Science Chapter 3 - Chemical Reactions And Equations SSC, SCIENCE PART I, NEW SYLLABUS, FOR BOARD EXAM 2020,

Question 1:

Choose the correct option from the bracket and explain the statement giving reason.
(Oxidation, displacement, electrolysis, reduction, zinc, copper, double displacement, decomposition)

a. To prevent rusting, a layer of ........ metal is applied on iron sheets.
b. The conversion of ferrous sulphate to ferric sulphate is ........ reaction.
c. When electric current is passed through acidulated water ........ of water takes place.
d. Addition of an aqueous solution of ZnSO4 to an aqueous solution of BaCl2 is an example of ....... reaction.

 

ANSWER:

a. To prevent rusting, a layer of zinc metal is applied on iron sheets.
b. The conversion of ferrous sulphate to ferric sulphate is oxidation reaction.
c. When electric current is passed through acidulated water of electrolysis water takes place.
d. Addition of an aqueous solution of ZnSO4 to an aqueous solution of BaCl2 is an example of a double displacement reaction.

Question 2:

Write answers to the following.
a. What is the reaction called when oxidation and reduction take place simultaneously? Explain with one example.
b. How can the rate of the chemical reaction, namely, decomposition of hydrogen peroxide be increased?
c. Explain the term reactant and product giving examples.
d. Explain the types of reaction with reference to oxygen and hydrogen. Illustrate with examples.
e. Explain the similarities and differences in two events, namely adding NaOH to water and adding CaO to water.

ANSWER:


a. The process in which oxidation and reduction occurs simultaneously is called redox reaction.
Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.

For example: If we add stannous chloride solution to the yellow solution of ferric chloride then light green ferrous chloride solution and stannic chloride solution are produced.

Before the reaction, 3Cl atoms were attached to each iron atom. After the reaction, only two chlorine atoms are attached. That is one negative chlorine atom is released. Therefore, reduction of FeCl3 happened or this reactor on the other hand, before reaction two chlorine atoms were attached with each atom of tin (stannum). Due to the above reaction the number of chlorine attached to tin atom increases to four. That is, oxidation of SnCl2has taken place. Therefore, in this reaction oxidation of one substance and reduction of another substance take place simultaneously. This is called redox reaction.

b.The rate of decomposition of hydrogen peroxide can be increased by having the reaction occurs in the presence of iodide ions. The reaction is proceed by two step mechanism:


c. Reactant: The substance which take part in a chemical reaction are called reactants.

Product: the substance which forms as a result of chemical reaction is called products.

The new substance produced as a result of chemical reaction is called products.

For example: When two sodium atoms react with two chlorine atoms(reactants), they give a completely new compound (product) i.e. sodium chloride (two atoms).


d. There are three main types of chemical reactions with reference to oxygen and hydrogen:

1.Combination Reaction
When two atoms react to form a compound, it is known as combination reaction.
For example: 

2. Decomposition Reaction
When a compound breaks into simple molecular substances from which it is made up of, it is known as decomposition reaction.
For example: 

3. Oxidation and reduction reaction:
Oxidation:
(i) The addition of oxygen to a substance is called oxidation.
(ii) The removal of hydrogen to a substance is called oxidation.

Reduction:
(i) The addition of hydrogen to a substance is called reduction.
(ii) The removal of oxygen to a substance is called reduction.
For example:
In the above reaction, copper oxide is changing to Cu. That is, oxygen is being removed from copper oxide. So, copper oxide is being reduced to copper.
In the above reaction, H2 is changing into H2O. That is , oxygen is being added to hydrogen. So, hydrogen is being oxidised to water.

e. Chemical equations involved:

Similarities:

1) Both of the equations are exothermic. It means a lot of heat is evolved during the reaction.

2) Both reaction form strong basic solution.

Differences :

1) Sodium Hydroxide is strong base dissociates to form Na⁺ and OH⁻ ion. While Calcium oxide added water to form Calcium Hydroxide which further dissociates.

2) NaOH is a monoacidic base. and CaO is a Di-Acidic base.

3) NaOH, CaO should be added to water gradually with constant stirring.CaO on reacting with water produces a basic solution called Calcium hydroxide which is used for white washing and this reaction is more dangerous as compared to NaOH.

Question 3:

Explain the following terms with examples.
a. Endothermic reaction and exothermic reaction
b. Combination reaction
c. Balanced equation
d. Displacement reaction

ANSWER:


a. Endothermic reaction:
Endothermic reactions are chemical reactions in which the reactants absorb heat energy from the surroundings to form products. These reactions lower the temperature of their surrounding area, thereby creating a cooling effect. Physical processes can be endothermic as well – Ice cubes absorb heat energy from their surrounding and melt to form liquid water (no chemical bonds are broken or formed).
Endothermic reactions generally involve the formation of chemical bonds through the absorption of heat from the surroundings. On the other hand, exothermic reactions involve the release of heat energy generated from bond-breakage.
The terms ‘endo’ and ‘exo’ have Greek roots, meaning ‘within’ and ‘out’ respectively. As the names suggest, the primary difference between endothermic and exothermic reactions is that the former absorbs heat from the surroundings whereas the latter involves a release of heat.
An exothermic reaction is a chemical reaction that releases energy in the form of heat and light. It is the opposite of an endothermic reaction. Exothermic Reaction means "exo" meaning releases and "thermic" means heat. So the reaction in which there is release of heat with or without light is called an exothermic reaction.

Expressed in a chemical equation: reactants → products + energy.
For example: Combustion is an exothermic reaction.
The chemical reaction between zinc granules and dilute sulphuric acid is exothermic reaction.

b. Combination reaction:
Those reactions in which two or more substances combine to form single substance is called combination reaction
For example: Magnesium and oxygen combine, when heated, to form magnesium oxide.

c. Balanced equation:

A balanced equation is an equation for a chemical reaction in which the number of atoms for each element in the reaction and the total charge are the same for both the reactants and the products. In other words, the mass and the charge are balanced on both sides of the reaction.

The balanced equation is:
Both the left and right sides of the equation have 4 Fe, 6 O, and 3 C atoms. When you balance equations, it's a good idea to check your work by multiplying the subscript of each atom by the coefficient. When no subscript is cited, consider it to be 1. It's also good practice to cite the state of matter of each reactant. This is listed in parentheses immediately following the compound. For example, the earlier reaction could be written as:

where s indicates a solid and g is a gas state of matter.

d. Displacement reaction:

Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound. Both metals and nonmetals take part in displacement reactions.
Chemical reactivity of metals is linked with their relative positions in the activity series.

A metal placed higher in the activity series can displace the metal that occupies a lower position from the aqueous solution of its salt.


Question 4:

Give scientific reasons.
a.When the gas formed on heating limestone is passed through freshly prepared lime water, the lime water turns milky.

ANSWER:

Limestone is made up of calcium carbonate.When calcium carbonate is heated Carbon dioxide is evolved. Lime water is made up of Calcium hydroxide.
When carbon dioxide is passed through lime water, the Carbon dioxide reacts with calcium hydroxide to form Calcium carbonate which is a white precipitate, then lime water turns milky as there is formation of calcium carbonate.


b. It takes time for pieces of Shahabad tile to disappear in HCl, but its powder disappears rapidly.

ANSWER:

It takes time for pieces of Shahabad tile to disappear in HCl, but its powder disappears rapidly because in pieces of the tile, the surface atoms can only react with HCl but in powdered form all the atoms can react with HCl. This makes the difference in their reactivity. Powders are simpler substances of the pieces and they take lesser time for the reaction as compared to the whole piece of tile, which is a compound.

For example: powdered salt react and water will dissolve, but salt rocks and water will not dissolve.


c.While preparing dilute sulphuric acid from concentrated sulphuric acid in the laboratory, the concentrated sulphuric acid is added slowly to water with constant stirring.

ANSWER:

While preparing dilute sulphuric acid from concentrated sulphuric acid in the laboratory, the concentrated sulphuric acid is added slowly to water with constant stirring because sulphuric acid (H2SO4) reacts very vigorously with water, it is a highly exothermic reaction. If you add water to concentrated sulphuric acid, it can boil and bump out due to which you may get a nasty acid burn.Water is a good absorber of heat, so we add acid to the water, slowly and with constant stirring.


d. It is recommended to use air tight container for storing oil for long time.

ANSWER:

It is recommended to use air tight container for storing oil for a long time in order to avoid the problem of rancidity. Rancidity is the condition produced by aerial oxidation of fats and oils marked by unpleasant smell and taste.

Question 5:

Observe the following picture a write down the chemical reaction with explanation.


ANSWER:

i. The given picture represents the electrochemical reaction which takes place during the corrosion of iron. (rusting) 

ii. Different regions on the surface of iron behave as anode and cathode. 

iii. In the anode region, Fe is oxidized to Fe2+

iv. In the cathode region, O2 is reduced to form water. 

V. When Fe2+ ions migrate from the anode region, they react with water and further get oxidized to Fe3+ ions. 

vi. Fe3+ ions form an insoluble hydrated oxide (Fe2O3.H2O), which is deposited as reddish brown layer on the surface. It is called rust. 


Question 6:

Identify from the following reaction the reactants that undergo oxidation and reduction.

ANSWER:

a. Fe + S → FeS
In a reaction, Fe is changing to FeS. That means, iron loses electrons to form FeS. Loss of electrons from a substance is called oxidation, so iron undergoes oxidation.

b. 2Ag2O → 4 Ag + O2
In a reaction, silver oxide is changing to silver. That is, oxygen is being removed from silver oxide. Removal of oxygen from a substance is called reduction, so silver oxide undergoes reduction.

c. 2Mg + O2 → 2MgO
In a reaction, magnesium is changing to magnesium oxide. That means, oxygen is being added to magnesium. Addition of oxygen to a substance is called oxidation, so magnesium undergoes oxidation.

d. NiO + H2 → Ni + H2O
In a reaction, Nickel oxide is changing to nickel. That is, oxygen is being removed from nickel oxide. Removal of oxygen from a substance is called reduction, so nickel oxide undergoes reduction.In a reaction, hydrogen is changing to H2O. That is,oxygen is being added to hydrogen. Addition of oxygen to a substance is called oxidation, so hydrogen undergoes oxidation.

Question 7:

Balance the following equation stepwise.

ANSWER:


a. H2S2O7(l) + H2O(l) → H2SO4(l)
Step1. Count the number of each atom in reactant side:
H= 4
S=2
O=8
Step2. Count the number of each atom in product side:
H= 2
S=1
O=4
Step3.Then balance the number of each atom in an equation by multiplying reactant and product side with numeral value:
If we multiply product side by 2, then the number of atoms in product and reactant side gets balance.
H2S2O7(l) + H2O(l) → 2H2SO4(l)

b. SO2(g) + H2S(aq) → S(s) + H2O(l)      
Step1. Count the number of each atom in reactant side:
H= 2
S=2
O=2
Step2. Count the number of each atom in product side:
H= 2
S=1
O=2
Step3.Then balance the number of each atom in an equation by multiplying reactant and product side with numeral value:
If we multiply H2S by 2 in the reactant side and S by 3 and H2O by 2 in the product side, then the number of atoms in product and reactant side gets balance.
SO2(g) + 2H2S(aq) → 3S(s) + 2H2O(l) 

c. Ag(s) + HCl(aq) → AgCl ↓+ H2
Step1. Count the number of each atom in reactant side:
H= 1
Ag=1
Cl=1
Step2. Count the number of each atom in product side:
H= 2
Ag=1
Cl=1
Step3.Then balance the number of each atom in an equation by multiplying reactant and product side with numeral value:
If we multiply Ag by 2 and HCl by 2 in the reactant side and AgCl by 2 in the product side, then number of atoms in product and reactant side gets balance.
2Ag(s) + 2HCl(aq) → 2AgCl ↓+ H2

d. NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + H2O(l)
Step1. Count the number of each atom in reactant side:
Na= 1
H=3
O=5
S=1
Step2. Count the number of each atom in product side:
Na= 2
H=2
O=5
S=1
Step3.Then balance the number of each atom in an equation by multiplying reactant and product side with numeral value:
If we multiply NaOH by 2 in the reactant side and H2O by 2 in the product side, then the number of atoms in product and reactant side gets balance.
2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) +2H2O(l)

Question 8:

Identify the endothermic and exothermic reaction.

ANSWER:

Identify the endothermic and exothermic reaction.
Heat is released in the product side, as it mentioned in the above reaction. So, It is an exothermic reaction because heat is evolved in exothermic reaction.
Heat is given in the product side to break the compound into simpler substances, as it mentioned in the above reaction. So, It is an endothermic reaction because heat is supplied in exothermic reaction.

Heat is released in the product side, as it mentioned in the above reaction. So, It is an exothermic reaction because heat is evolved in exothermic reaction.

Heat is provided in the product side to break the compound into simpler substances, as it mentioned in the above reaction. So, It is an endothermic reaction because heat is supplied in exothermic reaction.

Question 9:


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