CIE IGCSE Chemistry

Revision Notes

7.2 Reversible Reactions

Reversible Reactions

Reversible reactions

  • Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when all of the reactants are used up
  • In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again
  • It is said that the reaction can occur in both directions: the forward reaction (which forms the products) and the reverse direction (which forms the reactants)

 

Chemical equations for reversible reactions

  • When writing chemical equations for reversible reactions, two arrows are used to indicate the forward and reverse reactions
  • Each one is drawn with just half an arrowhead – the top one points to the right, and the bottom one points to the left

Example

  • The reaction for the Haber Process which is the production of ammonia from hydrogen and nitrogen:
N2 + 3H2 ⇌ 2NH3

 

Hydrated and anhydrous salts

  • Hydrated salts are salts that contain water of crystallisation which affects their molecular shape and colour
  • Water of crystallisation is the water that is stoichiometrically included in the structure of some salts during the crystallisation process
  • A common example is copper(II) sulfate which crystallises forming the salt copper(II) sulfate pentahydrate, CuSO4.5H2O
  • Water of crystallisation is indicated with a dot written in between the salt molecule and the surrounding water molecules
  • Anhydrous salts are those that have lost their water of crystallisation, usually by heating, in which the salt becomes dehydrated

 

Dehydration of Hydrated Copper (II) Sulfate:

Hydrated Copper (II) Sulfate ⇌ Anhydrous Copper (II) Sulfate     +     Water

 

Dehydration-of-hydrated-Copper-II-Sulfate, IGCSE & GCSE Chemistry revision notesDiagram showing the dehydration of Hydrated Copper (II) Sulfate

 

Explanation:

  • When anhydrous copper (II) sulfate crystals are added to water they turn blue and heat is given off (exothermic); this reaction is reversible
  • When Copper (II) Sulfate crystals are heated in a test tube, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube
  • The form of Copper (II) Sulfate in the crystals is known as Hydrated Copper (II) Sulfate because it contains water of crystallisation
  • When Hydrated Copper (II) Sulfate is heated, it loses its water of crystallisation and turns into anhydrous Copper (II) Sulfate:
CuSO4.5H2O (s) ⇌ CuSO4 (s) + 5H2O (l)

 

Dehydration of Hydrated Cobalt (II) Chloride:

Hydrated Cobalt (II) Chloride ⇌ Anhydrous Cobalt (II) Chloride + Water

 

Dehydration-of-Hydrated-Cobalt-II-Chloride, IGCSE & GCSE Chemistry revision notesDiagram showing the dehydration of Hydrated Cobalt (II) Chloride 

 

Hydration of Cobalt(II) Chloride

  • When anhydrous blue cobalt(II) chloride crystals are added to water they turn pink and the reaction is reversible
  • When the cobalt(II) chloride crystals are heated in a test tube, the pink crystals turn back to the blue colour again as the water of crystallisation is lost
  • The form of cobalt(II) chloride in the crystals that are pink is known as hydrated cobalt (II) chloride because it contains water of crystallisation
  • When hydrated cobalt(II) chloride is heated, it loses its water of crystallisation and turns into anhydrous cobalt(II) chloride:
CoCl2.6H2O (s) ⇌ CoCl2 (s) + 6H2O (l)

Exam Tip

Both the hydration of CoCl2 and  CuSO4 are chemical tests which are commonly used to detect the presence of water. You should remember the equations and colour changes:

  • CoCl2 + 6H2O ⇌ CoCl2.6H2O Blue to pink
  • CuSO4 + 5H2O ⇌ CuSO4.5H2O White to blue
Extended Only

The Concept of Equilibrium

Reversible reactions and equilibrium

  • We have already seen that a reversible reaction is one that occurs in both directions
  • When during the course of reaction, the rate of the forward reaction equals the rate of the reverse reaction, then the overall reaction is said to be in a state of equilibrium

Characteristics of a reaction at equilibrium

  • It is dynamic eg the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate
  • The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)
  • It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel

 

Equilibrium in open & closed systems, IGCSE & GCSE Chemistry revision notesEquilibrium can only be reached in a closed vessel which prevents reactants or products from escaping system

 

The reaction between H2 and N2  in the Haber process

  • When only nitrogen and hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
  • As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease, so the rate of the forward reaction will decrease
  • However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase (ammonia will decompose to reform hydrogen and nitrogen)
  • Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:
3H2 (g) + N2 (g) ⇌ 2NH3 (g)

 

Dynamic-Equilibrium, IGCSE & GCSE Chemistry revision notesDiagram showing when the rates of forward and backward reactions become equal

 

The position of equilibrium

 

  • Equilibrium position refers to the relationship between the concentration of reactants and products at the equilibrium state
  • When the position of equilibrium shifts to the left, it means the concentration of reactant increases
  • When the position of equilibrium shifts to right, this means the concentration of product increases

Effect of catalyst on equilibrium position

  • The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached
  • This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)
  • As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst

 

Effect catalyst on equilibrium position, IGCSE & GCSE Chemistry revision notesDiagram showing the effect of catalyst on equilibrium position

Extended Only

Le Chatelier’s Principle

  • Le Chatelier’s Principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change.
  • The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration. 

 

Effects of temperature

Effects of temperature table, IGCSE & GCSE Chemistry revision notes

 

Example:

Iodine Monochloride reacts reversibly with Chlorine to form Iodine Trichloride

ICl              +              Cl2              ⇌             ICl3

Dark Brown                                                         Yellow

When the equilibrium mixture is heated, it becomes dark brown in colour. Explain whether the backward reaction is exothermic or endothermic:

  • Equilibrium has shifted to the left as the colour dark brown means that more of ICI is produced
  • Increasing temperature moves the equilibrium in the endothermic direction
  • So the backward reaction is endothermic

 

Effects of pressure

Effects of pressure table, IGCSE & GCSE Chemistry revision notes

 

Nitrogen Dioxide can form Dinitrogen Tetroxide, a colourless gas

2NO2             ⇌              N2O4

Brown Gas                    Colourless Gas

Predict the effect of an increase in pressure on the position of equilibrium:

    • Number of molecules of gas on the left =    2
    • Number of molecules of gas on the right =    1
  • An increase in pressure will cause equilibrium to shift in the direction that produces the smaller number of molecules of gas
  • So equilibrium shifts to the right

 

Effects of concentration

 

Example:

Iodine Monochloride reacts reversibly with Chlorine to form Iodine Trichloride

ICl              +              Cl2              ⇌             ICl3

Dark Brown                                                         Yellow

Predict the effect of an increase in concentration on the position of equilibrium:

  • An increase in the concentration of ICl or Cl2 causes the equilibrium to shift to the right so more of the yellow product is formed
  • A decrease in the concentration of ICl or Cl2 causes the equilibrium to shift to the left so more of the dark brown reactant is formed

Exam Tip

When the conditions at equilibrium are changed, the system always responds by doing the opposite.

For example if the concentration is increased the system tries to reduce it by changing the direction of the reaction or if the temperature is increased the system will try to reduce the temperature by absorbing the extra heat.

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now