1. Activation energy Activation energy and energy profile
Background (Scientific context)

2. Activation energy and energy profile
Two basic requirements for an effective collision
the particles collide in the right orientation.
the colliding particles possess sufficient kinetic energy.
Activation energy, Ea
Activation energy is the minimum energy required for a chemical reaction to occur.
It is an energy barrier that the reactant particles have to overcome before a reaction can occur.

3. Only those reactant particles with energy equal to or greater than the activation energy can react to form the product.
Fast reactions have low activation energy; slow reactions have high activation energy.

4. Energy profile
The energy change of chemical species during the course of a reaction can be represented in an energy profile.
activated complex
Potential energy Ea
reactants H
products
Reaction coordinate
An energy profile of a single-step exothermic reaction.

5. Activated complex
is the species that exists at the point of maximum energy;
forms in the transition state that lies between the reactants and products of a chemical reaction;
contains partially broken and partially
formed bonds;
is highly unstable and can never be isolated.
The difference in energy between the reactants and the activated complex is the activation energy, Ea.

6. A reaction is exothermic if the energy of the
products is less than the energy of the reactants.
Potential energy Ea
reactants
H (negative)
products
Reaction coordinate
Energy profile of an exothermic reaction showing Ea and H (negative).

7. A reaction is endothermic when the energy of the
products is greater than the energy of the reactants.
Potential energy Ea
products
H (positive)
reactants
Reaction coordinate
Energy profile of an endothermic reaction showing Ea and H (positive).

8. Step 1: CD  intermediate Step 2: intermediate + E  CE + D
activated complex 1
activated complex 2
Ea1
Ea2
Potential energy
intermediate
+ E
CD + E
CE + D
Reaction coordinate
Energy profile of a two-step reaction between C–D and E.
Step 1: CD  intermediate
Step 2: intermediate + E  CE + D
______________________________________________________
Overall reaction: CD + E  CE + D

9. Ea1 > Ea2
The energy barrier for the first step is larger, so fewer particles possess sufficient energy to react.
The first step is slower. It limits the rate of the overall reaction.
The first step is regarded as the rate-determining step.

11. Rates of reaction

12. Measuring rates of reaction
Calcium carbonate + hydrochloric acid
calcium chloride + water + carbon dioxide
Example

15. Factors that affect the rate of reaction
surface area of a solid reactant.
concentration or pressure of a reactant.
temperature.
nature of the reactants.
presence/absence of a catalyst.

16. 1.

18. 2.

19. 3. Elephant toothpaste

20. 4.

22. Lab. Report Request Form due 14/10 (Wed)
You should finish your research question, background, materials list and methods (IV, DV, CV) by 14/10

23. Lab. Report
Day 1: Do the experiment (29/10) Day 2: Report Due (30/10)