What is a chemical reaction Class 10

Class 10th Science
Notes on Chemical Reactions and Equations

1.  Chemical reactions- The transformation of chemical substance into a new chemical substance by making and breaking of bonds between different atoms is known as Chemical Reaction.

2.  Signs of a chemical reaction- These factors denote that a chemical reaction has taken place- change of state of substance, change of color of substance, evolution of heat, absorption of heat, evolution of gas and evolution of light.

3.  Chemical Equation: The representation of chemical reaction by means of symbols of substances in the form of formulae is called chemical equation. E.g. - H2 + O2 ⇒ H2O

4.  Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both left and right sides of the reaction.

      *Note- According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.

5.  Types of Chemical Reactions-

                      I.  Combination- When two elements or one element and one compound or two compounds combines to give one single product.

                     II.  Decomposition- Splitting of a compound into two or more simple products.

                    III.  Displacement- It takes place when a more reactive metal displaces a less reactive metal.

                    IV.  Double displacement- Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

                     V.  Precipitation- The insoluble compound called precipitate forms in this reaction.

                    VI.  Exothermic- Reactions which produce energy are called exothermic reaction. Most of the decomposition reactions are exothermic.

                   VII.  Endothermic- Reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic.

                  VIII.  Oxidation: Gain of oxygen or removal of hydrogen or metallic element from a compound is known as oxidation.

                     IX.  Reduction: Addition of hydrogen or removal of oxygen from a compound is called reduction.

                       X.  Redox- A chemical reactions where oxidation and reduction both take place simultaneously are also known as redox reaction.

                             Eg - NaOH + HCl ⇒ NaCl + H2O

6.  Rusting- When iron reacts with oxygen and moisture forms a red substance called rust.

7.  Rancidity- Oils and fats when get oxidized on exposure to air show a change in taste and smell.

8.  Corrosion- Metals when attacked by oxygen, water, acids, gases, present in air changes its surface which is called corrosion.

Take a look at the different processes taking place around you:

• Plants preparing food by the process of photosynthesis
• Iron structures turning reddish brown with time due to corrosion (rusting)
• Dirt in clothes getting removed on washing with soap and water
• Cake batter rising and getting cooked on baking
• Burning of wood at a campfire

All of these processes that we observe in our day to day lives are a result of Chemical Reactions.

What are Chemical Reactions?

It is a process by which new substances with new properties are formed .

In a chemical reaction one or more substances combine to form new substances.

Example : 

Hydrogen reacts with Oxygen to form Water

Hydrogen + Oxygen     →        Water

        (Reactants)                   (Product)

Note : When Hydrogen and Oxygen react, they form a new substance Water, hence it is a chemical reaction

Is chemical reaction a result of Physical change or chemical change?

Chemical Reaction results in the formation of new substances. 

Thus we can say, it occurs because of chemical changes

Explanation

Based on the above explanation of physical and chemical changes, we can confirm that chemical reactions occur as a result of chemical changes , as;

• New products are formed as a result of chemical reactions and
• The reaction is usually not reversible under the same conditions

Note : If the products can react to produce the reactants, then the reaction will be considered reversible.

Do elements change because of chemical reaction ?

The simple answer is, No

Atom of one element does not change into another element

They only rearrange to form new substances

Example

Magnesium + Oxygen → Magnesium Oxide

During this reaction, Magnesium Oxide is formed which contains only Magnesium and Oxygen element (and not any new element )

Note- Magnesium ribbon burns with a dazzling white flame and changes into white powder. The white powder formed is Magnesium Oxide.

What are reactants and products?

The substances which undergo chemical change in a chemical reaction are called reactants .

The substances produced as a result of chemical reaction are called products

Example

Magnesium + Oxygen → Magnesium Oxide

In this case Magnesium and Oxygen are called Reactants

Magnesium Oxide is a Product

Magnesium + Oxygen → Magnesium Oxide

           (Reactants)                 (Product)

How to find whether chemical change has taken place?

If a chemical change takes place, it usually leads to following:

• Change in state
• Change in color
• Evolution of Gas (Gas being formed)
• Change in Temperature
• Formation of precipitate

Example- Change in color of substances and evolution of gas in a chemical reaction

When we heat green coloured Ferrous Sulphate crystals in a boiling tube,

We observe that the green colour of Ferrous Sulphate changes .

We can also observe smell on burning of sulphur

This is because  on heating,

Ferrous Sulphate breaks down into Ferric Oxide (solid) , Sulphur dioxide (gas) and Sulphur Trioxide(gas)

2FeSO 4 → Fe2O 3 ​+ SO 2 ​+ SO 3

                                                 (Reactant)           (Products)

​

Example- Change in Temperature

Calcium Oxide (Quick Lime) reacts with Water to produce Calcium Hydroxide (Slaked Lime) and Heat

CaO + H 2 O → Ca(OH) 2

                                                         (Reactants)      (Product)

Example- Formation of precipitate

When Sodium Sulphate solution is mixed with Barium Chloride solution

It forms Barium Sulphate precipitate and Sodium Chloride

(Insoluble solid is called precipitate)

Note : This Barium Sulphate is formed because of the reaction of positively charged ions of Barium and negatively charged ions of sulphur

Intext Question - Page 6 Q1, 

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Page 2

A chemical equation is the representation of a chemical reaction .

In the form of symbols and formulae

Steps of Writing a Chemical Equation

Step 1

First we write chemical equation in word

Example

Let us consider the following chemical reaction;

Magnesium burns with Oxygen gas to form white colored powder called Magnesium Oxide

Writing in words;

Magnesium + Oxygen → Magnesium Oxide

Magnesium + Oxygen → Magnesium Oxide

Note (for the graphic designer) : Make an image of this word equation

While writing a chemical equation, it should be noted that;

1. Reactants are written on the left (LHS)
2. Products are written on the right (RHS)
3. There is a '+' sign between the reactants or products if there is more than one reactant/product.
4. There is an arrow '→' between the reactants and products which shows the direction of the reaction
5. If there is any additional force provided to the reaction , like heat or pressure, then it is to be written above or below the arrow .

Now writing a word equation can be tedious, and we can make a word equation shorter by using the unique symbols of the elements

Step 2

We write symbols instead of words

Symbol for Magnesium is Mg

Symbol for Oxygen gas is O 2

Formula for Magnesium Oxide  

First we check the valency,

Valency of Magnesium is 2

Valency of Oxygen is 2

Formula for Magnesium Oxide is MgO as shown below

Writing Equation by using chemical formula

Magnesium + Oxygen → Magnesium Oxide

              Mg + O 2         →          MgO

 Mg + O 2         →          MgO

Note (for the graphic designer) : Make an image of this equation

This is our required chemical equation.

Step 3

Check and balance the equation

Starting with Oxygen,

No. of atoms of Oxygen is not balanced

2 in the reactants side and 1 in the product side

So we multiply product by 2

Mg + O 2 → 2 MgO

Mg + O 2 → 2 MgO

We again check number of elements

Now,

No. of atoms of Magnesium is not balanced

1 in the reactants side and 2 in product side

We try to make both same by multiplying reactant by 2

2 Mg + O 2 → 2MgO

2 Mg + O 2 → 2 MgO

Note (for the graphic designer) : Make an image of this equation

We again check number of elements

Step 4

Writing symbols of different states;

Following symbols are normally used

Explanation

In our case

Magnesium burns with Oxygen to form white colored powder called Magnesium Oxide

Magnesium is solid, Oxygen is gas and Magnesium Oxide is also solid

Hence we write equation as

2Mg (s) + O 2 (g) → 2MgO (s)

2 Mg (s) + O 2 (g) → 2 MgO (s)

Note (for the graphic designer) : Make an image of this equation

Note: Aqueous state (aq) means presence of a substance as a solution in water

Another examples:

Green Plant make food by photosynthesis

following equation takes place

Carbon Dioxide + Water → Glucose + Oxygen

6CO 2 (g) + 6H 2 O (l) → C 6 H 12 O 6 (aq) + 6O 2 (g)

This glucose is aqueous solution (glucose is present in water)

Step 5

Sometimes extra information is given in the form of temperature, pressure, catalyst etc.

Catalyst : A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.

For example

Potassium Chlorate is heated in the presence of Magnesium Dioxide catalyst, it decomposes to produce Potassium Chloride and Oxygen gas

                   MnO 2

2KClO 3         →       2KCl + 3O 2

                     △

Temperature -  

Endothermic Reaction : Reaction in which heat energy is used is called Endothermic Reaction.

For example

When Calcium Carbonate is heated, it decomposes into Calcium Oxide and Carbon Dioxide

CaCO 3 (s) → CaO(s) + CO 2 (g)

Temperature -  

Endothermic Reaction : Reaction in which heat energy is used is called Endothermic Reaction.

For example

When Calcium Carbonate is heated, it decomposes into Calcium Oxide and Carbon Dioxide

CaCO 3 (s) → CaO(s) + CO 2 (g)

Pressure-

For example

When Carbon Monoxide reacts with Hydrogen gas at a pressure of 250 atmospheres, Methanol is formed.

CO + 2H 2 →  CH 3 OH

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