--- marp: true theme: default html: true --- # Why does heat speed up reactions? Years 9 and 10 Chemistry · Reaction rates --- ## Today **We are learning to** particles react only when they collide with enough energy (the activation energy). Collision theory explains why temperature, concentration, surface area and catalysts change the rate of a reaction. - 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. --- ## Why it matters > 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. --- 💬 Discussion ## Engage: predict **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 --- ✍️ Activity ## Explore it {{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fcollision-theory%2Fstep-2&code=widget%3A+reaction%0Ashow%3A+H2+x15%2C+Cl2+x15%0Areact%3A+H2+%2B+Cl2+-%3E+2+HCl%0Aea%3A+3%0Agoal%3A+compare%0Acaption%3A+%22Run+it+at+one+temperature%2C+then+reset%2C+change+the+temperature+and+run+it+again.%22%0A}} [Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/collision-theory/?full=step-2#step-2) --- ✅ Check ## What happened? * Much faster. Particles move faster, so they collide more often and, more importantly, far more of the collisions have enough energy to react. --- ✅ Check ## 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 * Answer: They didn't have enough energy: they were below the activation energy --- ✅ Check ## What heating does **What happens when the temperature goes up?** Groups: True · False Particles move faster · Collisions happen more often · A larger share of collisions has enough energy · Particles get bigger · The activation energy gets smaller * 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 --- ✅ Check ## 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 * Answer: Many more collisions have enough energy to react --- ✅ Check ## 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 * Answer: Change only the temperature, keep the acid and the chips the same, time the fizzing, and repeat each temperature --- ✅ Check ## Four factors **Each change speeds up a reaction. Match it to the reason, in collision terms.** | | | |---|---| | `Higher temperature` | A. It lowers the activation energy, so a larger share of collisions succeed | | `Higher concentration` | B. Particles move faster: they collide more often and more of the collisions have enough energy | | `Larger surface area (smaller pieces)` | C. More reacting particles in the same space, so they collide more often | | `A catalyst` | D. More particles of the solid are exposed, so more collisions can happen at once | * 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 --- ✍️ Activity ## Elaborate: concentration {{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fcollision-theory%2Fstep-4&code=widget%3A+reaction%0Ashow%3A+H2+x8%2C+Cl2+x8%0Areact%3A+H2+%2B+Cl2+-%3E+2+HCl%0Aea%3A+2.5%0Atarget%3A+HCl+8%0Aslider%3A+off%0Acaption%3A+Low+concentration.+Note+the+time.%0A}} [Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/collision-theory/?full=step-4#step-4) --- ✅ Check ## 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 * Answer: More particles in the same space collide more often --- ✅ Check ## 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 * Answer: Powder exposes far more particles to the acid, so there are more collisions each second --- ✍️ Activity ## Elaborate: a catalyst {{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fcollision-theory%2Fstep-5&code=widget%3A+reaction%0Ashow%3A+H2+x15%2C+Cl2+x15%0Areact%3A+H2+%2B+Cl2+-%3E+2+HCl%0Aea%3A+3%0Acatalyst%3A+on%0Aslider%3A+off%0Agoal%3A+compare%0Acaption%3A+%22Run+it+without+the+catalyst%2C+then+reset%2C+turn+the+catalyst+on+and+run+it+again.%22%0A}} [Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/collision-theory/?full=step-5#step-5) --- ✅ Check ## 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 * Answer: The catalyst lowered the activation energy, so more collisions had enough energy --- ## 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. --- ✅ Check ## Exit ticket **Why does crushing a tablet make it dissolve and react faster in water?** * Crushing increases the surface area, so more particles are exposed and collisions happen more often.