Temperature Change

• KeywordsExothermic – A change that gives out

heat energy. Temperature of surroundings and reaction increases.

Endothermic- A change that takes in heat energy from the surroundings, so the temperature of the surroundings decreases.

Real lifeSelf Heating Drink Cans

• Exothermic reactions can be used for everyday purposes. For example, hand warmers and self-heating cans for drinks (such as coffee) use exothermic reactions.

Making and Breaking Bonds

• In chemical reactions, bonds in the reactants are broken and new bonds are formed.

• It takes energy to break bonds.

• Energy is released when bonds are made.

• Exothermic Reactions- Less energy is needed to break the bonds than is released in making bonds in the products.

• Endothermic Reactions- More energy is needed to break the bonds than is released in making the bonds in the products

Let’s now apply this to an actual exampleMagnesium reacts with iron oxide to make magnesium oxide and iron

Mg + FeO MgO + Fe

Mg Fe O FeOMg

This bond must break

This bond must be made

Above is an energy diagram which describes the energy changes during this reaction. You need to understand what these diagrams show!

Energy

Time

Energy to break the FeO bond Energy for making the

MgO bond

Mg + FeO MgO + Fe

Energy

Time

Mg + FeO MgO + Fe

This is the ENDO’ part of the reaction

This is the EXO’ part of the reaction

We say this is an exothermic reaction (overall) since the exothermic part is greater than the endothermic part

Exothermic reactions - a summary

Energy

Progress of reaction

Reactants

Products

In exothermic reactions the energy of the reactants is greater than the energy of the products

Endothermic reactions - a summary

Energy

Progress of reaction

Reactants

Products

In endothermic reactions the energy of the products is greater than the energy of the reactants