Mini-Unit 12: Reaction Rates and EquilibriumIntroduction to Rates of Reaction

Reaction RatesRxn

speed:Examples

Very fast

Moderate

Very slow

Mg and hydrochloric acid, sodium and water, burning a match, combustion of fuel, etc.Food spoiling, rusting of iron, baking a cake, food digestion, cooking, etc.

Forming diamonds, weathering of rocks, setting concrete, silver tarnishing, etc.

RatesA rate measures changes that occur within an interval of time.

In chemistry, the rate of a reaction is expressed

as the amount of reactant changing per unit time. (Example:

mol/sec)

Collision TheoryCollision theory: atoms, ions, and molecules can react to form products when they collide with one another, provided they have enough kinetic energy.•If they do not have enough kinetic energy they will collide then bounce apart.•Similarly, bonds holding the molecules together can break apart if supplied with enough energy.

Activation Energy (Ea)The minimum energy colliding

particles must have in order to react.•During a reaction there is an in-between state called the activated complex or transition state which is the unstable arrangement of atoms at the peak of the activation energy barrier. (Exists for a very short amount of time, ~10-13 seconds.)

Energy Changes in a Reaction

En

erg

y

Time

Energy of reactants

Energy of products

Ea o

f fo

rward

rx

n

Ea

Enthalpy (ΔH) of reaction

Factors That Affect Reaction Rates

The rate of a chemical reaction depends on:1.Temperature2.Concentration3.Particle size4.Adding a catalyst

TemperatureIncreasing the temperature raises the frequency of collisions and the number of particles that have enough kinetic energy to get over the energy barrier.

ConcentrationIncreasing the concentration causes an increase in the frequency of collisions.

Particle SizeThe smaller the particle size, the larger the surface area for a given mass of particles.

Adding a CatalystThe addition of a catalyst lowers the energy barrier of the activated complex.Example:

2H2(g) + O2(g) 2H2O(l)Pt

Catalyst: Increases the rate of a reaction by lowering the Ea.

Inhibitor: A substance that decreases the rate of a reaction by increasing Ea.