Chemical Reactions · Grade 10

Why does one flask warm up while another goes cold?

Because breaking bonds costs energy and forming bonds releases it, and the difference between the two totals decides. If more was released than spent, the surplus leaves as heat and the reaction is exothermic. If breaking cost more, the shortfall is taken from the surroundings and the flask cools — an endothermic reaction.

Learning objectives

Two halves to every reaction

Two things happen in every reaction. Bonds in the reactants break — which always costs energy, without exception — and then new bonds form in the products, which always releases energy.

The final account is the difference. If the new bonds released more than the breaking cost, the surplus spreads into the surroundings as heat and the flask is warm to the touch. Burning is the familiar example.

If breaking cost more than was released, the shortfall is drawn from the surroundings and the flask goes cold. A medical cold pack works exactly this way: a salt dissolving in water absorbs energy and the temperature drops.

Why burning needs a match

If burning releases energy, why does fuel not ignite by itself? Because releasing energy requires bonds to break first, and that first break needs an initial push.

That push is called the activation energy. Picture a hill between the reactants and the products: even if the products sit lower, you have to get over the hill to reach them.

On an energy profile the height of the hill is the activation energy, and the difference between the starting and finishing heights is the overall energy change. The two are independent: a reaction can release a great deal of energy and still demand a tall hill.

What a catalyst does and does not do

A catalyst lowers the hill. It offers the reaction a route needing less starting energy, so more particles manage to get over and the reaction runs faster.

It does not change the starting height or the finishing height. An exothermic reaction with a catalyst releases exactly the same amount of energy, only sooner, and the catalyst leaves the reaction as it entered it.

Worked examples

  1. The temperature in the flask fell from 22 to 15 degrees during the reaction. Which kind is it?

    1. The flask cooled
    2. Energy was taken from the surroundings into the reaction

    Answer: Endothermic

  2. Breaking the bonds cost 400 units and forming them released 550. What happens to the temperature?

    1. More was released than was needed
    2. The difference is 150 units
    3. The surplus goes to the surroundings

    Answer: The flask warms — an exothermic reaction

  3. A catalyst is added to an exothermic reaction. Will more heat be released?

    1. The catalyst changes only the height of the hill
    2. The heights of reactants and products are unchanged
    3. The difference between them stays as it was

    Answer: No. Exactly the same energy, sooner

Common mistakes

Thinking that breaking bonds releases energy
Breaking always costs and forming always releases. Swapping those two inverts every energy profile and every conclusion drawn from one.
Assuming an exothermic reaction starts by itself
Even a reaction that releases a lot of energy needs an initial push. That is why fuel needs a match and does not ignite in the bottle.
Thinking a catalyst increases the energy released
It shortens the route without moving the destination. The same reaction releases exactly the same energy, in less time.

What to remember

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