c - Reversible reactions and equilibria (copy)

3c (3.17-3.22)

Reversible reactions

Reaction where products of reaction can react with each other and convert back to original reactants

i.e. can go both ways

Symbol for reversible reactions

⇌

Example of reversible reaction

Thermal decomposition of ammonium chloride

• Ammonium chloride is white solid
  When heated, it breaks down into gases ammonia + hydrogen chloride - forward reaction

• If you let it cool, ammonia + hydrogen chloride react to re-form the solid - backward reaction

Another example is dehydration of copper(II) sulfate

Dynamic equilibrium

• If reversible reaction occurs in closed system, a state of equilibrium will always be reached

• Equilibrium = conc of reactants and products reach a certain balance and stay there
  Closed system = reactants/products can’t escape

• It is a DYNAMIC EQUILIBRIUM - meaning reactions are still taking place in both directions, but overall effect is nil because forward + reverse reactions cancel each other out

Characteristics of reaction at dynamic equilibrium

• Forward + reverse reactions occur at exact same rate

• Conc of reactants + products remain constant

Position of equilibrium

The relative amount of reactants and products

• Depends on temp + pressure of reacting mixture

• If you deliberately alter temp + pressure, you can move position of equilibrium to give more product + less reactants - position shifts to the right

Effect of temp on position of equilibrium

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

• If you raise temp, endothermic reaction increases to use up extra heat

• If you reduce temp, exothermic reaction increases to give out more heat

Effect of pressure on position of equilibrium

Most gaseous reactions have more moles of gas on one side than the other

• If you raise pressure, it encourages reaction which produces fewer moles of gas

• If you lower pressure, it encourages reaction which produces more moles of gas

Effect of catalyst on position of equilibrium

No effect because it speeds up forward + backward reaction by the same amount

Effect of catalyst on position of equilibrium

No effect because it speeds up forward + backward reaction by the same amount