CHEMICAL EQUILIBRIUM -I

 

Week:                                                                   Date:                                                     Time:

Period:                                 Duration: 1 HR 20 MIN.                                                  Average age of learners: 16YEARS

Subject:                               CHEMISTRY                                                                         Class: SS TWO

Topic:               CHEMICAL EQUILIBRIUM -I

Sub topic:  INTRODUCTION TO CHEMICAL EQUILIBRIUM

Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials:

Entry behavior: The students have been taught energy change and chemical reaction.

Behavioural objective: At the end of the lesson the students should be able to:

1.       Define chemical equilibrium.

2.       Mention some properties in the state of chemical equilibrium.

3.       State le chatelier’s principle

4.       Apply factors affecting chemical equilibrium

CONTENT

CHEMICAL EQUILIBRIUM -I

 An Equilibrium is establish when a system is close i.e. from which neither reactant nor product escapes, a condition with a minimum free energy (G) and a maximum entropy (S).

In order words, an equilibrium is established when the value of enthalpy change (ΔH) equal the product T ΔS i.e ΔG=0.

From the equation ΔG = ΔH - T ΔS.

At equilibrium the proportion of the reactants and the products must be the same in the equilibrium mixture.

Types of chemical equilibrium

Homogeneous equilibrium: All the reactants and products of any reaction under equilibrium are in same physical state. Example:  N₂(g) +  3H₂(g)   2NH₃ (g) 

Heterogeneous equilibrium:Physical state of one or more of the reacting species may differ i.e. all the reactants and products are not in same physical state.Example
2NaHCO₃(s)   Na₂CO₃(s) + CO₂(g) + H₂O(l)? 

PROPERTIES OR CHARACTERISTICS IN THE STATE OF CHEMICAL EQUILIBRIUM

I.                    The reaction is a reversible reaction.

II.                  It must take place in a closed system.

III.                The value of free energy change, ΔG, is zero.

IV.                Both the forward and backward reactions are occurring at the same rate.

EQUILIBRIUM IN A REVERSIBLE REACTION

In a reversible reaction a state of equilibrium can only be reached when there is an equal proportion of the reactant and product in the equilibrium mixture e.g

Fe(s)  +  H2O(g)                      Fe3O4(s)  +  H2(g)

At equilibrium: a. the concentration of the reactants and the products does not change.

                                b. an equilibrium can be approach from either the reactants end or product end.

LE CHATELIER’S PRINCIPLE: It states that if a system is in equilibrium and one of the condition is altered then the position of equilibrium will shift towards accommodating the effect.

Factors affecting the position of equilibrium

I.                    Effect of temperature

II.                  Effect of concentration

III.                Effect of pressure

IV.                Effect of catalyst

EFFECT OF TEMPERATURE ON EQUILIBRIUM MIXTURES

In general an increase in temperature of a system bring about an increase in the number of collision of the molecules of reactant which also bring about an increase in the frequency of collision between the molecules of reactant which leads to an increase in the rate of forward reaction. In such case more of product will be form than the reactant in the equilibrium will be shifted from left to right e.g  Boiling of water   H₂O_(l)                 H₂O_(g).

b. With a decrease in temperature the frequency of collision between the molecules decreases which brings about the production of little amount of the product since more of the product will be composed at low temperature to yield more of reactant.

Hence, the equilibrium mixture will contain more of the reactant than the products and so the position of equilibrium will be shifted from right to left i.e. equilibrium mixture will contain more of the water liquid than water in the example stated above.

Exothermic

i. An increase in temperature of a system in equilibrium where forward reaction is exothermic will bring about decrease in the number of collision between the molecules of reactant therefore the position of equilibrium will be shifted towards the direction that will favour the formation of the reactant i.e. the position will be shifted from right  to left eg N_2(g)  +  3H_2(g)                   2NH_3(g)

This implies that it will have more of N2 and H2 in the equilibrium mixture than ammonia.

ii. With a decrease in temperature of a system in equilibrium whose forward reaction is exothermic, the equilibrium position will shift from left to right since the forward reaction is favoured by a low temperature.

EFFECT OF CONCENTRATION:     A   +  B             C + D        [N_2(g)  +  3H_2(g)                   2NH_3(g)]

An increase either A or B lead to an increase in the number of collision between the molecules of A and B which brings about an increase in the rate of the forward reaction such that the position of equilibrium will be shifted from left to right, to have more of C and D in the equilibrium mixture than A and B.

A decrease in the concentration of A and B will lead to a decrease in the number of collision between the molecules of A and B which can be altered for by combining more of C and D together to compensate for the low concentration of A and B thereby, shifting the position of equilibrium from right to left.

EFFECT OF PRESSURE: Pressure effect is more pronounced in gaseous state than either a solid or liquid states.

This is because gaseous molecule can be easily compressed at high pressure than molecules of either solid or liquid. Let us considered the effect of pressure on the following system:

(a)    When volume increase from left to right e.g

     N2O4(g)                  2NO2(g)

      1 vol                 2 vol

An increase in pressure will shift the position of equilibrium towards smaller volume i.e. the position of equilibrium will shift from right to left.

A decrease in pressure will shift the position of equilibrium towards bigger volume i.e. the position of equilibrium will shift from left to right.

(b)   Increase in the volume from right to left

 N_2(g)  +  3H_2(g)                   2NH_3(g)

  1vol      3vol                   2vol

When pressure is increased volume decreases according to Boyle’s law.

An increase in pressure of this system in equilibrium will shift the position of equilibrium from left to right 1.e ammonia will be more in the equilibrium mixture than either nitrogen or hydrogen.

An decrease in pressure of this system in equilibrium will shift the position of equilibrium from right to left 1.e nitrogen and hydrogen will be more than ammonia.

(c.) No change in volume

                H_2(g) + I_2(g)                2HI_(g)

                1vol       1vol              2vol

Pressure increases or decreases has no effect on the position of equilibrium since the volume does not change.

EFFECT OF CATALYST: The major work of a catalyst is to lower the energy barrier of both forward and backward reaction. In effect, the equilibrium mixture is reached whether a catalyst is used or not.

In order word, the rate of forward reaction is promoted as well as the rate of backward reaction.

EQUILIBRIUM CONSTANT AND EQUILIBRIUM LAW

                aA   + bB               cC + dD

                K_c =   , where a,b,c and d = numbers of moles

Kc = equilibrium constant in term of concentration. The expression above is the equilibrium law. In the reaction above the reaction is a dynamic equilibrium with the concentration of the reactants and the products expression in moldm-3 such relationship is called the equilibrium law. Example of such relationship are as follows

                H_2(g) + I_2(g)                  2HI_(g)

 K_c =    =

 Kc has no unit because the volume does not change.

Condition necessary for the equilibrium constant

i.                     If the value of Kc is large then the reaction is proceeding to completion i.e. product is more favour as equilibrium.

ii.                   If the value of Kc is small, equilibrium is reached before the product is form.

iii.                  If the value of Kc is negative, reactant are more favour at equilibrium i.e. at equilibrium the reactant are favour.

PRESENTATION

i.The teacher explains collision theory.

ii. The teacher explains reaction profile and activation energy.

iv. The students define activation energy.

iii. The teacher explains types of chemical reaction.

EVALUATION

 The teacher evaluates the lessons by asking the following questions:-

1.       Define chemical equilibrium.

2.       Mention some properties in the state of chemical equilibrium.

3.       State le chatelier’s principle

4.       Apply factors affecting chemical equilibrium

5.                       Consider the following equilibrium reaction:

 3Fe_(s)+4H₂O_(g)                            FeSO₄+4H_2(g).

                  H=-150kJmol-.Explain the effect of the following factors on the position of equilibrium:

i.      Decrease in temperature.       ii. Increase in pressure              iii.     Removal of hydrogen

ASSIGNMENT

1.Consider the following reaction equation:

CO_(g)  +  2H_2(g)                 CH3OH_(g)           201KJmol-1

Predict the effect of each of the following factors on the position of equilibrium:

I.                    Decrease in temperature     II. increase in pressure                  III. Increase in concentration of CO

2a.State three features of an equilibrium reaction.

b.            H_2(g) + CO_2(g)                        H₂O_(g)  +  CO_(g)

I.             why is this reaction regarded as homologous?

II.            Explain briefly the effect of an increase in pressure on the:

i.              equilibrium position.  ii. reaction rate.