Rate of ReactionCambridge IGCSE Chemistry: Revision notes

The rate of reaction is the speed at which reactants are converted into products. Five key factors affect reaction rate:

Changing any of these variables allows us to control how fast a reaction proceeds.

Collision theory states that a reaction only occurs when reactant particles collide with sufficient energy. Two conditions must be met:

1. Particles must collide with each other
2. Collisions must have at least the activation energy (Ea)

The activation energy is the minimum energy required for a reaction to occur.

Key variables in collision theory:

• Number of particles per unit volume: Higher concentration/pressure = more particles in the same space
• Frequency of collisions: More particles per unit volume = more collisions per second
• Kinetic energy of particles: Higher temperature = particles move faster = more energetic collisions

A reaction is more likely if:

• There are more particle collisions per second
• A greater fraction of collisions have energy ≥ Ea

Increasing concentration (solutions):

• More solute particles dissolved in the same volume
• Number of particles per unit volume increases
• Particles are closer together → collision frequency increases
• Rate of reaction increases

Increasing pressure (gases):

• Gas particles are forced into a smaller volume (or more gas in same volume)
• Number of particles per unit volume increases
• Particles are closer together → collision frequency increases
• Rate of reaction increases

Key point: Both concentration and pressure work by the same mechanism—they increase the number of particles per unit volume, which increases collision frequency without changing the kinetic energy or activation energy.

Increasing surface area (solids):

• A powder has a larger surface area than the same mass of solid chunks
• More surface area exposed = more sites where reactant particles can collide with the solid
• Frequency of collisions increases (more solid is accessible)
• Rate of reaction increases

Increasing temperature:

• Particles move faster in all states
• Kinetic energy of particles increases
• Two effects:
  • Collision frequency increases slightly (particles move faster)
  • Much more importantly: a greater fraction of collisions have energy ≥ Ea
• Rate of reaction increases significantly

Important: Temperature typically has a much larger effect on rate than concentration because it affects the kinetic energy and the proportion of successful collisions, not just frequency.

A catalyst is a substance that:

• Increases the rate of reaction
• Is unchanged at the end of the reaction (not consumed)
• Works by lowering the activation energy (Ea)

How catalysts work (collision theory): A catalyst provides an alternative pathway with a lower Ea. Even at the same temperature, a greater fraction of collisions now have enough energy to react. The catalyst itself is not used up because it is regenerated.

Enzymes:

• Are biological catalysts (proteins)
• Dramatically increase reaction rates in living cells
• Are very specific—each enzyme catalyses only one type of reaction
• Work by lowering Ea (same mechanism as other catalysts)
• Can be denatured by heat or extreme pH, losing their catalytic activity

Key distinction: A catalyst speeds up a reaction but does not change the starting materials, final products, or overall energy change (ΔH) of the reaction.

Method 1: Measuring change in mass

• Used when a gas is produced in a reaction
• Equipment: balance, flask, reactants
• Procedure: Record the mass of the reaction vessel at regular time intervals as gas escapes
• Data collected: Mass decreases over time; plot mass against time
• Rate = gradient of the graph (steeper = faster)
• Advantages: Easy, direct measurement; no gas collection equipment needed
• Disadvantages: Gas must escape freely; difficult to keep conditions constant

Method 2: Measuring formation of a gas

• Used for reactions producing gas (e.g., acid + carbonate)
• Equipment: gas syringe, measuring cylinder, or inverted burette
• Procedure: Collect gas volume at regular time intervals
• Data collected: Volume of gas increases over time; plot volume against time
• Rate = gradient of the graph (steeper = faster)
• Advantages: Precise; gas volume is easy to measure
• Disadvantages: Gas syringe can stick; readings may be inaccurate if syringe is not vertical

Interpreting rate graphs:

• A steeper gradient = faster reaction rate
• A flatter gradient or horizontal line = slower reaction or reaction has finished
• Comparison: If one curve is steeper than another at the same time, that reaction was faster

When evaluating methods, consider:

Accuracy and precision:

• Does the method directly measure what you want (e.g., rate)?
• Are measurements precise enough (e.g., can you read a scale reliably)?
• Are there sources of error (e.g., gas syringe sticking, difficulty sealing flasks)?

Repeatability and reproducibility:

• Can the method be repeated with the same results?
• Are conditions (temperature, concentration) easy to keep constant?
• Can you take multiple readings?

Practical considerations:

• Is the equipment simple to use and readily available?
• Can you take readings frequently enough to plot a good graph?
• Can you keep the experiment safe (e.g., pressure building up)?

Data quality:

• Do you get enough data points to draw a reliable conclusion?
• Is the gradient of the graph steep enough to calculate accurately?

Example evaluation: "The change in mass method is simple and requires basic equipment, but it is difficult to keep the system sealed while allowing gas to escape. The gas syringe method is more precise and allows frequent readings, but the syringe can stick, making it less reliable."