Rates of reaction

• Rate of reaction


  The rate of change of the reactants into the products in a chemical reaction
• Factors affecting rate of reaction

  • Surface area or size of solid particles
  • Concentration of solutions
  • Temperature
  • Presence of a catalyst
• Other factors, such as light and pressure, may affect the rate of some chemical reactions, but these are not studied practically at GCSE
• Reactions used in rate of reaction experiments

  • Metal reacting with dilute acid (e.g. Mg + 2HCl → MgCl2 + H2)
  • Calcium carbonate reacting with dilute hydrochloric acid (CaCO3 + 2HCl → CaCl2 + H2O + CO2)
  • Catalytic decomposition of hydrogen peroxide solution (2H2O2 → 2H2O + O2)
  • Reaction between sodium thiosulfate solution and hydrochloric acid (producing a sulfur precipitate)
• Measuring a change in mass

  1. Measure mass of reaction mixture at regular intervals

  2. Cotton wool stops liquid from splashing out

  3. Smaller particles have larger surface area-to-volume ratio, leading to higher reaction rate
• Measuring gas volume


  1. Collect gas in gas syringe over time

  2. Volume of gas produced indicates rate of reaction
• Particle size


  Smaller particles have higher reaction rate
• Reaction progress


  Reaction rate is highest at start and decreases as reactants are used up
• Measuring time for reaction to complete

  1. Time when mass reaches minimum

  2. Time when gas volume reaches maximum

  3. Time when gas production stops

  4. Time when solid disappears
• Measuring production of solid precipitate

  1. Mix solutions in flask on white paper with black cross

  2. Time until cross is obscured by solid precipitate
• Calculating rate


  Rate = 1 / time
• The units of rate are s⁻¹
• Planning an investigation


  • Make an initial prediction
  • Ensure investigation is a fair test
• Predictions are often written as "as quantity one increases, quantity two increases/decreases"
• Stopwatch


  Instrument used to measure time
• White paper


  Surface used to observe precipitate
• Cross


  Visual aid to observe precipitate
• Time at start of reaction

  00:00
• Figure 23.5 Measuring production of a solid precipitate
• Rate


  Value calculated from time for reaction to be completed
• The units of rate are s¹
• Planning an investigation


  • Making an initial prediction
  • Ensuring the investigation is a fair test
• Prediction


  Written in the form: as quantity one increases, quantity how increases/decreases
• Fair test


  Investigation where one variable is changed (independent variable), one variable is measured (dependent variable) and all other variables are kept the same (controlled variables)
• Validity


  Part of the overall design of the experiment, ensuring the range of conditions tested is sufficient
• Reliability


  Whether the same result could be obtained if the experiment were repeated
• Anomaly


  A piece of data that does not match the pattern shown in the rest of the investigation
• Activation energy


  The minimum energy required for a reaction to occur
• As temperature increases


  The rate of most reactions increases
• As temperature increases


  Particles have more energy and move faster, leading to more collisions and more successful collisions
• As concentration increases


  The number of particles increases, leading to more collisions and more successful collisions
• The decomposition of hydrogen peroxide using manganese oxide, MnO2, as a catalyst can be used to study the effect of a catalyst
• Catalysts


  • Manganese(IV) oxide
  • Iron
• Catalyst


  A substance that increases the rate of a chemical reaction without being used up
• Catalysts work by providing an alternative reaction pathway of lower activation energy, which increases the number of successful collisions in a given period of time