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?

It is a common observation that most of the reactions when carried out in a closed vessel do not go to completion under given set of conditions of temperature and pressure. In fact in such cases, in the initial state, only the reactants are present but as the reaction proceeds, the concentration of reactants decreases and that of products increases. Finally a stage is reached when no further change in concentration of reactants and products take place. This state at which the concentration of reactants and products do not change with time is called a state of chemical equilibrium.

The equilibrium can be established in physical processes as well as in chemical processes. Hence, equilibrium can be classified into following two categories:

Physical equilibrium

Equilibrium set up in physical processes is called physical equilibrium. For example, when liquid water is placed in a closed vessel at room temperature, it starts evaporating. As the reaction continues, more and more molecules of water escape to the vapor state and the level of water decreases continuously. But after sometime the level of water stops decreasing because the molecules of water vapours collected over the liquid water strike with the surface of liquid water and get condensed. This process of condensation acts in opposite direction to the process of evaporation. Finally a stage comes when the rate of evaporation of water becomes equal to the rate of condensation of vapours, and this stage is called equilibrium stage. So at equilibrium,

Rate of evaporation = Rate of condensation

or

H₂O (l)            H₂O (g)

Chemical equilibrium

The equilibrium set up in a chemical process is called a chemical equilibrium. For example- nitrogen gas can react with hydrogen gas to produce ammonia. This reaction can be shown as:

N₂ (g) +3H₂ (g)            2NH₃ (g)

This reaction is forward reaction as the reactants are converted into products.  However it is found that as soon as ammonia gas is produced in the above reaction it starts breaking into nitrogen gas and hydrogen gas in the reverse direction as given below:

2NH₃ (g)            N₂ (g) +3H₂ (g)

This reaction is exactly opposite the first reaction so it is called backward reaction. As this reaction is taking place in both the directions so equilibrium state will be attained in this reaction when the rate of forward reaction will become equal to the rate of backward reaction i.e. at equilibrium,

Rate of formation of ammonia = Rate of decomposition of ammonia

Now we can represent the forward reaction and backward reaction in equilibrium with each other as:

N₂ (g) +3H₂ (g)            2NH₃ (g)

Characteristics of chemical equilibrium

1) Chemical equilibrium is dynamic in nature

The term ‘dynamic’ means continuous activity. it means that after attaining the state of equilibrium the reaction does not stop though it may appear as if it has stopped. In fact, both the forward and backward reactions continue even after attaining the equilibrium. But the rate the forward reaction becomes equal to the rate of backward reaction. This means that if some products are formed from the reactants than an equal amount of products are converted back into reactants. As a result of this the concentration of both the reactants and products become constant. Since both the forward reaction and backward reaction are still going on so it is concluded that equilibrium is dynamic in nature.

2) The equilibrium can be attained only if the reaction is taking place in a closed vessel

For the establishment of equilibrium it is necessary that the vessel in which the reaction is taking place should be closed, otherwise the products formed during the forward reaction will escape into the environment and the backward reaction will not take place. Consequently it will not be possible to attain the equilibrium. Let us understand it with the help of an example.

Suppose liquid water is present in an open vessel which starts evaporating. Now as the vessel is open thus, the vapours formed will escape out of the vessel into the environment. Hence there will be no condensation of water vapours, and it will not be possible to attain the equilibrium.

3) The equilibrium can be attained from either the reactant side or from the product side.

4) At equilibrium the concentration of reactants and products do not change.

Test your understanding and answer these questions:

1. What are equilibrium reactions?
2. What is chemical equilibrium? Give characteristics of chemical equilibrium.

Irreversible reactions

Generally we found that once reactants are converted into product then the product cannot be again converted into reactants. So, irreversible reactions are those reactions in which the products manufactured by reaction in reactants cannot be converted back in reactants. These reactions are represented by putting single headed arrow () between reactants and products. For example,

The cooking gas (L.P.G.) used in the kitchen is mainly butane gas, which on combustion with air produces carbon dioxide and water as products.

C₄H₁₀(g) + O₂ (g)            CO₂(g) + H₂O (g) + Heat

In this reaction we cannot combine carbon dioxide and water to remanufacture butane so this is an irreversible reaction.

Reversible reactions

In these reactions the products manufactured can be again converted into the reactants from which they are formed. These reactions are represented by putting a double headed arrow () between reactants and products. For example,

       1) Nitrogen gas can react with hydrogen gas to produce ammonia. This reaction can be shown as:

N₂ (g) +3H₂ (g)            2NH₃ (g)

However it is found that as soon as ammonia gas is produced in the above reaction it starts breaking into nitrogen gas and hydrogen gas in the reverse direction as given below:

2NH₃ (g)            N₂ (g) +3H₂ (g)

Now because the reaction is taking place in both the directions i.e. in forward and in backward direction so we conclude that this reaction is reversible in nature, and it can be represented as:

N₂ (g) +3H₂ (g)            2NH₃ (g)

       2) Dehydration of hydrated salt of copper sulphate is also an example of reversible reaction. The chemical formula of hydrated copper sulphate is CuSO₄.5H₂O. In the formula of copper sulphate 5 molecules of water of crystallization are present. Due to the presence the water molecules in copper sulphate crystals its colour is blue

When we heat the crystals of copper sulphate then the 5 molecules of water evaporates and the blue crystals of copper sulphate changes into white colour as given below:

However this reaction can be reversed by allowing the white crystals of copper sulphate to cool down. On cooling the crystals of copper sulphate will absorb the water vapors present in air and blue colour will reappear.

So, this reaction can be represented as:

       3) The process of transportation of oxygen gas by protein hemoglobin present in blood in our body is also reversible reaction. When we breathe in oxygen in our body then the hemoglobin of blood attaches the oxygen gas with itself to make a complex called oxyhemoglobin in our body. Then this oxygenated blood reaches every part of body. There this oxyhemoglobin complex breaks down and oxygen is given to body tissues and cells.

Oxygen + Hemoglobin            Oxyhemoglobin

In this way we conclude that transportation of oxygen in our body is a reversible reaction.

Test your understanding and answer these questions:

1. What are reversible reactions? Give example.
2. What are irreversible reactions? Give example.

There are a number of factors which affect the rate of reaction such as-

    1. concentration of reactants
    2. Temperature
    3. Catalyst
    4. Radiations
    5. Surface area of reactants

Let us briefly discuss these factors

1. Concentration of reactants

It is has been observed that the rate of chemical reaction is directly proportional to the concentration of the reactants. In the starting of a reaction when the concentration of reactants is maximum, the rate of reaction is also maximum. But as the reaction precedes to completion the concentration of reactants decreases as some amount of reactants is converted into products. So, the rate of reaction also decreases. This can be understood with the help of following example, when a piece of wood if burnt in air having less amount of oxygen say 30% , it will burn slowly but if it is burnt is air having large amount of oxygen say 100% then it will burn rapidly because the concentration of oxygen is more.

2. Temperature

In general, an increase in the temperature increases the rate of reaction or a decrease in temperature decreases the rate of reaction. This can be understood by taking following examples. We generally observe that milk is spoilt earlier in summer than in winter. It is due to the reason that in summer the temperature is high as compared to winters. So the microorganisms responsible for spoiling the milk are more active in summer as compared to the winter. In the same way we keep fruits and vegetables in refrigerators so that these can remain fresh for a long time because inside the refrigerator the temperature is low so the food spoiling microorganisms cannot easily spoil the food because their activity is reduced. But if we keep these fruits and vegetable in open outside the refrigerator then it is spoilt very soon due to high activity of microorganism because the temperature is high.

When we react calcium carbonate with hydrochloric acid to produce carbon dioxide at normal temperature then this reaction takes place very slowly and a very few bubbles of carbon dioxide are produced. However, when we raise the temperature slightly more, then this reaction takes place very fast and a large no. of bubbles of carbon dioxide is produced.

In general for every 100C raise in the temperature the rate of most chemical reactions becomes double.

3. Catalyst

Catalysts are the substances which change the rate of chemical reactions without undergoing any change in them. It means that catalysts are those substances which help in changing the rate of chemical reactions but they don’t themselves undergo any change in the end of chemical reaction. Due to this reason these are also known as marriage broker in Chinese language. For example, In the manufacture of ammonia, iron is used as a catalyst to increase the rate of reaction.

N₂ (g) +3H₂ (g)            2NH₃ (g)

Similarly, reaction of Sulphur dioxide and oxygen to produce Sulphur trioxide takes place in the presence of catalyst nitrogen monoxide (NO).

2SO₂ (g) + O₂ (g)            2SO₃ (g)

Types of catalysts:

Normally catalysts are used to increase the rate of a particular reaction. The catalysts which are used to increase the rate of a chemical reaction are known as Positive catalysts. But sometimes catalysts can also be used to slow down the rate of reaction. For example, glycerin is sometimes added to hydrogen peroxide to slow down it decomposition. The catalysts which slow down the rate of a particular reaction are known as Negative catalysts.

How catalysts work?

For a reaction to take place, the molecules must have a certain energy called activation energy. If the activation energy is higher, then the reaction will be slow. A catalyst works by lowering the activation energy of the chemical reaction.

Imagine the activation energy as a hill that you have to ride over. And the use of a catalyst is like taking another path, which avoids the hill. So a catalyst increases the rate of a chemical reaction by providing an alternate path having less activation energy.

4. Radiations

The rate of some chemical reactions increases in the presence of light. For example,

The process of photosynthesis for preparation of food in plants takes place only in the presence of light of sun. In this process plants convert the carbon dioxide and water into starch and oxygen in the presence of sunlight with the help of green colouring pigment chlorophyll

Reaction of hydrogen with chlorine also takes place in the presence of sunlight.

The process of photography is also a photochemical reaction. Actually the photographic film is coated with silver bromide (AgBr) which undergoes chemical reaction when exposed to sunlight.

 

5. Surface area of reactants

The rate of chemical reactions also increases by increasing the surface area of reactants. For example, a log of wood burns slowly but if it is cut into small wooden chips, the burning takes place rapidly. This is due the reason that the total surface area of wooden chips is more as compared to the surface area of wooden log.

Test your understanding and answer these questions:

1. Explain the factors that effect the rate of chemical reaction?

Most of the chemical reactions are accompanied by energy changes. In some reactions, energy is given in the form of heat and light. While in some other chemical reactions heat and light energy is absorbed. So, chemical reactions can be classified into following categories depending upon absorption or liberation of heat or light energy.

    1. Exothermic reactions
    2. Endothermic reactions
    3. photochemical reactions

Exothermic reactions

The chemical reactions in which energy is liberated in the form of heat are called exothermic reactions. In these reactions the energy required to break down the chemical bonds in reactants is usually less than the energy released during formation of chemical bonds in products. For example, when methane gas is burnt in the presence air then a lot of heat is produced.

CH₄(g) + 2O₂ (g)           CO₂(g) + 2H₂O (l) + Heat

Another example of exothermic reaction is reaction of iron oxide with aluminium to produce iron and aluminium oxide. This reaction is called thermite reaction and this reaction is used in welding of iron rails.

Endothermic reactions

The chemical reactions in which energy is absorbed are called endothermic reactions. In these reactions the energy required to break down the chemical bonds in reactants is more than the energy released during formation of chemical bonds in products. For example, when barium hydroxide and ammonium thiocynate are mixed together the reaction mixture becomes very cold which shows that it has absorbed energy so it is an endothermic reaction.

Another example of endothermic reaction is evaporation of water which takes place by absorption of heat energy.

Photochemical reactions

The chemical reactions which take place in the presence of light are known as photochemical reactions. Following are the examples of photochemical reactions-
The process of photosynthesis for preparation of food in plants takes place only in the presence of light of sun. in this process plants convert the carbon dioxide and water into starch and oxygen in the presence of sunlight with the help of chlorophyll pigment

Reaction of hydrogen with chlorine also takes place in the presence of sunlight.

The process of photography is also a photochemical reaction. Actually the photographic film is coated with silver bromide (AgBr) which undergoes chemical reaction when exposed to sunlight.

Test your understanding and answer these questions:

1. Define endothermic reactions. Give examples.
2. Define exothermic reactions. Give examples.
3. What do you understand by photochemical reactions. give examples.

Average rate of chemical reaction

It may be defined as the change in concentration of a reactant or product of a chemical reaction in a given interval of time. So

Let us take an example to understand this. When acidified hydrogen peroxide (H₂O₂) is added to a solution of potassium iodide (KI) then iodine is liberated.

H₂O₂ (aq) + 2KI (aq) + H₂SO₄ (aq)  2H₂0 (l) + I₂ (aq) + K₂SO₄ (aq)

Here initially the concentration of iodine is zero. But with the passage of time, it increases and the reaction solution becomes brownish. Concentration of iodine can be measured at different intervals of time by titration against sodium thiosulphate.

If concentration of iodine rises from 0 to 10^-5 mol L^-1 in 10 seconds.

Instantaneous rate of reaction

It may be defined as the change in concentration of a reactant or product of a chemical reaction at a given instant. So, we can calculate the instantaneous reaction rate of above reaction at any instant by using the following formula
       Instantaneous reaction rate = 

Where d[I₂] = small change in concentration of iodine

and        dt  = small change in time

Test your understanding and answer these questions:

1. What is average rate of reaction?
2. What is instantaneous rate of reaction?