– What makes “superglue” bond instantly while PVA glue does not?

– What factors determine how quickly food spoils?– Why do “glow sticks” last longer when stored in

the freezer?

Rates of reaction

Rate = change in concentration (of reactant or product)time

Methods for measuring rate of reaction

Collect a gas over time Follow a change in mass over time

150g

Formation of a precipitate or colour change over time.

Factors affecting rate

• Concentration (pressure with gases)

• Temperature• Catalysts • Surface Area

How doChemical ReactionsHappen?

In Contact• Reactions don’t happen unless the substances are in contact.

Why?• The particles of the reactants need to get together so that they can react.

How does that work then?

Reactant particlescollide

Product particlesformed

REACTION

Is it really that simple?

YesWell, sort of.

not really.

OK, not quite!

Not all collisions are effective

• Paper burns• Paper + oxygen carbon dioxide + water +

nitrogen

• The paper in this room isn’t burning.• It doesn’t have enough energy to

burn.

• If we make it hotter it will catch fire.• Paper burns on its own at 250 ºC

A collision but with no effect

Reactant particlescollide

Product particles not formed as there is

not enough energy

Activation Energy • Reactions only happen if the particles

have enough energy.

• The minimum amount of energy needed to start a reaction is called the Activation Energy

• The amount of activation energy needed is different for each reaction.

• But

• Every reaction has activation energy, they all need a little push to get started.

More than the activation energy.

Reactant particlescollide

Product particlesformed

REACTION

More than the activation energy.

C

O OCO O

Effective collisions, a reaction.

The Collision Theory• Particles are constantly moving

• For a chemical reaction to take place the reactant particles must collide first

• For the collision to be successful the particles must have the right amount of energy

• The minimum amount of energy required for an effective collision is called the activation energy

Successful collisions

When two molecules collide a reaction might take place if:

- The molecules have enough energy to overcome the activation energy

- The molecules collide in the correct orientation

Concentration

• More particles in the same volume means they are closer together so greater chance of colliding.

• More collisions means more collisions with energy greater than activation energy.

• More frequent successful collisions means increased rate of reaction.

Pressure• When the pressure is

increased the same number of molecules occupy a smaller volume

• For gaseous reactants this has the same effect as increasing the concentration.

• More collisions take place, so more collisions with energy greater than activation energy.

• More frequent successful collisions means increased rate of reaction.

Catalysts

A catalyst speeds up a reaction without being consumed by the overall reaction.

Catalysts

• Catalysts reduce the activation energy needed for a reaction

• They do this by offering an alternate route for the reaction to take (for example via an intermediate)

• Lower activation energy means more successful collisions

• More successful collisions means faster rate

Catalysts are very useful in industry

- They often require lower temperatures than uncatalysed reactions, which reduces the energy demand and carbon dioxide emissions from fossil fuels.

- They allow different reactions to be used with a better atom economy and reduced waste.

- They are often enzymes which allow processes to operate at lower temperatures and pressures

Examples of economically important catalysts

Iron in ammonia production

Ziegler– Natta catalyst in poly(ethene) production

Platinum/palladium/rhodium in catalytic converters