--- title: "Hotter or colder?" subtitle: "Year 8 Chemistry · Chemical change · Lesson 9" --- # Hotter or colder? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Snap an instant ice pack and it goes freezing cold. Shake a hand warmer and it gets hot. Neither is plugged in. Where does the heat go, and where does it come from? :::teacher-note **In one sentence:** Students explain exothermic and endothermic reactions as energy released to or absorbed from the surroundings, and classify everyday reactions (AC9S8U07, nodes 4 and 5). **Students should be able to say:** I can explain what exothermic and endothermic mean. I can say which way energy moves in each, between the reaction and its surroundings. I can give an everyday example of each. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **An instant ice pack contains two chemicals that react when you snap it. It goes cold. What is happening?** ☐ The reaction gives out energy, so the pack cools down ☐ The reaction takes in energy from the pack and your hand ☐ The chemicals were stored in a freezer ☐ The reaction makes 'cold', which flows into your hand My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** The reaction takes in energy from its surroundings (the pack, the air, your hand), so they get colder. A reaction that takes in energy is endothermic. There is no 'cold energy': cold just means less thermal energy. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: exothermic and endothermic :::keep-together ### Energy out **In an exothermic reaction, where does the energy go?** ☐ Into the reaction mixture, so it gets colder ☐ Out to the surroundings, so they get hotter ☐ It is destroyed ::: :::teacher-note **Answer:** Out to the surroundings, so they get hotter Exo = out. Energy stored in the reactants is released to the surroundings. ::: :::keep-together ### Sort them **Exothermic or endothermic?** Groups: Exothermic (gets hotter), Endothermic (gets colder) - Wood burning: {{field-15}} - Respiration in your cells: {{field-15}} - A hand warmer (iron rusting quickly): {{field-15}} - An instant ice pack: {{field-15}} - Photosynthesis (taking in light energy): {{field-15}} - Bicarb soda reacting with vinegar and fizzing: {{field-15}} ::: :::teacher-note **Answer:** Wood burning → Exothermic (gets hotter); Respiration in your cells → Exothermic (gets hotter); A hand warmer (iron rusting quickly) → Exothermic (gets hotter); An instant ice pack → Endothermic (gets colder); Photosynthesis (taking in light energy) → Endothermic (gets colder); Bicarb soda reacting with vinegar and fizzing → Endothermic (gets colder) ::: :::keep-together ### Stopping on its own **On the board, the nitrogen and oxygen only reacted at 600 K, and the temperature fell. Why might an endothermic reaction stop on its own?** ☐ It runs out of atoms ☐ It takes in energy and cools its surroundings, so fewer collisions have enough energy to react ☐ Endothermic reactions are always slow ::: :::teacher-note **Answer:** It takes in energy and cools its surroundings, so fewer collisions have enough energy to react Unless energy keeps coming in (a hot engine, sunlight for photosynthesis), an endothermic reaction cools itself down. ::: :::keep-together ### Where the energy comes from **Burning gas in a stove gives out a lot of energy. Where was that energy before?** ☐ Stored in the gas and oxygen (chemical energy), released as their atoms rearrange into new particles ☐ In the match or spark that lit it ☐ In the air around the flame ☐ Nowhere: burning makes new energy ::: :::teacher-note **Answer:** Stored in the gas and oxygen (chemical energy), released as their atoms rearrange into new particles A fuel and oxygen together store chemical energy. In an exothermic reaction their atoms rearrange into new particles that store less, and the difference is given out to the surroundings. ::: :::typical ## Elaborate: hand warmers and ice packs :::keep-together ### Which product **A sports trainer needs something to reduce swelling on a sprained ankle, at a game far from a freezer. Which product, and why?** ☐ A hand warmer: its reaction is exothermic ☐ Either: both change temperature ☐ An instant ice pack: its endothermic reaction takes energy from the ankle ::: :::teacher-note **Answer:** An instant ice pack: its endothermic reaction takes energy from the ankle Endothermic for cooling, exothermic for warming: the direction of energy transfer is the design choice. ::: ::: :::wider ## Elaborate: why burning keeps going :::keep-together **Wood is exothermic when it burns, yet a log doesn't catch fire on its own. You need a match. Why, and why does it keep burning once lit?** ☐ The match adds fuel ☐ Wood is endothermic until it is hot ☐ Reactions need enough energy to start; once lit, the energy given out heats the next bit enough to react ::: :::teacher-note **Answer:** Reactions need enough energy to start; once lit, the energy given out heats the next bit enough to react Collisions need enough energy to react (activation energy, Year 10). An exothermic reaction supplies that energy to its own next steps, so it keeps going. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Exothermic** reactions give out energy to their surroundings, so the surroundings get hotter: burning, respiration, hand warmers. - **Endothermic** reactions take in energy from their surroundings, so the surroundings get colder: instant ice packs, photosynthesis, bicarb and vinegar. - Energy is never made or destroyed; it moves between the reaction and its surroundings. - There is no "cold energy": colder means less thermal energy. ## Exit ticket :::keep-together **Burning candle wax: exothermic or endothermic? Explain using the word surroundings.** {{lines-3}} ::: :::teacher-note **Answer:** Exothermic: the reaction gives out energy to the surroundings, which get hotter (and light is given out). :::