14.0     DEFINITION OF EQUILIBRIUM

Equilibrium can be defined as a state of a system where there is no observable change in the properties of the system with respect to time. A saturated solution is an example of system in equilibrium. There are two types of equilibrium:

• STATIC EQUILIBRIUM – It is attained when a system in a state of equilibrium is in a state of rest (a stationary state). A good example is a balanced seesaw.
• DYNAMIC EQUILIBRIUM – It is an intermediate stage of a reversible reaction when both the forward and backward reactions are occurring at the same rate, thereby producing no net change. Such equilibrium only occurs in a closed system. It is further divided into two:
• PHYSICAL EQUILIBRIUM – It involves observable change in physical properties.
• CHEMICAL EQILIBRIUM – It involves observable change in chemical properties.

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14.1     REVERSIBLE REACTION

A reversible reaction is one that proceeds in both directions (forward and backward) under suitable conditions. Examples include:

• The reaction between ammonia gas and chlorine gas

NH₄Cl _(s) ⇌ NH_{3 (g)} + Cl _(g)

• Action of Steam on Iron

3Fe _(s) + 4H₂O _(g) ⇌ Fe₃O_{4 (s)}​ + 4H_{2 (g)}​

• Dissociation of Nitrogen Tetroxide

N₂​O_{4 (g)} ​⇌ 2NO₂​ _(g)

            14.2     PROPERTIES OF A SYSTEM IN EQUILIBRIUM

• For equilibrium to be established, the system must be closed.
• Equilibrium position can be approached from either the forward or backward reaction.
• A chemical reaction at equilibrium is dynamic in nature.

14.3     LE CHATELIER’S PRINCIPLE

Le Chatelier’s Principle states that if an external constraint such as a change in temperature, pressure or concentration, is imposed on a chemical system in equilibrium, the equilibrium will shift so as to annul or neutralize the constraint.

In the chemical industry, Le Chatelier’s Principle helps to define optimum conditions for chemical processes, reduce undesired reversibility and predict the effect of altered factors on the equilibrium.

14.4     FACTORS GOVERNING EQUILIBRIUM POSITION

Equilibrium position is dependent on three factors: Concentration, temperature, and pressure (for gases).

CONCENTRATION – The effect of change in concentration is as followed:

• Increasing the concentration of reactants and decreasing that of the products shifts the equilibrium position to the right (forward reaction is favoured).
• Decreasing the concentration of reactants and increasing that of the products shifts the equilibrium position to the left (backward reaction is favoured).

TEMPERATURE – The effect of change in temperature is as followed:

• For an endothermic reaction (∆H = +ve), increase in temperature shifts the equilibrium position to the right (forward reaction is favoured), while a decrease in temperature shifts the equilibrium position to the left (backward reaction is favoured).
• For an exothermic reaction (∆H = -ve), increase in temperature shifts the equilibrium position to the left (backward reaction is favoured), while a decrease in temperature shifts the equilibrium position to the right (forward reaction is favoured).

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PRESSURE (For Gases) – The effect of change in temperature is as followed:

• Increase in pressure shifts the equilibrium position towards the side with fewer moles of gas.
• Decrease in pressure shifts the equilibrium position towards the side with more moles of gas.

CATALYSTS – They speed up the attainment of equilibrium but do not affect the position of equilibrium. They do not shift the equilibrium since they equally affect both the forward and reverse reactions. It only lowers the activation energy, enabling the equilibrium position to be attained at a faster rate.

            14.5     EQUILIBRIUM CONSTANT (K)

The Equilibrium Constant (K) is a measure of the ratio of the concentration of products to the concentration of reactants at equilibrium. It varies with temperature: the higher the temperature, the higher the equilibrium constant, while the lower the temperature, the lower the equilibrium constant.

For the general reaction: mA + nB ⇌ pC + qD.