reversible reactions – shown as:

the rate of the forward reaction = the rate of the reverse reaction concentrations of the substances in the equilibrium mixture remain constant (T const)

 Equilibrium can only be reached in a closed system  The position of equilibrium can be achieved from either direction

 3H 2 (g) + N 2 (g)  2NH 3 (g)

 Page 2 of notes

3H 2 (g) + N 2 (g) 2NH 3 (g)  State expression for K c  Insert units in for each term  Cancel as appropriate  3H 2 (g) + N 2 (g) 2NH 3 (g)

 Write the expression for K c  Check units are moles per litre (mol dm -3 )  Insert values  Determine units  See notes page 3&4

 Page 5 of notes  These are moles - to be converted into concentration need to divide by volume

 NO!  BUT  Equilibrium will shift so that the equilibrium constant DOES stay the same.

 E.g.  A = B + C ΔH = - 57 kj mol -1  What happens to the eq m position and value of K if [B] is increased?

 E.g.  A = B + C ΔH = - 57 kj mol -1  What happens to the eq m position and value of K if [B] is increased?  Equilibrium shifts to the left  K stays the same

 NO!  Changing pressure causes a change in concentration in mol per litre.  Equilibrium must shift to keep K the same if moles of gas change.

 YES!  This is the only factor that does affect K  If temp increased then equilibrium shifts in endothermic direction (to minimise change)  This has corresponding effect on K c

 If temp increased then equilibrium shifts in endothermic direction  E.g.  A = B + C ΔH = - 57 kj mol -1  What happens to the eq m position and value of K if temp is increased?

 A catalyst will increase the rate of forward and backward reactions by the same amount.  Position of equilibrium unaffected BUT increased rate has great industrial significance.

 If K c is large then equilibrium lies to the right  i.e. lots of products relative to reactants (not worth making expensive changes)  Small K c = small conversion rate – may be financially viable to operate at high pressures and temperatures  Sometimes exothermic reactions are operated at elevated temps – why?

 See q’s and examples on pages 7&8  Followed by questions pages 10-14