Exothermic and endothermic reactions

What is an exothermic reaction? Include examples

• A reaction where energy is transferred FROM the reacting chemicals TO their surroundings

• This often heats up the surroundings, causing temperature to increase

eg. Combustion and neutralisation reactions

What is an endothermic reaction? Include examples

• A reaction where energy is transferred FROM the surroundings TO the reacting substances (stored in the products of the reaction)

• This often cools down the surroundings causing the temperature to decrease

eg. thermal decomposition, reactio. between sodium hydrogen carbonate and citric acid in sherbet sweets

How do reusable handwarmers work?

• They use energy released during crystallisation of the supersaturated solution of the salt sodium ethanoate

How do disposable handwarmers work?

• They heat up surroundings for longer and use energy released when iron is oxidised to form iron (III) oxide

How do self-heating cans work?

• They use calcium oxide + water —> calcium hydroxide to transfer energy to the surroundings

How do cold packs work?

• They usually contains ammonium nitrate (a salt) and water

• When ammonium nitrate dissolves, it absorbs energy from the surroundings, making the surroundings colder

In an exothermic reaction how much energy do the reactants have relative to the products? State whether it is a negative or positive energy change.

• The reactants have more energy than the products (because they transfer energy form their chemical store to the surroundings in the form of thermal energy)

• This is a negative energy change

In an endothermic reaction how much energy do the reactants have relative to the products? State whether it is a negative or positive energy change.

• The reactants have less energy than the products (because energy is transferred from the surroundings to the reacting chemicals)

• This is a positive energy change

What is activation energy? How is it represented on an energy profile diagram?

• The minimum energy required to start a reaction

• The arrow from in line with the reactants line, to the highest point on the curve