1. Heat of Reactions

2. The Nature of Energy Energy - the ability to do work or produce heat.
Potential energy
Energy of position
Kinetic energy
Energy of motion
E = ½ mv2

3. The Nature of Energy
Law of conservation of energy- Energy can be converted from one form to another but can neither be created or destroyed.

4. The Nature of Energy State function
Property of the system that changes independent of path
Is this a state function?

5. Temperature and Heat
Temperature - a measure of the random motions of the components of a substance.
Hot water Cold water
(90. oC) (10. oC)

6. Temperature and Heat
Heat - flow of energy between two objects due to a temperature difference between the objects.
Heat is the way in which thermal energy is transferred from a hot object to a colder object.
Hot water Cold water
(90. oC) (10. oC)
Water Water
(50. oC) (50. oC)

7. Exothermic and Endothermic Processes
System – part of the universe on which we focus attention
Surroundings – everything else in the universe
Burning a match

8. Exothermic and Endothermic Processes
Exothermic – energy flows out of the system
Endothermic – energy flows into the system

9. DH = Change in enthalpy (heat)
-DH = exothermic Rx
+DH = endothermic Rx

10. Thermodynamics Thermodynamics – study of energy
First law of thermodynamics
Energy of the universe is constant

11. Measuring Energy Changes
The common energy units for heat are the calorie and the joule.
Calorie – the amount of energy (heat) required to raise the temperature of one gram of water 1oC.
Joule – 1 calorie = joules

12. Thermochemistry (Enthalpy)
Enthalpy (Heat), H – energy function
At constant pressure H is equal to the energy that flows as heat.
H = heat

13. Calorimetry
Enthalpy, H is measured using a calorimeter.

14. Hess’s Law
For a particular reaction, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps.
Example:
N2(g) + 2O2(g)  2NO2(g) H = 68 kJ

15. ΔHf = (S DH products) – (S DH reactants)
Standard enthalpy of formation (ΔHf ) – The change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states
ΔHf = (S DH products) – (S DH reactants)