CHEMICAL REACTIONS AND EQUATIONS _ Part 2

Q. What is chemical equation?

Ans. A chemical equation is symbolic representation of chemical reaction. It uses chemical symbols and formulae to present the reactants and products.

For example

2H2 + O2 > 2H2O

Above mentioned example is a symbolic expression of a chemical equation. It has two sides – LHS and RHS. Both the sides are separated by arrow ( à) symbol. Substances on LHS are Reactants, while substances on RHS are Products.

In the above chemical reaction, 2H2O and O2 ( on LHS ) are Reactants, whereas 2H2O ( on RHS ) are Products.

The equation tells us when 2 molecules of hydrogen react with 1 molecule of oxygen; it produces 2 molecules of water.

Law of conservation of mass

Q. What is law of conservation of mass?

Ans.  Law of conservation of mass states that in any system, mass can neither be created nor be destroyed. It comes out from Einstein’s theory of Relativity. The law is much useful in creating a balanced chemical equation.

skeletal equation

Q. What is a Skeletal equation?

Ans. A Skeletal equation states which substances are reactants and which are products only. It is not a balanced chemical equation. It needs to be further balanced in terms of number of elemental atoms on both the sides. It is only a rough expression of reactants and products.

For example

Fe + H2O > Fe2O4 + 4H2

In the above mentioned reaction we can clearly see that number of atoms of Fe, H, O are not equal in both the sides.

As it only tells us the reactants and products without keeping the atoms of elements equal. It is skeletal equation.

balanced chemical equation

Q. What is a balanced chemical equation?

Ans. A balanced chemical equation is that equation in which number of elemental atoms is equal on both the sides of an equation.

For example

2H2 + O2 à 2H2O

Let’s understand the above example more closely,

·       H2 means one molecule of hydrogen has two hydrogen atoms.

·       O2 means one molecules of oxygen has two oxygen atoms.

·       2H2 means two molecules of hydrogen, and two hydrogen moles means four hydrogen atoms

So, LHS has 4 atoms of hydrogen and two atoms of oxygen. Now on RHS,

·       There are two molecules of water

·       One molecule of water has 2 hydrogen atoms and 1 oxygen atom,

·       Two molecule of water has 4 atoms of hydrogen and 2 atoms of oxygen.

So, LHS = 4 hydrogen atoms, 2 oxygen atoms

RHS= 4 hydrogen atoms, 2 oxygen atoms.

Thus we can say above mentioned equation is a balanced chemical equation.

NCERT QUESTION (page 13)

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

Q1. Why does the color of copper sulphate change when an iron nail is dipped in it?

Ans.

The color of copper sulphate changes because iron replaces copper in this process the quantity of copper sulphate decreases. It results change in colour in the solution. Iron also gets brownish.

Fe + CuSO₄ à FeSO₄ + Cu

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

Q2. Give an example of double displacement reaction other than the one given in Activity 1.10.

Ans.

When Sodium Thiosulfate ( Na₂S) reacts with hydrochloric acid (HCl) it forms Sodium Chloride (NaCl) and Hydrogen Sulfide (H₂S).

Na₂S + HCl à NaCl + H₂S

In the above example,

Sodium (S) replaces Chlorine (Cl) resuting

Sodium Thiosulfate becomes Hydrogen Sulfate.

Na₂S - H₂S

Chlorine (Cl) replaces Sodium resulting

Hydrochloric acid ( HCl) becomes Sodium Chloride (NaCl)

HCl - NaCl

As two elements are displacing each other simultaneously, this is an example of double displacement reaction.

Identify the substances that are oxydised and the substances that are reduced in the following reaction:

Q3. Identify the substances that are oxydised and the substances that are reduced in the following reaction:

(1) 4Na + O₂ > 2Na₂O

(2) CuO + H₂  > Cu + H₂O

Ans.

(1) Substances that are oxidised are, Na ; H₂

(2) Substances that are reduced are, O₂ ; CuO

CHEMICAL REACTIONS AND EQUATIONS – Part 1 (NCERT Questions)

Q1.  A solution of a substance ‘X’ is used for white washing.   (1). Name the substance ‘X’ and write its formula.   (2). Write the reaction of the substance ‘X’ named in (1) above with water.

Ans(1). Substance ‘X’ is CaO (Calcium oxide or quick lime).

Ans(2). Reaction of the substance ‘X’ is as follows:

CaO + H2O + Ca(OH)2

In the above example,

Calcium oxide reacts with water.

The product we get is Calcium hydroxide.

Q2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double the amount collected in the other? Name the gas.

Ans. In the Activity 1.7 the process of electrolysis of water is being performed.

This process of electrolysis gives two molecules of hydrogen (H2) at one electrode and one molecule of oxygen (O2) at the other electrode.

2H2O = 2H2 + O2

So, in electrolysis of water

    -  2 molecules of hydrogen are collected at electrode-1.

    - 1 molecule of oxygen is collected at electrode-2.

Thus, molecules collected at electrode-1 is double than that of electrode-2.

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Chemical Reactions and Equations - Part 1

Q. What is a chemical reaction?

Ans. A chemical reaction is a chemical change by interaction in two or more substance(s) to form new substance(s).

These interacting substances are called reactants and newly formed substances are called products.

It involves breaking of old bonds and making of new bonds in the presence of heat and light.

Q. How will you identify a chemical reaction has been taken place?

Ans. A chemical reaction can be identified by the following observations:

          Change in color

        Change in State

         Evolution of gas ,water and other substances

         Change in temperature

Q. How can a chemical equation be balanced?

Ans. Hit and Trial method is used to balance a chemical equation.

The basic concept of Hit and Trial method is that the total number of atoms should be maintained on both sides of a chemical equation.

Q. How many kinds of chemicai reactions are there ?

Ans. There are different kinds of chemical reactions. It depends upon the type of chemical change occurred. They are,

·       Decomposition Reaction

·       Combination Reaction

·       Displacement Reaction

·       Neutralization Reaction

·       Oxidation and Reduction Reaction

·       Exothermic and endothermic Reaction

Q. What is a Decomposition reaction?

Ans.  A Decomposition reaction is that chemical reaction in which a compound is broken down and forms two or more products.

For Example

Thermal Decomposition Reaction – In this reaction heat energy is used for decomposition reaction.

 2FeSO4 ____Heat_____> FE2O3 + SO2 + SO3

Q.  What is Displacement Reaction ?

Ans.  Displacement reaction is of two types -

    1. Single Displacement Reaction

It is a reaction wherein a more reactive element displaces less reactive element from the compound.

For example

Zn + CuSo4  -----> ZnSo4 + Cu 

2. Double Displacement Reaction

In this type of reaction two different ions are displaced by one another in the reactant compound.

For example

Fe CuSo4 ----> FeSo4 + Cu

Q. What is neutralisation reaction ?

Ans.  It is that chemical reaction where in an acid reacts to base in purpose of neutralization thus

 It is called neutralization reaction.

NaOH + HCl ---à NaCl + H2O

Q. What is Oxidation Reaction?

Ans.  In Oxidation Reaction either oxygen is added or hydrogen is removed from a compound.

For example

4Na + O2__>  2Na2O

In the above reaction sodium is added to Oxygen and in turn gives out sodium oxide. 

H2 + F2  __> 2HF

Q.  What is Reduction Reaction ?

Ans.  Generally it is a chemical process like the oxidation in reduction reaction either oxygen is removed or hydrogen is added to a substance.

 for example

2NA + H2 -à 2NaH

Q. What are Redox Reactions? Give example.

Ans.  Redox reactions are those reactions wherein oxidation and reduction happen simultaneously.

That is to say, on the one side a substance gains oxygen and loses hydrogen while on the other side a substance loses oxygen and gains hydrogen.

For example:

CuO + H2 ...> Cu + H2O

Let’s understand the above example closely.

1.  Copper oxide and Hydrogen react with one another.

2.  Copper oxide loses oxygen  (Reduction)

3.  Hydrogen gains oxygen   (Oxidation)

Thus two reactions, i.e. Reduction and Oxidation taking place simultaneously. This type of reaction is called Redox reaction.

exothermic reaction

Q. What are exothermic reactions?

 Ans. Exothermic reactions are those reactions which give out energy in the form of heat, light etc.

 For example

C + O2 à CO2 + Energy (393 Kj)

In the above example,

·       Carbon and Oxygen react.

·       The reaction produces Carbon dioxide and liberates 393 Kilojoules of energy.

Q. What is Endothermic reaction?

Ans. Endothermic reaction is that chemical reaction in which energy is absorbed in the form of heat, light etc.

For example

N2 + O2 à 2NO

In the above example,

·       Nitrogen (N2) and oxygen (O2) react.

·       During the reaction process, they absorb some amount (180.5 Kj) of energy.

·       After the reaction, the product we get is Nitric oxide (2NO).

Thus we see while Exothermic reaction liberates energy, Endothermic reaction absorb energy.

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