4.2 - Factors Affecting Rate of Reaction

Five Factors

Factor	Effect on rate of reaction
Temperature of reactants	Higher temperature $\to$ higher rate
Concentration of reactants	Higher concentration $\to$ higher rate
Size of solid reactants	Smaller size $\to$ higher rate
Presence of catalyst	Catalyst present $\to$ higher rate
Pressure	Higher pressure $\to$ higher rate for reactions involving gaseous reactants

Experiment 4.1 — Temperature of Reactants

Aim: To study the effect of temperature of reactants on rate of reaction.

Problem statement: How does temperature of reactants affect the rate of reaction?

Hypothesis: The higher the temperature of reactants, the higher the rate of reaction.

Variables

• Manipulated: temperature of sodium thiosulphate solution.
• Responding: time taken until ‘X’ is no longer visible.
• Constant: concentration and volume of sodium thiosulphate solution; concentration and volume of sulphuric acid; size of conical flask.

Materials

• $0.2{\text{mol dm}}^{-3}$ sodium thiosulphate solution;
• $1{\text{mol dm}}^{-3}$ sulphuric acid;
• white paper with an ‘X’ at the centre.

Apparatus

• $250{\text{cm}}^3$ conical flask;
• $50{\text{cm}}^3$ and $10{\text{cm}}^3$ measuring cylinders;
• stopwatch, thermometer, Bunsen burner, tripod stand and wire gauze.

Procedure

1. Measure and pour $50{\text{cm}}^3$ of $0.2{\text{mol dm}}^{-3}$ sodium thiosulphate solution into a clean, dry conical flask.
2. Leave the solution for $5$ minutes.
3. Measure and record its temperature.
4. Place the flask on the ‘X’ on the white paper.
5. Quickly add $5{\text{cm}}^3$ of $1{\text{mol dm}}^{-3}$ sulphuric acid and start the stopwatch simultaneously.
6. Observe the ‘X’ from the mouth of the flask.
7. Stop the stopwatch once the ‘X’ is no longer visible.
8. Record the time, $t$, and calculate $\frac{1}{t}$ in ${\text{s}}^{-1}$.
9. Repeat using sodium thiosulphate solution heated to $35^{\circ }\text{C}$, $40^{\circ }\text{C}$, $45^{\circ }\text{C}$ and $50^{\circ }\text{C}$.

Recording and analysis

• Record the room-temperature trial and the four heated trials.
• Plot:
  • temperature against time;
  • temperature against $\frac{1}{\text{time}}$.
• Operational definition: the rate of reaction is indicated by the time taken until the ‘X’ is no longer visible; a shorter time or larger $\frac{1}{t}$ represents a higher rate.
• Conclusion: increasing the temperature of reactants increases the rate of reaction; the hypothesis is accepted.

Experiment 4.2 — Concentration of Reactants

Aim: To study the effect of concentration of reactants on the rate of reaction.

Problem statement: How does concentration of reactants affect the rate of reaction?

Hypothesis: The higher the concentration of reactants, the higher the rate of reaction.

Variables

• Manipulated: concentration of sodium thiosulphate solution.
• Responding: time taken until ‘X’ is no longer visible.
• Constant: volume of sodium thiosulphate solution; concentration and volume of sulphuric acid; size of conical flask.

Materials

• Sodium thiosulphate solutions of $0.20$, $0.16$, $0.12$, $0.08$ and $0.04{\text{mol dm}}^{-3}$;
• $1{\text{mol dm}}^{-3}$ sulphuric acid, distilled water and white paper with an ‘X’ at the centre.

Apparatus

• $250{\text{cm}}^3$ conical flask;
• $50{\text{cm}}^3$ and $10{\text{cm}}^3$ measuring cylinders;
• stopwatch.

Procedure

1. Measure and pour $50{\text{cm}}^3$ of $0.20{\text{mol dm}}^{-3}$ sodium thiosulphate solution into a clean, dry conical flask.
2. Place the flask on the ‘X’.
3. Quickly add $5{\text{cm}}^3$ of $1{\text{mol dm}}^{-3}$ sulphuric acid and start the stopwatch simultaneously.
4. Observe the ‘X’ from the mouth of the flask.
5. Stop the stopwatch once the ‘X’ is no longer visible.
6. Record $t$ and calculate $\frac{1}{t}$.
7. Repeat with each of the other stated concentrations of sodium thiosulphate solution.

Recording and analysis

• Plot:
  • concentration of sodium thiosulphate solution against time;
  • concentration of sodium thiosulphate solution against $\frac{1}{\text{time}}$.
• A higher concentration gives a shorter time and a larger $\frac{1}{t}$.
• Conclusion: increasing the concentration of reactants increases the rate of reaction; the hypothesis is accepted.

Experiment 4.3 — Size of Solid Reactants

Aim: To study the effect of size of solid reactants on rate of reaction.

Problem statement: How does the size of reactants affect the rate of reaction?

Hypothesis: The smaller the size of solid reactants, the higher the rate of reaction.

Variables

• Manipulated: size of marble.
• Responding: time taken to collect $30.00{\text{cm}}^3$ of gas.
• Constant: temperature; mass of marble; concentration and volume of hydrochloric acid.

Materials

• Small pieces and large pieces of marble chips;
• $0.1{\text{mol dm}}^{-3}$ dilute hydrochloric acid.

Apparatus

• $250{\text{cm}}^3$ conical flask and $50{\text{cm}}^3$ measuring cylinder;
• rubber stopper with delivery tube, burette and basin;
• electronic balance, retort stand with clamp and stopwatch.

Apparatus arrangement: an inverted, water-filled burette is clamped vertically in a basin of water. A delivery tube connects the stoppered conical flask to the mouth of the burette so displaced water measures the gas collected.

Procedure

1. Fill the burette and basin with water. Invert and clamp the burette vertically in the basin.
2. Adjust the water level; observe and record the initial burette reading, $V_0$.
3. Pour $40{\text{cm}}^3$ of $0.1{\text{mol dm}}^{-3}$ dilute hydrochloric acid into a clean, dry conical flask.
4. Weigh and add $2\text{g}$ of large pieces of marble chips.
5. Immediately stopper the flask, place the other end of the delivery tube under the burette and start the stopwatch.
6. Stop the stopwatch when $30.00{\text{cm}}^3$ of carbon dioxide is collected; record the time.
7. Repeat with $2\text{g}$ of small pieces of marble chips.

Result and conclusion

• Small pieces take a shorter time than large pieces to produce the same $30.00{\text{cm}}^3$ of carbon dioxide.
• Therefore, smaller solid reactants react at a higher rate; the hypothesis is accepted.

Experiment 4.4 — Presence of Catalyst

Aim: To study the effect of presence of catalyst on rate of reaction.

Problem statement: How does the presence of a catalyst affect the rate of reaction?

Hypothesis: Presence of catalyst increases the rate of reaction.

Variables

• Manipulated: presence of catalyst.
• Responding: time taken to collect $30.00{\text{cm}}^3$ of gas.
• Constant: temperature; volume and concentration of hydrochloric acid.

Materials

• Small pieces of zinc;
• $0.1{\text{mol dm}}^{-3}$ dilute hydrochloric acid;
• $0.5{\text{mol dm}}^{-3}$ copper(II) sulphate solution.

Apparatus

• $250{\text{cm}}^3$ conical flask and $50{\text{cm}}^3$ measuring cylinder;
• rubber stopper with delivery tube, burette and basin;
• electronic balance, retort stand with clamp, spatula and stopwatch.

Procedure

1. Prepare the inverted, water-filled burette and record $V_0$.
2. Pour $40{\text{cm}}^3$ of $0.1{\text{mol dm}}^{-3}$ dilute hydrochloric acid into a clean, dry conical flask.
3. Weigh and add $2\text{g}$ of zinc pieces.
4. Immediately stopper the flask, connect it to the burette and start the stopwatch.
5. Stop the stopwatch when $30.00{\text{cm}}^3$ of hydrogen gas is collected; record the time.
6. Repeat by replacing the acid with a mixture of $40{\text{cm}}^3$ of $0.1{\text{mol dm}}^{-3}$ dilute hydrochloric acid and $5{\text{cm}}^3$ of $0.5{\text{mol dm}}^{-3}$ copper(II) sulphate solution.

Safety: A mixture of hydrogen and air in the burette can explode when ignited. Do not ignite the gas in the burette.

Result and conclusion

• The mixture containing copper(II) sulphate solution takes a shorter time to collect the same volume of hydrogen gas.
• Copper(II) sulphate solution acts as the catalyst.
• The presence of a catalyst increases the rate of reaction; the hypothesis is accepted.

Effect of Pressure

• Pressure affects reactions that involve gaseous reactants.
• The rate of reaction usually increases when pressure increases.
• The rates of reactions involving solid or liquid reactants are normally not affected by pressure.