Factors Affecting Reaction Rates - Chemistry

1. Fundamental Concepts

Definition: Reaction rate is the speed at which reactants are converted into products.

Factors Affecting Reaction Rates: Concentration, temperature, surface area, catalysts, and pressure (for gases).

2. Key Concepts

Factor	Change	Effect on Rate	Microscopic Explanation
Concentration	Increase	Increase	More particles per unit volume Collision Frequency increases.
Temperature	Increase	Significant Increase	1. Increases collision frequency. 2. More importantly: Increases the Fraction of Particles with energy Effective Collisions increase drastically.
Surface Area	Increase	Increase	(For heterogeneous reactions only). More surface exposed more frequent collisions.
Pressure (gases)	Increase	Increase	Higher pressure increases gas concentration → collision frequency increases.
Catalyst	Add	Increase	Lowers the Activation Energy () by providing an alternative reaction pathway.

3. Examples

Easy

Question: Why does milk spoil faster at room temperature than in a refrigerator?

Answer: Temperature. The higher temperature increases the kinetic energy of the molecules and bacteria, causing the chemical reactions of spoilage to occur much faster.

Medium

Question: Magnesium reacts with hydrochloric acid (). Propose two ways to increase the rate of gas production in this reaction.

Answer:

Increase Concentration: Use a more concentrated solution (more ions available to collide).

Increase Surface Area: Use magnesium powder instead of a magnesium strip (more surface area exposed for collision).

Hard

Question: Reaction X has an activation energy () of 40 kJ/mol, and Reaction Y has an of 80 kJ/mol. If the temperature is increased by 10°C, which reaction will experience a larger percentage increase in rate?

Answer: Reaction Y.

Reasoning: Reactions with higher activation energies are more sensitive to temperature changes. Increasing the temperature provides a much larger proportion of molecules that can overcome the higher barrier in Reaction Y compared to Reaction X.

4. Problem-Solving Techniques

Macro to Micro Link:
- When asked to explain a macroscopic change (e.g., "Why does increasing temperature..."), always answer using microscopic terms (e.g., "kinetic energy," "collision frequency," or "fraction of effective collisions").
- Avoid: "It goes faster because it's hotter." (This is circular reasoning).
Potential Energy Diagrams:
- Identify the (distance from Reactants to the Peak).
- A Catalyst lowers the peak but does not change the energy of the Reactants or Products ( remains the same).
Frequency vs. Effectiveness:
- Concentration/Surface Area mainly affect Collision Frequency.
- Temperature mainly affects the Effectiveness (fraction of collisions with ).
Units Check:
- Ensure your final answer for rate calculations is in Molarity per second (M/s).