NCERT Solutions for Class 10 Science Chapter 1 Chemical Reactions and Equations

Intext exercise 1

1. Why should a magnesium ribbon be cleaned before burning in air?

 Ans:

Magnesium is a kind of a reactive metal. It will react with oxygen and form a white layer of magnesium oxide (MgO) on its surface. Magnesium oxide is a stable compound and will prevent the further reaction of the metal. So, a magnesium ribbon is cleaned before burning in air to remove the layer of magnesium oxide from its surface.

2. Write the balanced equation for the following chemical reactions.

i) Hydrogen + Chlorine $\to $ Hydrogen chloride

Ans: A balanced equation consists of the same number of moles on the reactants and the products side. The balanced equation for the given reaction is:${{H}_{2(g)}}+C{{l}_{2(g)}}\to 2HC{{l}_{(g)}}$

ii) Barium chloride + Aluminium sulphate $\to $ Barium sulphate + Aluminium chloride

Ans: A balanced equation consists of the same number of moles on the reactants and the products side. The balanced equation for the given reaction is:$3BaC{{l}_{2(s)}}+A{{l}_{2}}{{(S{{O}_{4}})}_{3(s)}}\to 3BaS{{O}_{4(s)}}+2AlC{{l}_{3(s)}}$

iii) Sodium + Water $\to $ Sodium hydroxide + Hydrogen 

Ans:

A balanced equation consists of the same number of moles on the reactants and the products side. The balanced equations for the given reaction is $2N{{a}_{(s)}}+2{{H}_{2}}{{O}_{(l)}}\to 2NaO{{H}_{(aq)}}+{{H}_{2}}_{(g)}$ 

3. Write a balanced chemical equation with state symbols for the following reactions.

i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

Ans: A balanced chemical equation has the same number of moles of reactants and products. The balanced chemical equation is:$BaC{{l}_{2(aq)}}+N{{a}_{2}}S{{O}_{4(aq)}}\to BaS{{O}_{4(s)}}+2NaC{{l}_{(aq)}}$ 

ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Ans: A balanced chemical equation has the same number of moles of reactants and products. The balanced chemical equation is:$NaO{{H}_{(aq)}}+HC{{l}_{(aq)}}\to NaC{{l}_{(aq)}}+{{H}_{2}}{{O}_{(l)}}$ 

Intext exercise 2

1. A solution of a substance 'X ' is used for white washing.

i) Name the substance 'X' and write its formula.

Ans: Substance ‘X’ is calcium oxide also known as quicklime that is used in white washing. Its chemical formula is CaO.

ii) Write the reaction of the substance 'X' named in (i) above with water. 

Ans:Calcium oxide reacts with water in a combination reaction to from calcium hydroxide also called slaked lime, the reaction is:

$Ca{{O}_{(s)}}+{{H}_{2}}{{O}_{(l)}}\to Ca{{(OH)}_{2(aq)}}$ 

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas. 

Ans:

Water consists of hydrogen and oxygen atoms. It is a combination of two hydrogen and one oxygen atom. In electrolysis, hydrogen moves towards the cathode and oxygen towards the anode. So, the ratio of hydrogen and oxygen is 2:1. And hydrogen is double the amount of oxygen. 

$2{{H}_{2}}O\to 2{{H}_{2}}+{{O}_{2}}$  

(Image Will Be Updated Soon)

Intext exercise 3

1. Why does the color of copper sulphate solution change when an iron nail is dipped in it?

 Ans:

When an iron nail is dipped in a solution of copper sulphate, then the more reactive metal displaces the less reactive metal. As a result the blue solution of copper sulphate gets faded and forms a green compound known as ferrous sulphate.

\[\underset{Iron}{\mathop{F{{e}_{(s)}}}}\,+\underset{\begin{smallmatrix} 

 Copper\text{ }sulphate \\ 

 (Blue) 

\end{smallmatrix}}{\mathop{CuS{{O}_{4(aq)}}}}\,\to \underset{Ferrous\text{ }sulphate}{\mathop{FeS{{O}_{4(aq)}}}}\,+\underset{\begin{smallmatrix} 

 Copper \\ 

 (Green) 

\end{smallmatrix}}{\mathop{C{{u}_{(s)}}}}\,\] 

2. Give an example of a double displacement reaction other than the one given in Activity 1.10. 

Ans:

A double displacement reaction consists of two ions replacing each other’s position from the reactants to form new compounds in the products. A double displacement reaction occurs when Potassium iodide reacts with lead nitrate to form lead iodide (yellow precipitate) and potassium nitrate. 

\[2KI+Pb{{(N{{O}_{3}})}_{2}}\to Pb{{I}_{2}}\downarrow +2KN{{O}_{3}}\] 

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions

i) $\text{4N}{{\text{a}}_{\text{(s)}}}\text{+}{{\text{O}}_{\text{2(g)}}}\to \text{2N}{{\text{a}}_{\text{2}}}{{\text{O}}_{\text{(s)}}}$ 

Ans: Any species is said to be oxidised when it loses electrons and increases the oxidation state, while any species is said to be reduced when it gains electrons and decreases the oxidation state. Sodium (Na) is oxidised and oxygen gets reduced.

\[4\text{Na}+{{\text{O}}_{2}}\to 2\text{N}{{\text{a}}_{2}}\text{O}\] 

ii) $\text{Cu}{{\text{O}}_{\text{(s)}}}\text{+}{{\text{H}}_{\text{2(g)}}}\to \text{Cu(s)+}{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{(l)}}}$ 

Ans: Any species is said to be oxidised when it loses electrons and increases the oxidation state, while any species is said to be reduced when it gains electrons and decreases the oxidation state. Copper oxide (CuO) is reduced to copper (Cu) as it loses oxygen and hydrogen $\left( {{H}_{2}} \right)$is oxidized to water $({{H}_{2}}O)$ 

    \[CuO+{{H}_{2}}\xrightarrow{\Delta }Cu+{{H}_{2}}O\] 

NCERT exercises

1. Which of the statements about the reaction below are incorrect?

$\text{2Pb}{{\text{O}}_{\text{(s)}}}\text{+}{{\text{C}}_{\text{(s)}}}\to \text{2P}{{\text{b}}_{\text{(s)}}}\text{+C}{{\text{O}}_{\text{2(g)}}}$ 

a. Lead is getting reduced.

b. Carbon dioxide is getting oxidised.

c. Carbon is getting oxidised.

d. Lead oxide is getting reduced. 

Options

i) (a) and (b)

ii) (a) and (c) 

iii) (a),(b) and (c)

iv) all

Ans:

Incorrect option is (i) (a) and (b)

2. $\text{F}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{+2Al}\to \text{A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{+2Fe}$ 

The above reaction is an example of a _______________

 Options:

a) Combination reaction.

b) Double displacement reaction.

c) Decomposition reaction.

d) Displacement reaction.

 Ans:

(d) The given reaction is an example of a single displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.

a. Hydrogen gas and iron chloride are produced.

b. Chlorine gas and iron hydroxide are produced.

c. No reaction takes place,

d. Iron salt and water are produced. 

Ans:

(a) Hydrogen gas and iron chloride are produced. The reaction is as follows:

\[F{{e}_{(s)}}+2HC{{l}_{(aq)}}\to FeC{{l}_{2(aq)}}+{{H}_{2}}\uparrow \] 

4. What is a balanced chemical equation? Why should chemical equations be balanced? 

Ans:

A balanced chemical equation consists of the equal number of moles of the elements in the reactants and that in the products.

Chemical equations should be balanced, as according to the law of conservation of mass, mass can neither be created nor can be destroyed. Therefore, in a chemical reaction, the total mass of reactant should be equal to the total mass of products. Hence, a balanced chemical equation has total mass of reactants equal to total mass of products.

5. Translate the following statements into chemical equations and then balance them.

a) Hydrogen gas combines with nitrogen to form ammonia.

Ans: $3{{H}_{2(g)}}+{{N}_{2(g)}}\to 2N{{H}_{3(g)}}$

b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

Ans:$2{{H}_{2}}{{S}_{(g)}}+3{{O}_{2(g)}}\to 2{{H}_{2}}{{O}_{(l)}}+2S{{O}_{2(g)}}$

c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

Ans:$3BaC{{l}_{2(aq)}}+A{{l}_{2}}{{(S{{O}_{4}})}_{3(aq)}}\to 2AlC{{l}_{3(aq)}}+3BaS{{O}_{4(s)}}$

d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas 

Ans:$2{{K}_{(s)}}+2{{H}_{2}}{{O}_{(l)}}\to 2KO{{H}_{(aq)}}+{{H}_{2(g)}}$

6. Balance the following chemical equations

a)$\text{HN}{{\text{O}}_{3}}+\text{Ca}{{(\text{OH})}_{2}}\to \text{Ca}{{\left( \text{N}{{\text{O}}_{3}} \right)}_{2}}+{{\text{H}}_{2}}\text{O}$ 

Ans: $2HN{{O}_{3}}+Ca{{(OH)}_{2}}\to Ca{{\left( N{{O}_{3}} \right)}_{2}}+2{{H}_{2}}O$

b)$\text{NaOH}+{{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}\to \text{N}{{\text{a}}_{2}}\text{S}{{\text{O}}_{4}}+{{\text{H}}_{2}}\text{O}$ 

Ans: $2NaOH+{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{4}}+2{{H}_{2}}O$

c)$\text{NaCl}+\text{AgN}{{\text{O}}_{3}}\to \text{AgCl}+\text{NaN}{{\text{O}}_{3}}$ 

Ans: \[NaCl+AgN{{O}_{3}}\to AgCl+NaN{{O}_{3}}\]

d)$\text{BaC}{{\text{l}}_{\text{2}}}\text{+}{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\to \text{BaS}{{\text{O}}_{\text{4}}}\text{+HCl}$ 

Ans:$BaC{{l}_{2}}+{{H}_{2}}S{{O}_{4}}\to BaS{{O}_{4}}+2HCl$

7. Write the balanced chemical equations for the following reactions.

a) Calcium hydroxide + Carbon dioxide $\to $ Calcium carbonate + Water

Ans: $Ca{{(OH)}_{2}}+C{{O}_{2}}\to CaC{{O}_{3}}+{{H}_{2}}O$

b) Zinc + Silver nitrate $\to $Zinc nitrate +Silver

Ans: $Zn+2AgN{{O}_{3}}\to Zn{{\left( N{{O}_{3}} \right)}_{2}}+2Ag$

c) Aluminium + Copper chloride $\to $Aluminium chloride + Copper

Ans: $2Al+3CuC{{l}_{2}}\to 2AlC{{l}_{3}}+3Cu$

d) Barium chloride + Potassium sulphate$\to $Barium sulphate + Potassium chloride

Ans:$BaC{{l}_{2}}+{{K}_{2}}S{{O}_{4}}\to BaS{{O}_{4}}+2KCl$ 

8. Write the balanced chemical equation for the following and identify the type of reaction in each case.

a) Potassium bromide (aq) + Barium iodide (aq) $\to $Potassium iodide (aq) + Barium bromide(s)

Ans: $2KB{{r}_{(aq)}}+Ba{{I}_{2(aq)}}\to 2K{{I}_{(aq)}}+BaB{{r}_{2(aq)}}$ ; Double displacement reaction

b) Zinc carbonate (s) $\to $Zinc oxide (s) + Carbon dioxide (g)

Ans: $ZnC{{O}_{3(s)}}\to Zn{{O}_{(s)}}+C{{O}_{2(g)}}$ ; Decomposition reaction

c) Hydrogen (g)+ Chlorine (g)$\to $Hydrogen chloride (g)

Ans: ${{H}_{2(g)}}+C{{l}_{2(g)}}\to 2HC{{l}_{(g)}}$ ; Combination reaction

d) Magnesium (s) + Hydrochloric acid (aq) $\to $Magnesium chloride (aq) + Hydrogen (g) 

Ans:$M{{g}_{(s)}}+2HC{{l}_{(aq)}}\to MgC{{l}_{2}}_{(aq)}+{{H}_{2}}_{(g)}$ ; Single displacement reaction

9. What does one mean by exothermic and endothermic reactions? Give examples. 

Ans:

Exothermic Reaction: The reactions in which heat is evolved along with the formation of new products are called exothermic reactions. In these reactions the energy of the reactants is more than that of the products, so energy is released to complete the reaction. 

Energy of reactants > Energy of products

Example: Complete combustion of methane gas produces carbon dioxide and water along with heat and light.  

\[C{{H}_{4}}(~g)+2{{O}_{2}}(~g)\to C{{O}_{2}}+2{{H}_{2}}O+heat+light\] 

Another example of exothermic reactions is respiration and decomposition of vegetable matter.

Endothermic Reaction: The reactions in which energy is absorbed by the reactants to carry on the reaction are called endothermic reactions. In this type of reaction, the energy of the reactants is less than that of the products, so energy is needed and absorbed.

Energy of reactants <Energy of products

Example: Process of Photosynthesis, where plants absorb sunlight in the presence of carbon dioxide and water and make their food in the form of glucose and release oxygen. 

\[6C{{O}_{2(g)}}+6{{H}_{2}}{{O}_{(l)}}\xrightarrow{sunlight}{{C}_{6}}{{H}_{12}}{{O}_{4(aq)\text{ }}}+6{{O}_{2(g)}}\] 

10. Why is respiration considered an exothermic reaction? Explain. 

Ans:

Exothermic reactions are the reactions in which heat is going to be released. These reactions result in the formation of energy as the energy of reactants is greater than that of products. Respiration is the process in which the glucose from our body combines with oxygen in the cells to provide us with energy. The glucose is broken down through the process of digestion that along with oxygen provides us with energy and hence respiration is an exothermic reaction. The reaction that happens is as follows:

\[{{C}_{6}}{{H}_{12}}{{O}_{6(aq)}}+6{{O}_{2(g)}}\to 6C{{O}_{2(g)}}+6{{H}_{2}}{{O}_{(l)}}+energy\] 

11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions. 

Ans:

A decomposition reaction consists of a single reactant that breaks down into two or more simpler products. 

Example: Decomposition of calcium carbonate to give calcium oxide and carbon dioxide as,

\[CaC{{O}_{3}}+\text{ }Energy\text{ }\to CaO+C{{O}_{2}}\] 

A combination reaction consists of two reactants that combine or mix together to form a single product. 

Example: Combination of Calcium oxide and carbon dioxide to form one single product calcium carbonate

\[CaO+C{{O}_{2}}\to CaC{{O}_{3}}+\text{ }energy\text{ }\] 

Hence, decomposition reactions are the opposite of combination reactions.

12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

 Ans:

Decomposition reactions are the reactions where a reactant breaks down into two or more products.

a) Decomposition by heat:

Ferrous sulphate decomposes to give ferrous oxide, sulphur dioxide and sulphur trioxide as,

\[2FeS{{O}_{4(s)}}\xrightarrow{\Delta }F{{e}_{2}}{{O}_{3(s)}}+S{{O}_{2(g)}}+S{{O}_{3(g)}}\]

b) Decomposition by light:

Silver chloride decomposes in light to form silver and chlorine as,

\[2AgC{{l}_{(s)}}\xrightarrow{Light}2A{{g}_{(s)}}+C{{l}_{2(g)}}\]

c) Decomposition by electricity:

Water decomposes in presence of electricity to form hydrogen and oxygen gases as,

\[2{{H}_{2}}O\xrightarrow{Electricity}2{{H}_{2}}+{{O}_{2}}\]

13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Ans:

A displacement reaction is a reaction in which a more reactive element displaces a less reactive element from its compound. The elements at the top in the activity series can replace the elements at the bottom. 

 Example: Zinc is more reactive than copper and replaces copper from copper sulphate as:

\[CuS{{O}_{4(aq)}}+Z{{n}_{(s)}}\to ZnS{{O}_{4(aq)}}+C{{u}_{(s)}}\] 

Double displacement reaction is the type of reaction where two compounds react in the way that there is exchange of positive and negative ions and new compounds are formed as products. 

Example: On mixing sodium sulphate solution with barium chloride solution, a white precipitate of barium sulphate is formed as:

\[N{{a}_{2}}S{{O}_{4(aq)}}+BaC{{l}_{2(aq)}}\to BaS{{O}_{4(s)}}+2NaC{{l}_{(aq)}}\] 

14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved. 

Ans:

The equation of recovery of silver from silver nitrate solution in silver refining is as follows:

\[2AgN{{O}_{3(aq)}}+C{{u}_{(s)}}\to Cu{{(N{{O}_{3}})}_{2(aq)}}+2A{{g}_{(s)}}\]

Here displacement of silver by copper happens as copper is more reactive than silver.

15. What do you mean by a precipitation reaction? Explain by giving examples. 

Ans:

The type of reaction where an insoluble substance called a precipitate is formed when there is exchange of ions between the reactants is called a precipitation reaction. 

Example 1: On mixing sodium carbonate solution with calcium chloride solution, a white precipitate of calcium carbonate is formed.

\[N{{a}_{2}}C{{O}_{3(aq)}}+CaC{{l}_{2(aq)}}\to CaC{{O}_{3(s)}}\downarrow +2NaC{{l}_{(aq)}}\] 

Example 2: On mixing sodium sulphate solution with barium chloride solution, a white precipitate of barium sulphate is formed.

\[N{{a}_{2}}S{{O}_{4(aq)}}+BaC{{l}_{2(aq)}}\to BaS{{O}_{4(s)}}\downarrow +2NaC{{l}_{(aq)}}\]

16. Explain the following in terms of gain or loss of oxygen with two examples each.

a) Oxidation

Ans: Oxidation:  A type of reaction in which oxygen or an electronegative species is added to a substance or there is removal of hydrogen or a positive species from a substance is called oxidation.

For example:

$C{{O}_{2}}+{{H}_{2}}\to CO+{{H}_{2}}O$ (Addition of oxygen to${{H}_{2}}$)

$2Cu+{{O}_{2}}\to 2CuO$ (Addition of oxygen to Cu)

b) Reduction 

Ans: Reduction: A type of reaction in which hydrogen or an electropositive species is added to a substance or oxygen or an electronegative species is removed, is called reduction.

For example:

$C{{O}_{2}}+{{H}_{2}}\to CO+{{H}_{2}}O$ (Oxygen is removed from$C{{O}_{2}}$)

$CuO+{{H}_{2}}\xrightarrow{\Delta }Cu+{{H}_{2}}O$ (Oxygen is removed from CuO)

17. A shiny brown-coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black-coloured compound formed.

Ans:

Copper (Cu) is ‘X’ and the black colour compound is copper (II) oxide (CuO). Copper reacts with oxygen to form a black colour compound called copper (II) oxide as: 

\[2Cu+{{O}_{2}}\to 2CuO\] 

18. Why do we apply paint on iron articles?

 Ans:

Iron is a reactive metal and can react with moisture and air. Iron articles, if exposed for a long time in moisture or air, can get corroded and form rust. So, paint is applied on iron articles to prevent rusting and form a protective layer for exposure to air and moisture.

19. Oil and fat containing food items are flushed with nitrogen. Why?

 Ans:

Oils and fat containing items are perishable and can be spoiled when exposed to oxygen. This is because oil and fats are reactive and can react with oxygen easily and get oxidised. To prevent oxidation, these items are flushed with nitrogen gas. Nitrogen is an inert gas and cannot react with oils or fats easily. So, food items with oils and fats are kept in packets having nitrogen gas that increases the shelf life and makes these items last for a long time.

20. Explain the following terms with one example each.

a) Corrosion

Ans: Corrosion: Corrosion is defined as a process where a substance, generally metal deteriorates and forms a layer of oxide on its surface. Metals get converted into their hydrated oxides or sulphides. Ex; Iron, Cu, Ag

\[4Fe+3{{O}_{2}}+n{{H}_{2}}O\to \underset{Hydrated\,iron\,oxide}{\mathop{2F{{e}_{2}}{{O}_{3}}.n{{H}_{2}}O}}\,$\]

\[2Ag+{{H}_{2}}~S\to A{{g}_{2}}~S+{{H}_{2}}\] 

b) Rancidity

Ans: Rancidity: It is a process in which food items like fats and oils are oxidised. This results in the change in taste and smell of the food item and is called rancidity. For example, when any fried food is exposed to air for a long time then it has a change in smell and taste and it becomes rancid.

Rancidity can be avoided by:

• Adding antioxidants like BHA (Butylated tri hydroxy anisole)

• Refrigerate the food items

• keep food items in airtight containers

• Adding nitrogen to food packets to prevent oxidation.

NCERT Solutions for Class 10 Science Chapter 1 – A Brief Overview

Introduction

Every day in our life, we go through different kinds of physical and chemical changes. A physical change is a change that can be reversed like the melting of wax, freezing of water, etc. and chemical change is a change that cannot be reversed like digestion of food, rusting of iron, etc. All chemical changes are associated with chemical reactions and they are represented by equations. In this segment, we will study the different kinds of chemical changes represented by chemical reactions and the chemical equations.

You can Find the Solutions of All the Science Chapters below.

NCERT Solutions for Class 10 Science

• Chapter 1 - Chemical Reactions and Equations

• Chapter 2 -  Acids, Bases and Salts

• Chapter 3 - Metals and Non-metals

• Chapter 4 - Carbon and Its Compounds

• Chapter 5 - Periodic Classification of Elements

• Chapter 6 - Life Processes

• Chapter 7 - Control and Coordination

• Chapter 8 - How do Organisms Reproduce?

• Chapter 9 - Heredity and Evolution

• Chapter 10 - Light Reflection and Refraction

• Chapter 11 - Human Eye and Colourful World

• Chapter 12 - Electricity

• Chapter 13 - Magnetic Effects of Electric Current

• Chapter 14 - Sources of Energy

• Chapter 15 - Our Environment

• Chapter 16 - Management of Natural Resources

Chemical Reactions 

A chemical reaction is a change in which one more substance(s) are reactants(s) react (s) to form new compound substance(s) with entirely different properties.

Reactants are substances like molecules, ions, atoms that react. They undergo a chemical change in the reaction and form a new substance as a result of the reaction. This new substance formed is called a product. 

         Ex:         2Na(s)      +      2H2O (l)       →  2NaoH(aq)    +      H2(g) ↑

        Sodium         Water       Sodium hydroxide     Hydrogen

| ____________ | |_____________|

In the above chemical reaction, sodium and water are the reactants and the products formed as a result of the reaction are Sodium hydroxide and Hydrogen.

Identification of Chemical Reaction

The following observations can be identified as changes during a chemical reaction:

(i) Change in state (ii) Change in the colour

(iii) Evolution of a gas (iv) Change in temperature

(v) Formation of a precipitate

Chemical Equations

A chemical equation is a shorthand form for a chemical change represented by symbols. The reactants and the products obtained as a resultant of a chemical change are represented by symbols and formulas.

Ex: Methane + Oxygen → Carbon dioxide + Water

  |_________|                   |_____________|

      Reactants                   Products

This equation is called the word equation. The word equation can be changed into a chemical equation by writing the formulae and the symbols of the substance instead of their names. 

Ex: CH4(g)       +     2O2(g) →     CO2(g)        +      2H2O(l)

  Methane     Oxygen     Carbon dioxide    Water

Writing a Chemical Equation

A chemical equation shows a change of reactants to final products through an arrow symbol () that is placed between them.

The reactants are written with a plus sign (+) between them on the left-hand side of the arrow. Similarly, the final products are written on the right-hand side with a plus sign (+) between them. 

The arrowhead pointing towards the products showing the direction of the reaction.

Ex:        NH4OH  +  HCL   ------------------->  NH4Cl            +  H2O

    |________________| Arrow indicates     |_______________|

            Reactants     to yield         Products

Represents Direction

In the above reaction, the number of Ammonium Hydroxide and Hydrogen Chloride is reacting to form the products Ammonium Chloride and Water. If you observe, in the above equation the number of atoms in the reactants and the products are not the same on both sides. This kind of unbalanced equation is called a skeletal chemical equation.

Balanced Chemical Equations

When the total number of atoms of each element is equal on both sides of the equation then the equation is known as a balanced chemical equation. 

The balancing of a chemical equation is based on the law of conservation of mass. According to the law of conservation of mass, ‘mass can neither be created nor be destroyed during a chemical reaction.’

The method of hit and trial can be used for balancing a chemical equation. In this method, we keep making trials to balance the equation by using the smallest whole number of coefficients. The number of atoms of each element never changes, before and after a chemical reaction in this method.

Balancing of a Chemical Equation

There are many steps involved in balancing a chemical equation. 

1. The first step is to write an unbalanced equation and enclose the formulae in brackets.

Ex: (Na)     +    (H2O)       →   (NaOH)                          +    (H2)

Sodium      Water     Sodium Hydroxide         Hydrogen

2. The second step is to make a list of the number of atoms of different elements as present in an unbalanced equation.

3. Next step is to balance the first element. From the above table, it is known that atoms in Hydrogen are not balanced. So, first we need to balance it.

Thus, the balanced equation is (Na) + 2(H2O) → 2(NaOH) + (H2)

4. Then balance the second element. We examine the obtained equation and select the next unbalanced element. In the above chemical equation, Na is still not balanced. To balance the number of atoms in Na,

Thus, after the balancing the Na, we get the final chemical equation:

2Na + 2H2O → 2NaOH + H2

5. Further, we have to balance the other elements. If we further examine the reaction, no element is found to be unbalanced. This method that we used is called the hit and trial method.

6. Finally, we need to check the accuracy of the equation. To check where the equation is correct, we further tabulate the number of atoms of each element separately.

The above table clearly reveals that the obtained equation is a balanced equation.

Example: Balance the following equation C2H6 + O2 →  CO2 + H2O

Sol.   Step 1: Enclose all the symbols or formulae in brackets.

(C2H6 )+ (O2 ) →  (CO2 )+ (H2O)

Step 2: Count the number of atoms in each element on both the sides of the equation.

Step 3: From the above table, it is known that hydrogen has the maximum number of atoms. So to balance hydrogen atoms, multiply H2O by 3.

(C2H6 )+ (O2 ) →  (CO2 )+ 3(H2O)

Step 4: Count further the number of atoms in each element on both sides.

Now, to balance the atoms in carbon, multiply CO2 molecules by 2. 

(C2H6 )+ (O2 ) →  2(CO2 )+ 3(H2O)

There are 7 O-atoms on RHS. To make 7 O-atoms at LHS, we have to write 72 before O2 but we can use only whole number to balance the equation, so we write 72before  O2 and multiply the whole equation by 2.

[ C2H6 +  7/2 O2  →  2CO2 + 3H2O ] x 2

Thus, after removing the brackets we get the final and correct chemical equation

2C2H6 + 7O2  →  4CO2 + 6H2O

Making a Chemical Equation More Informative

The facts that remain unexplained in a chemical equation are

1. The physical state of the substances like the changes in the colour during the reaction.

2. The reaction conditions.

3. The changes in the evolution or the absorption of energy.

4. Completion of the reaction.

These Limitations Can Be Overcome by Adding the Following Information 

1. The physical states of the reactants and products can be represented by using the symbols, (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution, alongwith their respective formulae. Liberation of gas and precipitation can be represented by and respectively. 

2. The  circumstances under which reaction takes place like temperature, pressure, catalyst, etc. can be mentioned in the equation.

3. Evolution of heat or absorption of heat can be indicated by writing [+Heat] on the right hand side of the left hand side of the equation, respectively.

Types of Chemical Reactions

The chemical reactions depend upon the type of chemical changes taking place and they are classified as:

1. Combination Reaction

When two or more reactants are combined to form a single substance in a chemical reaction, then the reaction is called a combination reaction.

Ex: Calcium Oxide reacts very fast and strongly with water to form calcium hydroxide (slaked lime). The reaction given below is highly exothermic, because a lot of heat is liberated during the reaction.

        CaO(s) + H2O(l) → Ca(OH)2 (aq) + Heat

        Calcium oxide Water         Calcium

                            (Quick Lime)         Hydroxide

    (slaked lime)

2. Decomposition Reaction

This is a reaction in which a single reactant breaks down to form two or more products. This reaction is opposite to combination reaction. On the basis of the energy form required for the reaction, this reaction is classified into three types:

1. Thermal decomposition: This reaction uses the heat energy for decomposition of the reactant.

Ex: Ferrous Sulphate ( FeSO4.7H2O) when heated loses water of crystallisation and forms dehydrated FeSO4 and gives ferric oxide, sulphur dioxide and sulphur trioxide on decomposition.

heat

2FeSO4 (s)              →        Fe2O3 (s)       +        SO2 (g)    +     SO3 (g)

Ferrous Sulphate Ferric oxide Sulpur dioxide   Sulphur trioxide

2. Electrolysis: These reactions use the electrical energy for the decomposition of the reactant molecules.

Ex: When electric current is passed through water, it decomposes to give water and hydrogen.
Electric current

2H2O (l) →    2H2 (g)    +     O2 (g)

Water                 Hydrogen    Oxygen

3. Photolysis or photochemical decomposition: These reactions use light energy for the purpose of decomposition.

Ex: Silver chloride decomposes to produce silver metal and chlorine gas when exposed to sunlight.

sunlight

2AgCl (s)          →  2Ag (s) + Cl2 (g)

                        Silver bromide       Silver       Bromine

  (Pale yellow)   (Greyish white)   (yellowish green)

3. Exothermic and Endothermic Reactions

A reaction can be exothermic or endothermic depending on whether heat is liberated or absorbed during a reaction.

1. Exothermic reactions: The reactions in which the products formed along with the liberation of heat, are called exothermic reactions or combustion reactions. Respiration is an exothermic process. The decomposed vegetable matter that turns into compost is also an exothermic process.

Ex: Burning of magnesium ribbon

2Mg (s) + O2 (g) → 2MgO (s)  +  Heat

Magnesium   Oxygen     Magnesium oxide

Endothermic reactions: The reactions which take place by the absorption of heat/ energy (either in the form of light or electricity), are called endothermic reactions. Photosynthesis is an endothermic process. All decomposition reactions are endothermic reactions because the reactions require some form of energy like heat, light or electricity.

sunlight

6CO2 (aq) + 12H2O (l) →  C6H12O6 + 6O2 + 6H2O

                                                        Chlorophyll   Glucose

4. Displacement Reaction

When a more reactive element takes the place of  a less reactive element from its compound, it is called displacement reaction. This reaction is of two kinds:

1. Single Displacement Reaction: This is a type of chemical reaction where an element reacts with a compound and displaces another element in that compound is called single displacement.

Ex: Zinc is more reactive than copper and displaces Cu from CuSO4 solution and forms a new product zinc sulphate and copper metal.

 Zn         +       CuSO4  →                ZnSO4                 +       Cu 

Zinc metal    Copper sulphate       Zinc Sulphate   Copper metal

(Grey)          solution (blue)          solution (colourless)    (Brown)

2. Double Displacement Reaction: The reaction in which two different ions or groups of atoms in the molecules of a reactant are displaced by each other is called a double displacement reaction. In this reaction, a precipitate is produced so It is also called a precipitation reaction.

Ex: When sodium sulphate is added to barium chloride, a curdy white precipitate of barium sulphate and a solution of sodium chloride are formed as end products. 

Na2SO4 (aq)  + BaCl2 (aq) →  BaSo4 + 2NaCl (aq)

Sodium Sulphate      Barium Chloride  Barium Sulphate   Sodium Chloride 

Note: The reaction in which acid or acidic oxide react with a base or basic oxide to form salt and water is called neutralization reactions.

2NaOH      +       H2SO4                          →                Na2SO4 +        H2O

Sodium Hydroxide  Sulphuric Acid Sodium sulphate  Water

5. Oxidation and Reduction Reactions Oxidation

This reaction is a process in which oxygen is added to a substance or removed from a substance or a substance loses an electron(s). If a substance gains oxygen during a reaction then the substance is said to be oxidized. If a Oxygen is lost from a substance during a reaction, it is said to be reduced. 

The substance that oxidizes the other substance is called an Oxidizing agent. The substance  that brings a reduction in another substance is called a Reduction agent.

i) 2Cu   +   O2 →   2CuO

Copper  Oxygen   Copper oxide               

ii) 2H2S            +               O2      →  2S + 2H2O

Hydrogen sulphide   Oxygen  Sulphur   Hydrogen  oxide

iii) Zn →  Zn2+ + 2e-

       Zinc

Redox Reactions

When a reaction has both oxidation and reduction  taking place simultaneously then the reaction is called a redox reaction. 

1. In the reaction below, the copper (II) oxide is losing oxygen and is being reduced while oxygen is added to hydrogen and water is formed.

            Reduction

                                            ________________

              |                 |

                                               CuO       +   H2 →      Cu  +       H2O

        Copper  Hydrogen   Copper      Water

  Oxide       |_________________|

Oxidation

2. In the reaction below, HCL is oxidised to HCL is oxidized to Cl2 whereas, MnO2 is reduced to MnCl2

Reduction

  ________________ 

| |

MnO2 (s) + 4HCL → MnCl2 (aq) + Cl2 (g) + 2H2O

        (Conc.)

Oxidation

|_________________|

Effects of Oxidation Reactions in Everyday Life

Corrosion

The phenomenon due to which the upper surface of the metals get eaten away when they come in contact with air, water and chemicals present in the atmosphere, then it is called corrosion.

Ex: When iron substances are new then they shine but get coated with a reddish brown powder when left for sometime.

Rusting is basically the process of corrosion of iron. The rusting of iron is a redox reaction. Painting, galvanizing, and electroplating are few methods to prevent corrosion.

Corrosion can cause damage to the body of a car, bridge, iron railing, ships because they are made up of iron.

Rancidity

In this process,  the oil and fat which are volatile in nature present in the food materials slowly get oxidized resulting in the change of smell and taste in them.

The methods to prevent rancidity are:

1. Keep food in air-tight containers to prevent air from getting in.

2. Refrigeration of cooked food. 

3. Packing of certain food items in packets containing nitrogen gas.

4. Avoid keeping food under the direct sunlight. 

5. Rancidity can also be prevented by adding antioxidants like BHA and BHT.