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.
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 $H^+$ 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.
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.
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. | $Mg + 2HCl \rightarrow MgCl_2 + H_2$ |
| Pressure (for gases) | Compressing gases increases particle collisions by reducing space between particles. | $N_2 + 3H_2 \rightarrow 2NH_3 \text{ (Haber process)}$ |
| Surface Area | Increasing surface area exposes more particles, allowing for more collisions. | $CaCO_3 + 2HCl \rightarrow CaCl_2 + CO_2 + H_2O$ |
| Temperature | Higher temperature increases kinetic energy, leading to more frequent and successful collisions. | Combustion reactions |
Self review
- Which factor affects the frequency of collisions but not their energy?
- How does this differ from the effect of temperature?
