1. Reaction Kinetics & Potential Energy Diagrams

2. Reactions that Release Energy
These reactions are Exothermic
the system loses heat
The surroundings heat up

3. Reactions that Absorb Energy
These reactions are Endothermic
system gains heat
the surroundings cool down

4. Graphing Heat in Reactions
We’ve already looked at one graphical representation…
q = mHv
q = mCΔT
Temperature
q = mHf
Heat (Joules) or Time

5. What Heating Curves Tell Us
Heating Curves tell us how temperature changes when energy is absorbed or released.
For Example:
Energy Added  Kinetic Energy  ΔTemp
Energy Added  Potential Energy  Δ Phase
Heating Curves do NOT tell us how much energy was needed to get the rxn started.
Heating Curves do NOT tell us how the energy used to start the reaction compares to the energy produced by the reaction.

6. However, only a small fraction of collisions produces a reaction. Why?
We know that molecules must collide to react.
However, only a small fraction of collisions produces a reaction. Why?
Particles must collide with the correct orientation and the right amount of energy
Particles lacking the correct site or necessary kinetic energy to react bounce apart unchanged when they collide.

7. Activation energy is the amount of energy needed to cause a reaction.
How Reactions Happen
Activation energy is the amount of energy needed to cause a reaction.
Collisions must equal or exceed the activation energy

8. Why are we learning this?
Energy diagrams show how a reaction works.
Energy diagrams display
The energy of reactants and products
The energy needed for a reaction to occur
The energy absorbed or released due to reaction

9. Energy Diagram Parts Potential Energy of the reactants
Potential Energy of the products
Activation Energy E – the amount of energy required to reach the transition state or activated complex from the reactant
Transition State, Ts (Activated Complex) – when bonds break and form and reactants become products
Enthalpy, DH – science word for heat, Energy of products – energy of reactants

10. Types of Energy Diagrams
Endothermic – the reaction absorbs heat so DH is positive
The reactants have less potential energy than the products
A B AB
DH > 0
A + B

11. Types of Energy Diagrams
Exothermic – the reaction releases heat so DH is negative
The reactants have more potential energy than the products
A B
A + B
DH < 0 AB

12. A POTENTIAL ENERGY DIAGRAM: C + O2 → CO2 + 395 KJ
Products have lower energy than reactants
Energy released
Exothermic
ΔH = -395kJ
(Heat of Reaction)
C + O2
Energy
395kJ
CO2
Reactants
Products

13. A POTENTIAL ENERGY DIAGRAM: CaCO3 + 176kJ → CaO + CO2
Products have higher energy than reactants
Energy absorbed
Endothermic
ΔH = +176kJ
(Heat of Reaction)
CaO + CO2
Energy
176kJ
CaCO3
Reactants
Products

14. Potential Energy Diagrams
One thing that is not represented in these simple examples…
Activation energy
Minimum amount of energy required to start a reaction

15. Endothermic with Activation Energy
However, adding 150kJ won’t make the reaction happen
100 kJ
250 kJ
Products have 350kJ of PE
Activation Energy
ΔH = Heat of Reaction
150 kJ
Reactants have 200kJ of PE

16. Endothermic with Activation Energy
Activated Complex – temporary, unstable, intermediate union of reactants
100 kJ
250 kJ
150 kJ
450 kJ
ΔH = 150kJ
Activation Energy

17. Exothermic with Activation Energy
Activation energy = 10kJ
Activation energy
Reactants
40kJ
ΔH = 25kJ ΔH
Products15kJ

18. Catalyst: something that reduces the amount of activation energy needed to start a reaction – speeds up a reaction
Activation energy with a catalyst

20. Endothermic Reaction with a Catalyst

21. Exothermic Reaction with a Catalyst

22. Five Factors Affecting Reaction Rate
Nature of the Reactants: what the reactants are (solids, liquids or gases)
Temperature:
Increasing temperature always increases the rate of a reaction.
Surface Area:
Increasing surface area increases the rate of a reaction

23. Five Factors Affecting Reaction Rate
Concentration:
Increasing concentration USUALLY increases the rate of a reaction
Presence of Catalysts
Catalyst: A substance that speeds up a reaction, without being consumed itself in the reaction