Rate of Reaction

Collision Theory

• Collision theory explains that for a chemical reaction to occur, particles must collide with enough energy and in the correct orientation.
• Four main factors affect how often and how effectively particles collide:
  1. Number of particles per unit volume
     • Higher concentration (in solutions) or higher pressure (in gases) means more particles in the same volume.
     • This increases the frequency of collisions between reactant particles.
  2. Frequency of collisions
     • The more often particles collide, the greater the chance they will have successful collisions that lead to a reaction.
  3. Kinetic energy of particles
     • Higher temperature means particles move faster and have more kinetic energy.
     • More particles will have energy greater than or equal to the activation energy.
  4. Activation energy (Eₐ)
     • The minimum energy particles must have when colliding for a reaction to occur.
     • Even if particles collide, if their energy is less than Eₐ, no reaction will happen.

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course

Catalyst and Activation Energy

• Catalyst:
  • A substance that increases the rate of a chemical reaction without being used up or chemically changed.
  • Works by providing an alternative reaction pathway with a lower activation energy.
  • Eₐ decreases → more particles have sufficient energy to react.
  • Remains unchanged chemically and in mass after the reaction.
• Enzyme:
  • A biological catalyst found in living organisms.
  • Speeds up metabolic reactions at body temperature.
  • Works on a specific substrate due to its active site shape.

Factors Affecting Rate of Reaction (Collision Theory Applied)

(a) Concentration of Solutions

• Increase in concentration → more particles per unit volume → higher collision frequency → faster rate.
• Decrease in concentration → fewer collisions → slower rate.

(b) Pressure of Gases

• Increase in pressure → gas particles are forced closer together → more collisions → faster rate.
• Decrease in pressure → fewer collisions → slower rate.

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course

(c) Surface Area of Solids

• Greater surface area (e.g., powdered solid vs large lump) → more particles are exposed for collisions → faster rate.
• Smaller surface area → fewer available collision sites → slower rate.

(d) Temperature

• Higher temperature → particles move faster → more frequent collisions and more particles with E ≥ Eₐ → faster rate.
• Lower temperature → slower moving particles → fewer collisions and fewer particles above Eₐ → slower rate.

(e) Catalyst

• Adding a catalyst → lowers Eₐ → increases reaction rate without being consumed.
• Removing a catalyst → reaction returns to original slower rate.

Practical Methods for Investigating Rate of Reaction

Method 1: Change in Mass of a Reactant or Product

• Used when a gas is produced.
• Place reaction mixture on a balance; record mass loss over time.
• Example: Reaction between calcium carbonate (CaCO₃) and hydrochloric acid (HCl).
  • CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
• Graph: Mass vs Time → steeper slope = faster reaction.

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course

Method 2: Measuring Volume of Gas Produced

• Use a gas syringe to collect gas over time.
• Example: Magnesium reacting with hydrochloric acid:
  • Mg + 2HCl → MgCl₂ + H₂↑

Method 3: Observing Precipitate Formation

• Used when one product is an insoluble solid.
• Example: Sodium thiosulfate + hydrochloric acid produces sulfur precipitate → solution turns cloudy.
  • Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O
• Measure time taken for a cross under the flask to disappear from view.

Interpreting Data from Rate Experiments

• Graphs:
  • Steep slope = faster reaction rate.
  • Slope flattens = reaction slows as reactants are used up.
  • Horizontal line = reaction complete.
• Comparing curves:
  • Same final level but different slopes → same amount of product but different rates.
  • Higher curve final level → more product formed (different quantities of reactants).

Key Points to Remember

• Collision theory explains why reaction rates change.
• Catalysts lower Eₐ and speed up reactions without being consumed.
• Rate of reaction can be measured by:
  1. Change in mass
  2. Volume of gas produced
  3. Time for a visual change (e.g., precipitate)
• Temperature, concentration, pressure, surface area, and catalysts all influence rates.

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course