C5.2 Controlling reactions

• What is the focus of section C5.2 in the study material?

  Controlling reactions
• What is the first topic discussed under C5.2?

  Rate of reaction
• How is the mean rate of reaction calculated?

  By measuring the change in concentration of reactants or products over time
• What is the significance of measuring the volume of gas in a reaction?

  It helps to determine the rate of reaction
• What are the steps to measure the volume of hydrogen gas produced in a reaction with hydrochloric acid?

  1. Place hydrochloric acid in a conical flask connected to a gas syringe.
  2. Add magnesium ribbon to the acid and stopper the flask.
  3. Start a stopwatch and record the volume of hydrogen produced at regular intervals.
  4. Plot the results on a graph for analysis.
• Why is it important to stopper the flask immediately after starting the reaction?

  To ensure that all hydrogen gas produced is collected
• Why is it more useful to measure the volume of hydrogen produced rather than its mass?


  Volume measurements provide a direct indication of the gas produced during the reaction
• What does the term 'instantaneous rate of reaction' refer to?

  The rate of reaction at a specific moment in time
• How can you calculate the mean rate of reaction from a graph?

  By determining the slope of the line on the graph
• Why might results after a certain time be ignored when calculating the mean rate of reaction?

  Because they may not accurately represent the reaction's behavior
• What factors can affect the rate of a chemical reaction?

  • Temperature
  • Concentration of reactants
  • Pressure (for gases)
  • Surface area of solid reactants
  • Presence of catalysts
• How does temperature affect the rate of reaction?

  Reactions generally occur faster at higher temperatures
• What is the purpose of using a cross on paper during a reaction investigation?

  To visually determine when the reaction has occurred
• How can varying the temperature of a reaction mixture affect the results?

  It can change the rate at which the cross disappears
• What happens to the rate of reaction if the temperature is decreased?

  The rate of reaction decreases
• Why do reactions go faster at higher concentrations?

  Because there are more reactant particles available to collide
• How does increasing pressure affect the rate of reaction for gases?

  It increases the rate of reaction by making particles collide more frequently
• What is the relationship between particle size and reaction rate?

  • Smaller particles have a larger surface area.
  • Larger surface area leads to more frequent collisions.
  • Faster reactions occur with powdered substances compared to larger chunks.
• Why do reactions go faster with powdered reactants?

  Because powdered reactants have a larger surface area for collisions
• What are catalysts?

  Substances that increase the rate of a reaction without being consumed
• What are biological catalysts called?


  Enzymes
• How do catalysts work?

  By lowering the activation energy required for a reaction
• What are the steps to investigate the effect of a catalyst on a reaction?

  1. Place the catalyst in a conical flask with reactants.
  2. Start the stopwatch as the reaction begins.
  3. Measure the volume of product produced at intervals.
  4. Plot the results on a graph for analysis.
• Why is it important to keep the concentration of reactants constant when investigating a catalyst?

  To ensure that the results are due to the catalyst alone
• What is the effect of adding a catalyst on the rate of reaction?

  It increases the rate of reaction
• How can you investigate the use of copper powder as a catalyst?

  By measuring the rate of reaction in the presence and absence of copper powder