Factors Affecting Reaction Rates - Comprehensive Notes

Factors Affecting Rate of Reaction

• The greater the frequency of successful collisions between reactant particles, the greater the reaction rate.
• Temperature, concentration, pressure, and the use of catalysts affect reaction rate.

Rate of Reaction

• The rate of a reaction is a measure of how quickly a reactant is used up or a product is formed.

Collision Theory

• For a chemical reaction to occur:
  • Reactant particles must collide with each other.
  • The particles must have enough energy to react.
• A collision that produces a reaction is called a successful collision.
• Activation Energy:
  • The minimum amount of energy needed for a collision to be successful.
  • This is different for different reactions.

Concentration and Pressure

• The greater the frequency of successful collisions, the greater the rate of reaction.
• If the concentration of a reacting solution or the pressure of a reacting gas is increased:
  • The reactant particles are closer together.
  • The frequency of collisions between reactant particles increases.
  • Therefore, the rate of reaction increases.
• Note that the mean energy of the particles does not change.
• The frequency of collisions increases, so the frequency of successful collisions also increases.

Graphs

• The rates of two or more reactions can be compared using a graph of mass or volume of product formed against time.
• The gradient of the line is equal to the rate of reaction.
• The faster reaction at the higher concentration or pressure:
  • Gives a steeper line.
  • Finishes sooner.

Rates and Surface Area to Volume Ratio

• For a given mass of a solid, large lumps have smaller surface area to volume ratios than smaller lumps or powders.
• If a large lump is divided or ground into a powder:
  • Its total volume stays the same.
  • The area of exposed surface increases.
  • The surface area to volume ratio increases.

Lumps Versus Powders

• The greater the frequency of successful collisions, the greater the rate of reaction.
• If the surface area to volume ratio of a reacting solid is increased:
  • More reactant particles are exposed at the surface.
  • The frequency of collisions between reactant particles increases.
  • Therefore, the rate of reaction increases.
• Note that the mean energy of the particles does not change.
• The frequency of collisions increases, so the frequency of successful collisions also increases.

Rates and Temperature

• The greater the frequency of successful collisions, the greater the rate of reaction.
• If the temperature of the reaction mixture is increased:
  • Reactant particles move more quickly.
  • The energy of the particles increases.
  • The frequency of successful collisions between reactant particles increases.
  • Therefore, the rate of reaction increases.
• Note that the mean energy of the collision’s changes when the temperature changes:
  • The frequency of collisions increases.
  • The proportion of collisions that are successful also increases.

Temperature, Pressure and Concentration Graphs

• The rates of two or more reactions can be compared using a graph of mass or volume of product formed against time.
• The gradient of the line is equal to the rate of reaction.
• The faster reaction at the higher temperature:
  • Gives a steeper line.
  • Finishes sooner.

Catalysts

• A catalyst is a substance that:
  • Speeds up the rate of a reaction.
  • Does not alter the products of the reaction.
  • Is unchanged chemically and in mass at the end of the reaction.
• Only a very small mass of catalyst is needed to increase the rate of a reaction.
• However, not all reactions have suitable catalysts.
• Catalysts only affect the rate of reaction - they do not affect the yield of the reaction.
• A catalyzed reaction produces the same amount of product as an uncatalyzed reaction but it produces the product at a faster rate.
• Different substances catalyze different reactions.

Common Catalysts

• These catalysts are transition metals or compounds of transition metals.

How Catalysts Work

• A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalyzed reaction.
• This does not change the frequency of collisions.
• However, it does increase the frequency of successful collisions because a greater proportion of collisions now exceeds this lower activation energy.
• The effect of a catalyst on the activation energy is shown on a chart called a reaction profile.
• This shows how the energy of the reactants and products change during a reaction.

Reaction Profile

• A reaction profile for a reaction with and without a catalyst

Enzymes

• An enzyme is a biological catalyst.
• Enzymes are important for controlling reactions in cells.
• They are also important in industry.
• The use of enzymes allows some industrial reactions to happen at lower temperatures and pressures than traditionally needed.
• Yeast is a single-celled fungus.
• The enzymes in yeast are used to produce wine, beer, and other alcoholic drinks by fermentation of sugars.

Factors Affecting Reaction Rate

• Temperature:
  • Causes the number of particles to increase and so increases the chance of collision.
• Pressure:
  • Increases the speed at which the particles are moving and so increases the chance of successful collisions.
• Concentration:
  • Lowers the energy needed for a successful collision.
• Catalyst:
  • Gives a large surface area for particles to collide on and so increases the chance of collision.
• Reducing Particle Size (larger surface area):
  • Reduces the area in which the particles can collide and so has the same effect as increasing temperature.