UNIT 2 CHEMISTRY · PEARSON AAQ · D1.2.1
Factors Affecting Reaction Rate
FREE CHEMISTRY REVISION · TOPIC 33
Factors Affecting Reaction Rate
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · D1.2.1
What you will learn
Predict and compare the effects of concentration, gas pressure, temperature, solid surface area and catalysis on reaction rate. Distinguish how quickly a reaction proceeds from how much product can ultimately form. Design fair comparisons and connect observations to particles.
1. What does reaction rate describe?
Reaction rate describes how rapidly reactants are used up or products are formed. A faster reaction makes a given amount of product in less time, or makes more product in a fixed early time interval. It does not automatically give a larger final yield. Always identify the quantity monitored and the time interval.
Suitable observations include gas volume collected, mass lost when gas escapes, concentration change, colour intensity or the time to reach a defined endpoint. Gas-volume rate may be expressed in cm³ s⁻¹; a concentration rate in mol dm⁻³ s⁻¹. State what the measured rate represents. A mass-loss method requires the gas to leave the apparatus; mass is still conserved when the escaped gas is included.
For a fixed endpoint, 1/time is a useful relative-rate measure if the same amount of change is required each time. Its unit is s⁻¹. It is not automatically a concentration rate, and unequal endpoints cannot be compared fairly by reciprocal time.
2. Concentration: more reacting particles per volume
Increasing the concentration of a dissolved reactant usually increases the reaction rate because more reacting particles occupy each unit volume. A reacting particle has more opportunities to meet another appropriate particle. At unchanged temperature, greater concentration does not by itself increase the particles’ mean kinetic energy.
For marble chips reacting with acid, compare equal masses and chip sizes using acid of different concentrations, the same solution volume and the same temperature. If the acid remains in excess in both tests and the same carbonate amount is limiting, the final CO₂ amount can be the same even though the initial rates differ. If changing concentration changes the limiting reagent, the final amount may change too.
Do not assume that doubling concentration always doubles rate. The relationship depends on the reaction and experimental conditions. Quantitative rate laws are determined from evidence; simply counting particles is not a proof of a particular reaction order.
3. Pressure: specify gases and the method of change
For a gaseous reactant, compressing a fixed amount into a smaller volume at constant temperature increases its concentration and partial pressure. Reactant particles are closer together, increasing collision opportunities per unit volume and usually increasing reaction rate. Their mean kinetic energy is unchanged if temperature is held constant.
| Change | Particle consequence | Rate prediction |
|---|---|---|
| Compress reacting gases at fixed temperature | More reactant particles per unit volume | Usually faster |
| Increase gas temperature in a rigid vessel | Faster particles and higher pressure; concentration initially unchanged | Temperature effect must be considered |
| Add inert gas at constant volume and temperature | Reactant concentrations and partial pressures unchanged | No increase expected from extra reactant encounters alone |
A higher total pressure is not always evidence of a higher reactant concentration. Pressure is relevant here to reactions involving gases. It is not a useful general explanation for speeding a reaction between two liquids or solids under ordinary laboratory conditions.
4. Temperature: faster motion and more successful encounters
Increasing temperature increases particles’ mean kinetic energy. Particles move faster and collisions occur more frequently. More importantly for many reactions, a larger fraction of encounters has enough energy to overcome the activation barrier, so successful collisions become more frequent. The detailed activation-energy explanation is developed in D1.2.2.
Heating does not lower the activation energy of the same uncatalysed pathway. Individual particles have a range of energies: it is incorrect to say every particle acquires the same energy or that every collision becomes successful. There is no universal rule that rate doubles for every 10 °C increase; use supplied data.
In a fair temperature comparison, keep initial concentration, amounts, particle size and catalyst condition the same. Allow solutions to reach the intended temperature before timing. Changing temperature can also alter gas volume and solubility, so gas measurements should be compared under consistent collection conditions.
5. Surface area and surface-area-to-volume ratio
A solid reacts at accessible surfaces where other reactants can make contact. Breaking the same mass of a solid into smaller pieces increases the total exposed surface area, so more sites are available simultaneously. The amount of solid and the total number of its atoms have not increased.
For a cube of side a, area = 6a², volume = a³, and SA:V = 6/a. Smaller similar particles therefore have a greater surface-area-to-volume ratio. This is particularly useful when a solid reacts with a solution or gas. Crushing a completely dissolved solute does not preserve a separate reacting solid surface.
For equal masses of calcium carbonate in excess hydrochloric acid, powder usually gives a faster initial CO₂ production rate than large chips. With complete reaction and the same limiting carbonate amount, the theoretical final CO₂ amount is unchanged.
6. Catalysis: faster reaction, unchanged final catalyst
A catalyst increases the reaction rate without altering the products of the reaction and is chemically unchanged and unchanged in mass at the end. It provides an alternative reaction pathway with a lower activation energy. It is not an additional stoichiometric source of the product.
The catalyst can participate in intermediate steps, but it is regenerated overall. “Unchanged at the end” does not mean it never interacts with reactants. For a solid catalyst, reactions often occur at its surface; increasing accessible catalyst area may increase rate. A dissolved catalyst can act throughout a solution.
For hydrogen peroxide decomposition, manganese(IV) oxide can increase the rate of oxygen production: 2H₂O₂ → 2H₂O + O₂. The catalyst does not change these reaction products or, for a fixed limiting peroxide amount under comparable completion conditions, the theoretical oxygen amount. Dry, recovered catalyst is used for a fair mass comparison; apparent loss during transfer is not chemical consumption.
Catalysts do not supply a higher temperature or increase the mean kinetic energy of the reacting mixture. They may become contaminated or lose activity in practice; this is different from being consumed by the overall catalytic reaction.
7. Read the observation without confusing rate and extent
A steeper product-volume graph means faster product formation at that moment. A plateau means no further measurable gas is being collected. It does not by itself prove that every reactant is gone: a limiting reactant may be exhausted, reaction may have stopped or gas may no longer be collected effectively.
8. Five worked comparisons
Example 1 — concentration and a fixed endpoint
The same colour-change endpoint is reached in 50.0 s with a dilute reactant and 20.0 s with a more concentrated reactant. Relative rates are 1/50.0 = 0.0200 s⁻¹ and 1/20.0 = 0.0500 s⁻¹. The second relative rate is 0.0500/0.0200 = 2.50 times the first. More reactant particles per unit volume can explain the increase; these data do not establish a universal proportionality.
Example 2 — compress a gas without heating it
A fixed reacting gas sample is compressed from 2.00 dm³ to 1.00 dm³ at the same temperature. Its amount per volume doubles, and its ideal-gas partial pressure doubles. More collision opportunities per unit volume make a faster reaction likely. Mean kinetic energy has not doubled. Do not calculate an exact rate factor unless reaction data support it.
Example 3 — calculate exposed surface area
One cube of side 2.00 cm has area 6 × 2.00² = 24.0 cm² and volume 8.00 cm³. Eight cubes of side 1.00 cm have combined area 8 × 6 × 1.00² = 48.0 cm² and combined volume 8.00 cm³. The accessible area doubles; SA:V rises from 3.00 to 6.00 cm⁻¹. This predicts more surface encounters, not twice as much product.
Example 4 — compare measured gas rates
Run A produces 18.0 cm³ gas in the first 12.0 s; run B produces 30.0 cm³ in the same interval. Mean volume rates are 18.0/12.0 = 1.50 cm³ s⁻¹ and 30.0/12.0 = 2.50 cm³ s⁻¹. B has the greater early mean rate. These readings alone do not reveal which factor changed or the final gas amount.
Example 5 — catalyst evidence
A peroxide sample gives 40.0 cm³ oxygen eventually with or without a catalyst, but reaches 20.0 cm³ in 8.0 s with the catalyst and 40.0 s without it. The catalyst increases rate while the same limiting amount gives the same final gas volume. Recovering the same dry catalyst mass with unchanged chemical identity supports regeneration; it is not enough merely to observe bubbles.
Common misconceptions
- Higher concentration means faster particles. At fixed temperature, it means more reacting particles per unit volume.
- Higher total pressure always means a faster reaction. Check reactant partial pressures and how pressure was changed.
- Powder contains more reactant than the same mass of chips. The accessible area differs; the amount does not.
- Heating lowers activation energy. Heating changes the energy distribution; catalysis changes the pathway.
- A catalyst is used up or changes the products. It is regenerated overall and does not alter the products in the specified reaction.
- A faster reaction always produces more product. Distinguish rate from the limiting amount and final extent.
Exam tips
- Name the changed factor and state what is controlled.
- Link the observation to reactant particles, collision opportunities or exposed sites.
- For temperature include energy as well as collision frequency.
- For catalysts state lower activation energy and regeneration.
- Use units and equal endpoints when comparing measured rates.
- Qualify final-yield comparisons with the same limiting amount and completion conditions.
Check your understanding
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