Problems on Chemical Equilibrium

Equilibrium is the state of the process when the properties of the system like concentration, pressure, and temperature do not show any change with time.

In other words, equilibrium is a state of the body where neither the inner energy nor the movement of the body shifts with time. It is the point when the net superficial force and the torque working on the body about the COM or any other point are zero.

Equilibrium is categorised as unstable, stable, and neutral equilibrium.

Stable equilibrium: An example of stable equilibrium is a ball kept at the bottom of a hemisphere.

Unstable equilibrium: If we consider a ball kept on top of a sphere, it is an example of unstable equilibrium. The ball will roll away from the topmost point if we slide it.

Neutral equilibrium: This refers to the state when the body does not move towards or away from the equilibrium point. 

Chemical Equilibrium

Before going to the equilibrium in physical processes, it is important to understand the equilibrium in the chemical processes. In such processes, the two opposing processes occur at an equal rate. The mixture of reactants and products at the equilibrium state is the equilibrium mixture. It is represented as:

aA + bB = xX + yY

Here,

A, B are reactants

X, Y are products

chemical equilibrium, condition in the course of a reversible chemical reaction in which no net change in the amounts of reactants and products.

Types of chemical equilibrium

The law of chemical equilibrium is of two types: homogeneous equilibrium and heterogeneous equilibrium.

Homogeneous equilibrium

When all the products and reactants of the chemical equilibrium are in a similar phase, it is known as homogeneous equilibrium. Moreover, there are two types of homogeneous equilibrium:

1. Reactions in which the amount of molecules in the reactants is equal to the number of molecules in the products. For example: O2 (g) + N2 (g) ⇌ 2NO (g)

2. Reactions in which the total amount of reactant molecules is not equal to the number of molecules in the products. For example: Cl2 (g) + CO (g) ⇌ COCl2(g)

Heterogeneous Equilibrium

When the products and reactants of chemical equilibrium are present in different phases, it is known as heterogeneous equilibrium. For example:  

CO2 (g) +  CaO (s)⇌CaCO3 (s)

Hence, the various types of chemical equilibrium are based on the phases of the products and the reactants.

Examples of Chemical Equilibrium

In a chemical reaction, the reactants are changed into the products. After the reaction starts, the rate of the backward and forward reactions may be similar. Then, the reactants that were converted are again made by the reverse reaction. As a result, the products and the reactants are in chemical equilibrium.

• H2 + N2 ⇌ 2NH3

• PCl2 + PCl3 ⇌ PCl5

• 2NO2 ⇌ N2O4

Units of equilibrium constant:

 K has no unit for a reaction where the reactants and products are equimolar.

In general, unit of K = [M]∆n

Where, M= mol litre-1 and ∆n= number of moles of gaseous products- number of moles of gaseous reactants.

Similarly, the unit of Kp = [atm]∆n

 Problems on Chemical Equilibrium

Problem-1: At 300K, 6.00 moles of PCl5 were allowed to reach equilibrium in a 1 L closed reaction vessel. You must determine the composition of the mixture at equilibrium. Given that the reaction’s Kc is one,

❖     Step 1: Create a balanced reaction with the concentration you want to determine.

PCl5(g)  ⇋ PCl3(g)+ Cl2(g)

❖ Step 2: Calculate the molarity. The amount of PCl5 used before the response is 6 moles, and the reaction vessel capacity is 1 L. As a result, the PCl5 concentration is 6/1 mole/litre = 6 M.

❖     Step 3: Record the effective concentration. Then determining the concentrations of the other substances in terms of x.

❖     Step 4: Fill values of the equilibrium concentration. 

❖     Step 5: Find the value of X. In other words, the chemically meaningful value of X is chosen.

X2 + X -6 = 0

The equation yields either X = 2 or X = -3.

❖     Step 6: Using the value of x, calculate the equilibrium concentration values for each chemical.

As a result,

[PCl5] = 6–X = 6–2 = 4 M

X = 2 M = [PCl3] = [Cl2].

Problem-2: Calculate △G for ice melting at -10 ° C. 

Solution- T = -10 ° C + 273 = 263 K 

△G = △H – △TS 

= 6.01 k Jmol – (263 K) 0.022 k J mol 

= 6.01 k Jmol – 5.79 k Jmol

= 0.022 k J mol 

Therefore, at -10 ° C, △G = 0.022 k J mol 

 Conclusion

In this article, we have discussed the concept of equilibrium and equilibrium reactions, as well as their classifications. We learned that equilibrium refers to a state in chemistry that shows no net change in the concentrations of the products or the reactants. Moreover, an equilibrium reaction is a chemical reaction between the reactants before and after the reaction is complete (i.e., a thermodynamic equilibrium state). An equilibrium reaction example is water evaporating to form vapour. We also discussed the concept of chemical equilibrium and the factors that affect chemical equilibrium.