20.1-Reversible Reactions

Reversible Reaction

• In some chemical reactions, the products of the reaction can react together to produce the original reactants.

• These reactions are called reversible reactions.

• They can be represented in the following way:

  • A + B ⇌ C + D

• The symbol ⇌ has two half arrowheads, one pointing in each direction.

• It is used in equations that show reversible reactions:

  • the forward reaction is the one that goes to the right

  • the backward reaction is the one that goes to the left

• The reaction mixture may contain reactants and products, and their proportions may be changed by altering the reaction conditions.

Energy changes in reversible reactions

• If a reaction is exothermic in one direction, it will be endothermic in the other direction.

• The same amount of energy is transferred in both the forwards and reverse reaction.

Copper sulfate

• Blue copper sulfate is described as hydrated.

• The copper ions in its crystal lattice structure are surrounded by water molecules.

• This water is driven off when blue hydrated copper sulfate is heated, leaving white anhydrous copper sulfate.

  • This reaction is reversible:

  • hydrated copper sulfate ⇌ anhydrous copper sulfate + water

  • CuSO4.5H2O(s) ⇌ CuSO4(s) + H2O(l)

• The forward reaction is endothermic and the reverse reaction is exothermic.

20.2-Le Chatelier’s Principle

Reversible reactions try to counteract changes

• Le Chatelier’s Principle is the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change

• It can be used to predict the effect of any changes you make to a reaction system

Such as changes to the temperature

• All reactions are exothermic in one direction and endothermic in the other

• If you decrease the temperature the equilibrium will move in the exothermic direction to produce ore heat

• This means you’ll get more products for the exothermic reaction and fewer products for the endothermic reaction

• If you raise the temperature, the equilibrium will move in the endothermic direction to try and decrease it

• You’ll now get more products for the endothermic reaction and fewer products for the exothermic reaction

Pressure

• Changing the pressure only affects an equilibrium involving gases

• If you increase the pressure, the equilibrium tries to reduce it, it moves in the direction where there are fewer molecules of gas

• If you decrease the pressure, the equilibrium tries to increase it, it moves in the direction where there are more molecules of gas

• You can use the balanced symbol equation for a reaction to see which side has more molecules of gas

Practice Questions:

• Melting of wax, freezing of ice, boiling water which evaporates as steam and condenses back to water are examples of

  • Reversible reactions

• All reactions are ____ in one way and ___in the other

  • Exothermic, endothermic

• What does changing pressure affect

  • Only affects an equilibrium involving gases

• What happens if you increase the pressure?

  • The equilibrium tries to reduce it, and it moves in the direction where there are fewer molecules of gas

• What is Le Chatelier’s Principle?

  • The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change