Energy changes in chemical reactions

What happens in a chemical reaction?

Atoms are rearranged to make new compounds Bonds are broken. Bonds are formed.

Products Reactants Substances formed Starting substances Remind students that: the starting substances are called reactants the substances formed by the reaction are called products the arrow represents a chemical change. Note the position of the reactants on the left-hand side of the arrow and the products on the right-hand side of the arrow.

A water molecule Oxygen atom Hydrogen atoms An example is the reaction of water to produce hydrogen gas and oxygen gas. Point out the features of the water molecule: oxygen atom in red hydrogen atoms in blue.

A water molecule Chemical bonds Blue lines represent chemical bonds. Remind students that energy is stored as potential energy in these bonds.

Decomposition of water Hydrogen Water Oxygen Explain the diagram represents the decomposition reaction of water. Focus questions: What is the reactant? What are the products? What does decomposition mean? How does this describe the reaction? What has to happen first? Where would there be energy changes in this reaction?

Decomposition of water The bonds in the water molecule break. Focus questions: Does water under normal daily circumstances do this reaction? Why do you think this? Tell students that it takes energy to break chemical bonds so this is an energy change. What happens next?

Decomposition of water Hydrogen Oxygen Bonds between hydrogen atoms are made. Bonds between oxygen atoms are made. In both cases there is a change in where the energy is being stored and used.

Chemical reactions involve: bond breaking and bond making Tell students to record this section of the slideshow.

Chemical potential energy is the energy stored by molecules in their bonds. Remind students that they identified and investigated chemical potential energy in Year 8.

Overall, energy changes in a chemical reaction can be measured as changes n temperature.

If the temperature of a reaction mixture increases: overall energy has been released by the reaction exothermic. Ask students to add examples of exothermic reactions they have observed.

If the temperature of a reaction mixture increases: overall energy has been released by the reaction exothermic. HEAT OUT Tell students that the word exothermic comes from Greek words meaning ‘out’ and ‘heat’, so the word tells you what is happening in the reaction.

If the temperature of a reaction mixture decreases: overall energy has been absorbed by the reaction endothermic. Ask students to add examples of endothermic reactions they have observed.

If the temperature of a reaction mixture decreases: overall energy has been absorbed by the reaction endothermic. HEAT IN Tell students that this word is also from Greek words and means ‘in heat’.