During nuclear reactions high energy radiations called nuclear radiations are continuously produced. These radiations effect human beings, animals, plants and buildings very seriously. Actually the nuclear radiations consist of high energy gamma rays which have high penetrating power. Thus, when a person is exposed to these radiations, these radiations penetrate into his body cells and cause deadly diseases such as cancer and leukemia (blood cancer) by disrupting cell membranes, reducing effectiveness of enzymes and changing constitution of chromosomes or genes.

The nuclear radiations cause defects in eyes and reduce ability of body to fight against infections. These rays also affect various organs of the body such as lungs, kidneys and liver. The ill effects of nuclear radiations can be classified into two types: somatic effects and genetic effects. The somatic effects affect only the body of that particular person who is exposed to nuclear radiations whereas genetic effects are passed from one generation to next generation.

During explosions of nuclear bombs, the effects of nuclear radiations are highly destructible. A nuclear bomb may cause death and destruction of millions of human beings, animals and plants. Any accidental leakage of radiations from the nuclear reactor can also cause disaster. One such accident has taken place in nuclear plant of Chernobyl located in U.S.S.R. This accident took place in April 26, 1986. This accident led to many deaths and resettlement of about 2 lakh people.

The other important source of nuclear radiations is nuclear waste. Nuclear wastes are those substances which are produced in nuclear reactor during nuclear reaction and have the ability to emit harmful nuclear radiations. Generally, nuclear wastes are produced when

1. mining and processing of ore is done.
2.  radioactive materials are used in nuclear reactors.
3.  radio isotopes are used in medical, industrial and research applications.

 The safe disposal of nuclear wastes is a very serious problem. Usually, nuclear wastes are stored in a very strong concrete container and dumped into sea. Otherwise, nuclear wastes are also stored in thick lead containers and buried deep in the earth.

Test Your Understanding and Answer These Questions:

1. What is radiation poisoning?
2. What is nuclear waste?
3. What are somatic effects of radiations?
4. What are genetic effects of radiations?

The nature of a substance that weather it is acidic or basic can be determined in terms of concentration of hydrogen ion or hydroxide in it. If an aqueous solution has an equal concentration of hydrogen ions and hydroxide ions, then it is neither acidic nor basic, such a solution is said to be neutral. Now, if the aqueous solution has more concentration of hydrogen ions than hydroxide ions then it will be acidic solution. On the other hand, if an aqueous solution has more concentration of hydroxide ion than hydrogen ion then it will be basic in nature.

In 1909, Sorensen devised a scale known as pH scale on which the acidic nature as well as basic nature of solutions can be expressed only by considering the hydrogen ion concentration in them. The pH of a solution may be defined as:

       The ph of a solution is the negative logarithm of hydrogen ion concentration i.e.

       pH = – log [H⁺]

For calculating the pH of the solution, we have to use the concentration of hydrogen ions in moles/litre. We will now find out the pH of pure water.

pH of pure water

The concentration of hydrogen ions in pure water is 10^-7 M which means [H⁺] = 10^-7

        pH = -log [H⁺]

             = -log [10^-7]

             = – [-7]                         [because log [10^-7]= -7]

    so pH = 7

Thus pH of water is 7. Whenever the pH of a solution is 7, it will be a neutral solution. Such solutions have no effect on litmus paper.

In the similar manner we can also find out the pH of acidic solutions as well as basic solutions by substituting different values of hydrogen ion concentrations [H⁺] in above relation. It is found that for acidic solution the pH value is always less than 7 and for basic solution the pH value is always more than 7. Thus, we can represent the pH scale in following way:

Test your understanding and answer these questions:

1. What is pH scale?
2. Who invented pH scale?

The word acid is derived from Greek word which means ‘sour’. These have generally sour taste and they can change blue litmus into red. On the other hand the bases are the substance which have bitter taste and which can convert red litmus into blue. When bases are dissolved in water these are given a special name alkali.

Concepts of acids and bases

       Following are the three concepts of acids and bases-

    1. Arrhenius concept
    2. Bronsted- Lowry concept
    3. Lewis concept

1. Arrhenius concept

Acid

According to Arrhenius an acid is a substance which dissociates in an aqueous solution to give hydrogen ions (H⁺). For example, HCl, HNO₃, CH₃COOH, H₂SO₄ etc. out of these acids some acids are strong acids while others are weak. The strength of an acid depends upon its degree of dissociation in water. The acids which can completely dissociate into ions on adding with water thus produce a large number of H⁺ ions are called strong acids e.g. HCl, HNO₃ and H₂SO₄ are strong acids because they dissociate into ions completely. On the other hand the acids which partially dissociate into ions on addition with water and give small number of H⁺ ions are called weak acids e.g. CH₃COOH is a weak acid. The dissociation of acids in water can be shown by following equations:

Strong acids:

HNO₃           H⁺ + NO_3^–

H₂SO₄           2H⁺ + SO_4^2-

HCl           H⁺ + Cl_^–

Weak acid:

CH₃COOH            H⁺ + CH₃COO^–

Base

According to Arrhenius a base is a substance which dissociate in an aqueous solution to give hydroxyl ions (OH^–). For example, NaOH, KOH, Ca(OH)₂ are bases because these can dissociate in water to give hydroxyl ions as given below-

Strong bases:

The bases which completely dissociate into water and give a large number of hydroxyl ions (OH^–) are called as strong base. For example, NaOH and KOH.

NaOH           Na⁺ + OH^–

KOH           K⁺ + OH^–

Weak bases:

The bases which partially dissociate into ions on addition of water and give a small number of hydroxyl ions (OH^–) are called weak bases. For example, Ca(OH)₂.

Ca(OH)₂            Ca²⁺ + 2OH^–

2. Bronsted-Lowry concept

According to Bronsted Lowry concept an acid is defined as a substance which has a tendency to donate a proton (H⁺) to any other substance and a base is a substance which has a tendency to accept proton (H⁺) from any other substance. In other words, an acid is proton donor and a base is proton acceptor. In terms of this definition, acids and bases are inter-related with each other as shown below:

Acid            Base + proton

Examples:

HCl + NH₃            NH_4⁺ + Cl ^–
Acid   base                                      

CH₃COOH + NH₃            NH_4⁺ + CH₃COO ^–
Acid       base                                          

In these examples HCl and CH₃COOH lose a proton so these act as acids while NH₃ accepts a proton so it acts as a base.

3. Lewis concept

According to Lewis concept an acid is a substance which can accept a pair of electrons from other substances while a base is a substance which can donate a pair of electrons to other substances. In other words an acid is an electron pair acceptor while a base is an electron pair donor. For example,

NH₃ + BF₃            NH₃-BF₃
base                                  acid   

Test your understanding and answer these questions:

1. Give arhenius concept of acids and base?
2. Give bronsted lowry concept of acid and base.
3. Give Lewis concept of acid and base.

A compound which conducts electricity when dissolved in water or in molten state is called an electrolyte. For example, the aqueous solution of sodium chloride (NaCl), copper sulphate (CuSO₄), sodium hydroxide (NaOH) and silver nitrate (AgNO₃) are electrolytes. Actually when an electrolyte is dissolved in water then it splits into ions which are responsible for conduction of electricity.

Let us understand it more deeply, the table salt consists sodium ion (Na⁺) and chloride ion (Cl^–). In solid state the sodium ions are bound with chloride ions very strongly with the help of strong electrostatic forces. So, in the solid sodium chloride the ions are not free to move, thus it cannot conduct the electricity through itself in solid state. But when table salt is dissolved in water then the forces holding together the two ions are broken and the sodium ions and chloride ions become free to move in the aqueous solution. As both the ions have become free now so they can conduct electricity. This process of breaking sodium chloride into ions when dissolved in water can be represented as

This process of breaking compounds into ions by the action of water is called dissociation or ionization.

Now we have learnt that an electrolyte is a substance which conducts electricity when dissolved in water or in molten state by breaking into ions. But all the electrolytes do not ionize to the same extent. Depending on the extent of ionization, the electrolytes can be classified into two types.

    1. Strong electrolytes
    2. Weak electrolytes

Strong electrolytes

The electrolytes which ionize almost completely into ions in aqueous solution are called as strong electrolytes. E.g. HCl, H₂SO₄, HNO₃, NaOH, KOH, NaCl, KCl₃ etc. Usually all ionic compounds are strong electrolytes.

The equation for the ionization reactions of strong electrolytes are written with only single headed arrow directed to the right. For example:

or

HCl +H₂O           H₃O⁺ + Cl^–

Weak electrolytes

The electrolytes which ionize to a small extent in aqueous solution are called as weak electrolytes. E.g. CH₃COOH, NH₄OH, HCN etc. Usually all the covalent compounds are weak electrolytes.

In such cases, the molecules are in equilibrium with their ions. The ionization of such electrolytes is represented with the double headed arrows. For example:

CH₃COOH + H₂O            H₃O⁺ + CH₃COO^–

NH₃ + H₂O            NH₄⁺ + OH^–

Differences between strong electrolytes and weak electrolytes

S No.	Strong electrolytes	Weak electrolytes
1.	The electrolytes which ionize almost completely into ions in aqueous solution are called as strong electrolytes.	The electrolytes which ionize to a small extent in aqueous solution are called as weak electrolytes.
2.	The equation for the ionization reactions of strong electrolytes are written with only single headed arrow directed to the right.	The ionization of such electrolytes is represented with the double headed arrows.
3.	Usually all ionic compounds are strong electrolytes.	Usually all the covalent compounds are weak electrolytes.
4.	Examples:- HCl, H2SO4, HNO3, NaOH, KOH, NaCl, KCl3 etc.	Examples :- CH3COOH, NH4OH, HCN etc.

Non electrolytes

The compounds which do not conduct electricity in molten state or in aqueous solution are known as non-electrolytes. For example, sugar, urea, glycerin etc.

Test your understanding and answer these questions:

1. What are electrolytes? Give examples.
2. Give differences between strong electrolytes and weak electrolytes.
3. What are non-electrolytes?

It has been found experimentally that at a particular temperature, when equilibrium is achieved, the ratio between the concentration of products and reactants becomes constant. This can be represented with the help of following formula:

Example:- In reaction

N₂ (g) +3H₂ (g)            2NH₃ (g)

       Here the concentration of each term [NH₃], [N₂] and [H₂], is raised to the power equal to the stoichiometric coeffient and Kc is called equilibrium constant.

Importance of equilibrium constant Kc: Magnitude of equilibrium constant Kc indicates the extent of a chemical reaction. Larger the value of Kc, higher will be the concentration of products at equilibrium. Smaller value of Kc indicates the lower concentration of the products at equilibrium.

Test your understanding and answer these questions:

1. What is equilibrium constant? What is its importance?

Following are the factors which affect the equilibrium state of a reaction-

    1. Temperature
    2. Pressure
    3. Concentration of reactants and products
    4. Catalyst

The effect of these factors on chemical equilibrium can be understood with the help of Le chatelier’s principle.

Le chatelier’s principle

According to Le chatelier’s principle when the temperature, pressure or concentration of a reaction in equilibrium is changed then the reaction shifts in that direction in which the effect of these changes is reduced.

We can understand the Le chatelier’s principle by taking example of reaction of nitrogen with hydrogen to produce ammonia as

N₂ (g) +3H₂ (g)            2NH₃ (g)

Effect of change in temperature

We already know that there are two types of reactions

    1. exothermic reactions and
    2. endothermic reactions.

In a reversible reaction if forward reaction is of one type then the backward reaction will be of another type. Here we will discuss both the cases.

N₂ (g) +3H₂ (g)            2NH₃ (g)

When the forward reaction is exothermic:

According to Le chatelier’s principle if the temperature of the reaction is increased then the equilibrium will shift in that direction in which the effect of increased temperature is reduced i.e. the equilibrium will shift in backward direction, which means that on increasing the temperature the ammonia will decompose into nitrogen and hydrogen.

When the forward reaction is endothermic:

According to Le chatelier’s principle if the temperature of the reaction is decreased then the equilibrium will shift in that direction in which the effect of decreased temperature is reduced i.e. the equilibrium will shift in forward direction, which means that on decreasing the temperature the nitrogen and hydrogen will combine together to form ammonia. Thus

Effect of pressure

According to Le chatelier’s principle , an increase in pressure will favour the reaction in that direction in which the volume of reactants is reduced and decrease in pressure will favour the reaction in that direction in which the volume of reactants is increased.

In this reaction the volume of reactants is 4 units while the volume of products is 2 units. So according to Le chatelier’s principle an increase in pressure of this reaction will favour the forward reaction to form more ammonia while a decrease in pressure of the reaction will favour the backward reaction to form more nitrogen and hydrogen. Thus

Effect of change in concentration

1. Effect of change in concentration of reactants:- according to Le chatelier’s principle if we increase the concentration of reactants then the reaction will shift in forward direction

N₂ (g) +3H₂ (g)            2NH₃ (g)

An increase in concentration of nitrogen and hydrogen will result in shifting of reaction in forward direction in which more ammonia will be formed.

2. Effect of change in concentration of products:- if we increase the concentration of products (ammonia) then the reaction will shift in backward direction i.e. formation of nitrogen and hydrogen will take place.

Effect of catalyst

       A catalyst has no effect on equilibrium state of a reaction. It is added into the reaction mixture only to achieve the equilibrium state quickly because addition of a catalyst increases the rate of both the forward reaction and backward reaction equally.

Test your understanding and answer these questions:

1. What is Le chatelier’s principle? Explain.
2. What factors effect chemical equilibrium?