Factors Affecting Rates of Reaction (SQA National 5 Chemistry): Revision Note

Increasing rates of reaction

• There are four main ways to speed up a chemical reaction:

  1. Increase the temperature

  2. Increase the concentration of a reactant solution

  3. Increase the surface area of a solid reactant

  4. Use a catalyst

• All these factors work by increasing the frequency of successful collisions between reactant particles per second

How the factors look on a graph

• When you change the temperature, concentration or surface area, you change the speed of the reaction

• However, you still make the same total amount of product

• This has a predictable effect on the shape of a reaction rate graph:

  • Faster reactions:

    • Have a steeper curve

    • Finish quicker

    • This means that the curve on the graph levels out sooner

  • Slower reactions:

    • Have a less steep curve

    • Take longer to finish

    • This means that the curve on the graph takes longer to level out

Explaining the factors (collision theory)

1. Increasing concentration

• If you increase the concentration of a reactant, the reaction gets faster

• Why it gets faster:

  • A higher concentration means there are more reactant particles in the same volume

  • This leads to more frequent collisions between particles

  • This increases the number of successful collisions per second

  • The rate of reaction increases

2. Increasing temperature

• If you increase the temperature of the reaction, the reaction gets faster

• Why it gets faster:

  • A higher temperature gives the reactant particles more kinetic energy

  • This means the particles move faster

  • This leads to more frequent collisions

  • The collisions are also more energetic, so more of them are successful

  • The rate of reaction increases

3. Increasing surface area

• If you increase the surface area of a solid reactant, the reaction gets faster

• Why it gets faster:

  • Breaking a solid lump into a powder increases the surface area

  • This means more reactant particles are exposed at the surface

  • This leads to more frequent collisions between particles

  • This increases the number of successful collisions per second

  • The rate of reaction increases

4. Using a catalyst

• Adding a catalyst increases the rate of reaction

• Why it gets faster:

  • A catalyst provides an alternative reaction pathway that allows the reaction to happen more easily

  • This increases the number of successful collisions per second

  • The rate of reaction increases

How Catalysts Work

• A catalyst is a substance that speeds up a chemical reaction but is not used up in the process

  • This means a catalyst can be recovered chemically unchanged at the end of the reaction and used again

• Catalysts are very important in industry

  • They save money by allowing reactions to happen faster or at lower, less energy-intensive temperatures

• SQA example: The Haber process uses an iron catalyst to make ammonia

The catalyst "shortcut"

• You can think of a chemical reaction as a journey over a mountain.

  • The energy needed to get the reaction started is like the effort of climbing the mountain

• A catalyst works by providing an alternative reaction pathway that is easier to follow

  • It's like finding a tunnel that goes through the mountain instead of having to climb over the top

• Since the catalyst provides an easier route, more particles have enough energy to react at any given moment

• This increases the number of successful collisions per second

• Therefore, the rate of reaction increases