equilibrium 2 reaction yields. equilibrium very few reactions proceed unhindered to completion. some...
TRANSCRIPT
Equilibrium
• Very few reactions proceed unhindered to completion.
• Some begin reversing as soon as products are present.
• Examples of reversible reactions– Melting ice block
• H2O (s) H2O (l)
– Ni-Cad rechargeable batteries
Equilibrium
• Chemical reactions that consist of two opposing processes (forward and reverse reactions) will eventually reach an equilibrium.
• The state of equilibrium is characterized by the forward and reverse reactions proceeding at the same rate
• i.e. reactions do not stop ‑ we have a dynamic situation
Dynamic Equilibrium
• Characterized by the following criteria1. amounts and concentrations of substances
remain constant
2. total gas pressure remains constant
3. temperature remains constant
4. the reaction is incomplete (all substances involved in the reaction are present)
Equilibrium
rate
timeEquilibrium first established
N2 + 3H2 2NH3
2NH3 N2 + 3H2
Variation of the rates of the forward and reverse reactions with time
The Equilibrium Law
• For the general equilibrium xA + yB pC + qD
• It can be stated
= constant = K[C]p [D] q
[A]x [B]y
Equilibrium Law
• K allows for the evaluation of the concentration fraction at any time.
• When the system is at equilibrium the concentration fraction is constant ‑ so called the equilibrium constant (K).
• For a particular reaction, K is constant for all equilibrium mixtures (provided temperature remains constant)
Information From The Equilibrium Constant
• If K is about 104 to 10–4 there will be significant amounts of both reactants and products present at equilibrium
• If K is very large (> 104) the equilibrium mixture consists mostly of products
• If K is very small (< 10–4 ) the equilibrium mixture consists mostly of reactants
Le Chatelier's Principle
• Whenever a change is made to a system at equilibrium, the equilibrium position will shift to partially oppose the change
Disturbing Equilibrium
• There are 4 major means of disturbing a system at equilibrium
1. Adding or removing a reactant or product
2. Changing the pressure by changing the volume (gases only)
3. Dilution (for solutions only)
4. Changing the temperature
Disturbing Equilibrium
• Addition of a catalyst will increase both the rate of the forward and reverse reactions equally
• It will simply reduced the time taken to reach equilibrium.
Effect of Temperature on Equilibria
• As temperature INCREASES– For exothermic reactions, value of K decreases
and amounts of products decrease– For endothermic reactions, value of K increases
and amounts of products increase
Effect of Temperature on Equilibria
• The value of K depends on temperature• When stating a value of K, the temperature
at which the constant was calculated must also be stated
• Temperature is the only change that can be made to a system at equilibrium that will actually change the equilibrium constant (ie K is temperature dependant)
Effect of Adding Nitrogen
• Causes the rate of the forward reaction to increase
• More ammonia is formed [NH3] increases
• This causes the rate of the back reaction to increase to re form more N2 and H2
Effect of Adding Nitrogen
timeInitialequilibrium
Nitrogen added
System returns to equilibrium
[NH3]
[N2]
[H2]
conc
entr
atio
n
Effect of Adding Nitrogen
timeInitialequilibrium
Nitrogen added
System returns to equilibrium New equilibrium
established
[NH3]
[N2]
[H2]
conc
entr
atio
n
Effect of Adding Hydrogen
timeInitialequilibrium
Hydrogen added
System returns to equilibrium
[NH3]
[N2]
[H2]
conc
entr
atio
n
Effect of Adding Hydrogen
timeInitialequilibrium
Hydrogen added
System returns to equilibrium New equilibrium
established
[NH3]
[N2]
[H2]
conc
entr
atio
n
Effect of Adding Ammoniaco
ncen
trat
ion
timeInitialequilibrium
Ammonia added
System returns to equilibrium
[NH3]
[N2]
[H2]
Effect of Adding Ammoniaco
ncen
trat
ion
timeInitialequilibrium
Ammonia added
System returns to equilibrium New equilibrium
established
[NH3]
[N2]
[H2]
Effect of Changing Reactant / Product
• Addition of Reactant leads to more Products being formed (Nett Forward Reaction)
• Addition of Product leads to more Reactants being formed (Nett Back Reaction)
• Removal of Reactant leads to less Products being formed (Nett Back Reaction)
• Removal of Product leads to less Reactants being formed (Nett Forward Reaction)
Changing Pressure
• Pressure can be changed by increasing or decreasing the volume of the container while keeping the temperature constant.
• Need to examine 2 examples
Changing Pressure
• 2SO2(g) + O2(g) 2SO3(g)
– 3 gas particles 2 gas particles
• A nett forward reaction – involves a reduction in the number of gas particles,
– so a reduction in pressure
• A nett back reaction– Involves an increase in the number of gas particles
– So an increase in pressure
Changing Pressure
• 2SO2(g) + O2(g) 2SO3(g)
– 3 gas particles 2 gas particles
• Using Le Chatelier’s Principle• An increase in pressure will lead to
– Be adjusted by a reduction in pressure– A nett forward reaction will occur increasing
the amount of sulphur trioxide present at equilibrium
Changing Pressure•2SO2(g) + O2(g) 2SO3(g)
SO2 1
O2 1
SO3 5
TOTAL 7
Increased pressureNett forward
reaction
Changing Pressure
• N2O4(g) 2NO2(g)
– 1 gas particles 2 gas particles– Colourless Brown
• A nett forward reaction – involves an increase in the number of gas particles, – so an increase in pressure
• A nett back reaction– Involves a decrease in the number of gas particles– So a decrease in pressure
Changing Pressure
• N2O4(g) 2NO2(g)
• An equilibrium mixture of the gases was compressed
• Initially darkened - [NO2] increases
• Then colour of gas mixture fades– Nett backward reaction
Changing Pressure
• N2O4(g) 2NO2(g)
conc
entr
atio
n
Increase of pressure
Initial equilibrium
time
[N2O4]
[NO2]
Changing Pressure
• N2O4(g) 2NO2(g)
conc
entr
atio
n
Increase of pressure
Initial equilibrium
System returns to equilibrium
time
[N2O4]
[NO2]
Changing Pressure
• N2O4(g) 2NO2(g)
conc
entr
atio
n
timeIncrease of pressure
Initial equilibrium
System returns to equilibrium
New equilibrium established
[N2O4]
[NO2]
Adding an inert gas
• Total pressure of equilibrium system can be changed without changing the volume of the container by adding an inert gas
• There is no increase in concentrations of reactants or products
• No change in equilibrium
Dilution
• When dilution occurs, a net reaction results which produces the greater number of particles
• The effect of diluting the solution by adding water is– A net reaction in the direction that produces
more particles
Dilution
• Fe3+(aq) + SCN–
(aq) Fe(SCN)2+(aq)
– 2 particles in soln 1 particle in soln
• Dilution of this equilibrium will result in a nett back reaction
• Results in an increase of [Fe3+] and [SCN–]
Change in Temperature
• Using Le Chatelier’s Principle• Exothermic reaction can be written as
– Reactants Products + energy– Heating increases the energy of the substances– Principle says the reaction will oppose an
increase in energy by removing energy– A nett back reaction occurs– Less product and more reactants now present
Change in TemperatureExothermic
A + B C + D
conc
entr
atio
n
[A]
[B]
[C]
[D]
Initial equilibrium
time
Change in TemperatureExothermic
A + B C + D
conc
entr
atio
n
[A]
[B]
[C]
[D]
Initial equilibrium
Temperature increases
Temperature increases
System returns to equilibrium
time
Change in TemperatureExothermic
A + B C + D
time
conc
entr
atio
n
[A]
[B]
[C]
[D]
Initial equilibrium
Temperature increases
Temperature increases
System returns to equilibrium
New equilibrium established
Change in TemperatureEndothermic
A + B C + D
conc
entr
atio
n
[A]
[B]
[C]
[D]
Initial equilibrium
time
Change in TemperatureEndothermic
A + B C + D
conc
entr
atio
n
[A]
[B]
[C]
[D]
Initial equilibrium
Temperature increases
Temperature increases
System returns to equilibrium
time
Change in TemperatureEndothermic
A + B C + D
conc
entr
atio
n
[A]
[B]
[C]
[D]
Initial equilibrium
Temperature increases
Temperature increases
System returns to equilibrium
timeNew equilibrium established