7.2Factors Affecting Rate of Reactions
Different chemical substances have distinct chemical properties, causing reactions to proceed at different speeds. The speed at which reactants convert into products can be manipulated by changing specific physical and environmental conditions.
Learning Objectives
- Investigate factors affecting the rate of reactions through experiments, based on size of reactants, concentration of solutions, temperature of reaction mixtures and presence of a catalyst.
- Understand the effect of pressure on reactions involving gases.
- Interpret experimental data, calculate rates of reaction, and deduce relationships using graphical methods.
Factors Affecting the Rate of Reaction & Curve Comparisons
See how each factor changes particle exposure and how rate curves compare on a volume–time graph.
Surface Area
Smaller pieces expose more total surface area.
Concentration
More particles per unit volume increases collision frequency.
Pressure (Gases)
Compression raises gas concentration.
Rate Curves
Faster reactions are steeper but end at the same plateau.
Brief Explanation
The four main experimental factors and the special case of pressure.
1. Size of Reactants
When a fixed mass of solid is divided into smaller pieces, its total surface area increases while its total mass remains constant. The smaller the solid particles, the larger the exposed surface area and the higher the rate of reaction.
Example: A \(2\text{ cm} \times 2\text{ cm} \times 2\text{ cm}\) cube has a total surface area of \(24\text{ cm}^2\). Cutting it into two halves increases the total surface area to \(32\text{ cm}^2\), and into four pieces to \(40\text{ cm}^2\).
2. Concentration of Reactants
Concentration is the quantity of solute dissolved in a given volume of solution. The higher the concentration of a reacting solution, the higher the rate of reaction.
Dilution follows \(M_1V_1 = M_2V_2\), where \(M_1\) and \(V_1\) are the initial concentration and volume, and \(M_2\) and \(V_2\) are the diluted concentration and total volume.
3. Temperature
Most reactions proceed faster at higher temperatures. For reactions occurring around room temperature, each increase of \(10\ ^\circ\text{C}\) roughly doubles the rate of reaction.
The graph of \(\dfrac{1}{\text{time}}\) against temperature curves upwards, showing that the rate increases non-linearly with rising temperature.
4. Presence of Catalyst
A catalyst alters the rate of a chemical reaction without undergoing any chemical changes at the end of the reaction. Its chemical properties and total mass remain unchanged, although its physical properties may change.
Catalysts do not alter the final quantity or yield of products; they only shorten the time needed to reach the end of the reaction.
| Factor | Effect on rate | Key experimental method |
|---|---|---|
| Smaller solid size | Increases total surface area, so rate increases. | Marble chips + HCl; measure CO₂ volume over time. |
| Higher concentration | More particles per unit volume, so rate increases. | Na₂S₂O₃ + H₂SO₄; disappearing 'X' mark. |
| Higher temperature | Particles gain thermal energy, so rate increases. | Same thiosulphate system at different temperatures. |
| Catalyst | Provides an alternative pathway, so rate increases. | H₂O₂ decomposition with MnO₂; glowing splinter relights. |
| Higher pressure | Increases gas concentration, so rate increases. | Only for reactions involving gaseous reactants. |
| Factor | Manipulated variable | Responding variable | Main fixed variables |
|---|---|---|---|
| Size | Size of marble chips | Volume of CO₂ over time | Marble mass, temperature, HCl volume and concentration |
| Temperature | Temperature of Na₂S₂O₃ | Time for the ‘X’ mark to disappear | Reactant volumes and concentrations, flask size |
| Catalyst | Presence of MnO₂ | Glowing-splint observation / O₂ release rate | H₂O₂ volume and concentration, temperature, MnO₂ mass |
Try to Answer First
Answer mentally, then check the explanation.
Why does powdered calcium carbonate react much faster with hydrochloric acid than large marble chips of the same mass?
In the reaction between sodium thiosulphate solution and sulphuric acid, what causes the solution to turn cloudy, and how is the reaction rate determined?
A student adds manganese(IV) oxide powder to hydrogen peroxide solution. What happens to the mass and chemical identity of the manganese(IV) oxide at the end of the reaction?
Common Mistakes
Avoid these errors when explaining reaction rates.
Concept Misunderstandings
Compare each misconception with the correct fact.
A catalyst participates in the reaction, so some of it must be permanently used up or chemically converted.
Although a catalyst interacts during the reaction, it is chemically regenerated by the end. Its chemical properties and overall mass are strictly conserved.
Increasing the volume of acid used always increases the rate of reaction.
Increasing the volume of acid without changing its concentration does not alter the rate of reaction. Rate depends on concentration, not total volume.
Diluting an acid with water makes the reaction faster because there is more total liquid.
Adding water lowers the concentration of acid particles per unit volume, which actually decreases the reaction rate.
Summary
Key facts to remember.
- Factors increasing reaction rate: smaller solid reactant size, higher solution concentration, higher temperature, presence of a suitable catalyst, and higher pressure for gaseous reactants only.
- Acronym: CSTP — Concentration, Size, Temperature, Pressure.
- Catalyst properties: speeds up reaction, remains chemically unchanged, mass is conserved, does not change total product yield.
- Experimental measurements: gas syringe or inverted burette for gas volume over time, and disappearing cross method with Rate = 1/t for precipitate formation.
- On a volume–time graph, a faster reaction is steeper at the start but reaches the same final volume if reactant quantities are identical.
Short Activity
Complete all parts before checking your score.
A. Objective Quiz
What happens to the mass and chemical properties of a catalyst once the chemical reaction is complete?
For a chemical reaction occurring at room temperature, what is the approximate effect on the reaction rate if the temperature is increased by \(10\ ^\circ\text{C}\)?
B. Fill in the Blanks
In the experiment between sodium thiosulphate solution and sulphuric acid, the insoluble yellow solid that causes the solution to turn cloudy is ______.
Increasing pressure increases the rate of reaction only for reactions involving ______ reactants.
C. Match the Factor with Its Characteristic
Drag each factor on desktop. On mobile, tap a factor and then tap the matching box.
Answer Choices
Smaller solid reactant size Higher solution concentration Addition of a catalyst Higher pressureCharacteristic / Effect
Keywords
Tap or hover over a term to read its meaning.
