RATES OF REACTION

„The speed at which a reaction occurs.

„This can be affected by:

„ Concentration

„ Temperature

„ Surface Area of reactants

„ Catalysts

For a chemical reaction to occur, reactants need to collide with enough energy and with the correct orientation. This is called COLLISION THEORY.

The rate of a reaction can be measured by:

Measuring how

- much reactants are used up

- much product is formed

average reaction rate = Total mass of product / total time

Concentration

Concentration refers to the amount of a substance (solute) present in a specific volume of liquid (solvent). It shows how much solute is dissolved in the solvent.

Increasing the concentration of the reactants will increase the rate of reaction.

Surface Area

•The surface area is the total area of ALL the faces of a shape.

•The SMALLER the particles the LARGER the surface area.

How does surface area affect the rate of reaction?

•When we INCREASE surface area we INCREASE the rate of reaction.

•This occurs because there is more atoms exposed and therefore, they are more likely to collide with the correct orientation and the required energy.

Temperature

•Increasing the temperature increases the kinetic energy of the particles (meaning they get faster)

How does temperature affect the rate of reaction?

•At a higher temperature, particles have more energy. This means they move faster and are more likely to collide with other particles.

•When the particles collide, they do so with more energy, and so the number of successful collisions increases.

THEREFORE, INCREASING TEMPERTAURE INCREASES THE RATE OF REACTION.

Catalyst

•A catalyst is a chemical that is added to the reaction.

•It provides an alternate pathway which has a LOWER ACTIVATION energy.

•This means that more particles have the required energy for the reaction and therefore the reaction goes faster.

Enzymes

•Enzymes are biological catalysts

•They are used to increase the speed of reactions in the body

•Examples – enzymes are used in the digestive system to break down food

Lock and key model

•Enzymes work like a “lock and key”

•Only one enzyme will fit into the substrate (the chemical the enzyme works on)

Examples

•Amylase – Breaks down carbohydrates (starch) into simple sugars

•Protease – breaks down the protein in food into smaller molecules

•Lipase – breaks down fats in food

Enzymes work in the body

•Enzymes have an optimum temperature and an optimum pH

•If the temperature or pH changes from this optimum the reaction will SLOW down

•E.g. Amylase works in the mouth and then breaks down in the stomach

•Enzymes denature (breakdown) when:

• the temperature gets too high

• the pH goes too low/high