Chemical Equilibria Flashcards
Reversible reaction
A reversible reaction is one which can proceed in both the forward and reverse
directions.
Dynamic equilibrium
A dynamic equilibrium refers to a reversible reaction where the forward and reverse
reactions are continuing at the same rate and the concentrations of the reactants and
products are constant.
Characteristics of dynamic equilibrium
- The concentrations of the reactants and products are constant under given conditions of temperature, pressure and initial amount of substance
- Equilibrium can be achieved in either direction
- Equilibrium can only be achieved in a closed system (absolutely no exchange of material with the surroundings).
On the other hand, an open system may allow matter to escape or to enter which will not allow the system to reach equilibrium.
Types of equilibria
- Homogenous equilibria
○ Reaction mixture consists of reactants and products which are in the same phase/physical state- Heterogeneous equilibria
○ Reaction mixture consists of reactants and products which are in different phases/physical states
- Heterogeneous equilibria
In what cases is the concentration excluded in the Kc expression
(i) concentration of water (for equilibria in aqueous medium; water is a solvent)
When water is present as a solvent, we assume the [H2O] remains practically constant
at 1.00 g cm−3 or 55.5 mol dm−3
. This is much larger than the concentrations of the
solutes and thus we can assume that [H2O] does not change significantly throughout
the reaction.
(ii) concentration of solids
Concentrations for a solid is related to their densities (which are constants).
(iii)concentration of immiscible liquids (for heterogeneous equilibrium only)
If an immiscible liquid is a reactant or product, its concentration is also related to its
density which is a constant and is not included in the equilibrium constant expression.
General Kc
Kc =
product of concentration of products at equilibrium raised to the appropriate powers/
product of concentration of reactants at equilibrium raised to the appropriate powers
where the appropriate power is the stoichiometric coefficient of that species in the equation
for the reaction.
Equilibrium Constant in terms of Partial Pressure, Kp
- Like Kc, the equilibrium constant, Kp, also has a constant value at a given temperature.
- The units for Kp are dependent on the balanced equation.
- Concentration of solids and liquids are NOT included in the expression for Kp.
SIGNIFICANCE OF THE VALUES OF Kc AND Kp
Value of Kc (or Kp) is large (10^5):
More products than reactants are present.
Forward reaction is almost complete.
Equilibrium position lies to the right.
More reactants than products are present.
Forward reaction does not proceed to any
appreciable extent.
Equilibrium position lies to the left.
Position of equilibrium is dependent on standard Gibbs free energy of reaction
When equilibrium is reached,
∆GƟ = – RT ln K (where K = Kc).
Recall that ∆GƟ of a reaction indicates the thermodynamic spontaneity of a reaction.
The more negative the ∆GƟ, the more spontaneous the reaction is likely to be.
Kc is large:
∆GƟ is a negative value
Reaction is spontaneous in the forward reaction.
Position of equilibrium lies more to the right hand side of the reaction equation.
Kc is small:
∆GƟ is a positive value
Reaction is not spontaneous
in the forward reaction.
(It is spontaneous in the
backward reaction)
Position of equilibrium lies more to the left hand side of the reaction equation.
Kc is 1:
∆GƟ = 0
Both forward and backward
reactions are at
equilibrium.
Concentrations of both
reactants and products are the same.
A particular value of K is constant for a given temperature. This means that K is only affected
by temperature changes and not changes in concentration of reactants or products or even with
the use of a catalyst.
Le Chatelier’s Principle
Le Chatelier’s Principle states that when a system in equilibrium is disturbed, the system
will shift the equilibrium position to counteract the change imposed to re-establish the
equilibrium.
The position of equilibrium is disturbed when the following changes is made to a system in equilibrium:
* change in concentration or partial pressure of the reactants or products;
* change in total pressure by varying the volume (or adding an inert gas);
* change in temperature.
Addition of catalyst
- A catalyst increases the rate of a reaction by providing an alternative reaction pathway having
lower activation energy, Ea. - It remains chemically unchanged at the end of the reaction.
- It will speed up both the forward and backward reactions by the same extent, and so it has no
effect on the equilibrium position and the value of the equilibrium constant, K. But it will enable
the equilibrium state to be reached more quickly (i.e., reaction reaches equilibrium earlier).
3 most important factors to consider to design an efficient procedure for the process
- Rate of reaction
The rate of conversion from the reactants to the products must be as fast as possible. - Yield of the product
The reaction must occur to produce as much of the required products as possible (high percentage
yield). - Economic (Production cost)
The cost can be minimised by
a) using cheap reactants (air, water)
b) using a catalyst to increase the reaction rate
c) avoid very high temperature and pressure to reduce operating and maintenance cost of
equipment
d) recycling of the unreacted raw materials
Haber process
The Haber Process is an example of industrial application of chemical equilibrium.
3H2(g) + N2(g) ⇌ 2NH3(g) H = −92.0 kJ mol−1
- According to Le Chatelier’s Principle, high pressure favours a high yield of NH3(g), since the
equilibrium position will shift to the right to reduce the total number of moles of gas molecules.
However, a very high pressure will incur high maintenance and operating costs. - As the production of ammonia (forward reaction) is exothermic, so a low temperature favours a
high yield of NH3(g), as the equilibrium position will shift to the right to produce heat. However, a
low temperature would mean a slow reaction, i.e. it takes a long time to reach equilibrium.
Therefore, a moderately high temperature alongside the use of a catalyst is needed for an optimal
rate of reaction.
Bearing in mind all these considerations, the optimal conditions used for the Haber Process are:
450 °C, 200 atm and an iron catalyst.
