Equilibrium in Chemistry

What is the equilibrium?

Let us look at the reaction for the formation of ammonia from nitrogen and hydrogen. I,e N2 (g) + 3H2 (g)-----------> 2NH3 (g)   where    G =- 33.1 kJ.

From the above chemical equation, the negative sign represents the standard free energy, G0. In chapter six, we learned that the negative sign for G shows that the reaction is spontaneous under standard conditions. The standard conditions are defined as 298K and (1)one-atmosphere pressure. But spontaneous reactions are not always fast.  Practically, the reaction that leads to the formation of ammonia is slow, and it is recommended that it should be carried through temperatures above 298K and the pressure must be more than one atmosphere to increase the yield of ammonia. The word equilibrium refers to the point of balance between the 2 opposing processes.  Note that the concentration of the product, NH3, is zero at the start and gradually increases with time. In this reaction, the reactants, H2 and N2, are used up in the reaction, so their concentrations gradually decrease. However, after sometimes, we expect that the concentration of hydrogen, nitrogen, and ammonia remains constant. All the concentrations become constant and not all the reactants in the reaction have been converted into products.


Table Of Contents

1. What is the equilibrium?

2. Equilibrium in physical and chemical processes

3. What are the general characteristics of physical equilibrium?

4. Dynamic nature of Equilibrium

5. Equilibrium constant

Equilibrium in physical and chemical processes

A reversible reaction refers to the reaction that occurs in both the forward and the reverse direction simultaneously. There are some chemical reactions whose reactants completely react to form products. One example of a reversible reaction is the reaction between sulfur and oxygen to yield sulfur trioxide. The double arrows used in the equation shows that it is reversible.


In the first reaction, sulfur dioxide reacts with oxygen to yield sulfur trioxide. In the second reaction, which is read from right to left, sulfur trioxide decomposes into oxygen and sulfur dioxide. The first reaction is known as the forward reaction while the second reaction is referred to as the reverse reaction.  The rate of the reverse process, the formation of sulfur dioxide and oxygen from sulfur trioxide are zero. Sulfur trioxide, the product of the forward reaction, then begins to form. An increase in the concentration of sulfur trioxide leads to the formation of a small amount of sulfur dioxide and oxygen in what is referred to as a reverse reaction. The reverse reaction speeds up as the concentration of sulfur trioxide becomes higher with time. The forward reaction is simultaneously slowing down because sulfur dioxide and oxygen form sulfur trioxide.

The chemical equilibrium refers to a state in which the forward and backward reaction takes place at the same rate. The equilibrium position indicates whether the components on the left or right side of a reversible reaction are at higher concentration.  The concentrations of reactants and products are only equal when the system is at equilibrium. This doesn't mean that the amount /concentration of reactants and products are always equal.


What are the general characteristics of physical equilibrium?

The system to be physically equilibrium, the system must be a closed one and dynamic in nature. The measurable property should not undergo a change with time. Instead, they must reach a constant At equilibrium, there exists an expression involving the concentration of the substance which becomes constant at a given temperature.


Dynamic nature of Equilibrium

We should note that the equilibrium is a state of action but not stationary. In a given chemical reaction, a dynamic equilibrium is established when a reversible reaction leads to change in its ratio of either reactants or products. In dynamic equilibrium, the substances move between the chemicals at an equal rate. This implies that at equilibrium, the reaction does not stop but the system acquires constant observable properties because of the equal rates of forward and backward reactions.


Equilibrium constant

Generally, the position of equilibrium is expressed in terms of numerical values. The numerical values in the equation are related to the number of reactants and products at equilibrium. Consider the hypothetical reaction in which x mole with its reactant X and y moles with their reactant Y react to yield z moles of product Z and p mole of product P at equilibrium.

xX   + yY  <------------>zZ + pP

In chemistry, the equilibrium constant, Keq refers to the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the value of n moles of the substance understudy in the reaction. Thus the equilibrium-constant expression has the general form.

    [Z]z ×[X]x /  [P]p × [Y]y

The exponents in the equilibrium-constant expression are the coefficients from the balanced chemical equation.  The [ ] indicates the concentrations of substances in moles per litre. Equilibrium constants provide valuable chemical information. Because equilibrium constant is always expressed as a ratio of products to reactants, where if the value of Keq is greater than one implies that products are favoured over reactants. Conversely, Keq value less than one means that the reactants are favoured over products. Equilibrium.





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