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 (
- Particles must collide with the correct orientation.
Hint
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.
Example
In the reaction between magnesium and hydrochloric acid, increasing the concentration of HCl results in more frequent collisions between
Hint
Concentration changes only affect reactions in solutions or gases. For pure solids or liquids, concentration does not influence the reaction rate.
Analogy
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.
Example
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.
Surface Area: Maximizing Collision Sites
- In heterogeneous reactions (where reactants are in different phases, such as a solid reacting with a liquid), collisions can only occur at the surface of the solid.
- By breaking a solid into smaller pieces or grinding it into a powder, you increase its surface area, exposing more particles to potential collisions.
- Effect on reaction rate: Larger surface area → more collision sites → faster reaction.
Example
Magnesium powder reacts with hydrochloric acid much faster than a magnesium ribbon because the powder has a larger surface area.
Tip
To speed up heterogeneous reactions, use finely divided solids or stir the reaction mixture to expose fresh surfaces.
Temperature: Energizing Particles
- Temperature is one of the most powerful factors influencing reaction rates.
- When temperature increases, particles move faster because they gain kinetic energy. This leads to two critical effects:
- More frequent collisions because particles are moving faster.
- Higher proportion of successful collisions because more particles have energy equal to or greater than the activation energy.
- Effect on reaction rate: Higher temperature → more frequent and successful collisions → much faster reaction.
Example
Baking cookies at a higher temperature cooks them faster because heat increases the rate of chemical changes in the dough.
Analogy
Think of temperature as turning up the speed on a treadmill. At higher speeds, more runners (particles) have the energy to reach the finish line (overcome the activation energy).
Common Mistake
Don’t confuse temperature with heat. Heat is the energy transferred, while temperature measures the average kinetic energy of particles.
Summary Table: Factors Affecting Reaction Rates
| Factor | How it Works | Example Reaction |
|---|---|---|
| Concentration | More particles increase collision frequency, leading to a higher reaction rate. | |
| Pressure (for gases) | Compressing gases increases particle collisions by reducing space between particles. | |
| Surface Area | Increasing surface area exposes more particles, allowing for more collisions. | |
| Temperature | Higher temperature increases kinetic energy, leading to more frequent and successful collisions. | Combustion reactions |
Reflection
Self review
- Which factor affects the frequency of collisions but not their energy?
- How does this differ from the effect of temperature?
