The rate of reaction is a core concept in chemical kinetics. It helps you understand how fast reactants turn into products and why some reactions occur instantly while others take hours or even years. In IB Chemistry, reaction rate calculations and explanations appear throughout Topic 6 and Paper 2. This article gives you a clear, exam-friendly explanation of what the rate of reaction is and how it is measured.
What Is Rate of Reaction?
Rate of reaction is the change in concentration of a reactant or product per unit time.
In simpler words, it measures how fast a reaction happens.
Chemically, rate can be expressed as:
- The disappearance of reactants, or
- The appearance of products
For example, if the concentration of a reactant decreases quickly, the reaction is fast. If it decreases slowly, the reaction is slow.
How Rate of Reaction Is Measured
You can measure reaction rate in several ways, depending on the reaction type.
1. Change in mass
Used when gas is released.
Example: Reaction between metal and acid.
2. Change in volume of gas produced
Using a gas syringe to measure gas production over time.
3. Change in color intensity
Useful for reactions involving colored ions or indicators.
4. Conductivity changes
If the reaction involves ions being formed or consumed.
5. pH changes
Used for acid–base reactions.
6. Precipitate formation
Time taken for a solid to form or obscure a cross under a flask.
IB students often perform these experiments in Paper 3 practicals.
The Rate Equation
The simplest form is:
Rate = Δconcentration / Δtime
Units commonly used:
- mol dm⁻³ s⁻¹
- g s⁻¹
- cm³ s⁻¹
Collision Theory: Why Reactions Happen at a Certain Rate
To understand rate, you need collision theory:
A reaction occurs only if particles:
- Collide
- Collide with enough energy (greater than activation energy)
- Collide with correct orientation
Increasing the frequency or energy of collisions increases the reaction rate.
Factors That Affect Rate of Reaction
IB Chemistry focuses on five major factors:
1. Temperature
Increasing temperature increases particle kinetic energy, causing more frequent and energetic collisions.
This increases reaction rate significantly.
2. Concentration
Higher concentration → more particles in a given volume → more collisions per second → faster reaction.
3. Pressure (for gases)
Increasing gas pressure reduces volume, increasing collision frequency.
4. Surface Area
More surface area means more exposed particles available for collision.
For example, powdered solids react faster than large chunks.
5. Catalyst
A catalyst provides an alternative reaction pathway with lower activation energy.
This increases the number of successful collisions without being consumed.
These factors are commonly tested in data-analysis questions.
Rate Graphs in IB Chemistry
IB exams often include graphs showing:
- Concentration vs. time
- Volume vs. time
- Mass vs. time
Key interpretations:
- Steeper slope = faster rate
- As reaction proceeds, slope decreases because reactants are used up
- Rate eventually reaches zero when reaction stops
Understanding these graphs is crucial for Paper 2.
Initial Rate vs. Average Rate
- Initial rate is the reaction rate at the very beginning, when concentration is highest.
Commonly determined using tangents on graphs. - Average rate is calculated over a longer time interval.
Useful for comparing overall reaction progress.
Initial rate is preferred in kinetic studies because it avoids complications from changing concentrations.
FAQs
Why do reactions slow down over time?
As reactants are used up, fewer particle collisions occur. Lower concentration means a smaller chance of colliding with the required energy and orientation, reducing reaction rate.
Why do catalysts not affect equilibrium?
Catalysts speed up forward and reverse reactions equally. They change the rate but not the position of equilibrium because they do not alter enthalpy change.
Can rate ever be negative?
No. Rate is defined as a positive value. The decrease in reactant concentration is expressed mathematically, but the rate itself is always reported as positive.
Conclusion
The rate of reaction measures how quickly reactants turn into products. It depends on factors like temperature, concentration, surface area, catalysts, and pressure for gases. By understanding collision theory and mastering rate calculations, you gain a deeper insight into how reactions behave, helping you perform better in both practical experiments and IB Chemistry exams.
