OVS SCH4U1 Chapter 21 (21.4)

Introduction to equilibrium

Define a reversible reaction

a chemical reaction that occurs both forewards (reactant —> product) and reverse (product —→ reactant) simultaneously

when do reactions go to completion? (Non-reversible reactions)

the product of the reaction is released as soon as it is formed, the reverse reaction can never be reached

What are the three types of non-reversible reaction products?

1. formation of a (insoluble) precipitate

   1. AgNO3(aq) + NaCl(aq) → NaNO3(aq) + AgCl(s)

      1. no double arrow

2. formation of a gas

   1. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

      1. open system

3. formation of a molecular compound (water)

   1. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

      1. dissociation constant

However if we consider the net ionic equation Ag+ + Cl− → AgCl, then the reverse reaction would be AgCl → Ag+ + Cl−.

The Keq for the reverse reaction is 1.8 × 10−10. For all practical purposes, the reaction goes to completion.

Silver nitrate also forms a precipitate with NaI. What would this precipitate be?

Would the reaction between HBr and KOH also be considered irreversible?

Define Chemical equilibrium

state of a system where the rate of the foreward reaction = rate of the reverse reaction

equilibrium between reactants and products

what are the five conditions of a system at equilibrium are:

1. system must be closed

   1. no new material in, no removal of material

   2. kept constant

2. system must remain at the same temperature and pressure

3. equilibrium is a dynamic process

   1. forward and reverse are taking place

4. The rates of the forward and reverse reaction must be equal

5. reactants =/= products

   1. once equilibrium is attained, the amount of product and reactant will be constant

phase equilibrium vs solution equilibrium

phase equilibrium

• a substance is in equilibrium between two states

solution equilibrium

• a solid substance isn in a saturated solution

  • rate of dissolution = recrystallization