Chemistry C2: Rates, Equilibrium and Organic Chemistry

Three kinds of prediction

Chemistry is worth learning because it is predictable, and this unit is three different kinds of prediction stacked together. The first is how FAST. Reactions happen when particles collide hard enough to react, so anything that makes collisions more frequent or more forceful makes the reaction go faster, and you can work out the effect of a change before making it. The second is how FAR. Some reactions run to a stop, with one of the starting substances used up entirely. Others can run in both directions at once, and instead of stopping they settle at a balance point where the forward and backward changes are happening at the same speed. Predicting where that balance sits is a different question from predicting how quickly it is reached, and keeping the two apart is most of what this part of the unit is testing. The third is what a compound will DO. Carbon compounds are so numerous that memorising them individually is hopeless, so they are sorted into families, and every member of a family behaves in much the same way. Learn the family and you have learned all of them. Everything that follows is one of those three questions.

Five words used precisely

Two of these get swapped for each other constantly, and the difference between them is worth a mark on its own.

What does each change do?

Why does heating do more?

Heating a reaction mixture usually speeds it up more than making the solution more concentrated does. What is the best explanation?

• Heating makes collisions more frequent AND makes more of them energetic enough to react, while concentration only makes them more frequent
• Heating adds more particles to the mixture
• Heating makes the particles weaker so they break apart more easily
• Heating acts as a catalyst

Reactions that stop, and reactions that settle

Not every reaction runs until something is used up. Telling the two behaviours apart is what the middle third of this unit is about.

Work out the mean rate

A reaction gives off 72 cubic centimetres of gas, and it takes 45 seconds to finish. Work out the mean rate of the reaction, in cubic centimetres of gas per second.

Decide what to change

You want a reaction to go faster. Put these five steps into the order that gets you a reasoned answer rather than a guess.

• State what is actually reacting, and which of them is a solid, a liquid or a gas
• Ask what is limiting the collisions at the moment
• List the changes that would act on that limit
• Choose the change that acts on the limit, and say which part of collision it affects
• Check whether the change alters how much product you end up with, as well as how fast

How fast is not how much

This is the confusion that costs the most marks in this unit, and it is easy to fix once it is named. Rate answers the question how quickly. Yield answers the question how much. They are independent, and a change can affect one without touching the other. A catalyst is the clearest case. It gives the reaction an easier route, so more of the collisions succeed and the reaction reaches its finishing point sooner. It does not change WHERE that finishing point is. Add a catalyst to a reversible reaction and you arrive at the same balance, in less time. Nothing about the amounts at equilibrium is different, because the catalyst speeds the backward change by as much as the forward one. So a sentence like the catalyst increased the amount of product is wrong, however reasonable it sounds, and an examiner will read it as a misunderstanding rather than as a slip. The correct version is one word different: the catalyst increased the RATE at which the product formed. Watch for the same confusion in the other direction too. Powdering a solid makes a reaction finish faster and leaves the final amount exactly as it was, because the same mass of solid is present either way. Whenever you write a sentence about a change to a reaction, check which of the two questions it is answering.

Complete the reaction sentences

How much happens in a given time is _____. A substance that speeds a reaction without being used up is _____. A reaction that can run forwards and backwards, settling at a balance rather than stopping, is _____. A family of carbon compounds that share a pattern and behave in much the same way is _____.

One family, learned once

The point of sorting carbon compounds into families is that you stop learning compounds and start learning patterns. Take the alcohols. Every member of the family carries the same reactive part, and the members differ only by how long the carbon chain is. That one fact predicts a great deal. Because they share the reactive part they undergo the same kinds of reaction, so knowing what one alcohol does with an acid tells you what the others do with an acid: they form the sweet-smelling compounds called esters, which is the reaction used to make many flavourings and perfumes. Because they differ in chain length their physical properties change steadily along the family, so the boiling point rises as the chain gets longer, in a smooth and predictable way rather than at random. And because the pattern is regular you can place an unfamiliar member without ever having met it: given its position in the family you can say roughly how it will boil and confidently what it will react with. The alkanes and the alkenes work the same way, each with its own reactive arrangement and its own set of behaviours. That is the whole idea of a homologous series, and it is why organic chemistry is a much smaller subject than the number of carbon compounds would suggest.

True of a homologous series

Which THREE of these are true of the members of a homologous series?

• They share the same reactive part, so they undergo the same kinds of reaction
• They differ from each other by the length of the carbon chain
• Their physical properties change gradually along the family rather than at random
• They all boil at the same temperature
• They all react at exactly the same rate as each other

Find the sentence that confuses the two

Five sentences from a write-up about speeding up a reaction. Tap the ONE that confuses how fast with how much.

• Grinding the solid into a powder exposed more surface, so collisions could happen in more places.
• Heating the mixture made the particles move faster, so collisions were both more frequent and more energetic.
• Adding the catalyst increased the amount of product that was finally formed.
• The reaction finished in less than half the time once both changes had been made.
• The same mass of solid was used each time, so the final amount of product was unchanged.

Explain a rate change

A rate explanation earns marks for naming the mechanism, not just the change. Choose the option in each gap.

The reactions run

Five quick questions on rates, balance and families. Three lives.

Three decisions about one reaction

A reaction between a solid and a solution is running too slowly for a process to be worth using. You have been asked to advise on it, and you are told the reaction is reversible. Work through three decisions.

• What do you establish first?
• The solid is supplied in large lumps and the solution is dilute. What do you recommend, and how do you justify it?
• You are asked whether adding a catalyst would also increase the amount of product. What do you say?

Explain how to speed it up honestly

A reaction between a solid and a solution is running too slowly. Explain two changes that would speed it up, how each one works, and whether either would change the amount of product formed.

• Name two changes, and say which state of substance each one acts on
• For each, explain what it does to the collisions between particles
• Say which change acts on two things at once, and why that matters
• State clearly whether either change alters the amount of product, and why
• Finish by explaining what a catalyst would and would not do here