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Reversible reactions
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Some reactions can go both ways. Changing the conditions can reverse them.
The ⇌ symbol
A reversible reaction can go forwards and backwards. This is shown by ⇌ in an equation.
A ⇌ B means A can turn into B, and B can turn back into A.
A ⇌ B means A can turn into B, and B can turn back into A.
Hydrated copper(II) sulfate
CuSO4·5H2O ⇌ CuSO4 + 5H2O
Heating blue hydrated copper(II) sulfate drives off the water, leaving white anhydrous copper(II) sulfate (dehydration).
Adding water turns it blue again, and the mixture gets hot.
Heating blue hydrated copper(II) sulfate drives off the water, leaving white anhydrous copper(II) sulfate (dehydration).
Adding water turns it blue again, and the mixture gets hot.
Ammonium chloride
NH4Cl ⇌ NH3 + HCl
Heating white solid ammonium chloride at the bottom of a test tube breaks it down into ammonia and hydrogen chloride gases.
At the cooler top of the tube, the gases react again to re-form white ammonium chloride.
Heating white solid ammonium chloride at the bottom of a test tube breaks it down into ammonia and hydrogen chloride gases.
At the cooler top of the tube, the gases react again to re-form white ammonium chloride.
What would you see if you heated blue copper(II) sulfate crystals, let them cool, then added water?
- Heating removes water: blue turns white.
- Adding water reverses the reaction: white turns blue.
- The reverse reaction is exothermic, so it gets warm.
Answer: Blue to white on heating; white back to blue, with warming, when water is added.
Reversible reactions are shown by ⇌. Heating blue hydrated copper(II) sulfate gives white anhydrous copper(II) sulfate; adding water reverses it. Ammonium chloride decomposes on heating and re-forms in the cooler part of the tube.
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