Rates and equilibrium, Year 11: reversible reactions and equilibrium

A reversible reaction

HintIt can run in either direction.

The whyIn a reversible reaction the change can go forwards or backwards, depending on the conditions. Its equation is written with the double arrow ⇌ in place of the usual one-way arrow.

reversible

HintThe two half-arrows point in opposite directions for a reason.

The whyOne half-arrow stands for the forward reaction and the other for the reverse reaction. An ordinary one-way arrow is used when a reaction goes in one direction only.

Cooling

HintDo the opposite of what drove the forward reaction.

The whyThe direction of a reversible reaction depends on the conditions. Here, heating breaks the solid down into two gases, and when the gases meet a cooler part of the tube they combine again into the white solid.

C and D

HintRead the equation from right to left.

The whyThe forward reaction turns A and B into C and D. The reverse reaction runs the other way, so its starting materials are C and D and its products are A and B.

ammonia; hydrogen chloride

HintThe names of both gases can be worked out from the two parts of the solid's name.

The whyThis is AQA's example of a reversible reaction: NH4Cl ⇌ NH3 + HCl. On cooling, the two gases react to form solid ammonium chloride again.

Endothermic

HintWhatever energy one direction gives out, the other direction must take back in.

The whyGoing forwards, the products store less energy than the reactants, and the difference is transferred to the surroundings. Going backwards means climbing back up the same energy gap, so that energy has to be taken in.

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HintThe two directions cross the same energy gap.

The whyThe same amount of energy is transferred in each direction: given out one way, taken in the other. Energy is conserved, so going there and back leaves the total unchanged.

It turns blue and it gets hot

HintThe reverse reaction undoes both the colour change and the energy change.

The whyAdding water runs the reaction backwards: hydrated copper sulfate re-forms, so the blue colour returns. Because the forward direction took energy in, the reverse direction gives the same amount out, and the temperature rises.

Equilibrium

HintThe word also describes a balance that is not tipping either way.

The whyEach substance is being made by one reaction exactly as fast as it is being used up by the other. Nothing appears to change, although both reactions are still going on.

The gas escapes, so it cannot take part in the reverse reaction

HintAsk what the backward change needs as its starting materials, and where they have gone.

The whyThe reverse reaction needs the products to stay and collide with each other. If one of them drifts away, the reverse reaction can never become as fast as the forward one, and the forward reaction just carries on.

dynamic

HintIt is the opposite of 'static'.

The whyAt equilibrium the forward and reverse reactions have not stopped: they carry on at equal rates, so their effects cancel. The word marks the difference from a reaction that has simply ended.

No: the amounts stay constant but need not be equal

HintUnchanging is one thing; identical is another.

The whyEqual rates mean that the amount of each substance stops changing. The mixture might be mostly products, mostly reactants or anything in between, depending on the reaction and the conditions.

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HintCompare the speeds of the two directions at equilibrium.

The whyAt equilibrium the forward and reverse reactions have exactly the same rate. The product is made and used up equally fast, which is why its amount does not change.