CHEMICAL REACTION -II

 Week:                                                                   Date:                                                     Time:

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

Subject:                               CHEMISTRY                                                                         Class: SS TWO

Topic:                                    CHEMICAL REACTIONS - II

Sub topic:  COLLISION THEORY AND TYPES OF CHEMICAL REACTION

Reference materials:

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

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

(3) INTERNET

Instructional materials: Sodium hydroxide, glucose, water, 2 beakers, thermometer.

Entry behavior: The students have been taught basic concept in chemical reaction.

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

1.       Explain collision theory.

2.       State the types of chemical reaction.

3.       Differentiate between activation energy and activation complex

4.       Give examples of endothermic and exothermic reaction

CONTENT

COLLISION THEORY AND TYPES OF CHEMICAL REACTION

COLLISION THEORY

For a chemical reaction to occur, the reacting species must collide with one another. This collision must be strong enough to break the existing bond so that new ones can be formed. Only particles with a certain minimum amount of energy will have a successful collision that will result in bond breakage followed by the formation of the new product. This maximum amount of energy required for a reaction to occurs is called activation energy. If the energy of the colliding reactant particles is equal to or more than the activation energy reaction occurs, but if the energy of the colliding reactant particles is less than the activation energy, no reaction will occur.

REACTION PROFILE AND ACTIVATION ENERGY

Reaction profile is the path in which the reactants take before it formed the products.

During chemical reaction there is an energy barrier which has to overcome before a reaction can be proceed to the product. This minimum energy is known as ACTIVATION ENERGY (the minimum energy which reactant molecule must posses before chemical reaction could take place). When the activation energy is acquire, the reactant molecule will form a complex with a high energy complex known as ACTIVATED COMPLEX. The activation complex soon decomposes to form the products.

 


TYPES OF CHEMICAL REACTIONS

There are two types of chemical reactions involves heat. They are :

I.                    Exothermic reaction               II. Endothermic reaction

EXOTHERMIC REACTION: This is a reaction in which heat is being liberated or given up to the surrounding and it is represented by ∆H = negative.

Examples of exothermic reactions are: Neutralization reaction, dissolution of sodium hydroxide in water, dilution of concentrated acid, production of tetraoxosulphate (vi) by contact process, production of ammonia by harber process, reaction of sodium metal with water etc.

Energy level diagrams illustrate the above discuss.

 


 Exothermic reaction

From the diagrams above, the heat of reactions ∆H can be deduced. Eaf represents the activation energy of the forward reaction; Eab represents the activation energy of the backward reaction (in which the products of the forward reaction become the reactants.

Enthalpy change is the difference between Eaf and Eab.

I.e., ∆H = Eaf - Eab.

In fig (above), the reactants are at higher total energy than the products (Eaf < Eab). This means that, to balance the energy, excess of energy has to be released as heat to the surroundings - the reaction is exothermic (∆H = - ve).

From the energy level (or energy profile) diagrams above:

 I.e., Energy of products - Energy of reactants. Or Eaf - Eab. But if energy of products and Eaf are less than energy of reactants and Eab, then ∆H is negative, i.e., the reaction is exothermic.

ENDOTHERMIC REACTION: This is the type of reaction in which heat is being absorbed from the surrounding i.e (∆H = +ve).

 Example of endothermic reaction are esterification reaction and dissolution of sodium thiosulphate (NaSO3) in water, because the test-tube feel cooler on addition of water.

   


 


Endothermic reaction

 

In figure (above), the reactants are at lower total energy than the products (Eaf > Eab) hence, to balance the energy, heat energy is absorbed by the system (to be able to reach the energy level of the products) – the reaction is endothermic (∆H = +ve).

 Activation energy, Ea is the energy difference between the activated complex and energy of reactants. It is the energy required by reactant molecules to become activated complex, hence it is always endothermic.

The heat of reaction (or enthalpy change, ∆H) is the difference between the energy levels of products and reactants.

I.e., Energy of products - Energy of reactants. Or Eaf - Eab. If energy of products and Eaf are greater than energy of reactants and Eab, ∆H is positive, i.e., the reaction is endothermic.

Example 1



In the diagram above, determine the (i). activation energy (ii). activation energy for the backward reaction (iii). enthalpy of the reaction.

Solution

Notice that when a question states activation energy, without indicating whether it is for the forward or backward reaction, it should be taken for the forward reaction.

(i). The activation energy is for the forward reaction. This is the energy difference between the activated complex and the reactants. I.e., 300 - 150 = 150 kJ/mol

(ii). Here, the reactants are QS I.e., 300 - 100 = 200 kJ/mol

(iii). The enthalpy of the reaction is the energy difference between the products and the reactants.

I.e., 100-150 = - 50 kJ.

2. in an equilibrium reaction between gases Q2 and R2 to form QR, the energy content of the reactants is 100kJ and that of the product is 54kJ. The energy content of the activated complex is 210kJ. Determine the: I. activation energy of the reaction. II. enthalpy change of the reaction, III. write a balance equation of the reaction IV. State whether the reaction is exothermic or endothermic. V. give reason for the answer in (iv) above.

Solution

I. activation energy of the reaction =delta H activated complex – delta H reactant = 210-100 = 110kJ

II. Enthalpy change of the reaction = Hp –HP = 54 – 100 = -46 kJ mol-1

III. Write a balance equation of the reaction: Q2(g) + R2(g)                2QR(g)

IV. State whether the reaction is exothermic or endothermic- exothermic

V. give reason for the answer in (iv) above- because energy is given out or delta H is negative or energy of product is lower than energy of reactant.

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.       Explain collision theory.

2.       State the types of chemical reaction.

3.       Differentiate between activation energy and activation complex

4.       Give examples of endothermic and exothermic reaction

ASSIGNMENT

1.Define activation energy

Sketch and label an energy profile diagram for the following reaction: 

A +B            C+D;      H =- XKJmol-1

2a.          Consider the reaction represent by the equation below:

Na2S2O3aq + 2HClaq            2NaClaq + H2O(L) + SO2(g) + S(s)

i.              List 2 factors that can affect the rate of this reaction.

ii.             Which of the products can be readily used to measure the rate of the reaction. Give a reason for your answer.

iii.            Name two instruments that can be used to measure factors in (a) i above.

b.            Which of the following will dissolve faster? 10g of NaOH pellets in 100cm3 of water, 10g of NaOH powder in 50cm3 of water give reason for your answer.

3ai.         Draw energy profile diagram for the reaction:

                H2(g) + I2(g)             2HI(g)   H = -3KJmol-1

ii.If the concentration of HI increase from 0 to 0.00moldm-3 in 50 seconds. What is the rate of the reaction?

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