• Rate of a chemical reaction: The amount of reactant used up or product made in a given time.

Factors Affecting Rate of Reaction

Concentration

• Increasing concentration of reactants increases rate of reaction

• More particles per unit volume, so the collision rate between reacting particles increases

  → The successful collision rate increases, which results in an increased rate of reaction.

Temperature

• Increasing temperature increases the rate of reaction

• The average kinetic energy of particles increase

• Particles are moving faster and colliding more ften and so more particles have an energy greater than or equal to the activation energy

  • Increased rate of succesful collisions and so increased rate of reaction

Surface Area

• Decreasing the particle size (increasing surface area) increases the rate of reaction

• This is because there are more reactant particles exposed to collide, so the collision rate increases, therefore the successful collision rate increases, resulting in an increased rate of reaction

• Large surface area can be dangerous'

  • For example, flour dust and wood dust have large surface areas, and are combustible. A spark from a machine, or a lit match, can cause an explosion. This also applies to gases from mines.

Pressure

• Increasing the pressure in a gaseous system increases the rate of reaction

• The distance between particles is reduced under pressure

• There are more particles per unit volume, so the collision rate increases, therefore the successful collision rate increases, resulting in an increased rate of reaction.

Presence of Catalyst

• Catalyst: is a substance (usually a transition metal) which speeds up a chemical reaction, but remains unchanged at the end

• Adding a catalyst increases the rate of reaction

• A catalyst lowers the activation energy for a reaction

• More particles will have an energy greater than or equal to the activation energy, therefore successful collision rate increases resulting in increased rate of reaction

• Enzymes are protein molecules. They are biological catalysts which speed up reactions but remain chemically unchanged at the end

• Enzymes function best at optimum temperature and pH level otherwise they may denature and completely stop functioning

Photochemical Reactions

Light Causing a Chemical Reaction

Photochemical Reaction: reaction which needs light to take place

• The higher the light intensity the higher the rate of the reaction.

• Photosynthesis: light provides energy for the reaction and chlorophyll is a dye that absorbs light. carbon dioxide + water → (light + chlorophyll) → glucose + oxygen

• Use of silver salts in photography:

  • Silver halide salts are used in black and white photography

  • AgCl is sensitive to light & breaks down to form metallic silver Ag+ -> Ag

  • Appears black

• Brighter the light on the film, the faster the reaction & the darker that part of the photograph appears i.e. improves efficiency & accuracy of photos