1. AP Chemistry Factors that Affect Reaction Rate and Potential Energy Diagrams

2. Collision Theory of Reactions
A successful reaction depends upon the effective collisions between molecules.
Reactant molecules must collide with enough energy to break bonds
Not every collision will result in a reaction.
The minimum energy required to break the bonds of the reactants and start a reaction is called the Activation Energy

3. Examples of Activation Energy
Scraping a match on a rough surface to light it.
Using that lighted match to light a candle.
Lightening strike causes oxygen (O2) to form ozone (O3)

4. Reaction Rates
The rate of a reaction can be increased, in general, by increasing the chances for effective collisions between molecules.
Likewise, you can slow a reaction down by reducing the chances of collisions.
Molecular Orientation in a collision can affect whether a reaction will occur.
There are also four other main factors that affect reaction rates.

5. 1. Temperature
Higher temperature molecules move faster  Molecules will have more energy
There will be more effective collisons
Molecules collide with the minimum energy (activation energy) more often.

6. 2. Concentration Higher Concentration = More particles
More particles = more possible collisions
More collisions = faster reaction!
For a gas - Increase Pressure Volume decreases  concentration increases.

7. 3. Surface Area Particle size determines surface area
Smaller particles  more surface area.
More surface area means more area for collisions to occur
MORE COLLISIONS = FASTER REACTION!

8. 4. Catalysts
A catalyst is a substance that increases the rate of a reaction without being used up in the reaction.
It increases the rate of the reaction by lowering the activation energy for a reaction
Low activation energy means = faster reactions
There is a greater chance for an effective collision if the activation energy is lower.

9. Exothermic vs. Endothermic Reactions & Enthalpy
Enthalpy(H) – the amount of heat a substance has at a given temperature and pressure
In a reaction, we look at the change in enthalpy of the reactants vs. the products (ΔH)
Reactions can absorb heat or release heat as they progress.

10. Endothermic Reactions
An endothermic reaction absorbs heat (+ΔH ) , we can feel a beaker become colder as a reaction proceeds as heat is taken in from the surroundings.
**Energy required to break reactant bonds is more than the energy released when product bonds form
A + B + Energy/heat  C + D

11. Exothermic Reactions
An exothermic reaction releases heat (-ΔH), we can feel the heat given off to the surroundings as the reaction happens.
**Energy required to break reactant bonds is less than the energy released when product bonds form.
A + B  C + D + Energy/heat

12. Potential Energy Diagrams
A graphic picture of a reaction which combines the ideas of enthalpy change (ΔH) and reaction rate.
Potential energy diagrams show the following
The starting potential energy of the reactants
The ending potential energy of the products
The activation energy (minimum energy required for the reaction to work- or to break bond in reactant molecules)

13. ΔHrxn = Heat energy released/absorbed in reaction
Reverse reaction
Activation energy
ΔHrxn = Heat energy released/absorbed in reaction

14. Potential Energy Diagram with Catalyst
With a catalyst – The Activation Barrier is Lowered
This Increases Reaction Rate
Note - Enthalpy , ΔH Remains the same!

15. If the forward Ea = 20 kJ and the ΔH = 50 kJ, what is the Reverse activation energy?
If the forward Activation energy is 125kJ and the Reverse activation energy is 45 kJ, what is the enthalpy for the reaction?