10.02.1 Exo- and Endothermic Reactions

Syllabus

- Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.
- If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.
- Exothermic reactions transfer energy to the surroundings and raise its temperature.
- Exothermic reactions include combustion, many oxidation reactions and neutralisation.
- Everyday uses of exothermic reactions include self-heating cans and hand warmers.
- An endothermic reactions take in energy from the surroundings and lower its temperature.
- Endothermic reactions include thermal decompositions and the reaction of Citric Acid and Sodium Hydrogencarbonate.
- Some sports injury packs are based on endothermic reactions.

Students should be able to:

- distinguish between exothermic and endothermic reactions on the basis of the temperature change of the surroundings
- evaluate uses of exothermic and endothermic reactions given appropriate information.

Limited to measurement of temperature change. Calculation of energy changes (ΔH) is not required.

Required practical 4: investigate the variables that affect temperature changes in reacting solutions such as, eg acid plus metals, acid plus Carbonates, neutralisations, displacement of metals.

What does this mean?

Exo- & Endo-thermic Reactions

Most common reactions are exothermic

They release energy, making us feel warmer.

A few are endothermic and take in energy, making us feel colder.

Before any substances can react their existing bonds have to break.

This takes in energy - is endothermic.

When new bonds form energy is released - exothermic.

So every reaction has an endothermic and then an exothermic part.

ENDOTHERMIC REACTIONS

If it take more energy to break the old bonds than is released by the new ones, the reaction will be endothermic overall.

This reaction is endothermic - Energy in is greater than energy out.

The bonds in the reactants were stronger than the bonds in the products.

And the Products are above the Reactants on an Energy Profile diagram

Exothermic Reactions

If it take less energy to break the bondsin the reactants than is released by the new bonds that form in the products, the reaction will be exothermic overall.

This reaction is exothermic - Energy in is less than energy out.

The bonds in the reactants were weaker than the bonds in the products.

So, the Energy Profile shows the Products below the Reactants.

The energy we put in at the start is less than the energy released.

Exothermic Reactions and their uses.

Since Exothermic Reactions release energy they are often used as sources of energy - in most cases as fuels.

Fuels are burned - the chemical name for burning is combustion.

Oxidation is a word that can mean to add Oxygen to a substance.

Combustion is a form of oxidation, but some oxidations don't involve flames.

And some of these reactions can be used to raise temperatures too.

Self-heating cans & hand-warmers often work in this way.

Endothermic Reactions and their uses.

Endothermic Reactions absorb energy and this means they'll stop if the source of energy is removed.

Cooking is endothermic - once the source of heat is removed the cooking process stops.

Photosynthesis is endothermic - put a plant in the cupboard and the lack of light will eventually kill it.

Sports injury packs have two chemicals that react endothermically to cool down an inflamed area.

Citric Acid (like the acid in lemons) and Sodium Hydrogen Carbonate (found in baking powder) react endothermically.

These two are often mixed as solids (they won't react because acids aren't actually acidic until dissolved) and icing sugar is added.

When the sherbet is placed in the your mouth the citric acid dissolves in your saliva.

Now it can dissociate and make Hydrogen (H+) ions (see Year 9).

These react with the Sodium Hydrogencarbonate to produce a cooling effect that's pleasant in the mouth.

The Icing sugar just makes it taste nice