Speed or Rates of Reaction

Speed/Rates of Reaction

  • The rate of a reaction can be measured in two ways:

    • By the rate at which a reactant is used up.

    • By the rate at which a product is formed.

  • Key factors affecting the rate of a reaction:

    • Temperature

    • Concentration

    • Pressure of reacting gases

    • Surface area of reacting solids

    • Use of catalysts

  • Reactant particles must collide with sufficient energy for a reaction to occur.

    • Frequent collisions and higher proportion of energetic collisions lead to a greater reaction rate.

Measuring Rates of Reaction

  • Two main methods for measuring reaction rates:

    • Measuring the change in amount of a reactant used.

    • Measuring the change in amount of a product formed.

  • Measurement techniques:

    • Mass: Measured with a balance for solids, liquids, and gases.

    • Volume: Typically measured using a gas syringe or upside-down measuring cylinder/burette.

Example of Measuring Gas Production

  • Record the mass or total volume at regular intervals, then plot a graph:

    • Vertical axis: Collisions and reactions

    • Horizontal axis: Time

  • Example calculation: If 24 cm³ of hydrogen gas is produced in 2 minutes,

    • Mean rate of reaction = 24 cm³ / 2 min = 12 cm³ hydrogen/min.

Factors Affecting Reaction Rate

  • To increase the rate of a reaction, consider:

    • Increasing temperature

    • Increasing concentration of dissolved reactants

    • Increasing pressure of reacting gases

    • Breaking solid reactants into smaller pieces

    • Using a catalyst

  • Understanding graphs:

    • The steeper the line on a graph, the greater the rate of reaction.

    • Reactions are fastest at the beginning when reactant concentrations are highest.

    • A horizontal line indicates the reaction has stopped.

Collision Theory and Reaction Conditions

  • For a chemical reaction:

    • Reactant particles must collide with sufficient energy (activation energy).

  • Effect of changes in concentration or pressure:

    • Increased concentration or pressure leads to more reactant particles in the same volume.

    • This increases the probability of collisions and thus increases the reaction rate.

Changing Particle Size

  • When solid reactants are broken into smaller pieces:

    • Surface area increases.

    • More particles are available for collisions, increasing the reaction rate.

Changing Temperature

  • Higher temperatures result in:

    • Increased motion of reactant particles.

    • More particles acquire enough energy to reach activation energy.

    • Increased collision frequency leading to a higher reaction rate.

Using Catalysts

  • Catalysts speed up reactions without being consumed.

    • They lower the activation energy needed for reactions to occur.

    • More collisions result in reactions when catalysts are present.

    • Different reactions require different catalysts, which are critical in industrial applications to reduce costs.