Chemical Equilibrium

Equilibrium State:

The state of process at which rate of forward reaction becomes equal to rate of backward reaction is called equilibrium State.

Rate of forward reaction (r_f) = rate of backward reaction (r_b)

Types of Equilibrium:

Physical equilibrium:

The equivalent state rate of forward process becomes equal to the rate of backward process in physical change is called physical equilibrium.

Example: Melting of ice into water.

Rate of melting of ice (r_f) = Rate of freezing of water (r_b)

Chemical equilibrium:

The equilibrium state in rate of forward reaction becomes equal to rate of backward reaction in reversible chemical reaction is called chemical equilibrium.

Chemical equilibrium are of two types:

Homogeneous Chemical Equilibrium

The chemical equilibrium of reversible reaction in all the reactant and the product are of same phase is called homogeneous chemical equilibrium.

Example:

i.3H₂ (g) + N₂ (g) ⇆ NH3(g)

ii.H₂ (g) + I₂ (g) ⇆ 2HI (g)

Heterogeneous Chemical Equilibrium:

The chemical equilibrium reversible reaction in when are more reactant and product r of different phases is called heterogeneous chemical equilibrium.

Example:

CaCO₃ (s) ⇆ CaO (s) + CO₂ (g)

Characteristics of equilibrium:

1. It is dynamic in nature.

2. It does not depend upon concentration of reactant and product.

3. It depends upon the temperature.

4. Catalyst does not affect equilibrium state.

5. Equilibrium state is achieved from either side.

6. It exist in only close vessel.

Types of chemical reaction:

Reversible Reaction:

The chemical reaction in which reactants combined to forms products and products combined to form reactants simultaneously under the same condition is called reversible reaction.

Example:

PCl₅ (g) ⇆ PCl₃ (g) + Cl₂ (g)

Irreversible reaction:

The chemical reaction in which the reactant combines to form the products and the product does not combine to form the reactant simultaneously under the same condition is called irreversible chemical reaction.

Example:

Zn + H₂SO₄ à ZnSO₄+ H₂

NaOH + HCl à NaCl + H₂O

Difference between reversible and irreversible chemical reaction:

Reversible Reaction:	Irreversible reaction:
Reactants combined to forms products and products combined to form reactants simultaneously under the same condition is called Reversible Reaction.	Reactant combines to form the products and the product does not combine to form the reactant simultaneously under the same condition is called irreversible chemical reaction.
Equilibrium exists.	Equilibrium doesn’t exists.
Reaction takes place only in close vessels.	Reaction takes place in both close and open vessels.
It is denoted by double headed arrow (⇆).	It is denoted by single headed arrow (à).

Law of Mass Action:

It states that, "rate of reaction is directly proportional to the product of molar concentration of reactants, each concentration being raised to the power equal to stoichiometric coefficient in balanced chemical equation."

Verification:

Let us consider a hypothetical reaction.

aA + bB ⇆cC + dD

According to law of mass action:

Rate of forward reaction (r_f) [A]^a[B]^b

Or, r_f = K_f [A]^a[B]^b……….(i)

Where, K_f is rate constant for forward reaction.

And

Rate of backward reaction (r_b) [C]^c[D]^d

Or, r_b = K_b [C]^c[D]^d……….(ii)

Where, K_b is rate constant for backward reaction.

Now, At Equilibrium state:

Rate of forward reaction (r_f) = rate of backward reaction (r_b)

i.e. K_f [A]^a[B]^b= K_b [C]^c[D]^d

Or, $\frac{{{K_f}}}{{{K_b}}} = {\rm{ }}\frac{{{{\left[ C \right]}^c}{{\left[ D \right]}^d}}}{{{{\left[ A \right]}^a}{{\left[ B \right]}^b}}}$

$\frac{{{K_f}}}{{{K_b}}}$ is replaced by another constant K_c called equilibrium constant in terms of concentration. So above reaction can be written as,

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

Equilibrium constant:

It is defined as the ratio between rate constant for forward reaction and rate constant for backward reaction.

Mathematically:

Equilibrium Constant (K_c) = $\frac{{{K_f}}}{{{K_b}}}$

Characteristics of equilibrium constant:

Its value is constant for particular reaction.

Its value is independent upon concentration of reactant and product.

Its value depend upon temperature.

Its value is not affected by the use of catalyst.

Equilibrium constants for forward and backward reactions are reciprocal of each other.

If k_b>k_f (k_c<1), Rate of forward reaction is less than date of backward reaction

If k_b<k_f (k_c>1), Rate of forward reaction is greater than the rate of backward reaction.

If k_b=k_f (k_c=1), Then equilibrium constant exists.

Relationship between K_P and K_C:

Let us considered the hypothetical reversible reaction

aA + bB ⇆ cC + dD

According to the law of mass action:

${K_c} = \frac{{{{\left[ C \right]}^c}{{\left[ D \right]}^d}}}{{{{\left[ A \right]}^a}{{\left[ B \right]}^b}}}$ ……….. (i)

From ideal gas equation:

PV = nRT

Or, $P = \frac{{nRT}}{V}$

Or, $P \propto \frac{n}{V}$ [Molar Concentration]

Thus, Equilibrium constant in terms of partial pressure (k_P) will be

${K_P} = \frac{{{{[{P_c}]}^c}{{[{P_d}]}^d}}}{{{{[{P_a}]}^a}{{[{P_b}]}^b}}}$ ……….. (ii)

Again from ideal gas equation:

PV = nRT

$P = \frac{{nRT}}{V}$

For Gas A: ${P_A} = \frac{{{n_A}RT}}{V}$

For Gas B: ${P_B} = \frac{{{n_B}RT}}{V}$

For Gas C: ${P_C} = \frac{{{n_C}RT}}{V}$

For Gas D: ${P_D} = \frac{{{n_D}RT}}{V}$

Substituting Value of P_A, P_B, P_C, P_Din Equation (i)

${K_P} = \frac{{{{[\frac{{{n_C}RT}}{V}]}^c}{{[\frac{{{n_D}RT}}{V}]}^d}}}{{{{[\frac{{{n_A}RT}}{V}]}^a}{{[\frac{{{n_B}RT}}{V}]}^b}}}$
${K_P} = \frac{{{{\{ [C]RT\} }^c}[{{\{ [D]RT\} }^d}}}{{{{\{ [A]RT\} }^a}{{\{ [B]RT\} }^b}}}$

${K_p} = \frac{{{{[C]}^c}{{[D]}^d}}}{{{{[A]}^a}{{[B]}^b}}} \times \frac{{{{(RT)}^c}{{(RT)}^d}}}{{{{(RT)}^a}{{(RT)}^b}}}$
${K_p} = {K_c} \times \frac{{{{(RT)}^c}{{(RT)}^d}}}{{{{(RT)}^a}{{(RT)}^b}}}$ [From equation (i)]

K_P= K_c × (RT)^(c+d)-(a+b)

If ∆n = (c+d)-(a+b. Then,

K_P= K_c × (RT)∆n

When ∆n = 0, then

K_P= K_c

Le Chatellier Principle:

It states that, "when a system in equilibrium is subjected to the change in pressure temperature and concentration then equilibrium will shift in such direction so as to notify the effect of change."

Effect of temperature:

If the temperature of the system is changed the equilibrium will shift in such a direction where the effect of temperature is changed.

Example: Exothermic reaction produce heat so equilibrium will shift in backward reaction when temperature is increased and vice versa.

Effect of pressure:

If pressure is changed then equilibrium will shift in such a direction where the effect of change pressure is consumed.

Example: equilibrium will shift to a lower volume if the pressure is increased and vice versa.

Effect of concentration:

If the concentration of reactant is increased then equilibrium will shift in forward direction and if the concentration of product is increased the equilibrium will shift in backward direction.

Application of Le Chatellier principle:

Physical process:

Le Chatellier principle easy used in reversible physical reaction. Shots as melting and freezing process vaporization and condensation process etc.

Example:

i. Melting of ice:

Reaction:

Effect of Temperature:

Since melting of ice endothermic reaction so equilibrium will shift in forward direction and more ice get melted when temperature is increased and vice versa.

Effect of Pressure:

Since volume of water is more than that of ice show equilibrium will shift in backward direction and more water will be frozen if pressure is increased and vice versa.

ii. Vaporization of water:

Reaction:

Effect of temperature:

Since, the vaporization of water is an endothermic process. So, equilibrium will shift in forward direction and more vaporization takes place when temperature is increased and vice versa.

Effect of pressure:

Since volume of water is less than that of vapor. So, equilibrium will ship it in backward direction and more condensation will occur when pressure is increased and vice versa.

2. Chemical Process:

Le Chatellier Principal is most beneficial and applicable in the manufacture of chemicals industrially such as ammonia SO₃, H₂SO₄ etc.

Manufactured of NH₃ by Harbor process:

Reaction:

Effect of temperature:

Since the manufacturer of ammonia is exothermic. Therefore on increasing temperature equilibrium will shift in backward direction while equilibrium will shift forward direction on decreasing the temperature. So low temperature is favorable for the production of ammonia.

Effect of pressure:

Since related volume of a product is lower than the relative volume of the reactant therefore equilibrium will shift in forward direction if the pressure is increased and more ammonia will be produced. But equilibrium will receive in backward direction if the pressure is decreased.

Effect of concentration:

If the concentration of any reactant is increased equilibrium will shift in forward direction and the more amount of ammonia will be produced and if the concentration of the ammonia is increased equilibrium will shift in backward direction so as to consume the increased concentration.

Effect of the catalyst and the promotor:

Equilibrium is not affected by the presence of the catalyst and promotor. However equilibrium will be achieved faster so catalyst and promoter increases the rate of production of ammonia.

Chemical Equilibrium Notes | Class 11 Chemistry Notes | NEB | NEPALI EDUCATE

Chemical Equilibrium

Equilibrium State:

Types of Equilibrium:

Physical equilibrium:

Chemical equilibrium:

Chemical equilibrium are of two types:

Homogeneous Chemical Equilibrium

Heterogeneous Chemical Equilibrium:

Characteristics of equilibrium:

Types of chemical reaction:

Reversible Reaction:

Irreversible reaction:

Difference between reversible and irreversible chemical reaction:

Reversible Reaction:

Irreversible reaction:

Law of Mass Action:

Equilibrium constant:

Characteristics of equilibrium constant:

Relationship between KP and KC:

Le Chatellier Principle:

Effect of temperature:

Effect of pressure:

Effect of concentration:

Application of Le Chatellier principle:

Physical process:

i. Melting of ice:

ii. Vaporization of water:

2. Chemical Process:

Manufactured of NH3 by Harbor process:

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Relationship between K_P and K_C:

Manufactured of NH₃ by Harbor process: