Chemical equilibrium refers to the state of a system in which the concentration of the reactant and the concentration of the products do not change with time and the system does not display any further change in properties.
Table of Content
- What is Chemical Equilibrium?
- Factors Affecting Chemical Equilibrium
- Reversible Chemical Reaction
- Rate of Chemical Reactions
What is Chemical Equilibrium?
When the rate of the forward reaction is equal to the rate of the reverse reaction, the state of chemical equilibrium is achieved by the system. When there is no further change in the concentrations of the reactants and the products due to the equal rates of the forward and reverse reactions, the system is said to be in a state of dynamic equilibrium.
A graph with the concentration on the y-axis and time on the x-axis can be plotted. Once the concentration of both the reactants and the products stops showing change, chemical equilibrium is achieved.
⇒ Check: Ionic Equilibrium
Examples of Chemical Equilibrium
In chemical reactions, reactants are converted into products by the forward reaction and the products may be converted into the reactants by the backward reaction. The two states, reactants and products are different in composition.
After some time of the start of the reaction, the rate of the forward and the backward reactions may become equal. After this, the number of reactants converted will be formed again by the reverse reaction such that the concentration of reactants and products do not change anymore. Hence, the reactants and products are in chemical equilibrium.
- N2O4 ⇌ 2NO2
- PCl5 ⇌ PCl3+PCl2
- N2+H2 ⇌ 2NH3
Types of Chemical Equilibrium
- Homogeneous Chemical Equilibrium
- Heterogeneous Chemical Equilibrium
In this type, the reactants and the products of chemical equilibrium are all in the same phase. Homogenous equilibrium can be further divided into two types: Reactions in which the number of molecules of the products is equal to the number of molecules of the reactants. For example,
- H2 (g) + I2 (g) ⇌ 2HI (g)
- N2 (g) + O2 (g) ⇌ 2NO (g)
Reactions in which the number of molecules of the products is not equal to the total number of reactant molecules. For example,
- 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
- COCl2 (g) ⇌ CO (g) + Cl2 (g)
In this type, the reactants and the products of chemical equilibrium are present in different phases. A few examples of heterogeneous equilibrium are listed below. CO2 (g) + C (s) ⇌ 2CO (g)CaCO3 (s) ⇌ CaO (s) + CO2 (g) Thus, the different types of chemical equilibrium are based on the phase of the reactants and products.
Factors Affecting Chemical Equilibrium
According to Le-Chatelier’s principle, if there is any change in the factors that affect the equilibrium conditions of the system, the system will counteract or reduce the effect of the overall change. This principle is applicable for both physical as well as in chemical equilibrium.
There are several factors like temperature, pressure, and concentration of the system which affect equilibrium. Some of the important factors affecting chemical equilibrium are discussed below.
Change in Concentration:
- The concentration of the reactants or products added is relieved by the reaction which consumes the substance which is added.
- The concentration of reactants or products removed is relieved by the reaction which is in the direction that replenishes the substance which is removed.
- When the concentration of the reactant or product is changed, there is a change in the composition of the mixture in chemical equilibrium.
Change in Pressure:
Change in pressure happens due to the change in the volume. If there is a change in pressure it can affect the gaseous reaction as the total number of gaseous reactants and products are now different. According to Le Chatelier’s principle, in heterogeneous chemical equilibrium, the change of pressure in both liquids and solids can be ignored because the volume is independent of pressure.
Change in Temperature:
The effect of temperature on equilibrium depends upon the sign of ΔH of the reaction and follows Le-Chatelier’s Principle.
- As temperature increases the equilibrium constant of an exothermic reaction decreases.
- In an endothermic reaction the equilibrium constant increases with increase in temperature.
Along with equilibrium constant, the rate of reaction is also affected by the change in temperature. As per Le Chatelier’s principle, the equilibrium shifts towards the reactant side when the temperature increases in case of exothermic reactions, for endothermic reactions the equilibrium shifts towards the product side with an increase in temperature.
Effect of a Catalyst:
A catalyst does not affect the chemical equilibrium. It only speeds up a reaction. In fact, catalyst equally speeds up the forward as well as the reverse reaction. This results in the reaction reaching its equilibrium faster.
The same amount of reactants and products will be present at equilibrium in a catalyzed or a non-catalysed reaction. The presence of a catalyst only facilitates the reaction to proceed through a lower-energy transition state of reactants to products.
Effect of Addition of an Inert Gas:
When an inert gas like argon is added to a constant volume it does not take part in the reaction so the equilibrium remains undisturbed. If the gas added is a reactant or product involved in the reaction then the reaction quotient will change.
Problems on Chemical Equilibrium
Q.1: The equilibrium constant KP for the reaction N2 (g) + 3H2 (g) ⇌ 2NH3 (g) is 1.6 × 10-4 atm-2 at 400oC. What will be the equilibrium constant at 500oC if the heat of the reaction at this temperature range is -25.14 kcal?
Equilibrium constants at different temperature and heat of the reaction are related by the equation,
log KP2 = –25140 / 2.303 × 2 [773 – 673 / 773 × 673] + log 1.64 × 10-4
log KP2 = –4.835
KP2 = 1.462 × 10–5 atm–2
Q.2: Given the equation, N2 (g) + 3H2 (aq) ⇌ 2NH3 (g), Find Q and determine which direction the reaction will shift in order to reach equilibrium.
Given, [N2] = 0.04M, [H2] = 0.09M, and K = 0.040
Since only nitrogen and hydrogen concentration is given, the can be assumed as the reactants and
ammonia as the product. Since ammonia concentration is not given it can be assumed to be zero.
As q is the ratio of the relative concentration of products to reactants, here Q =0.
Since K = 0.04 is larger than Q, nitrogen and hydrogen will combine to form product ammonia.
Reversible Chemical Reaction
When reactants react to form, in some cases or under some conditions, products also become capable of reacting to form back the reactants. The occurrence of such simultaneous reactions is called reversible reactions.
Initially, the forward reaction of the reactants will be comparatively faster. The backward reaction by the product molecules slowly picks up such that reactions never go to completion. After some time the forward and backward reaction become equal such that the concentration of reactant and product do not change any more. The reaction is in an equilibrium state.
Reversible reactions are dynamic, reactants and products coexist, the conversion will never be 100% and attainable from either reactant or product sides.
Also Read: Reversible and Irreversible changes
Examples of Reversible Reaction:
Hydrogen and nitrogen do not react much at normal conditions. At high pressure and temperature and in presence of iron catalyst they react to form ammonia. But the conversion is never 100%, as some ammonia molecules decompose to give back nitrogen and hydrogen.
N2 + 3H2 ⇌ 2NH3
The reaction is made of two reactions – a forward reaction to produce ammonia and a backward reaction to form the reactant. The reaction is dynamic and reversible. After some time, the number of molecules of ammonia formed and decomposed becomes the same, such that concentration of hydrogen, nitrogen and ammonia remains constant.
Rate of Chemical Reactions
The rate of a chemical reaction is the number of atoms or molecules undergoing conversion in unit time. It can be expressed in terms of either reactants or products.
Consider an irreversible chemical reaction involving ‘a’ molecules of reactant A, ‘b’ molecules of reactant B, giving ‘c’ molecules of product C and d molecules of product D.
aA + bB ⇌ cC + dD
Rate of the Reaction = r = −1/a (dA/dt) = −1/b (dB/dt) = 1/c (dCdt) = 1/d (dD/dt)
- a, b, c and d are the stoichiometric coefficients
- dA, dB, dC, and dD are the change in concentration.
- dt is the time taken for the change.
Since the reactant concentration decreases with time, the rate of the reaction with respect to the reactants has a negative sign. Since the concentration of the products increases with time, reaction rate with respect to the product has a positive sign.
For the production of nitric oxide from ammonia,
4NH3 (g) + 5O2 (g) → 4NO (g) + 6H2O (g)
Rate of the reaction = r = -¼ (dNH3/dt) = -⅕ (dO2/dt) = ¼ (dNO/dt) = ⅙ (dH2O/dt)