SS2 Chemistry Lesson Note on Chemical Equilibrium

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TOPIC: CHEMICAL REACTIONS

CONTENTS:

1. CHEMICAL EQUILIBRIUM:
2. CHARACTERISTICS OF A CHEMICAL SYSTEM AT EQUILIBRIUM,
3. LE CHATELIER’S PRINCIPLE AND FACTORS AFFECTING EQUILIBRIUM OF CHEMICAL REACTION
4. EQUILIBRIUM CONSTANT, KC

PERIOD 1:   CHEMICAL EQUILIBRIUM

Studies have shown that some reactions are such that all the reactants are never completely converted into products. In such reactions, the products are converted into the reactant as they are been formed.

A reaction is said to be reversible, if it can be made to proceed forward and backward, under a given set of conditions.

For a chemical reaction involving a gas to be reversible, it must be carried out in a closed vessel, i.e. an isolated or closed system. Two opposing arrows in a chemical equation represent a reversible reaction:

1. a) H₂O_(l) H₂O_(g)
2. b) N_2(g) + 3H_2(g) 2NH_3(g)
3. c) CaCO_3(s) CaO_(s)  + CO_2(g)

Such reactions usually have relatively low activation energies for the reverse reactions.

A reaction in which the products cannot readily be combined to give the reactants is said to be irreversible. In such a reaction, a single arrow is used pointing to the product(s), e.g. combustion reactions

1. a) C_(s) + O_2(g)     CO_2(g)
2. b) 2Mg_(s) + O_2(g) 2MgO_(s)

Such reactions usually have relatively high activation energies for the reverse reaction.

REACTIONS AT EQUILIBRIUM

Dynamic Equilibrium

When the rates of forward and backward reactions in a reversible reaction are equal, the reaction is said to be at equilibrium.  At equilibrium, both the forward and backward reactions are still going on, but because there rates are the same, the concentrations of the reactants and products do no longer change with time.  Hence, chemical equilibrium is dynamic and not static.

How to Identify a Reaction at Equilibrium

At equilibrium, certain observable properties become constant depending on the type of reaction. Such properties include;

• Concentrations of the reactants and products;
• Pressure of gases;
• Density or intensity of colour of a solution;
• Temperature of the system.

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