NO2(g) H=-112 kJ

18. Hess’s Law Practice
Calculate the heat of reaction, H, for the overall reaction:
2 S (s) + 3 O2 (g)  2 SO3 (g)
Given:
S (s) + O2 (g) SO2 (g)  ∆H  =  -297 kJ
2 SO3 (g)2 SO2 (g) + O2 (g)  ∆H  =  198 kJ 

19. PbCl2 (s) + Cl2 (g) → PbCl4 (l)
Hess’s Law Exit Slip
Calculate ΔH for this reaction:
PbCl2 (s) + Cl2 (g) → PbCl4 (l)
Given:
Pb (s) + Cl2 (g) → PbCl2 (s) ∆ H = kJ
Pb (s) + 2 Cl2 (g) → PbCl4 (l) ∆ H = kJ