Chapter 20: Reversible Reactions

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.

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.

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.

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

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

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

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