1. Ch 16 Reaction Energy

2. Thermochemistry
Thermochemistry: the study of the transfer of energy as heat that occurs during chemical reactions and changes in state.
Heat: q, is energy transferred from one object to another because of a temperature difference between them.
Heat always flows from a warmer object to a cooler object and will continue to flow until they are in equilibrium.

3. Endothermic Process: one that absorbs heat from the surroundings (+q).
Exothermic Process: one that releases heat to it’s surroundings (-q).
Calorimeter: the insulated device used to measure the absorption or release of heat in chemical or physical processes.

4. Specific Heat Heat flow is measured in joules (J).
One joule of heat raises the temperature of 1g of pure water °C.
Specific Heat: the amount of heat needed to increase the temp of 1 g of the substance 1°C or 1 Kelvin.
Water’s specific heat is 4.18 J/(g●K)

5. Calculating Specific Heat (C)
Divide the heat input, q (Joules) by the temperature change, ΔT (°C) times the mass of the substance, m (g).
C = q = heat m x ΔT mass x change in temp

6. Example
The temperature of a 95.4 g piece of copper increases from 25.0°C to 48.0°C when the copper absorbs 849 J of heat. What is the specific heat, C, of copper?
q = 849 J m = 95.4g
ΔT = (48.0°C -25.0°C)=23.0°C
C = q = J m x ΔT g x 23.0°C
C = J/(g x °C)

7. Example 2
How much heat, q, is required to raise the temperature of g of silver 45°C? The specific heat of silver is 0.24 J/(g x °C).
ΔT = 45°C m = g
C= 0.24 J/(g x °C)
q = C x m x ΔT
q = 0.24 J/(g x °C) x g x 45°C
q = 4320 J

8. ∆H = Hproducts - Hreactants
Enthalpy of Reaction
Enthalpy Change is the amount of energy absorbed by a system as heat during a process at constant pressure.
The enthalpy change is always the difference between the enthalpies of the products and the reactants and is called Enthalpy of Reaction.
∆H = Hproducts - Hreactants

9. ∆H is negative for an exothermic reaction because the system loses heat.
∆H is positive for an endothermic reaction because the system gains heat.

10. 2H2(g) + O2(g)  2H2O(g) ∆H = -483.6kJ
Example 3
Enthalpy of Reaction for the formation of water vapor.
What we already know:
2H2(g) + O2(g)  2H2O(g)
This equation does not tell us that energy is released as heat during the reaction.
Thermochemical Equation:
2H2(g) + O2(g)  2H2O(g) kJ
Writing with ∆H:
2H2(g) + O2(g)  2H2O(g) ∆H = kJ

11. 2H2O(g)  2H2(g) + O2(g) ∆H = +483.6kJ
Enthalpy of Reaction for the decomposition of water vapor.
What we already know:
2H2O(g)  2H2(g) + O2(g)
This equation does not tell us that energy as heat is absorbed during the reaction.
Thermochemical Equation:
2H2O(g) kJ  2H2(g) + O2(g)
Writing with ∆H:
2H2O(g)  2H2(g) + O2(g) ∆H = kJ

12. Compounds that release a large amount of energy as heat when they are formed are very stable.
Compounds that release a very small amount of energy as heat or absorb a large amount of energy as heat when they are formed are sometimes unstable and may decompose or react violently.

13. Ch 17.1
Reaction Kinetics

14. Chemical Reactions
Activation Energy: the minimum energy that colliding particles must have in order to react.
Activated Complex: an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier.
This is also called the Transition State.

15. Energy Diagrams ∆Eforward = energy of products – energy of reactants
∆Ereverse = energy of reactants – energy of products
Ea = energy of activated complex – energy of reactants
Ea’ = energy of activated complex – energy of products

16. ∆Eforward is positive for endothermic and negative for exothermic
∆Ereverse is negative for endothermic and positive for exothermic

17. Examples!! These are the sample problem A on page 566 and practice problem #3 on page 567.

18. Ch 17.2

19. Rate Influencing Factors
The rate of a chemical reaction depends upon temperature, concentration, particle size, and the use of a catalyst.
The nature of the reactants and their bonds is also a factor, but not one that can be easily changed so we will not talk about it.

20. TEMPERATURE CONCENTRATION
Raising the temperature speeds up the reaction and lowering the temperature slows down the reaction.
The higher the concentration, the more likely collisions will take place, which increases the reaction rate.
CONCENTRATION

21. PARTICLE SIZE CATALYST
The smaller the particle size, the more surface area, which increases the reaction rate.
Adding a catalyst will increase the rate of reaction, in some cases, better than increasing the temperature.
Inhibitor: a substance that interferes with the action of a catalyst.
These will slow down or even stop a reaction.
CATALYST