Learning Objective Describe energy transfers during a reaction To be able to: Describe energy transfers during a reaction Key Words: Exothermic, endothermic, energy

How do we know if a reaction has taken place? Tuesday, 21 May 2019 Exothermic and Endothermic Reactions Date and title in books. You need a pen, pencil & ruler. Starter Activity – 5 minutes Quick question How do we know if a reaction has taken place?

Explain how exothermic and an endothermic reactions happen (F) Target 9-1 Identify ways we know a reaction has taken place (F/H) Target 4-9 Describe an exothermic and an endothermic reaction (H) Target 5-9 Explain how exothermic and an endothermic reactions happen Extension: Application of Knowledge Target 8/9 Evaluate the uses and applications of exothermic and endothermic reactions Learning Outcomes

Energy 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 A reaction which gives out heat energy is called an exothermic reaction. Quick task: List any other exothermic reactions you can think of……

Endothermic Reactions Finish the sentence: Endothermic reactions…… How might a test tube feel if there is an endothermic reaction taking place inside it?

Uses for Reactions Everyday uses of exothermic reactions include self-heating cans and hand warmers. Endothermic reactions include thermal decompositions and themreaction of citric acid and sodium hydrogencarbonate. Some sports injury packs are based on endothermic reactions.

Breaking Bonds In any reaction, bonds are broken. Do you think breaking bonds needs energy or gives out energy? Breaking bonds ALWAYS needs energy. It is making new products and forming new bonds that can need or release energy!

Particles Colliding Hydrogen Chlorine Reactant particles collide Reactant particles move towards each other Product particles move apart

Activation Energy Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy (more next lesson!)

GCSE Question Look at the table. Reaction Start Temp (0C) End Temp (0C) A+B 19 27 C+D 20 25 E+F 17 Decide which is reaction is exothermic or endothermic. How can you tell? The volume of solution was the same in each reaction. Which had the largest energy change?