Chemical Equilibrium
Middle of collage
Chemical Equilibrium is depicted by a Physical Balance containing stop watch in left pan while measuring cylinder is in right pan. Balance is symbol of contentment and one rejoices after attaining equilibrium in any aspect of life.
The question is what do we understand by chemical equilibrium.
A Û B
Many chemical reactions give rise to products which are capable of reacting in such a way as to give back the original reactants:
For example, at 20 ºC, two molecules of the gas NO2 can react to form one molecule of another gas, N2O4. But this gas in turn is not very stable, and can dissociate at 20 ºC into two molecules of NO2.
On top of physical balance there is shown example of physical change
NaCl(s) → NaCl(aq)
Rate of conversion of NaCl(s) to NaCl(aq) Û Rate of conversion of NaCl(aq) to NaCl(s)
This is equilibrium is shown in figure below, a balance between forward and reverse reactions. In a sense, this means that a chemical reaction is never “done”; it never stops; it simply gets to a point where the rate of the forward reaction is equal to the rate of the reverse reaction
When the reaction is in equilibrium it means rate of forward reaction = rate of backward reaction
Even though rate of forward reaction = rate of backward reaction ,Concentration of component on both sides may not be equal.
Top of collage
There are two graphs of concentration vs Time to show chemical equilibrium for reaction AóB
In experiment (i)
Where B = NO2, A= N2O4
B = 100%, A = 0% at t = 0 kinetics in green region is dominated by reaction B(g)àA(g).When enough A forms the rate of B(g)→A(g),the forward reaction dominates. The net reaction still favours production of A for a short time..At 60 sec the reaction A(g) Û B(g) attains equilibrium level (Pink colour)
The same experiment can be summarized as follows
1. B(g) → A(g)
2. At t =0, B =100%, A =0%
3. As the reaction progresses
4. .[B] decreases and attains constant value
5. .[A] increases from zero to a constant value
6. As [A] and [B] are constant , Equilibrium is achieved
A « B
The double arrow implies a dynamic process.
K = Equilibrium constant = [Product]/ [Reactant]
.Larger K means more product at equilibrium
.Smaller K implies more reactants at equilibrium.
In (ii) experiment
.A = 100 % , B = 0 % At t=0.0
(A(g) → B(g)] with passing of time ,A begins to undergo conversion to B.
Conversion from A to B is faster than B to A till it reaches 60 sec.(It is shown by small and big arrow)
Pink colour portion of graph shows A equilibrium with B
In middle of collage are shown five flasks
Above figure shows five flask containing mixture of cis and trans difluroethane
1.80%cis +20% trans difluoroethene
2. 20%cis +80% trans difluoroethene
3 100% cis difluoroethene
4. 100% trans difluoroethene
Experiment (iii)
Four flasks containing mixture of different percentage composition of cis-and-trans- difluoroethene, each is heated to 623 K, it eventually changes into an equilibrium mixture of the isomers with the ratio [trans-C2H2F2]/[cis-C2H2F2] is always 0.5 at 623 K.33.3% of the trans isomer and 66.7% of the cis isomer. Equilibrium mixture is shown in fifth flask. (figure above).
Characterstics of reaction which is in chemical equilibrium
1.Concentration of species participating in the equilibrium does not change.
2.A chemical equilibrium can always be attained by approaching the equilibrium from more than one direction.
The above figure shows that for reaction , N2(g) + H2(g) <=> NH3(g) ,whichever direction we may start, equilibrium can be reached from either direction
.3.Presence or absence of catalyst does not affect the equilibrium concentrations of reactants .
4At constant Temperature the concentrations of various species involved in equilibrium mixture can be expressed by mathematical relationship.
These above mentioned four charactersics of chemical equilibrium can be expressed by following experiments which are monitored as Concentration vs Time(graph)
I close my collage lesson with a quote : Old chemist never die they just attain equilibrium.
Take home message is ;chemical equilibrium collage is in shape of bat which is a vertebrate animal,but still flies in sky by making equilibrium. This gives us the message that like bat always try to make equilibrium in life.