Every card in Rates and equilibrium, Year 11: reversible reactions and equilibrium
The whole deck, in order — so you can read it through before your child ever sees it.
← Back to Rates and equilibrium, Year 11: reversible reactions and equilibrium
- A reaction in which the products can react together to produce the original reactants
A reversible reaction
HintIt can run in either direction.
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.
- In an equation, the symbol ⇌ shows that the reaction is ____.
reversible
HintThe two half-arrows point in opposite directions for a reason.
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.
- Solid ammonium chloride is heated: ammonium chloride ⇌ ammonia + hydrogen chloride. Heating drives the forward reaction. What change in conditions makes the two gases re-form ammonium chloride?
Cooling
HintDo the opposite of what drove the forward reaction.
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.
- A reversible reaction is written A + B ⇌ C + D. Which substances are the reactants of the reverse reaction?
C and D
HintRead the equation from right to left.
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.
- When solid ammonium chloride is heated it breaks down into two gases: ____ and ____.
ammonia; hydrogen chloride
HintThe names of both gases can be worked out from the two parts of the solid's name.
WhyThis is AQA's example of a reversible reaction: NH4Cl ⇌ NH3 + HCl. On cooling, the two gases react to form solid ammonium chloride again.
- The forward reaction of a reversible reaction is exothermic. Is the reverse reaction exothermic or endothermic? (one word)
Endothermic
HintWhatever energy one direction gives out, the other direction must take back in.
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.
- In a reversible reaction, the forward reaction transfers 92 kJ of energy to the surroundings. How much energy, in kJ, does the reverse reaction take in from the surroundings for the same amounts of substances? (number only)
92
HintThe two directions cross the same energy gap.
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.
- Heating blue hydrated copper sulfate gives white anhydrous copper sulfate and water; this direction is endothermic. What two things would you notice when water is added to the white powder?
It turns blue and it gets hot
HintThe reverse reaction undoes both the colour change and the energy change.
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.
- The state reached by a reversible reaction in a sealed container when the forward and reverse reactions are happening at exactly the same rate
Equilibrium
HintThe word also describes a balance that is not tipping either way.
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.
- Equilibrium can only be reached in apparatus that stops reactants and products escaping. Why would a reversible reaction not reach equilibrium in an open container if one of its products is a gas?
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.
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.
- The equilibrium reached by a reversible reaction is described as ____, because both reactions are still going on.
dynamic
HintIt is the opposite of 'static'.
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.
- At equilibrium, must there be equal amounts of reactants and products in the container?
No: the amounts stay constant but need not be equal
HintUnchanging is one thing; identical is another.
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.
- A reversible reaction in a sealed flask is at equilibrium. In one second the forward reaction makes 100 molecules of product. How many molecules of product does the reverse reaction use up in that second? (number only)
100
HintCompare the speeds of the two directions at equilibrium.
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.
1★ GCSE-CHEM-RTE-0027
2★ GCSE-CHEM-RTE-0028
3★ GCSE-CHEM-RTE-0029
4★ GCSE-CHEM-RTE-0030
5★ GCSE-CHEM-RTE-0031
6★ GCSE-CHEM-RTE-0032
7★ GCSE-CHEM-RTE-0033
8★ GCSE-CHEM-RTE-0034
9★ GCSE-CHEM-RTE-0035
10★ GCSE-CHEM-RTE-0036
11★ GCSE-CHEM-RTE-0037
12★ GCSE-CHEM-RTE-0038
13★ GCSE-CHEM-RTE-0039
Keep what you learn
Here, nothing is saved. In your child’s own sky every card is scheduled — it comes back just before they’d forget it — and the professor who wrote it is one tap away.