Chemical Equilibrium Formulas for JEE

Chemical equilibrium occurs in reversible reactions when the rates of the forward and reverse reactions are equal, leading to a stable state where the concentrations of reactants and products remain constant over time. At equilibrium, the system continues to react, but there's no net change in the concentrations of substances involved.

1.0Equilibrium Constant   

At equilibrium, the concentrations of reactants and products can be described by the equilibrium constant (K), which is the ratio of the concentrations of products to reactants, with each concentration raised to the power of its coefficient in the balanced chemical equation.

For a general reaction:

$aA+bB\rightleftharpoons cC+dD$

The equilibrium constant expression would be:

$K=\frac{[C{]}^c[D{]}^d}{[A{]}^a[B{]}^b}$

Where:

• K is the equilibrium constant.
• a,b,c, and d are the stoichiometric coefficients of the balanced chemical equation.
• [A],[B],[C], and [D] represent the molar concentrations of the respective substances at equilibrium.

The equilibrium constant (K) remains constant at a given temperature for a specific reaction. It characterizes the position of equilibrium and provides information about the relative concentrations of reactants and products when the reaction reaches equilibrium.

2.0Reaction Quotient (Q)

The reaction quotient (Q) is similar to the equilibrium constant but is calculated using concentrations at any point during a reaction, not just at equilibrium. It helps determine the direction a reaction will proceed to reach equilibrium.

$Q=\frac{[\mathrm{C}{]}_{\mathrm{t}}^c[\mathrm{D}{]}_{\mathrm{t}}^d}{[\mathrm{A}{]}_{\mathrm{t}}^a[\mathrm{B}{]}_{\mathrm{t}}^b}$

Relationship between Equilibrium Constant and Reaction Quotient

• For a reaction at equilibrium, Q equals K. 
• When Q < K, the reaction will proceed in the forward direction to reach equilibrium.
•  If Q > K, the reaction will proceed in the reverse direction to establish equilibrium.

3.0Equilibrium Constant and Temperature

The equilibrium constant (K) can change with temperature. The relationship between K values at different temperatures can be described by the Van't Hoff equation:

$\ln \left(\frac{K_2}{K_1}\right)=-\frac{\Delta H}{R}\left(\frac{1}{T_2}-\frac{1}{T_1}\right)$

Where:

K₁ and  K₂ are equilibrium constants at temperatures T₁ And T₂, respectively.

ΔH is the change in enthalpy.

R is the gas constant. T₁ and T₂ are temperatures in Kelvin.

Equilibrium Constant and Partial Pressure - 

The equilibrium constant (K_p) can also be expressed in terms of partial pressures for gas-phase reactions.

For a reaction involving gases:

$aA(g)+bB(g)\rightleftharpoons cC(g)+dD(g)$

The equilibrium constant (K_p​) in terms of partial pressures is given by:

$K_p=\left(\frac{P_C^c\cdot P_D^d}{P_A^a\cdot P_B^b}\right)$

Where:

• Kp is the equilibrium constant in terms of partial pressures.
• PA​,PB,PC, and PD are the partial pressures of gases A,B,C, and D at equilibrium, respectively. a,b,c, and d are the coefficients of gases A,B,C, and D in the balanced chemical equation.

4.0Relation between Kp and Kc

It's important to note that Kp and Kc

​(equilibrium constant in terms of concentrations) are related by the ideal gas law when the gases behave ideally:

                                                      Kp = Kc⋅(RT)^Δn

Where:

• R is the gas constant ( R= 0.0821 L.atm.mol⁻¹.K⁻¹).
• T is the temperature in Kelvin.
• Δn is the difference in the total moles of gaseous products and reactants 

                                                 Δn = (c+d) − (a+b)

This relationship allows for the conversion between equilibrium constants expressed in terms of partial pressures (Kp) and concentrations (Kc​) for gas-phase reactions under certain conditions.

For a reaction at equilibrium, the standard free energy change (ΔG) is related to the equilibrium constant (K) by the following equation:

ΔG^∘  = − RT lnK

Where:

• ΔG^∘  is the standard Gibbs free energy change.
• R is the gas constant (0.0821 L.atm.mol⁻¹.K⁻¹).T is the temperature in Kelvin.
• K is the equilibrium constant.

This relationship between K and ΔG^∘ helps us understand how the position of equilibrium (as indicated by K) relates to the spontaneity of a reaction. If K is large, the reaction favors the products, and the standard free energy change is more negative, indicating a more spontaneous process.

5.0 Degree of Dissociation (α)

The degree of dissociation (α) is a measure of the extent to which a compound dissociates or breaks apart into its constituent ions or components in a solution or when undergoing a reaction. It's particularly applicable to weak electrolytes or partially dissociating compounds.

For a generic reaction of a substance AB dissociating into its constituents:

$AB\rightleftharpoons A+B$

The degree of dissociation is represented by α and is defined as the fraction of the initial concentration of the compound that dissociates:

Amount of substance dissociatedInitial concentration of substance

$\alpha =\frac{\text{ Amount of substance dissociated }}{\text{ Initial concentration of substance }}$​

Or, mathematically:

$\alpha =\frac{\text{ concentration of dissociated particles }}{\text{ Initial concentration of undissociated substance }}$

For instance, if a compound initially has a concentration C and x amount dissociates, leading to the formation of x amount of products, the degree of dissociation would be:

$\alpha =\frac{x}{C}$

This parameter is particularly relevant in equilibrium situations, especially for weak acids or bases, where the extent of dissociation can significantly influence the equilibrium constant and the concentrations of the species involved in the equilibrium.