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Speed Of Reaction

The Speed of Reaction is a measure of how quickly a reaction takes place.

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5 reaction-rate factors8 worked questions

Start with the essentials

Five factors to understand

The 5 factors affecting speed of reaction that we will cover in this topic are:

  • Temperature
  • Concentration of aqueous reactant (Only applies to aqueous reactants).
  • Pressure of gaseous reactant (Only applies to gaseous reactants)
  • Particle size of reactant (Only applies to solid reactants)
  • Presence of catalyst

Temperature

  1. Temperature increases.
  2. Average kinetic energy increases and reactant particles move faster.
  3. Particles collide more often; a greater proportion of collisions have enough energy to overcome the activation energy.
  4. The frequency of effective collisions increases.

Result: rate of reaction increases.

Question 1

CHECK YOUR UNDERSTANDING

The effects of a change in conditions on a chemical reaction are listed.

1. The total number of collisions per minute increased.

2. The number of effective collisions per minute increased.

3. The average energy of the particles increased.

4. The activation energy increased.

Which of the above effects are resulted by an increase in temperature?

A. 1, 2, 3 and 4

B. 1, 2 and 3 only

C. 1 and 2 only

D. 3 and 4 only

Answer & explanation · Question 1
Correct answer: B

An increase in temperature increases the average energy of the reactant particles. As a result, the reactant particles move faster, colliding into each other more often, which increases the total number of collisions per minute and increases the number of effective collisions per minute. However, the activation energy remains the same as it is not affected by a change in temperature.

Hence, answer is B (1, 2 and 3 only).

Concentration

Concentration of aqueous reactant (Only applies to aqueous reactants)

  1. Concentration of the aqueous reactant increases.
  2. The number of reactant particles per unit volume increases.
  3. Particles are closer together and the frequency of collisions increases.
  4. The frequency of effective collisions increases.

Result: rate of reaction increases.

Question 2

CHECK YOUR UNDERSTANDING

100 cm3 of 0.5 mol/dm3 dilute hydrochloric acid was added to 2 g of magnesium carbonate.

Which of the following changes would increase the initial rate of formation of carbon dioxide?

A. Adding 20 cm3 of deionized water to the original acid

B. Adding another 10 cm3 of 0.5 mol/dm3 hydrochloric acid

C. Adding 10 cm3 of 2 mol/dm3 dilute hydrochloric acid to the original acid

D. Lowering the temperature

Answer & explanation · Question 2
Correct answer: C

A: Incorrect. Adding 20 cm3 of deionized water to the original acid would decrease the concentration of the original acid, which would decrease the rate of reaction and decrease the initial rate of formation of carbon dioxide.

B: Incorrect. No. of mol of HCl in initial solution = (100/1000) × 0.5 = 0.0500 mol

No. of mol of HCl in final acid mixture = (10/1000 × 0.5) + 0.0500 = 0.0550 mol

Concentration of HCl in final acid mixture = 0.0550 / (110/1000) = 0.500 mol/dm3

As there is no increase in concentration, rate of reaction would not increase and initial rate of formation of carbon dioxide would not increase.

C: Correct. No. of mol of HCl in final acid mixture = (10/1000 × 2) + 0.0500 = 0.0700 mol

Concentration of HCl in final acid mixture = 0.0700 / (110/1000) = 0.636 mol/dm3

Adding 10 cm3 of 2 mol/dm3 dilute hydrochloric acid to the original acid would increase the concentration of the original acid, which would increase the rate of reaction and would increase the initial rate of formation of carbon dioxide.

D: Incorrect. Lowering the temperature would decrease the rate of reaction, which would decrease the initial rate of formation of carbon dioxide.

Pressure

Pressure of gaseous reactant (Only applies to gaseous reactant)

  1. The same amount of gas is compressed at unchanged temperature.
  2. The pressure and number of reactant particles per unit volume increase.
  3. Particles are closer together and collide more frequently.
  4. The frequency of effective collisions increases.

Result: rate of reaction increases.

Question 3

CHECK YOUR UNDERSTANDING

Which change would increase the speed of reaction between two gases?

A. A decrease in surface area of the catalyst

B. A decrease in temperature

C. A decrease in the size of the reaction flask

D. A decrease in the pressure of the gases

Answer & explanation · Question 3
Correct answer: C

A: Incorrect. A decrease in the exposed surface area of a solid catalyst would decrease the rate of reaction because fewer active sites are available for the gaseous reactants.

B: Incorrect. A decrease in temperature would decrease the speed of reaction between two gases.

C: Correct. For the same amount of gas at the same temperature, a decrease in the size of the reaction flask would increase the pressure of the gases and thus, increase the speed of reaction between two gases.

D: Incorrect. A decrease in the pressure of the gases would decrease the speed of reaction between two gases.

Particle size

Particle size of reactant (Only applies to solid)

  1. Particle size decreases, keeping the mass of solid the same.
  2. The total exposed surface area increases.
  3. There is a larger area of contact, so collisions with the solid occur more frequently.
  4. The frequency of effective collisions increases.

Result: rate of reaction increases.

Question 4

CHECK YOUR UNDERSTANDING

In the reaction between zinc and hydrochloric acid, the following changes could be made to the conditions.

1. Increase in concentration of the acid

2. Increase in particle size of the zinc

3. Increase in pressure of the system

4. Increase in temperature of the system

Which pair of changes will increase the rate of reaction?

A. 1 and 2

B. 1 and 4

C. 2 and 3

D. 3 and 4

Answer & explanation · Question 4
Correct answer: B

1: Correct. Increase in concentration of the acid will increase the rate of reaction.

2: Incorrect. Increase in particle size of the zinc will decrease the rate of reaction.

3: Incorrect. Increase in pressure of the system will only increase the rate of reaction for gaseous reactants. However, zinc and hydrochloric acid are solid and aqueous reactants respectively. Therefore, in the reaction between zinc and hydrochloric acid, increase in pressure of the system will not increase the rate of reaction.

4: Correct. Increase in temperature of the system will increase the rate of reaction.

Hence, Answer is B(1 and 4).

Catalyst

A catalyst is a substance that increases the rate of a chemical reaction but itself remains chemically unchanged at the end of the reaction.

Biological catalysts are known as enzymes.

  1. A suitable catalyst is present.
  2. An alternative reaction pathway has a lower activation energy.
  3. At the same temperature, more particles have enough energy to react by this pathway.
  4. The frequency of effective collisions increases.

Result: rate of reaction increases.

Without catalyst · solid lineWith catalyst · dashed line
Catalysed and uncatalysed energy profiles Energy is plotted against reaction progress. Both curves start at the same reactant energy and finish at the same lower product energy. The uncatalysed path has the higher peak. Both activation-energy arrows begin at the reactant level; the catalysed arrow is shorter. EnergyReaction progress ReactantsProductsEₐEₐ
View larger energy diagram

Swipe sideways to see the full diagram. Keyboard users can focus the diagram and use the arrow keys.

Catalysed and uncatalysed energy profiles Energy is plotted against reaction progress. Both curves start at the same reactant energy and finish at the same lower product energy. The uncatalysed path has the higher peak. Both activation-energy arrows begin at the reactant level; the catalysed arrow is shorter. EnergyReaction progress ReactantsProductsEₐEₐ
The catalysed pathway has a lower activation energy. Reactant and product energy levels, and the overall energy change, are unchanged. The horizontal axis is reaction progress, not time.

Question 5

CHECK YOUR UNDERSTANDING

How will the addition of a catalyst affect the energy of the particles and the activation energy of the reaction?

On smaller screens, swipe sideways within the table. Keyboard users can focus the table and use the arrow keys.

Compare the effects of adding a catalyst at unchanged temperature.
Option Energy of Particles Activation energy
A Remains the same Remains the same
B Remains the same Decreases
C Increases Remains the same
D Increases Decreases
Answer & explanation · Question 5
Correct answer: B

A catalyst decreases the activation energy required for the reaction to occur. The energy of particles remains the same as the temperature of the system remains the same.

Hence, answer is B.

Measuring reaction speed

There are many ways to measure the speed of a chemical reaction. The 2 commonly used methods that are covered in this topic are:

1. Measuring how much product is formed over time

Stoppered flask connected to a gas syringe to measure gas produced over time.
Stoppered flask connected to a gas syringe to measure gas produced over time.
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The volume of the gas produced is measured using the gas syringe over time.

2. Measuring the loss in mass as gas escapes over time

Flask containing calcium carbonate and hydrochloric acid on a balance, with cotton wool to prevent acid spray.
Flask containing calcium carbonate and hydrochloric acid on a balance, with cotton wool to prevent acid spray.
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The gas produced escapes from the solution and is represented by the loss in mass of the reaction vessel. Rate of gas production is then measured.

Question 6

CHECK YOUR UNDERSTANDING

When calcium carbonate is added to dilute hydrochloric acid, carbon dioxide gas is released.

Three sets of apparatus are shown.

Three setups: 1, cotton-wool flask on a balance; 2, flask with tap funnel and gas syringe; 3, flask with an open thistle funnel above the liquid connected to a measuring cylinder over water.
Three setups: 1, cotton-wool flask on a balance; 2, flask with tap funnel and gas syringe; 3, flask with an open thistle funnel above the liquid connected to a measuring cylinder over water.
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Which set(s) of apparatus is/are suitable, together with a stopwatch, for following the rate of this reaction?

A: 1 only

B: 2 only

C: 1 and 2 only

D: 1,2, and 3

Answer & explanation · Question 6
Correct answer: C

1: Correct.

The carbon dioxide gas released escapes from the solution and is represented by the loss in mass of the reaction vessel. This set of apparatus is an experimental setup for following the rate of the reaction between calcium carbonate and dilute hydrochloric acid by measuring the loss in mass.

2: Correct.

This set of apparatus is an experimental setup for following the rate of the reaction between calcium carbonate and dilute hydrochloric acid by collecting the carbon dioxide gas produced in the gas syringe and the rate of gas production is then measured.

3: Incorrect.

The volume of carbon dioxide gas produced cannot be accurately measured by the measuring cylinder because some gas can escape through the thistle funnel, whose stem is above the liquid level.

Hence, answer is C (1 and 2 only).

Interpreting graphs (Reactant vs products graph)

The rate of a reaction is usually not constant. It is usually fastest at the start of the reaction and slower towards the end of the reaction, eventually stopping altogether.

The magnitude of the gradient represents the rate of reaction. The steeper the curve, the faster the rate of reaction.

The typical graph for the rate of reaction where H2 gas is produced in an acid-metal reaction is shown below.

Gas volume increases steeply at first, then levels off as the reaction stops.
Gas volume increases steeply at first, then levels off as the reaction stops.
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The rate of reaction is the fastest at the beginning of the reaction (time = 0), when the concentration of reactants is the highest.

As the reaction progresses, reactants are continually being converted into products. Hence, the concentration of reactants decreases, resulting in a decreasing rate of reaction and the gradient gets less steep or more gentle.

The reaction eventually stops when the limiting reagent has been completely used up, resulting in gradient = 0.

Question 7

CHECK YOUR UNDERSTANDING

A student investigates the rate of reaction between marble chips and excess hydrochloric acid by measuring the loss in mass of the reaction flask.

The graph shows the results of two experiments, P and Q.

Mass of the reaction flask decreases to the same final mass for P and Q; Q initially falls more steeply and levels off earlier.
Mass of the reaction flask decreases to the same final mass for P and Q; Q initially falls more steeply and levels off earlier.
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Which change best explains the difference between P and Q?

A. A catalyst is added in P.

B. A higher temperature is used in P.

C. A lower concentration of hydrochloric acid is used in Q.

D. Smaller pieces of marble chips are used in Q.

Answer & explanation · Question 7
Correct answer: D

The magnitude of the gradient of the curve represents the rate of the reaction. The steeper the gradient, the faster the rate of reaction. Since Q has a steeper initial gradient than P, Q has a faster initial rate of reaction than P.

A: Incorrect.

Addition of a catalyst in P increases the initial rate of reaction in P but the graph shows that P has a slower initial rate of reaction than Q.

B: Incorrect.

Using a higher temperature in P increases the initial rate of reaction in P but the graph shows that P has a slower initial rate of reaction than Q.

C: Incorrect.

Using a lower concentration of hydrochloric acid in Q decreases the initial rate of reaction in Q but the graph shows that Q has a faster initial rate of reaction than P.

D: Correct.

Using smaller pieces of marble chips in Q increases the initial rate of reaction in Q and the graph shows that Q has a faster initial rate of reaction than P.

S-shaped graphs & autocatalysis

S-shaped gas-volume curve: initially slow, then faster, then levelling off.
S-shaped gas-volume curve: initially slow, then faster, then levelling off.
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What are three situations that can result in an S-shaped graph?

  1. A strip of magnesium may have an oxide layer that is insoluble in water; the reaction speeds up as the oxide layer reacts with the acid and is removed.
  2. A temperature rise due to an exothermic reaction can speed up the reaction in the first few minutes.
  3. One of the products of the reaction acts as a catalyst (‘autocatalyst’), hence the reaction speeds up as the first trace of products are formed.

Question 8

CHECK YOUR UNDERSTANDING

In autocatalysis, a chemical reaction is catalysed by one of its products. An example is the reaction between oxalic acid, (COOH)2, and acidified potassium manganate (VII).

5(COOH)2 + 6H+ + 2MnO4− → 2Mn2+ + 10CO2 + 8H2O

The reaction is very slow at room temperature, but it is catalysed by Mn2+ ions that are produced during the reaction.

Assume the oxalic acid is completely used up. Which graph correctly shows the changes in concentration of (COOH)2 as the reaction proceeds?

Concentration-time options: A rises sigmoidally; B rises then levels off; C falls steeply then levels at zero; D falls slowly, then steeply, then levels at zero.
Concentration-time options: A rises sigmoidally; B rises then levels off; C falls steeply then levels at zero; D falls slowly, then steeply, then levels at zero.
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Answer & explanation · Question 8
Correct answer: D

Since (COOH)2 is a reactant, it is used up in this reaction and its concentration should decrease as the reaction proceeds. Thus options A and B are eliminated. The magnitude of the gradient of the curve represents the speed of the reaction. The steeper the gradient, the faster the speed of reaction. After the reaction starts, Mn2+ ions that are produced during the reaction act as a catalyst. Thus, the reaction speeds up after the first trace of Mn2+ is formed and the gradient of the curve becomes steeper. As the reaction progresses, reactants are continually being converted into products. Hence, the concentration of reactants decreases, resulting in a decreasing rate of reaction and the gradient of the curve becomes gentler. The concentration of (COOH)2 eventually decreases to 0 when it has been completely used up.

Hence, answer is D.

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