The rate and extent of chemical change

• How can the rate of a chemical reaction be found?

  By measuring the quantity of a reactant used or the quantity of product formed over time
• What is the formula for the mean rate of reaction using reactants?

  $\text{Mean rate of reaction} =$$\frac{\text{quantity of reactant used}}{\text{time taken}}$
• What is the formula for the mean rate of reaction using products?

  $\text{Mean rate of reaction} =$$\frac{\text{quantity of product formed}}{\text{time taken}}$
• How can the quantity of reactant or product be measured?

  By mass in grams or by volume in cm³
• What are the units of rate of reaction?

  g/s or cm³/s
• What additional units for rate of reaction are required for Higher Tier students?

  Units for rate of reaction in mol/s
• What calculations should students be able to perform regarding the mean rate of a reaction?

  • Calculate the mean rate from given information about reactants or products
  • Draw and interpret graphs showing quantity of product formed or reactant used against time
  • Draw tangents to curves and use the slope as a measure of rate
  • Calculate the gradient of a tangent at a specific time (HT only)
• What does MS 1a refer to in skills development?

  Recognise and use expressions in decimal form
• What does MS 1c refer to in skills development?

  Use ratios, fractions, and percentages
• What does MS 1d refer to in skills development?

  Make estimates of the results of simple calculations
• What does MS 4a refer to in skills development?

  Translate information between graphical and numeric form
• What does MS 4b refer to in skills development?

  Drawing and interpreting appropriate graphs from data to determine rate of reaction
• What does MS 4c refer to in skills development?

  Plot two variables from experimental or other data
• What does MS 4d refer to in skills development?

  Determine the slope and intercept of a linear graph
• What does MS 4e refer to in skills development?

  Draw and use the slope of a tangent to a curve as a measure of rate of change
• What factors affect the rates of chemical reactions?

  Concentration of reactants, pressure of gases, surface area, temperature, and presence of catalysts
• How does increasing the concentration of reactants affect the rate of reaction?

  It increases the frequency of collisions, thus increasing the rate of reaction
• How does increasing the pressure of reacting gases affect the rate of reaction?

  It increases the frequency of collisions, thus increasing the rate of reaction
• How does increasing the surface area of solid reactants affect the rate of reaction?

  It increases the frequency of collisions, thus increasing the rate of reaction
• How does increasing the temperature affect the rate of reaction?

  It increases the frequency of collisions and makes them more energetic, thus increasing the rate of reaction
• What is collision theory?

  It explains how various factors affect rates of reactions based on particle collisions
• What is activation energy?

  The minimum amount of energy that particles must have to react
• How do catalysts affect the rate of chemical reactions?

  Catalysts increase the rate by providing a different pathway with lower activation energy
• What role do enzymes play in biological systems?

  Enzymes act as catalysts
• What are the characteristics of reversible reactions?

  • Products can react to produce original reactants
  • Represented as: A + B ⇌ C + D
  • Direction can be changed by altering conditions
• What happens if a reversible reaction is exothermic in one direction?

  It is endothermic in the opposite direction
• What is equilibrium in reversible reactions?

  • Occurs when forward and reverse reactions happen at the same rate
  • Reactants and products remain constant in concentration
• What is Le Chatelier’s Principle?

  It predicts how a system at equilibrium responds to changes in conditions
• What happens to the equilibrium if the concentration of a reactant is increased?

  More products will be formed until equilibrium is reached again
• What happens to the equilibrium if the concentration of a product is decreased?

  More reactants will react until equilibrium is reached again
• What happens to the equilibrium if the temperature of a system at equilibrium is increased for an endothermic reaction?

  The relative amount of products at equilibrium increases
• What happens to the equilibrium if the temperature of a system at equilibrium is decreased for an exothermic reaction?

  The relative amount of products at equilibrium increases
• How does an increase in pressure affect gaseous reactions at equilibrium?

  It shifts the equilibrium position towards the side with fewer molecules
• How does a decrease in pressure affect gaseous reactions at equilibrium?

  It shifts the equilibrium position towards the side with more molecules