How fast or slow is a reaction? And how can we make it go faster or slower?

Rate is always dependent on time, the best example is how fast a car is going.

Chemical reaction rates are determined by the increase in products over time.

rate = ∆products/∆t, where ∆ = change Because the products cannot form

unless the reactants are consumed, rate is also equal to the decrease in reactants over time.

Rate = ∆reactants/∆t = ∆products/∆t

Take the following reaction for example H2 + I2 2 HI

In order for a chemical reaction to occur, the molecules must collide and the collisions require a certain amount of energy. (see gas law simulator program)

Factors that affect frequency of collisions:• Temperature – affects speed of molecules • Pressure – affects distance between molecules• Concentration – affects interference from

molecules that do not participate in the reaction• Surface area – affects area available for collisions

to occur Note that temperature, pressure, & surface

area also affect the dissolving process.

The greater the frequency of collisions and the greater the force of those collisions, the faster the reaction will occur.

Some reactions can take a billionth of a second, others hundreds of years.

The energy needed for a collision to result in the formation of product(s)is called the “activation energy.” It is often abbreviated Ea. Think of the activation energy as being the energy of the collision.

A “catalyst” is a substance that increases reaction rate by lowering the required activation energy.

Catalysts are NOT consumed in the reaction; they can be used over and over again.

A biological catalyst is called a “enzyme,” and are present in all cells.