7.2 - Factors Affecting Rate of Reactions

Main Factors

For a given reaction, its rate can be changed by decreasing the size of a solid reactant, increasing concentration, increasing temperature, or adding a suitable catalyst. Different substances also react at different rates because they have different chemical properties.

Experiment 7.1 - Size of Reactants

Aim / problem: Investigate how the size of marble chips affects rate of reaction.

Hypothesis: The smaller the marble chips, the higher the rate of reaction.

Variable	Specification
Manipulated	Size of marble chips, $\mathrm{CaC}{\mathrm{O}}_3$
Responding	Rate of reaction
Fixed	Mass of marble chips; temperature, volume and concentration of HCl

• Materials: $80.0{\text{cm}}^3$ of $0.1{\text{mol dm}}^{-3}$ HCl; 5.0 g each of large and small marble chips.
• Apparatus: $250{\text{cm}}^3$ conical flask, inverted burette, basin, delivery tube, stopper, retort stand and clamp, $100{\text{cm}}^3$ measuring cylinder, electronic balance and stopwatch.

1. Put the acid in the flask and set the water-filled inverted burette reading to $50.00{\text{cm}}^3$.
2. Add 5.0 g of large marble chips, immediately seal the flask and start the stopwatch.
3. Swirl gently and record the burette reading every 30 seconds until the burette is filled with gas.
4. Repeat with 5.0 g of small marble chips under identical conditions.
5. Plot both gas-volume–time curves on the same axes. Compare the tangent gradients at $t=0$ and the completion times.

Conclusion: For the same mass, smaller pieces have a larger total surface area and react faster. Handle the corrosive acid with gloves and safety glasses.

Experiment 7.2 - Concentration

Aim / problem: Investigate how sodium thiosulphate concentration affects rate of reaction.

Hypothesis: The higher the $\mathrm{N}{\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ concentration, the shorter the time for the mark to disappear.

Variable	Specification
Manipulated	Concentration of $\mathrm{N}{\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ solution
Responding	Time for the mark to disappear
Fixed	Temperature; total mixture volume; concentration and volume of ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$; flask size

• Materials: $0.2{\text{mol dm}}^{-3}$ sodium thiosulphate, $1.0{\text{mol dm}}^{-3}$ sulphuric acid, distilled water and white paper marked with an X.
• Apparatus: $150{\text{cm}}^3$ conical flask, 10 and $50{\text{cm}}^3$ measuring cylinders and stopwatch.

1. Place $45.0{\text{cm}}^3$ sodium thiosulphate solution in the flask over the mark.
2. Quickly add $5.0{\text{cm}}^3$ sulphuric acid and start the stopwatch; swirl gently.
3. View vertically through the flask and stop timing when the mark disappears.
4. Repeat with sodium thiosulphate volumes of 40.0, 30.0, 20.0 and $10.0{\text{cm}}^3$, adding 5.0, 15.0, 25.0 and $35.0{\text{cm}}^3$ water respectively. Keep acid at $5.0{\text{cm}}^3$ and total mixture at $50.0{\text{cm}}^3$.
5. Calculate each diluted concentration using $M_1{V}_1=M_2{V}_2$ and use $1/t$ as the relative rate in ${\text{s}}^{-1}$. Plot $1/t$ against $M_2$.

• In $M_1{V}_1=M_2{V}_2$, $M_1$ is the original concentration, $V_1$ is the volume of stock solution, $M_2$ is its diluted concentration and $V_2$ is the volume after dilution with water ($45.0{\text{cm}}^3$), before adding acid.

Conclusion: Increasing reactant concentration increases rate. Keep the same flask, total volume, viewing position and disappearance endpoint so every trial reaches the same cloudiness.

Experiment 7.3 - Temperature

Aim / problem: Investigate how temperature affects rate of reaction.

Hypothesis: The higher the temperature of sodium thiosulphate solution, the shorter the time for the mark to disappear.

Variable	Specification
Manipulated	Temperature of $\mathrm{N}{\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ solution
Responding	Time for the mark to disappear
Fixed	Volume and concentration of ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$

• Materials: $50.0{\text{cm}}^3$ of $0.2{\text{mol dm}}^{-3}$ sodium thiosulphate, $5.0{\text{cm}}^3$ of $1.0{\text{mol dm}}^{-3}$ sulphuric acid and marked white paper.
• Apparatus: $150{\text{cm}}^3$ conical flask, 10 and $50{\text{cm}}^3$ measuring cylinders, stopwatch, thermometer, Bunsen burner, wire gauze and tripod stand.

1. Leave the sodium thiosulphate solution for 5 minutes, record its temperature and place the flask over the mark.
2. Quickly add acid and start timing. Swirl gently and view the mark vertically.
3. Stop timing when the mark disappears.
4. Repeat using sodium thiosulphate heated to 40, 45, 50 and $55{}^{\circ }\mathrm{C}$.
5. Plot $1/t$ against temperature.

Conclusion: Increasing temperature increases rate. Near room temperature, a $10{}^{\circ }\mathrm{C}$ increase approximately doubles the rate.

Experiment 7.4 - Presence of a Catalyst

A catalyst changes the rate of a reaction without undergoing chemical change at the end. Its physical form may change, but it does not alter the quantity of product formed.

Aim / problem: Investigate how the presence of a catalyst affects rate of reaction.

• Materials: 20-volume hydrogen peroxide solution, manganese(IV) oxide powder and distilled water.
• Apparatus: $10{\text{cm}}^3$ measuring cylinder, test tubes, test tube rack, glowing wooden splinter, filter funnel, filter paper, $150{\text{cm}}^3$ beaker, spatula and electronic balance.

Hypothesis: The presence of a catalyst increases the rate of reaction.

Variable	Specification
Manipulated	Presence of catalyst
Responding	Rate of reaction
Fixed	Mass of $\mathrm{Mn}{\mathrm{O}}_2$, temperature and volume of ${\mathrm{H}}_2{\mathrm{O}}_2$

1. Label two test tubes I and II, place them in a rack, and put $5.0{\text{cm}}^3$ of 20-volume hydrogen peroxide into each tube.
2. Add 0.5 g manganese(IV) oxide powder to tube II only.
3. Quickly place a glowing wooden splinter at the mouth of each tube and record the changes to the splinter.

• The mass of the catalyst is unchanged before and after the reaction; comparing its mass confirms this property.

Conclusion: Manganese(IV) oxide increases the decomposition rate but remains chemically unchanged.

Pressure and Summary of Effects

Change	Effect on rate
Smaller solid pieces / larger exposed surface area	Increases
Higher reactant concentration	Increases
Higher temperature	Increases
Suitable catalyst present	Increases
Higher gas pressure at constant temperature	Increases because gas concentration increases

• Pressure changes do not significantly affect rates involving solid and liquid reactants because their volumes change negligibly with pressure.
• For the calcium–water comparison, raising temperature changes how quickly hydrogen forms, while the same reactant quantities give the same final gas volume.

Interpreting Rate Graphs and Comparing Conditions

• Calcium and water: 0.7 g calcium reacts with $200{\text{cm}}^3$ water. Raising the water temperature by $10{}^{\circ }\mathrm{C}$ gives a steeper gas-volume–time curve that reaches its plateau sooner. Both trials produce $420{\text{cm}}^3$ hydrogen because their reactant quantities are identical.
• Antacid and excess hydrochloric acid: The following conditions illustrate the separate effects of concentration, volume and temperature.

Trial	Conditions and completion time
I	$50{\text{cm}}^3$ HCl, $1.0{\text{mol dm}}^{-3}$, $30{}^{\circ }\mathrm{C}$; 120 s
II	$50{\text{cm}}^3$ HCl, $2.0{\text{mol dm}}^{-3}$, $30{}^{\circ }\mathrm{C}$; 60 s
III	$100{\text{cm}}^3$ HCl, $2.0{\text{mol dm}}^{-3}$, $30{}^{\circ }\mathrm{C}$; 60 s
IV	$50{\text{cm}}^3$ HCl, $2.0{\text{mol dm}}^{-3}$, $40{}^{\circ }\mathrm{C}$; 30 s

• Comparing I and II shows that higher concentration increases rate. II and III show that doubling the volume of excess acid at the same concentration does not change rate. II and IV show that higher temperature increases rate. Crushing the tablet also increases rate by increasing its total exposed surface area.
• Adding a catalyst during a reaction: Adding a spatula of black powder to hydrogen peroxide at the fifth minute makes the oxygen-volume–time curve steeper. The powder acts as a catalyst.
• Comparing catalysts: For $50{\text{cm}}^3$ of 10-volume hydrogen peroxide, oxygen volumes at 10, 20, 30, 40, 50 and 60 s are:
  • Manganese(IV) oxide: 57, 82, 93, 100, 100 and $100{\text{cm}}^3$.
  • Copper(II) oxide: 12, 19, 25, 28, 30 and $31{\text{cm}}^3$.
  • Iron: 33, 47, 55, 58, 59 and $60{\text{cm}}^3$.
• Manganese(IV) oxide is the most effective of these catalysts because it gives the greatest oxygen volume in the same time. The different volumes at 60 s indicate different progress towards completion; they are not all final volumes.