Factors Affecting Reaction Rates
Collision Theory: The Foundation of Reaction Rates
- Chemical reactions happen when particles (atoms, ions, or molecules) collide with enough energy and the correct orientation to break and form bonds.
- As discussed in the previous section, according to collision theory, three conditions must be satisfied for a reaction to occur:
- Particles must collide.
- Collisions must have sufficient energy to overcome the activation energy ($E_a$).
- Particles must collide with the correct orientation.
- The rate of a reaction depends on two key factors: the frequency of collisions and the proportion of successful (or "effective") collisions.
- Each factor we’ll explore influences one or both of these aspects.
Concentration: Increasing Collision Frequency
- Increasing the concentration of reactants increases the number of particles in a given volume, leading to more frequent collisions.
- Effect on reaction rate: Higher concentration → more collisions per second → faster reaction.
In the reaction between magnesium and hydrochloric acid, increasing the concentration of $HCl$ results in more frequent collisions between $H^+$ ions and magnesium atoms, speeding up the production of hydrogen gas.
Hint- Concentration changes only affect reactions in solutions or gases.
- For pure solids or liquids, concentration does not influence the reaction rate.
- Picture a crowded room where people are constantly bumping into one another.
- The more people there are, the more frequent the collisions.
Pressure: Compressing Gaseous Reactants
- For reactions involving gases, increasing the pressure compresses the gas particles into a smaller volume, much like squeezing a balloon.
- This increases particle density, which is analogous to increasing the concentration of a solution.
- Effect on reaction rate: Higher pressure → more collisions per second → faster reaction.
In the synthesis of ammonia (Haber process), increasing the pressure speeds up the reaction between nitrogen and hydrogen gases.
Note- Pressure changes only affect reactions involving gases.
- For solids and liquids, pressure has little to no effect because they are nearly incompressible.
