Rate of Reaction

• Definition: The rate of a chemical reaction measures how quickly a reactant is consumed or a product is formed. It’s expressed as the change in the amount of a reactant or product per unit time.

Collision Theory

For a chemical reaction to occur:

• Collisions: Reactant particles must collide with each other.
• Energy: Collisions must have enough energy to be successful.

A successful collision leads to a reaction. The rate of reaction increases with more successful collisions. The minimum energy needed for a successful collision is called activation energy.

Factors Affecting the Rate of Reaction

1. Concentration of Reactants
   • Increasing reactant concentration raises the frequency of collisions, speeding up the reaction.
2. Temperature
   • Higher temperatures increase the kinetic energy of particles, leading to more frequent and energetic collisions, thus increasing the reaction rate.
3. Surface Area of Solid Reactants
   • Smaller solid particles have a larger surface area, allowing more collisions with other reactants, speeding up the reaction. For example, powdered solids react faster than lumps.
4. Use of a Catalyst
   • Catalysts speed up reactions without being consumed. They lower the activation energy, increasing the frequency of successful collisions.

Methods of Measuring Rates of Reactions

• Different methods are used based on the reactants and products involved:
  1. Measuring Volume of Gas Produced: Use a gas syringe to measure the volume of gas released at regular intervals.
  2. Measuring Change in Mass: Monitor mass loss as reactants convert to products.
  3. Time for Reactant to Disappear: Measure how long it takes for a specific mass of reactant to fully react.
  4. Time for Product Formation: Measure the time taken to form a certain amount of product.

Example: Measuring the Volume of Gas Produced

• When gas is produced, use a gas syringe:
  • Measure the gas volume at regular intervals (e.g., every 30 seconds).
  • Record the plunger position to determine the volume (e.g., if the plunger reaches 20 cm³, then 20 cm³ of gas has been collected).

Reversible and Irreversible Reactions

A. Reversible Reaction

• Definition: A reversible reaction is a chemical reaction where products can be converted back into reactants under suitable conditions.
• Example: Hydration and Dehydration of Copper(II) Sulfate
  • Dehydration: Heating blue hydrated copper(II) sulfate causes it to lose water, forming white anhydrous copper sulfate:
    CuSO4⋅5H2O (s)→CuSO4 (s)+5H2O (l)
    • (Blue crystals) → (White powder)
  • Hydration: Adding water to white anhydrous copper sulfate restores the blue color:
    CuSO4 (s)+5H2O (l)→CuSO4⋅5H2O (s)
    • (White powder) → (Blue crystals)
  • Overall Reaction:
    CuSO4⋅5H2O (s)⇌CuSO4 (s)+5H2O (l)

B. Irreversible Reaction

• Definition: An irreversible reaction is a chemical reaction in which products cannot be converted back into reactants.
• Example: Decomposition of Calcium Carbonate
  • When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide: CaCO3 (s)→CaO (s)+CO2 (g)
    • The carbon dioxide escapes into the air, and the reaction cannot be reversed.