Reaction Rates

Chemical Equation Review

Chemical Reactions

• Every reaction has a chemical equation.• The reaction of nitrogen monoxide with ozone

produces nitrogen dioxide and oxygen gas.

NO + O3 NO2 + O2

What is the evidence of a chemical reaction?

• Color changes• Bubbles are produced• An odor is produced• Formation of a solid or a gas• A flame occurs (heat is released)

Collision Model – Ineffective Collision

• Molecules must collide in order for a reaction to occur.

• Sometimes molecules collide, but no reaction occurs. This is called an ineffective collision.

• An ineffective collision does not produce new products, so a reaction has not occurred.

• In this example, NO and O3 try to react, but are unsuccessful.

Collision Model – Effective Collision

• Other times, the molecules collide, and a reaction occurs. This is called an effective collision. An effective collision does produce new products, so a reaction has occurred.

• In this example, the reactants are NO and O3 and the new products formed are NO2 and O2.

Rate of Reactions

• The rate of a reaction is the decrease in the concentration of the reactants OR the increase in the concentration of the products with time.

What factors affect the rate of the reaction?

1. Nature of the reactants2. Concentration (for liquids)3. Pressure (for gases)4. Temperature5. Surface Area6. Presence of a catalyst

Nature of the reactants

• Substances vary greatly in their tendency to react depending on their bond strengths and structures.

Concentration (for liquids)

• The rate of a reaction depends on the number of collisions that occur between reactants.

• Higher concentrations have more molecules per unit volume, so reaction rates typically increase as concentration increases.

Pressure (for gases)

• The rate of a reaction depends on the number of collisions that occur between reactants.

• Reaction rates typically increase as pressure increases.

Surface Area

• Reaction rates increase with increased surface area.

Click here for animation

• More particles are exposed, increasing the chances of reactant particles colliding. Therefore there are more collisions at any given time and there is an increase in reaction rate.

Temperature

• Temperature is directly proportional to kinetic molecular energy.

• If temperature is increased, then the kinetic molecular energy of the molecules will increase, creating an increase of high-energy collisions between the molecules, increasing the rate of reaction.

Energy

• Only collisions with enough energy react to form products. This energy is called activation energy (Ea). The activation energy is the amount of energy required for the reaction to occur. In this picture the activation energy is the amount of energy required for the man to get rock A over the mountain.

Activation Energy

• A minimum amount of energy is needed for a reaction to occur, the activation energy (Ea).

• If a given collision has an energy greater than the Ea, then the collision can result in a reaction.

• If a given collision has an energy less than the Ea, then the collision will not result in a reaction.

Presence of a catalyst

• Catalysts speed up a chemical reaction• Catalysts may be reused because they are not

consumed in the reaction.• In living organisms, catalysts are called

enzymes.

Catalysts and Activation Energy

• Catalysts work by lowering the activation energy (Ea) required for the collision to occur. Thus more collisions will have enough energy to allow a reaction and this in turn leads to an increase in the reaction rate.