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6.3.1 Reversible reactions

Definition

Reversible reaction

A reaction in which the products can react to re-form the original reactants, shown by the symbol ⇌\rightleftharpoons⇌.

  1. Most reactions go in one direction only, turning reactants into products.
  2. In a reversible reaction, the products can also react together to turn back into the original reactants.
  3. The symbol ⇌\rightleftharpoons⇌, made of two half-arrows, shows that a reaction is reversible.
  4. A general reversible reaction is written as A+B⇌C+D\text{A} + \text{B} \rightleftharpoons \text{C} + \text{D}A+B⇌C+D.
  5. The reaction from left to right is the forward reaction, and the reaction from right to left is the reverse reaction.
  6. Changing the conditions, such as heating a substance or adding water to it, can change the direction in which a reversible reaction goes.
  7. Two compounds show this especially clearly, because they change colour as the reaction changes direction: copper(II) sulfate and cobalt(II) chloride.

Hydrated and anhydrous compounds

Definition

Hydrated substance

A substance whose crystals contain water chemically combined within them, called water of crystallisation.

Definition

Anhydrous substance

A substance that contains no water of crystallisation.

  1. Many salts form crystals with water molecules chemically combined inside the crystal structure.
  2. This water is called water of crystallisation, and it is shown after a dot in the formula, as in CuSO4⋅5H2O\mathrm{CuSO_{4} \cdot 5H_{2}O}CuSO4​⋅5H2​O.
  3. Heating a hydrated salt drives off its water of crystallisation as steam and leaves the anhydrous salt.
  4. Adding water to the anhydrous salt turns it back into the hydrated salt.
  5. The hydrated and anhydrous forms of a salt often have different colours, so you can see the reaction change direction.
  6. Removing the water of crystallisation is a chemical change, because the water is chemically combined and a substance with a different formula is formed.

Copper(II) sulfate

  1. Hydrated copper(II) sulfate, CuSO4⋅5H2O\mathrm{CuSO_{4} \cdot 5H_{2}O}CuSO4​⋅5H2​O, forms blue crystals.
  2. Heating the blue crystals removes the water of crystallisation and leaves a white powder of anhydrous copper(II) sulfate, CuSO4\mathrm{CuSO_{4}}CuSO4​.
  3. Adding water to the white anhydrous copper(II) sulfate turns it blue again.
  4. The reversible reaction is CuSO4⋅5H2O⇌CuSO4+5H2O\mathrm{CuSO_{4} \cdot 5H_{2}O \rightleftharpoons CuSO_{4} + 5H_{2}O}CuSO4​⋅5H2​O⇌CuSO4​+5H2​O.
  5. Heating makes the reaction go in the forward direction, from blue to white.
  6. Adding water makes the reaction go in the reverse direction, from white to blue.
  7. The reverse reaction releases thermal energy, so the white powder becomes noticeably hot when water is dripped onto it.
  8. Because anhydrous copper(II) sulfate turns blue with water, it is also used to test whether a liquid contains water.

Cobalt(II) chloride

  1. Hydrated cobalt(II) chloride, CoCl2⋅6H2O\mathrm{CoCl_{2} \cdot 6H_{2}O}CoCl2​⋅6H2​O, is pink.
  2. Heating it removes the water of crystallisation and leaves blue anhydrous cobalt(II) chloride, CoCl2\mathrm{CoCl_{2}}CoCl2​.
  3. Adding water to the blue anhydrous cobalt(II) chloride turns it pink again.
  4. The reversible reaction is CoCl2⋅6H2O⇌CoCl2+6H2O\mathrm{CoCl_{2} \cdot 6H_{2}O \rightleftharpoons CoCl_{2} + 6H_{2}O}CoCl2​⋅6H2​O⇌CoCl2​+6H2​O.
  5. Cobalt(II) chloride paper is blue when it is dry and turns pink when it touches water, so it is also used to detect water.
  6. For both compounds, heating favours the anhydrous form and adding water favours the hydrated form.
Example

Following the colour changes

  • A strip of blue cobalt(II) chloride paper is left on a damp windowsill and slowly turns pink as it takes up water.
CoCl2+6H2O→CoCl2⋅6H2O \mathrm{CoCl_{2} + 6H_{2}O \rightarrow CoCl_{2} \cdot 6H_{2}O} CoCl2​+6H2​O→CoCl2​⋅6H2​O
  • The pink paper is then warmed gently in front of a heater and turns blue again as the water is driven off.
CoCl2⋅6H2O→CoCl2+6H2O \mathrm{CoCl_{2} \cdot 6H_{2}O \rightarrow CoCl_{2} + 6H_{2}O} CoCl2​⋅6H2​O→CoCl2​+6H2​O
  • The same substance changes back and forth because the reaction is reversible, and the conditions decide which direction it goes.
CoCl2⋅6H2O⇌CoCl2+6H2O \mathrm{CoCl_{2} \cdot 6H_{2}O \rightleftharpoons CoCl_{2} + 6H_{2}O} CoCl2​⋅6H2​O⇌CoCl2​+6H2​O
Practical

Reversing the dehydration of copper(II) sulfate

  • Aim: to show that the dehydration of hydrated copper(II) sulfate is reversible by heating the blue crystals and then adding water back to the white powder.
  • Apparatus: boiling tube, bung fitted with a delivery tube, clamp and stand, Bunsen burner, heat-resistant mat, spatula, test tube standing in a beaker of ice-cold water, dropping pipette, thermometer, hydrated copper(II) sulfate crystals and eye protection.
  • Method, heating the crystals:
    • Place about two spatulas of blue hydrated copper(II) sulfate in a clean, dry boiling tube.
    • Clamp the boiling tube near its mouth so that it slopes slightly downwards towards the bung, which stops condensed water running back onto the hot glass and cracking it.
    • Fit the bung and lead the delivery tube into the test tube standing in ice-cold water, so that the steam condenses there.
    • Heat the crystals gently at first, then more strongly, moving the flame along the tube until no blue colour remains.
    • Watch drops of colourless liquid collect in the cooled test tube.
    • Lift the delivery tube out of the collecting tube before you stop heating, so that cold liquid cannot be sucked back into the hot boiling tube.
  • Method, adding the water back:
    • Leave the boiling tube to cool completely on the heat-resistant mat.
    • Push a thermometer into the white powder and note the temperature.
    • Use a dropping pipette to add the collected liquid to the white powder a few drops at a time.
    • Record the colour change and the highest temperature reached.
  • Results: the blue crystals turn into a white powder as water is driven off, and adding the water back turns the powder blue again and makes it hot.
  • Watch out: a powder that still has blue patches has not been fully dehydrated, so keep heating until it is white all the way through.
  • Safety: wear eye protection, take care to avoid suck-back, point the boiling tube away from everyone, and wash your hands after the practical because copper(II) sulfate is harmful if swallowed and irritates the eyes.

Describing the direction of change

  1. When you describe a reversible reaction, state the condition that is changed and the direction in which the reaction then goes.
  2. Heating hydrated copper(II) sulfate makes the reaction go forwards, giving white anhydrous copper(II) sulfate and water.
  3. Adding water to anhydrous cobalt(II) chloride makes the reaction go backwards, giving pink hydrated cobalt(II) chloride.
  4. The colour change is the evidence for the direction of the reaction.
Exam technique

Describing hydrated and anhydrous compounds

  • Give both the starting colour and the final colour, for example “blue to white” when hydrated copper(II) sulfate is heated.
  • Use the terms hydrated and anhydrous accurately, and include the Roman numeral in each name.
  • When you write the equation, use ⇌\rightleftharpoons⇌ and not a single arrow.
  • When you are asked how to reverse the change, state the condition precisely, such as “add water” or “heat”.
Common Mistake
  • Do not mix up the colours, because copper(II) sulfate is blue when hydrated but cobalt(II) chloride is blue when anhydrous.
  • Do not describe dehydration as melting or drying, because the water is chemically combined in the crystals.
  • Do not say that adding water to anhydrous copper(II) sulfate dissolves it, because the colour change shows water combining with it to form the hydrated salt.
Self review
  • What does the symbol ⇌\rightleftharpoons⇌ show?
  • What is the colour change when hydrated copper(II) sulfate is heated?
  • What is the colour of anhydrous cobalt(II) chloride?
  • What happens to the temperature when water is added to anhydrous copper(II) sulfate?
  • Which condition makes the reaction for cobalt(II) chloride go in the reverse direction?
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6.3.1 Reversible reactions Revision Guide

  1. IGCSE
  2. /Chemistry
  3. /6.3.1 Reversible reactions

Revision notes for CIE IGCSE Chemistry 6.3.1 Reversible reactions: explanations and worked examples.

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