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Lesson · Grade 10 · Science

Activation energy: the push a reaction needs to start

  • Grade 10
  • Science
  • 1 min 39 s
  • British Columbia
  • BC10-SC-27

An energy diagram builds up step by step: wood and oxygen on the left, carbon dioxide and water lower down on the right, and the energy given out that makes burning exothermic, followed by the hill in between whose height is the activation energy. A ball for the particles rolls back at room temperature until a match flame pushes it over, and a dashed lower hill shows how a catalyst, such as an enzyme or a car's catalytic converter, lowers the activation energy without being used up.

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In one sentence

The question this lesson answers

Burning wood gives out lots of energy. So why doesn't a log burst into flames by itself?

Here is an energy diagram, with energy up the side and the progress of the reaction along the bottom.

Transcript

What the lesson says

Burning wood gives out lots of energy. So why doesn't a log burst into flames by itself?

Here is an energy diagram, with energy up the side and the progress of the reaction along the bottom.

On the left are the reactants, the substances we start with: wood and oxygen.

On the right are the products, the new substances made: mainly carbon dioxide and water.

The products have less energy. The difference is given out as heat and light: we call this exothermic.

So shouldn't an exothermic reaction start by itself? Yet wood can sit in air for years without burning.

In between is an energy hill: bonds must start to break before new ones form, and that takes energy.

The hill's height above the reactants is the activation energy, the minimum energy needed to start.

For wood the hill is high: at room temperature, almost no particles have enough energy to get over it.

A match gives the push: friction heats its head until it lights, and the flame heats the wood over the hill.

A catalyst lowers the hill by giving the reaction a different route, and it is not used up.

Enzymes in your body are catalysts, and so are the metals in a car's catalytic converter.

Even an exothermic reaction needs a push over the hill to start: its activation energy.

Curriculum

Where this fits

British Columbia, Grade 10

Science 10, Chemistry: energy change during chemical reactionsBC10-SC-27

Previous: Exothermic and Endothermic ReactionsNext: Why You Must Never Mix Cleaners

The outcomes and strands follow the British Columbia curriculum.

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