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Reversible Reactions

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

In this experiment, the water of crystallisation is removed from hydrated copper(II) sulfate by heating. Condensing the vapour produced in a second test tube collects the water. The white anhydrous copper(II) sulfate is then rehydrated and the blue colour returns.

hydrated copper sulfate ⇆ anhydrous copper sulfate + water

1. Set up the apparatus and place about 5 g of powdered hydrated copper(II) sulfate in the test tube. Make sure that the tube is clamped near the bung as shown.
2. Heat the blue copper(II) sulfate until it has turned white. Move the flame along the length of the test tube from time to time (avoiding the clamp), to prevent water condensing on the cooler regions and then running down on to the hot solid, possibly cracking the test tube.
3. Act quickly to prevent suck-back if the level of water collecting in the test tube reaches the end of the delivery tube. Lift the clamp stand so that the delivery tube does not reach into the water in the test tube.
4. Allow the anhydrous copper(II) sulfate to cool back to room temperature.
5. Holding the test tube containing anhydrous copper(II) sulfate in one hand, pour the collected water very slowly on to the white powder. What observations can you make?
6. Adapt the experiment to study the changes observed when solid hydrated cobalt(II) chloride is heated.

Record any observations that you made during the heating process and when the water was poured back on to the anhydrous copper(II) sulfate.

Heating copper sulphate; a reversible reaction
Questions in the video:
1. How do you know the reaction is reversible?
2. What would you expect to see if you add water to CuSO4?
3. When we add water to CuSO4, the reaction gets hot. What does this tell you about the reaction?

1. What evidence is there in your observations about the heats of reaction of the forward and reverse reactions?
2. From your knowledge of the cobalt chloride test for water, write a word equation for the reversible reaction used in that test.
3. What are the colours of solid hydrated cobalt chloride and anhydrous cobalt chloride?

  • Show Solutions
    1. The dehydration of the hydrated crystals is a thermal decomposition - it is an endothermic reaction
    The recombination with water generates heat - it is an exothermic reaction.
    2. hydrated cobalt chloride ⇆ anhydrous cobalt chloride + water
    3. hydrated cobalt chloride is pink, anhydrous cobalt chloride is blue.
The following video shows the dehydration and hydration of cobalt chloride.

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