--- title: "Why does heat speed up reactions?" subtitle: "Years 9 and 10 Chemistry · Reaction rates · Lesson 15" --- # Why does heat speed up reactions? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Milk goes off in days on the bench but lasts a week in the fridge. A glow stick in hot water glows brighter but fades sooner. Same idea: how often, and how hard, particles collide. :::teacher-note **In one sentence:** Students explain reaction rate with collision theory (collisions with at least the activation energy) and use it to explain temperature, concentration, surface area and catalysts (AC9S10U07, node 5, a hinge node). **Students should be able to say:** I can explain that particles react only when they collide with at least the activation energy. I can explain why more frequent or more energetic collisions make a reaction faster. I can name and explain at least two ways to increase the rate. **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 **A reaction takes 30 seconds at room temperature. You heat it by 100 °C. How long do you predict it takes?** ☐ Much faster, a few seconds ☐ Slower ☐ A little faster, maybe 25 seconds ☐ About the same My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Much faster. Particles move faster, so they collide more often and, more importantly, far more of the collisions have enough energy to react. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: collisions and energy :::keep-together ### Why most collisions fail **Most collisions on the board did not make a reaction. Why not?** ☐ The particles were too far apart ☐ They didn't have enough energy: they were below the activation energy ☐ Only the first collision of each particle counts ☐ The particles were the wrong colour ::: :::teacher-note **Answer:** They didn't have enough energy: they were below the activation energy A collision reacts only if it has at least the activation energy. The rest bounce off. ::: :::keep-together ### What heating does **What happens when the temperature goes up?** Groups: True, False - Particles move faster: {{field-15}} - Collisions happen more often: {{field-15}} - A larger share of collisions has enough energy: {{field-15}} - Particles get bigger: {{field-15}} - The activation energy gets smaller: {{field-15}} ::: :::teacher-note **Answer:** Particles move faster → True; Collisions happen more often → True; A larger share of collisions has enough energy → True; Particles get bigger → False; The activation energy gets smaller → False ::: :::keep-together ### The bigger effect **A 100 °C rise doesn't make the particles collide 10 times more often, yet the reaction can go 10 times faster. What explains the rest?** ☐ The container gets bigger ☐ The particles stick together for longer ☐ Many more collisions have enough energy to react ::: :::teacher-note **Answer:** Many more collisions have enough energy to react Temperature changes how hard particles hit far more than how often. That is why heat has such a big effect. ::: :::keep-together ### A fair test **You want to test how temperature changes the rate of marble chips reacting with acid. Which plan is a fair test?** ☐ Heat the acid and use stronger acid too, so the difference is bigger ☐ Change only the temperature, keep the acid and the chips the same, time the fizzing, and repeat each temperature ☐ Do one run at each temperature: one result is enough ::: :::teacher-note **Answer:** Change only the temperature, keep the acid and the chips the same, time the fizzing, and repeat each temperature Change one factor, measure the rate (time until the fizzing stops, or gas made each minute) and keep everything else the same. Repeats show how much the results vary. ::: :::keep-together ### Four factors **Each change speeds up a reaction. Match it to the reason, in collision terms.** A. It lowers the activation energy, so a larger share of collisions succeed B. Particles move faster: they collide more often and more of the collisions have enough energy C. More reacting particles in the same space, so they collide more often D. More particles of the solid are exposed, so more collisions can happen at once {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Higher temperature` | | | `Higher concentration` | | | `Larger surface area (smaller pieces)` | | | `A catalyst` | | ::: :::teacher-note **Answer:** Higher temperature → B. Particles move faster: they collide more often and more of the collisions have enough energy; Higher concentration → C. More reacting particles in the same space, so they collide more often; Larger surface area (smaller pieces) → D. More particles of the solid are exposed, so more collisions can happen at once; A catalyst → A. It lowers the activation energy, so a larger share of collisions succeed Every factor works through collisions: either more of them each second, or a bigger share with enough energy. ::: :::typical ## Elaborate: concentration :::keep-together ### Why faster **The concentrated reaction reached the target faster. Why?** ☐ More particles in the same space collide more often ☐ Each collision had more energy ☐ The activation energy was lower ::: :::teacher-note **Answer:** More particles in the same space collide more often Concentration changes how often particles collide, not how hard. ::: :::keep-together ### Surface area **A lump of chalk reacts slowly with acid; chalk powder fizzes fast. Which explanation fits collision theory?** ☐ Powder exposes far more particles to the acid, so there are more collisions each second ☐ Powder is a different substance ☐ Powder particles have more energy ::: :::teacher-note **Answer:** Powder exposes far more particles to the acid, so there are more collisions each second Surface area, like concentration, changes collision frequency. ::: ::: :::wider ## Elaborate: a catalyst :::keep-together ### How a catalyst works **With the catalyst, the reaction sped up but the temperature didn't change. How?** ☐ The catalyst lowered the activation energy, so more collisions had enough energy ☐ The catalyst reacted and was used up ☐ The catalyst gave the particles extra energy ::: :::teacher-note **Answer:** The catalyst lowered the activation energy, so more collisions had enough energy A catalyst provides a route with a lower activation energy. (In the model it simply halves the threshold; real catalysts often work on their surface.) ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Particles react only when they **collide** with at least the **activation energy**. Most collisions just bounce. - **Temperature:** faster particles collide more often and, much more importantly, harder. - **Concentration** and **surface area:** more collisions each second. - **Catalyst:** a lower activation energy, so more collisions succeed. It is not used up. ## Exit ticket :::keep-together **Why does crushing a tablet make it dissolve and react faster in water?** {{lines-3}} ::: :::teacher-note **Answer:** Crushing increases the surface area, so more particles are exposed and collisions happen more often. :::