Heat blue copper(II) sulfate crystals and they turn white; add water and they turn blue again. Some reactions can run in either direction, and in a closed container they settle into a balance where both directions happen at once.
A reversible reaction can go in either direction, shown by the symbol ⇌. Heating hydrated copper(II) sulfate (blue) drives off water to leave anhydrous copper(II) sulfate (white); adding water reverses it. Extended: in a closed system a reversible reaction reaches dynamic equilibrium, where the forward and reverse reactions happen at the same rate and the concentrations of reactants and products stay constant. Changing the conditions moves the position of equilibrium to oppose the change.
Fig. 1: reaching dynamic equilibrium in a closed system
Section 01
Reversible reactions.
Two colour changes appear in every syllabus. Both show water being removed and put back.
Copper(II) sulfate: CuSO4·5H2O(s) ⇌ CuSO4(s) + 5H2O(l). Heating turns blue hydrated crystals into white anhydrous powder; adding water turns it blue again, and it gets hot.
Cobalt(II) chloride: CoCl2·6H2O ⇌ CoCl2 + 6H2O. The hydrated form is pink, the anhydrous form blue. Blue cobalt(II) chloride paper turns pink when water is added.
Tests for water: anhydrous copper(II) sulfate turns from white to blue, and cobalt(II) chloride from blue to pink. These show water is present; checking the boiling point (100 °C) shows it is pure.
Energy: if one direction is endothermic, the reverse is exothermic by the same amount.
Stage 1 · Learn
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What does the symbol ⇌ show in an equation?
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Blue hydrated copper(II) sulfate is heated. What is seen?
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Which test shows that a liquid contains water?
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(Extended) At dynamic equilibrium:
Section 02
Dynamic equilibrium.
Extended. Start the simulator with reactants only in a closed box. Watch the forward rate fall and the reverse rate rise until they are equal.
Dynamic: both reactions are still happening; particles keep changing from reactants to products and back.
Equilibrium: the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations stay constant.
Closed system: nothing can enter or leave. In an open beaker a gas product escapes and equilibrium is never reached.
Section 03
Changing the conditions.
Extended. When a condition is changed, the position of equilibrium shifts in the direction that opposes the change.
Change
Position of equilibrium moves
increase concentration of a reactant
to the right (more product)
increase temperature
in the endothermic direction
decrease temperature
in the exothermic direction
increase pressure (gases)
to the side with fewer gas molecules
add a catalyst
no change; equilibrium is reached faster
Examiner trap
At equilibrium the concentrations are constant, not equal. There may be far more product than reactant, or the other way round. And a catalyst never moves the position of equilibrium: it speeds up the forward and reverse reactions equally.
Stage 2 · Exam
Exam-style questions
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(Extended) For N2O4(g) ⇌ 2NO2(g), the forward reaction is endothermic. What happens to the position of equilibrium when the temperature is increased?
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(Extended) For 2SO2(g) + O2(g) ⇌ 2SO3(g), what is the effect of increasing the pressure?
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(Extended) Why can a reversible reaction reach equilibrium only in a closed system?
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White anhydrous copper(II) sulfate is added to water. What is seen and what kind of change is it?
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Reversible reactions and equilibrium, mastered.
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