Reversible Reactions

1
Reversible Reactions

• Section 19.2
• Pages 539 - 544

2

• What is a completion reaction?

• A reaction where the amount of reactants
  decrease and the amount of products increase
  until there are no reactants left
• Cannot be changed back to the original reactants

3

• What is a reversible reaction?

• A reaction where the amount of reactants and
  products formed and reformed occur simultaneously
• Can be changed back to original reactants

4
Toothpick activity
5
Copy onto a sheet of paper½ R ? ?¼ P

Reactants Change Products Change
40 0




3 people / group 1 person R, 1 P and 1
recorder Exchange sticks according to rate (Round
up) Exchange ONLY what you have at each
round Record what was lost and what was gained
6
Round 1
Reactants Change Products Change
40 0
20 -20, 0 20 20, -0
15 -10, 5 25 10, -5
14 -8, 7 27 8, -8
14 -7, 7 27 7, -7
7
Round 2
Reactants Change Products Change
40 20
25 -20, 5 35 20, -5
21 -13, 9 39 13, -9
20 -11, 10 40 11, -10
20 -10, 10 40 10, -10
8
Round 3
Reactants Change Products Change
0 60
15 -0, 15 45 0, -15
18 -8, 11 42 8, -11
20 -9, 11 40 9, -11
20 -10, 10 40 10, -10
9
Equilibrium is Everywhere! It is why change
happens in the universe

• All processes move in the direction of
  equilibrium or INCREASED ENERGY DISPERSAL
  (INCREASED ENTROPY)
• Knowledge of thermodynamics allow us to predict
  the direction a process will move and the
  composition of the equilibrium state

10
At equilibrium

• Rate of forward reaction rate of backward
  reaction (not necessarily the concentrations)
• There is no net change in the actual amounts of
  the components
• Under the same conditions, at equilibrium the
  concentrations of both reactants and products
  remain constant
• At the same temp. the eq. constant is constant
  (will discuss on Friday)
• Eq. may be approached from different starting
  points
• At other temps. the value of the eq. constant
  differs (Friday)

11
When the rate of the formation of products equals
the rate of reformation of reactants

• Equilibrium

• LeChateliers Principle If a stress is
  applied to a system at equilibrium, the reaction
  will shift in a way to relieve the stress

In other words there will be a greater
concentration of either products or reactants
12
What Affects Equilibrium?

• Temperature

• Concentration

• Pressure

13

• N2O4 heat ?? 2NO2
• (light) (dark)
• Which way will the reaction shift if
• The reaction is heated?
• The reaction is cooled?
• The concentration of NO2 is increased?
• The pressure is increased? (an increase in
  pressure
• will shift the reaction to the side w/ the
  fewer moles of gas)

Shift towards the products
Shift towards the reactants
Shift towards the reactants
Shift towards the reactants
14
Try this one CoCl4-2 4H2O ??
Co(H2O)62 4Cl- heat
Stress Response
Apply heat
Cool
Add Water
Add Ag
Note AgCl will form a precipitate, so Ag
ions will use up the Cl- ionstherefore, the
products side will have less and the reaction
will shift toward the products