Reaction RateHow Fast Does the Reaction Go

Collision Theory

• Chemists believe that all chemical change (rearrangement of matter) occurs due to the collision of the atoms or molecules that are reacting.

Collisions must have enough energy to produce the reaction (must equal or exceed the activation energy).

Orientation of reactants must allow formation of new bonds

Molecules not correctly orientated, so no reaction

Molecules are correctly orientated, so reaction occurs

For example:2BrNO(l) 2NO (g) + Br2 (g)

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Reaction coordinate

Reactants

Products

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Reaction coordinate

Reactants

Products

Activation Energy - Minimum energy to make the reaction happen Ea

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Reaction coordinate

Reactants

Products

Activated Complex or Transition State

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Reaction coordinate

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Products

Overall energy change ΔH

Endothermic Reactions

Exothermic Reactions

Bimolecular Reaction

Things that affect Rate

1. Temperature

-Higher temperature faster particles.

-More and harder collisions.

-Faster Reactions.

Temperature & Reaction Rate

Things that affect rate

2. Concentration

-More concentrated closer together the molecules.

-Collide more often.

-Faster reaction.

Things that Effect Rate

3. Nature of reactants

a. Reactions are rapid at room temperature if no bonds are required

Ionic reactions:

Pb2+ + SO42- → PbSO4

b. Reactions are slow at room temperature when several bonds in the reacting substances must be broken

Molecular reactions

C2H5OH + 3 O2 2CO2 + 3H2O

4. Surface area

-Molecules can only collide at the surface.

Smaller particles bigger surface area.

Smaller particles faster reaction.

5. a)Catalysts- substances that speed up a reaction without being used up.(enzyme).

-Speeds up reaction by giving the reaction a new path.

-The new path has a lower activation energy.

-More molecules have this energy.

-The reaction goes faster.

b) Inhibitor- a substance that blocks a catalyst (raises the activation energy).

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Reaction coordinate

Reactants

Products

Endothermic Reaction witha Catalyst

Exothermic Reaction with a Catalyst

Catalysts Increase the Number of Effective Collisions

Reaction Mechanism

• Elementary reaction- a reaction that happens in a single step.

• Reaction mechanism is a description of how the reaction really happens.

• It is a series of elementary reactions.• The product of an elementary reaction is

an intermediate.• An intermediate is a product that

immediately gets used in the next reaction.

This reaction takes place in three steps

Ea

First step is fast

Low activation energy

Second step is slowHigh activation energy

Ea

Ea

Third step is fastLow activation energy

Second step is rate determining

Intermediates are present

Activated Complexes or Transition States

Mechanisms and rates

• There is an activation energy for each elementary step.

• Slowest step (rate determining) must have the highest activation energy.

A chemical example of a Reaction Mechanism

• H2 (g)+I2 (g) → 2HI (g)

• This reaction is very slow at room temperature but if the system is exposed to sunlight ( source of UV light ) the reaction proceeds rapidly.

• Step 1– I2 (g) →2I (g)

• Step 2– H2 (g)+2I (g) → 2HI (g)

• Overall: H2 (g)+I2 (g) → 2HI (g)

Mechanism with catalyst: Step rate

1. Cl + O3 → O2 + ClO Medium

2. O3 → O2 + O Fast

3. ClO + O → Cl + O2 Fast

Overall 2O3 → 3O2

Intermediates are ClO and O; catalyst is Cl