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Introduction
Equilibrium is the state of a process in which the system’s parameters, such as temperature, pressure, and concentration, remain constant across time.
If a reaction is in chemical equilibrium, the rate of forward and backward reactions will be the same.
However, reaction factors like temperature, solvent, and ionic strength can affect the value of the equilibrium constant.
A reactant and product mixture in equilibrium is referred to as an equilibrium mixture.
Chemical reactions and physical processes can both reach equilibrium.
Types of Equilibrium
Physical Equilibrium
When opposing processes in equilibrium involve simply physical changes, the equilibrium is referred to as physical equilibrium.
For instance, with the evaporation of water, there is an equilibrium between water and vapor.
Vaporized water Physical Equilibrium Characteristics –
- It is only conceivable in a closed system under specific parameters such as temperature, pressure, and so forth.
- The rate of forward and backward reactions is the same.
- All of the system’s measured attributes stay constant.
- When a physical process reaches equilibrium, one of its parameters maintains a constant value at a given temperature.
- At any point in time, the size of such values shows how far the physical process has progressed before achieving equilibrium.
Chemical equilibrium
When opposing processes in equilibrium entail chemical changes, the equilibrium is referred to as chemical equilibrium.
Equilibrium between acetic acid, water, acetate, and hydronium ions, for example. The equation is as follows: CH3CO2H + H2O⇔ CH3CO2 + H3O+ CH3CO2 + H3O+
Chemical Equilibrium Characteristics –
- It can be done from any angle or direction. It’s a living organism. At equilibrium, the concentrations of reactants and products stay constant.
- The presence of a catalyst can achieve this form of equilibrium.
- Ionic equilibrium is a chemical equilibrium that involves the equilibrium of ions in an aqueous solution.
Physical processes and chemical reactions can both achieve equilibrium. Dynamic equilibrium occurs when there is no change in the concentrations of reactants and products and the rate of forward reaction equals the rate of backward reaction.
Homogeneous equilibria – Reactants and products are present in the same phase in these forms of equilibrium.
Heterogeneous equilibria – Reactants and products are found in diverse phases in these types of equilibrium.
Reversible reaction – A reversible reaction is a chemical process that can occur in both forward and reverse directions under the same conditions.
Irreversible reaction – An irreversible reaction is a chemical process that only proceeds in one way.
Overall response rate – The overall reaction rate is obtained by subtracting the backward reaction rate from the forward reaction rate.
Overall reaction rate = forward reaction rate minus backward reaction rate It’s important to remember that at equilibrium, a reversible reaction’s overall rate becomes zero.
Law of mass action
In 1864, Norwegian scientists Peter wage and Cato gulberg proposed the law of crowd action.
At a given temperature, the law explains the link between the velocity of a chemical reaction and the molar concentrations of the reactants.
The rate of a chemical reaction at a given temperature is directly proportional to the product of the active masses of the reactants, according to the rule of mass action.
The term “active mass” refers to the molar concentration of a reactant per unit volume. It is measured in mol dm-3. Assume aA + bB = cC + dD for a generic reaction. ——————— (1)
where Kb = rate constant for the backward reaction at a given temperature
At equilibrium, the rate of forward reaction is equal to the rate of backward reaction. So, we can write –
For chemical reactions in equilibrium at a given temperature, Kc is referred to as the equilibrium constant or rate constant.
The product of reaction product concentrations raised to their separate stoichiometric coefficients in the balanced chemical equation divided by the product of reactant concentrations raised to their individual stoichiometric coefficients has a constant value at a given temperature.
This is referred to as the equilibrium law or chemical equilibrium law.
Constant of Equilibrium
A few key points about the equilibrium constant are stated below:
- The value of the equilibrium constant for a given reaction is always constant and is independent of the reactant concentrations with which we begin or the direction from which the equilibrium is approached.
- The value of the equilibrium constant is inverse when the reaction is reversed.
- When an equation with the equilibrium constant K is divided by 2, the resultant equation’s equilibrium constant is the square root of K (that is,√ K). When an equation with the equilibrium constant K is multiplied by 2, the new equation’s equilibrium constant is the square of K. (that is, K²)
- If an equation with an equilibrium constant K is stated in two steps (with an equilibrium constant K1 and an equilibrium constant K2),then K1 × K2 = k
- The addition of a catalyst to the process has no effect on the value of the equilibrium constant. Because the catalyst raises the speed of both the forward and backward reactions to the same extent, this is the case.
Equilibrium Constants in Practice The equilibrium constant’s key characteristics and applications are described below :
- Only when the concentrations of the reactants and products have reached a constant value at equilibrium can the equilibrium constant be calculated.
- Its value is independent of the reactant and product concentrations at the start.
- Temperature is a factor. At a particular temperature, it displays a unique value for a balanced equation.
- It is equal to the inverse of the forward reaction’s equilibrium constant for the reverse reaction. K is the equilibrium constant for a system.
Equilibrium Prediction of the Size of a Reaction Constant
If the value of Kc is large (Kc > 103) at equilibrium, the reaction will proceed close to completion or favour forward reaction.
If the value of Kc is low at equilibrium –(Kc > 10-3) Rarely will a reaction be completed, and if it is, it will favour backward response.
If the value of Kc is moderate (between 103 and 10-3), the reaction will proceed to equilibrium.
Solubility Product (ksp)
When a salt is dissolved in water then it starts breaking into ions and after sometime the solubility process attains equilibrium.
AgCl(s) ↔️ Ag+ (aq) + Cl– (aq) Ksp = [Ag+] [Cl–] = Q = I.P. Where Ksp = solubility product Q = reaction quotient (It is defined as Kc but the concentrations in Qc are not necessarily equilibrium values)
I.P. = Ionic productIf I.P. < Ksp, then forward reaction will dominate, and more salt can be dissolved If I.P. = Ksp, then saturation will take place and no more salt can be dissolved If I.P. > Ksp, then backward reaction will dominate and precipitation of solid salt will take place.
Predicting the Direction of a Reaction
- If Qc < Kc , net reaction goes from left to right. It means the reaction will proceed in the direction of reactants.
- If Qc > Kc , the net reaction goes from right to left. It means the reaction will proceed in the direction of products.
- If Q = K , no net reaction occurs. It means the reaction mixture is already at equilibrium.
Equilibrium Constant and Gibbs Free Energy (Keq and G) Relationship
If G is negative and K is 1, the reaction is spontaneous and moves ahead.
If G is positive and K is greater than one, the reaction is nonspontaneous and proceeds in a reserve or reverse direction.
When G equals 0, the reaction is in equilibrium.
The Principle of Le Chatelier
According to Le Chatelier’s principle, a change in any of the components that determine a system’s equilibrium conditions will lead the system to alter in such a way that the effect of the change is offset.
In other words, if a disturbance is put into an equilibrium mixture, it will respond in such a way that it undoes the disturbance, according to Le Chatelier’s principle.
Effect of temperature change – Low temperatures favour exothermic reactions, while high temperatures favour endothermic reactions.
Effect of pressure change – If the moles of reactants and products are the same, pressure change will have no effect.
When the pressure is increased, the reaction will move to the side with fewer moles, and vice versa. This concludes our discussion of “Equilibrium.”
Conclusion
The equilibrium constant is independent of the initial analytical concentrations of the reactant and product species in the mixture for a given set of reaction conditions.
As a result, established equilibrium constant values can be used to determine the composition of a system at equilibrium, given its beginning composition.
Physical processes and chemical reactions can both achieve equilibrium.
Dynamic equilibrium occurs when there is no change in the concentrations of reactants and products and the rate of forward reaction equals the rate of backward reaction.
