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C2 Rates and Energy

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Rates and Energy
Topic Quiz

Rates of Reaction

The rate of a reaction is basically just how fast the reactants are turning into products. We can measure rate in two ways:

  • how quickly the reactants are used up
  • how quickly the products are made

Rate of reaction = (reactant used/product made) divided by time


Three easy-to-do practicals can help us measure the rate:

  1. measuring the mass decreasing (if a gas is given off as a product)
  2. measuring increasing volume (if a gas is given off as a product)
  3. measuring how much light can pass through a solution (if a solid is made as a product)

Collision Theory

There are four factors that can affect the rate of a reaction:

  1. temperature
  2. surface area
  3. concentration/pressure
  4. catalysts

Reactions take place when particles collide together. A reaction doesn't just take place when they collide though, they need to have the right amount of energy. This is the activation energy.

If we increase numbers 1-3 above then what we are doing is providing either more energy, or more particles to collide. This leads to more frequent successful collisions and so the rate of reaction increases.


Surface Area

We can increase the surface area of a reactant by crushing it into a fine powder. This means that:

  • More particles are exposed to the other reactant(s)
  • There is a greater chance of the particles colliding
  • The rate of reaction increases as there are more frequent successful collisions.

Temperature

By increasing the temperature:

  • The reactant particles will move quicker
  • More particles will have the required activation energy (or greater)
  • There will be more frequent successful collisions
  • The rate of reaction increases

Concentration

Concentration and pressure can be considered as the same thing in terms of changing rate, as they have the same effect on the rate. Concentration refers to lqiuids or dissolved solids, and pressure refers to gases. By increasing either:

  • There are more reactant particles in the same volume
  • There are more frequent successful collisions
  • The rate of reaction increases

Catalysts

Catalysts increase the rate of reaction without being used up in the reaction. They work by lowering the activation energy needed, by providing an alternative pathway. Different reactions need different catalysts, and (like the other factors) they allow for more frequent successful collisions.


Exo/Endothermic

Exothermic reactions transfer energy (in the form of heat) to the surroundings. If you monitored this on a thermometer the surrounding temperature would increase.

Examples:

  • Combustion
  • Most oxidations
  • Neutralisation

Endothermic reactions take in energy (in the form of heat) from the surroundings. If you monitored this on a thermometer the surrounding temperature would decrease.

Examples:


Energy Changes

We covered reversible reactions in the last topic, but what we didn't mention was the energy changes of the forward or reverse reaction. The forward reaction refers to the reactants making products, and the reverse reaction where products make reactants.

Every reversible reaction has two parts: an exothermic, and an endothermic part. One of the forward/reverse reactions will be one, and vice versa. The same amount of energy absorbed by the endothermic reaction will be released by the exothermic reaction.


An easy example to think about is that of hydrated copper sulfate:


Rates Infographic

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Page last updated: 16/04/2017