Potential Potential Energy Energy

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Drill – 5/22/08

1. What is the general rate law format?

2. For the following rate law R = k[H2]2[O2]

If the concentration of H2 is tripled, what happens to the rate?

If the concentration of O2 is tripled, what happens to the rate?

•Will two molecules that never encounter each other react?–NO! In order for molecules to

react they must collide.

–Collision Theory states that in order for two atoms, ions, or molecules can react to form products when the collide IF they have enough energy.

This energy is called the ACTIVATION ENERGY. It is the minimum energy that colliding particles must have in order to react.

• Let’s consider the reaction between hydrogen and oxygen to form water:

2H2 + O2 2H2O

+

Reactants

Products

Ene

rgy

Time

Activated Complex - An unstable arrangement of atoms forms momentarily at the activation-energy barrier. Also called TRANSITION STATE.

Reactants

Products

Ene

rgy

Time

Activated Complex

Activation Energy

Heat of Reaction (ΔH)

Endothermic or Exothermic???

• Now let’s look at an endothermic reaction.

–What will have higher energy, the reactants or the products?

–Will the Heat of Reaction (ΔH) be positive or negative?

Reactants

Products

Ene

rgy

Time

Activated Complex

Activation Energy

Heat of Reaction (ΔH)

Endothermic or Exothermic???

Catalyst

• A CATALYST increases the rate of reaction by lowering the activation-energy barrier.

Reactants

Products

Ene

rgy

Time

Activation Energy without catalyst

Heat of Reaction (ΔH)

Does a catalyst affect the Heat of Reaction?

Activation Energy with catalyst

• How does the catalyst affect the magnitude of the activation energy?

• Does the catalyst change the amount of energy released in the reaction?

• Along which of the two reaction paths are reactants converted more rapidly to products?