Energy Changes Do as a demo if necessary.

Combustion of a match It is clear that heat and light are being released. So where does all that energy come from.

The conservation of energy: Energy cannot be created or destroyed The conservation of energy: Energy cannot be created or destroyed. It can only be transferred between energy stores. In chemistry: All chemical reactions involve a change in energy. But that energy is neither created nor destroyed. It is transferred from or to the surroundings.

Respiration and photosynthesis Make sure the reaction scheme has the key labels involving energy in and energy out

In exothermic reactions, energy is released to the surroundings and its temperature increases In endothermic reactions, energy is taken in from the surroundings and its temperature decreases

Exothermic Endothermic Energy Released to surroundings Taken in from surroundings Temperature of surroundings Increases Decreases Examples Combustion, respiration, self-heating cans, hand warmers Thermal decomposition, citric acid and sodium hydrogen carbonate, sports related cold-packs

A solid + a liquid where the temperature changes from: Generally students will say that the second one is endothermic as the temperature of the actual chemical is dropping. What is key here is that the temperature change is the temperature of the thermometer, which IS the surroundings. So if the thermometer is dropping, the surroundings must be losing energy so it must be endothermic. A thermometer jammed into a piece of wool. When I add a certain chemical to it the temperature changes from: 20°C  5°C A solid + a liquid where the temperature changes from: 20°C  40°C

Questions 1-6 from the mastery booklet If you finish the questions start reading the next section

Measuring the change

Questions 7-18

Reaction profiles

Answer Q19 and the challenge question

Bond energy/enthalpy

H H O O H H Molecules are stable – all atoms have full outer shells as they are bonded together Step 1: Atoms need to be pulled away from each other. This requires energy so is endothermic H O H O For this reaction, the amount of energy going in to step 1 is less than the amount released in step 2. The overall reaction is therefore exothermic. Step 2: New bonds are formed. This process is exothermic

Breaking bonds requires energy to be put in. It is endothermic Making bonds releases energy to the surroundings. It is exothermic

Calculating the change – page 118 in your text book

Worked examples from booklet Answer questions 20-30

Working out the overall energy change Hydrogen + water Nitrogen + hydrogen

Complete mastery booklet