Lesson Notes By Weeks and Term - Senior Secondary School 2

SUBJECT: CHEMISTRY

CLASS:  SS 2

DATE:

TERM: 2nd TERM

REFERENCES

• New School Chemistry for Senior Secondary School by O.Y.Ababio (6th edition)
• Calculations on Chemistry by E.U. Akusoba and G.O. Ewelukwa
• WASSCE past Questions and Answers on Chemistry
• UTME past Questions and Answers on Chemistry

 
WEEK ONE

TOPIC: RATE OF REACTION

CONTENT

•  Meaning of Rate of Chemical Reaction.
• Rate Curve. 
• The Collision Theory.
• Factors Affecting Rate of Chemical Reactions

MEANING OF RATE OF REACTION

The rate of a chemical reaction is the number of moles of reactants converted or products formed per unit time.          

Usually, rate of reaction is determined experimentally by measuring change in concentration of one of the components in the reaction with time. 

Thus, 

Rate of reaction = change in concentration of reactant or product (mol/dm3)

                                                         Time taken for the change (seconds)

The unit of the rate of reaction is mol/dm-3s-1 or g dm-3s –1.

Rate of reaction can also be expressed as:

Rate of reaction = change in number of mole or mass of reactant or product

                                                     Time taken for the change

Then the unit of rate is mols-1 or gs-1

EXAMPLE: When 0.5g of calcium trioxocarbonate (IV) was added to excess dilute hydrochloric acid, carbon (IV) oxide was evolved. The complete reaction took 5 minutes. What was the rate of reaction?

SOLUTION: 

Rate of reaction = change in mass of reactant

                                Time taken for the change

    = (0.5 – 0)g   = 0.5

        5x60         300

    = 1.67 x 10-3 gs-1

WAYS OF MEASURING REACTION RATE

Concentration is one of the properties of a reaction that can change with time.

The following properties can also change with time and can thus be used to measure rate of reaction.

1. Decrease in mass of reaction system
2.  Volume of gaseous product
3. Amount of precipitate formed
4. Change in colour intensity
5. Change in pH
6. Change in total gas pressure

RATE CURVE

The rate curve is a graphical illustration of the rate of a reaction.

The following graph illustrate rate curve

FEATURES OF RATE CURVE 

1. It passes through the origin. This is because there is no change in concentration or mass at the start of reaction.
2. It steeps at first, this because the rate is fast at the beginning.
3. It becomes less steep later. This is because the rate slows down.
4. It finally becomes horizontally. This is because the reaction has reached the end points.

The following can be determined from the rate curve

1. Average rate of reaction

Average rate = total number of mole / mass involved

    Time taken 

1. Rate at a particular instant during the reaction

              Rate at instant = Gradient at a point on the curve

When the rate of reaction has direct variation with concentration, then 

                    Rate of reaction   α[Concentration of A]

                                                    R α [A]

                                                    R = k[A]

Where k is called Rate constant

EVALUATION        

1. What is rate of reaction?
2. State two ways of measuring reaction rate

COLLISION THEORY

The collision theory states that for a chemical reaction to occur the reactant particles must collide and they must collide with a certain minimum amount of energy known as activation 

energy.

Reacting particles are in continuous motion, thus they possess energy and they also collide with one another. Not all collisions result into chemical reaction. Collisions, which result into chemical reaction, are called EFFECTIVE COLLISIONS. Minimum amount of energy required by reacting particles for chemical reaction to occur is called ACTIVATION ENERGY. Activation energy is the ENERGY BARRIER the reactants must overcome for reaction to occur. It is the minimum energy required for bond breaking for chemical reaction to occur.                

Chemical reaction occur only when the energy of the colliding reactant particles is equal to or more than the activation energy. Activation energy must be equal to energy barriers also for chemical reaction to occur.

Every reaction has its own energy of activation. Reactions with low activation energy have high rate of reaction and occur spontaneously. Reaction with high activation energy have low rate of reaction and are not spontaneous.

The graph below illustrates the concepts of the activation energy for endothermic and exothermic reactions.

EXOTHERMIC REACTION    ENDOTHERMIC REACTION

From graph, it can be seen that

1. Both exothermic and endothermic reactions generally require an initial input of energy to overcome the activation energy barriers. 
2. Exothermic reaction once started proceed without any further external energy supply e.g burning of kerosene 

3.Endothermic reaction proceeds with continuous supply of external energy e.g cooking of rice.

EVALUATION 

1. State collision theory.
2. Graphically differentiate exothermic reaction from endothermic reaction.

FACTORS AFFECTING RATE OF REACTION

From the collision theory, it can be seen that rates of reaction depends on the following features.

1. The energy of the particle.
2. The frequency of collision of the reaction.
3. The activation energy of the reaction.

These features of a chemical reaction are in turn affected by some factors, which can make them to change and consequently affect the rate of reaction. These are factors that affect the rate of reactions. Some important ones are:

1. Nature of reactants.
2. Concentration / pressure (for gases) of reactants.
3. Surface area of reactants
4. Temperature of reaction mixture
5. Presence of light 
6. Presence of catalysts

To study the effect of any one of these factors on rate of reaction all other factors must be kept constant.

EFFECT OF NATURE OF REACTANTS

If all other factors are kept constant, different substances will have different rate of reaction with dilute HCl, for example. When dilute HCl reacts with zinc, iron and gold under the same condition, hydrogen gas is evolved fast with zinc, slow with iron and no gas evolved with gold.

The difference in rate of reaction is due to the chemical nature of the elements as they naturally posses different amount of energy content.

EFFECT OF CONCENTRATION OF REACTANTS

The frequency of collision among particles is high when the particles are crowded in a small space, i.e high concentration. This leads to high effective collision and thus high rate of reaction. An increase or decrease in the concentration of the reactants will result in corresponding increase or decrease in effective collisions of the reactants and hence the reaction rate.

EFFECT OF SURFACE AREA OF REACTANTS

This is a very important factor to be considered when a solid is involved in a chemical reaction. Lumped solids offer small surface area of contact for reaction while powdered solids offer large surface area for reaction. Rate of reaction is slow with lumped solid but high with powdered solids.

EFFECT OF TEMPERATURE

Increasing the temperature of a system can lead to an increase in reaction rate in two ways. When heat is raised, energy in form of heat is supplied to the reactant particles, so that

1. The number of particles with energy equal to or greater than the activation energy increases.
2. The velocity of all the reactant particles increases due to the greater kinetic energy, leading to a higher frequency of collision.

As a result, the number of effective collisions increases and the reaction proceeds at a faster rate. Decreases in temperature leads to decrease rate of reactions.

EFFECT OF LIGHT 

Some reactions are influenced by light. The rate of reaction is high when the lights intensity is high, low when the intensity is low and does not proceed at all in the absence of light. Such reactions are known as photochemical reaction. Examples of photochemical reactions 

include.

1. Reaction between hydrogen and chlorine and 
2. Decomposition of hydrogen peroxide
3. Reactions between methane and chlorine
4. Photosynthesis in plant
5. Conversion of silver halides to grey metallic silver.

EFFECT OF CATALYST

A Catalyst is a substance, which alters the rate of a reaction, but itself does not undergo any change at the end of the reaction.

A positive catalyst increases the rate of reaction by lowering the activation energy of the reaction whereas, the one which increases the activation energy is known as a negative catalyst or an inhibitor.

The diagram below illustrates the energy profile for catalyzed and uncatalyzed exothermic and endothermic reactions 

EXOTHERMIC REACTION                                    ENDOTHERMIC REACTION

GENERAL EVALUATION/REVISION 

1. List and explain three factors that can affect the rate of reaction of the following reaction: CaCO3(s) + 2HCl(aq)→ CaCl2(aq) + H2O(l) + CO2(g)
2. In a chemical reaction, after 10seconds, 6moles from the initial concentration of 16moles of the reactant disappeared. Calculate the rate of the reaction.
3. State THREE characteristics of catalyst.
4. What is the volume in dm3 of 8g of oxygen gas at s.t.p?
5. Calculate the percentage of water in sodium trioxocarbonate (VI) heptahydrate

READING ASSIGNMENT: New School Chemistry for Senior Secondary School by O.Y.Ababio (6th edition) pages 250-261

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY 

1. The minimum amount of energy that colliding molecules must possess for their collisions to be effective is a. thermal energy b. collision energy 

1. activation energy d. kinetic energy   

2.Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

    The rate of evolution of hydrogen gas in the above reaction will be greatly increased if a. the zinc is in the form of pellets b. a smaller volume of H2SO4 is used c. the reaction flask is immersed in an ice bath d. the zinc is in powdered form   

1. The units of rate of reactions is  a. moldm-3s-1   b. mol-1s-1 c. mol-1 d. smol-1
2. If 2g of zinc granules was reacted with excess dilute HCl to evolve hydrogen gas which 

came to completion after 5min. Calculate the rate of the chemical reaction in ghr-1

1. 48ghr-1   b. 12ghr-1   c. 24ghr-d     d 240ghr-1

1. What do we do to increase the surface area of solid reactants to high rate of reaction? a. grinding them into powder   b. subjecting the reactants to high pressure  c. altering the directing of the reaction   d. using reactants of different densities.

SECTION B

1a.    Define rate of reaction

1. State the collision theory

2a.    Explain in terms of collision theory, how rate of gaseous reaction is affected by an increase in pressure

1. Give the reason why red-hot iron wool reacts more readily with oxygen than red-hot iron nail

WEEK TWO            DATE: _________

TOPIC: EXOTHERMIC AND ENDOTHERMIC REACTIONS

CONTENT

• Heat Content (Enthalpy) of a substance.
• Types of Heat of Reactions.
• Thermodynamics: First and Second Laws. 
• Entropy and Free energy

ENERGY

Energy is defined as the ability to do work. It exists in different forms like: heat, light, sound, electrical, potentials (stored), kinetic etc.

LAWS OF CONSERVATION OF ENERGY

Energy can be changed from one form to another. The total amount of energy before and after the change remains the same. This observation is stated in the law of conservation of energy which states that energy can neither be created nor destroyed but can be changed from one form to another.

There are types of energy such as chemical energy, heat energy, and light energy.

HEAT CONTENT (ENTHALPY) OF A SUBSTANCE

Heat content or Enthalpy of a substance is the characteristic internal energy possess by the substance, which is due to the structure and physical state of a substance. The potential energy is due to the structure while the kinetic energy is due to the physical state. Enthalpy of one substance is different from another. Total enthalpy cannot be measured but only enthalpy change. Generally, an enthalpy change (∆H) is the heat that would be exchanged with the surrounding, that is, it is the amount of energy involved in a reaction. 

Thus,

Enthalpy change = Heat of products – Heat of reactants

That is, ∆H = Hproducts - Hreactants

The enthalpy change of a given reaction is always written side by side with the given equation and it may be either a positive or negative value.

Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)      ∆H = -57.3kJ

Unit of enthalpy change(∆H) is Joules(J) or kilojoules(kJ)

EXOTHERMIC AND ENDOTHERMIC REACTIONS

EXOTHERMIC REACTION 

A chemical reaction in which heat is given off to the surrounding is known as exothermic reaction. When an exothermic reaction occurs, heat is liberated and transferred from the chemicals to the surroundings and the temperature of the reaction mixture rises. The reaction vessel will feel hot.

Examples of exothermic reactions include:

1. Reaction between calcium oxide and water 
2. Reaction between an acid and a base.
3. Combustion of fuel
4. Corrosion of metals
5. Respiration

In exothermic reaction, enthalpy change is negative since the heat content of the products is less than the heat content of the reactants.

ENDOTHERMIC REACTION

An Endothermic reaction is a type of reaction in which heat is absorbed from the surroundings. When an endothermic reaction occurs, heat energy is absorbed and transferred from the surrounding to the reactants and the temperature of the reaction mixture falls. The reaction vessel will feel cold.

Examples of endothermic reactions are:

1. Thermal decomposition of calcium trioxocarbonate (IV)
2. Thermal dissociation of ammonium chloride
3. Action of light on silver bromide in photographic film
4. Photosynthesis in plants 

In endothermic reaction, the heat content of the product is more than the heat content of the reactant; hence the enthalpy change is positive.

ENERGY LEVEL DIAGRAMS

Energy changes can be presented by diagrams, which shows at once whether reactions are exothermic or endothermic.

EVALUATION

1. What is enthalpy?
2. Giving TWO examples each, define endothermic and exothermic reactions.

HEAT OF REACTION AND CHEMICAL BONDS 

During chemical reactions, chemical bonds are broken, atoms are regrouped and new bonds are formed. Bond breaking requires energy and bond forming evolves energy. The minimum amount of energy required for bond breaking is called activation energy. While bond breaking is endothermic, bond forming is exothermic. Thus, heat of reaction comes from breaking and forming of chemicals bond. Heat reaction is negative [exothermic] when bond-breaking energy is less than bond forming energy. Heat of reaction is positive [endothermic] when bond-breaking energy is more than bond forming energy. 

TYPES OF HEAT CHANGES IN CHEMICAL REACTIONS

HEAT OF FORMATION

The amount of heat evolved or absorbed when one mole of a substance is formed from its elements is known as heat of formation [or enthalpy of formation].

The standard heat of formation of a substance(∆Hfθ) is the heat evolved or absorbed, when one mole of that substance is formed from its elements under standard conditions.

For the formation of 1 mole of liquid water, the equation is

H2(g)  + 1/2O2(g)→ H2O(1)          ∆Hfθ  = - 285kJmol-1 

Thus, ∆Hfθ of water = - 285kJmol-1 

HEAT OF NEUTRALIZATION

Neutralization is an exothermic reaction. The amount of heat evolved during a neutralization reaction in which one mole of water is formed is known as the heat of neutralization (or enthalpy of neutralization).The standard heat of neutralization ∆Hnθ  is the amount of heat evolved when 1 mole of hydrogen ions, H+, from an acid reacts with 1 mole of hydroxide ions, OH-, from an alkali to form 1 mole of water under standard conditions. Heat of neutralization is also known as heat of formation of one mole of water from its ionic components. 

     H+(aq)  +  OH-(aq)   →  H2O(l)            ∆Hnθ  = – 57.4kJmol-1

HEAT OF COMBUSTION 

Combustion reaction is always exothermic. The amount of heat evolved when one mole of a substance is burned completely in oxygen is known as the heat of combustion or enthalpy of combustion. The standard heat of combustion of a substance, ∆HCθ; is the heat evolved when one mole of the substance is burned completely in oxygen under standard conditions. 

A bomb calorimeter is usually used for accurate determination of heat of combustion.

Heat of combustion can be determined from the relation below:

Heat of combustion = Heat energy produced   x molar mass

    Mass burnt              1

When the heat evolved by the burning substance is used to raise the temperature of a known mass of water, then the expression for heat of combustion can be given as:

Heat of combustion =  mCâˆ†θ        x  molar mass 

                                   Mass burnt        1

Where m = mass of water

            C = Specific heat capacity of water

           âˆ†θ = change in temperature, that is, θ2 – θ1

HEAT OF SOLUTION

Heat of solution can be exothermic or endothermic. Heat of solution is the heat evolved or absorbed when one mole of a substance is dissolved in so much water that further dilution results in no detectable heat change.

Standard heat of solution, ∆Hsθ , is the amount of heat evolved or absorbed when 1 mole of substance is dissolved in so much water that further dilution results in no detectable heat change at standard conditions.  

EVALUATION

Define each of the following:

1. Heat of combustion
2. Heat of neutralization

THERMODYNAMICS

Thermodynamics is the study of relationship between heat and other forms of energy.

System in thermodynamics is any part of the universe chosen for thermodynamics consideration, i.e. the physical and chemical phenomenon or process occurring in a given 

environment.  A system can be isolated, closed or open. 

Surrounding is the environment in which a phenomenon or a process occurs.

The first law of thermodynamics states that energy can neither be created nor destroyed but may be converted from one form to another.

In thermodynamics, heat is represented by q and other forms of energy are referred to as work denoted by w.  The conditions or state of a chemical system is changed when:

1. Heat is evolved or absorbed, and / or
2. Work is done on or by the system

In any case, the internal energy, U, of the system is affected and it is changed.

From first law, heat is changed into internal energy of the system it may be represented by 

change in internal energy = Heat absorbed by the system + Work done by the system

i.e.     U    =    q    +    w

Work done by the system is negative since this lead to decrease in internal energy, therefore:

    U    =    q    -    w

For a gaseous system,    w  =  P  V

    U    =    q    -    P    V

U    =    H    -    P    V

    H    =    U    -    P    V

EVALUATION

1. State the first law of thermodynamic
2. Calculate: (a)     the heat adsorbed by a system when it does 72J of work and its internal energy decreases by 90J(b) U for a gas that releases 35J of heat and has 128J of work done on it.

SECOND LAW OF THERMODYNAMIC

The second law of thermodynamic states that a spontaneous process occurs only if there is an increase in the entropy of a system and its surroundings

Factors which determines the spontaneously of a process are:

1. enthalpy, H: The heat content of the substances involved
2. entropy, S: The measure of degree of disorderliness or randomness of a substance
3. free energy G: The energy which is available for doing work.

ENTROPY (S)

Entropy is the measure of degree of disorderliness or randomness of a system. The standard entropy change (∆Sθ) is a state function because it depends on the initial and final state of the system. That is:

∆Sθ = Sθproducts - Sθreactants

The S.Iunit of is JK-1mol-1

Entropy increases from solid to liquid to gaseous state because as you go from solid to liquid to gaseous state, randomness increases, that is; ∆Sθ tends to positive. 

For a reversible process at constant temperature,

    S    =    H/T

When ∆S is positive, there is increase in entropy.  When ∆S is negative there is decrease in the entropy of a system.

GIBB’S FREE ENERGY

The free energy of a system is the energy which is available for doing work in the system; that is, the driving force that brings about a chemical change.

The standard free energy change (∆Gθ) is a state function because it depends on the initial and final state of the system. That is:

∆Gθ = Gθproducts - Gθreactants

Free energy takes into account the effect of the enthalpy and entropy factors as represented in the equation below:

    G = H-TS

For a change at constant temperature,

         G =     H - T    S

NOTE:

1. When    G is negative, the reaction is spontaneous or feasible.
2. When   G is positive, the reaction is not spontaneous, unless the resultant effect of both      H and    S leads to a net decrease in     G
3. When    G is zero, the system is in equilibrium 

Example: The reaction:  C(s) +  O2(g)                 CO2(g)

is carried out at a temperature of 57oC.  If the enthalpy change is -500J and the entropy change is +15J.Calculate the free energy change

Solution:     

G =         H  - T   S 

   =    -5000  - (57 + 273)  x  (+15)

   =    -5000    - 330 x 15

   =    -5000    - (+4950)

   =    -5000    - 4950

   =    -9950J or -9.950kJ

GENERAL EVALUATION/REVISION

1. Define standard heat of formation and standard heat of neutralization
2. Define exothermic and endothermic reactions giving two example each
3. What is the number of oxygen atoms in 32g of the gas? [NA = 6.02 x 1023]
4. State the modern periodic law
5. The table below refers a portion of the periodic table:

I             II          III           IV           V         VI          VII             VIII

Lithium                        Carbon P

X                                                                         Q

Y        R

1. The most reactive metal
2. The most reactive non-metal
3. Name the family of the elements represented by P,Q and R
4. The element Q forms compounds with lithium and carbon. Write the formula of each compound formed

READING ASSIGNMENT

New School Chemistry for SSS by O.Y. Ababio(6th edition) pages 230- 242

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1. A chemical reaction in which heat is absorbed is said to be a. thermostatic b. 

isothermal c. exothermic d. endothermic

1. For the reaction: H3O+(aq)  +  OH-(aq) → 2H2O(l)  the heat change accompanying the process is heat of a. formation b. hydration c. neutralization d. combustion
2. A reaction in which ∆H is negative is a. isothermic b. endothermic c. adiabatic d. exothermic
3. How much heat will be liberated if 10g of hydrogen burns in excess oxygen according to the following equation:

 H2(g)  +  1/2O2(g) → H2O(l); ∆HCθ = -286Kj a. -1430kJ b. -2860kJ c. -572kJ d. -286kJ

1. Which of the following statement is TRUE about the dissolution of sodium hydroxide pellets in water? a. It is an endothermic process b. Heat of solution of sodium hydroxide is positive c. Heat is gained from the surrounding d. Heat of solution of sodium hydroxide is negative

SECTION B

1. State THREE factors that determine the spontaneity of a chemical reaction.
2. 0.92g of ethanol raised the temperature of 100g of water from 298K to 312.3K when burned completely. What is the heat of combustion of ethanol?