Collision theory
- The reactant molecules should collide with proper orientation and sufficient energy to start a reaction and form the product.
  - Such collisions are called effective collisions.
- Non-effective collisions do not produce products.
- The particles must possess sufficient amounts of energy to react.
  - If they collide with the correct orientation but with insufficient energy, they do not react.
Activation energy
- The minimum amount of energy required for particles to react is called activation energy.
- Activation energy on exothermic and endothermic reactions on enthalpy level diagrams are shown in the given graph:
Collision frequency
- It is the number of collisions happening per second
- If the frequency of effective collision increases, more products will be formed.
- As the reaction proceeds, the reactants get gradually converted to products. So, the concentration of reactants decreases while concentration of products increases with the time.
  - The rate of reaction or reaction rate is the change in concentration of reactants or products per unit of time.
  - The formula to calculate the rate of reaction is
    - Using a negative sign in A gives us a positive value. It’s because the change in concentration of A is negative.
Concentration versus time graph.
- Consider a concentration of reactant (A) at different times in a chemical reaction A ⟶ B.
  - The concentration of reactant A decreases with time while the concentration of product B increases with time.
- The plot of the concentration of both reactants and products with time can be represented in the same graph.
- The rate of reaction at a given time is the slope of the curve at that time.
  - It can be determined by drawing a tangent to the curve.
    - The steeper the slope, the faster the reaction.
- Initially, at t = 0, the gradient is the steepest, so the rate is the fastest.
- As time proceeds, the gradient becomes less steep.
- At the end, the gradient of the line is zero as the line is flat. At this point, the reaction has stopped.
Factors affecting the rate of reaction
- The rate of reaction can be altered by:
  - Changing the concentration and temperature of reactants
    - With higher concentrations of reactants, more product is formed because there are more reactant molecules at the start.
    - The number of successful collisions per second is increased at higher concentrations and temperatures.
    - The average speed of reactant particles increases with temperature.
  - Using catalysts
  - Change in concentration of reactants
  - Pressure
    - In gases, an increased pressure means more gas molecules in a given volume. So, this results in more collisions in any given time, and a faster rate of reaction.
    - Unlike gases, pressure change does not affect reactions in the solid or liquid phase. This is because there is no considerable change in the volume of solid or liquid when pressure is applied.
Measuring the rate of reaction graphically
- The concentration of either reactants or products is plotted against time.
- To find the reaction rate at a certain time (say 40 seconds), a tangent to the curve at this specific point is drawn.
- The tangent line is extended and a triangle is constructed with the tangent forming the hypotenuse.
- The slope of a line is calculated as the ratio of the change in x to the change in y.
- Finally, the slope of the line gives the rate of reaction at 40 seconds.

8.01 - Rate of Reaction

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