Rates, Energetics and Organic Chemistry

One unit, three questions

This unit looks like four separate topics: rates, energy changes, organic chemistry and electrolysis. They are really one thing looked at from different sides. Every one of them is a question about a REACTION, and there are only three questions. ⭐⭐ HOW FAST does the reaction go? The answer is collision theory. Reacting particles do nothing until they COLLIDE, and even then only if they collide often enough and with enough energy. So everything that speeds a reaction up, heating it, crowding the particles, breaking a solid into smaller pieces, or adding a catalyst, works by changing the collisions. ⭐ WHICH WAY DOES THE ENERGY GO? Breaking bonds takes energy in; making bonds gives energy out. If more comes out than goes in, the reaction releases energy and warms its surroundings, which is exothermic. If more goes in than comes out, it takes energy in and cools its surroundings, which is endothermic. WHAT DOES IT MAKE? Crude oil is separated into hydrocarbons; cracking breaks long chains into shorter, more useful ones and into alkenes; alkenes join into polymers; and electrolysis uses electricity to break a compound into its elements. ⚠️ So do not learn four topics. Learn three questions and ask them of any reaction: how fast, energy in or out, and what does it make? Carry those three questions through the whole module.

How fast, which way, what it makes

The whole unit fits into three questions about a reaction. The first is rates, the second energetics, the third the products.

Tap the two that speed it up

Tap the TWO changes that would make a reaction go FASTER.

• Warming the reaction mixture up.
• Grinding a solid reactant into a fine powder.
• Cooling the reaction mixture down.
• Using one large lump of the solid instead of powder.

Why heat speeds a reaction

Warming a reaction mixture makes the reaction go faster. Using collision theory, why?

• The particles move faster, so they collide more often AND each collision carries more energy, and more collisions succeed
• Warming creates extra particles for the reaction
• Warming removes the need for the particles to collide at all
• Warming acts as a catalyst

Words for reactions and energy

Six terms this unit turns on. The first four are the engine; the last two are what reactions make.

Match each factor to what it does to collisions

Two that are true of exothermic

Select the TWO statements that are true of an EXOTHERMIC reaction.

• It releases energy to its surroundings overall
• The surroundings get warmer
• It takes energy in from its surroundings overall
• The surroundings get colder

Bonds break in, bonds make out

A few habits carry the marks across the whole unit. ⭐⭐ FOR RATES, ANSWER THROUGH THE COLLISIONS. Do not just say a factor speeds the reaction up; say what it does to the collisions. Heating makes them more frequent and more energetic. A higher concentration or a larger surface makes them more frequent. A catalyst gives an easier path. Naming the factor is half a mark; explaining the collisions is the rest. ⭐⭐ FOR ENERGETICS, HOLD ONE RULE: breaking bonds takes energy IN, making bonds gives energy OUT. Then compare. If making the new bonds gives out more than breaking the old ones took in, energy is released overall and the reaction is exothermic and warms its surroundings. If it is the other way round, the reaction is endothermic and cools them. Burning and neutralising are exothermic; some dissolving and thermal breakdowns are endothermic. ⭐ FOR THE ORGANIC AND ELECTROLYSIS SIDE, ASK WHAT IT MAKES. Crude oil is separated by boiling point into hydrocarbon fractions. Cracking breaks the long, less useful chains into shorter fuels and into alkenes, and alkenes join into polymers. Electrolysis is a different way to drive a reaction: electricity breaks a compound into its elements, which needs the compound melted or dissolved so its particles can move. ⚠️ A fair word on the environment. Crude oil is a finite resource and plastics made from it can be hard to deal with as waste. State that plainly; do not invent figures and do not cast any industry as a villain. Keep the three questions to hand: how fast, energy in or out, and what does it make?

Order crude oil to plastic

Put these stages in the order that turns crude oil into a plastic.

• Crude oil arrives as a mixture of many hydrocarbons
• Fractional distillation separates it by boiling point into fractions
• Long, less useful chains are cracked into shorter alkanes and alkenes
• The alkenes are collected as reactive short molecules
• Many alkenes are joined together into a long-chain polymer

Build the energy sentence

Assemble the sentence that explains what makes a reaction exothermic.

Work out the rate

In a worked reaction, 24 units of gas are produced in 6 seconds. Work out the rate of reaction, in units of gas per second.

The reactions run

Five quick decisions about rates, energy and organic chemistry. Three lives.

A crowded, fast-moving room

Leave chemistry aside for a screen. Picture a room and ask how often the people in it bump into one another. In a near-empty room where a few people wander slowly, bumps are rare; you could stand there a long time and touch nobody. Now crowd the same room and get everyone moving quickly, and people knock into each other constantly. ⭐ Two things changed the number of bumps. CROWDING, more people in the same space, meant more chances to meet. And SPEED, everyone moving faster, meant they met more often and harder when they did. ⭐⭐ And notice a subtler point. Even in the busiest room, not every bump matters; a gentle brush passing changes nothing, while a firm knock does. So what counts is not just how OFTEN people meet but how HARD they meet when they do. One more thing. If you filled the spare corners of the room with statues that nobody interacts with, the number of real bumps between the moving people would not change at all, because the statues take no part. ⚠️ Hold that picture. A reaction is people meeting: crowd them, speed them up, or give more surface to meet on, and the meetings that matter happen more often; add something that takes no part and nothing changes.

Complete the rates and energy facts

A reaction happens only when particles meet in a _____, and only if they meet often enough and with enough energy. A substance that speeds a reaction up by offering an easier path, without being used up itself, is a _____. A reaction that releases energy and warms its surroundings is _____. A reaction that takes energy in and cools its surroundings is _____. A compound made of only hydrogen and carbon, such as those in crude oil, is a _____.

Judge three reactions

Three reactions come up in a lab. For each, reason from the three questions of the unit.

• A reaction between an acid and a solid is going too slowly, and a student wants to speed it up without changing what reacts.
• A reaction is carried out in a flask and the flask becomes warm to the touch, and the student must classify it.
• A refinery has too much of a long-chain hydrocarbon and not enough short fuels or material for plastics.

Explain how to speed a reaction

Explain how the rate of a reaction can be increased, using collision theory, and explain what makes a reaction exothermic or endothermic. You may use any reaction as your example.

• State that a reaction needs particles to collide with enough energy
• Give two ways to speed a reaction up and say what each does to the collisions
• Explain what a catalyst does and why it is not used up
• Explain the difference between an exothermic and an endothermic reaction using bonds breaking and making
• Finish by saying how you could tell, in the lab, whether a reaction was exothermic