3. Chemical Reactions and Equations | Part 1 | Science I | Class 10

The blog presents the short Questions and Answers of the Chapter 3, Chemical Reactions and Equations, from Science Part I of Class 10 from Maharashtra State Board. These can be very useful for students preparing for SSC Board exams as well as competitive exams, as it includes the objective type basic concepts & fundamentals.

Q1. What is a chemical reaction ?

Ans. A chemical reaction is a process in which some substances undergo bond breaking and are transformed into new substances by formation of new bonds.

Q2. What are reactants ?

Ans. The substances taking part in chemical reaction are called reactants.

Q3. What are products ?

Ans. The substances formed as a result of a chemical reaction by formation of new bonds are called products.

Q4. Give an example of chemical reaction ?

Ans. Formation of carbon dioxide gas by combustion of coal in air is a chemical reaction. In this reaction coal (carbon) and oxygen (from air) are the reactants while carbon dioxide is the product.

Carbon + Oxygen −−→> Carbon dioxide

Q5. How are chemical equations written ?

Ans. Chemical equations can be written in following two ways :
• Word Equation :

copper sulphate + zinc dust −−→> Aqueous solution of zinc sulphate + copper

• Chemical Equation :

CuSO4 + Zn −−→ ZnSO4 + Cu

Q6. What are the rules for writing a chemical equation ?

Ans. The rules for writing a chemical equation are :
(a) In a chemical equation the reactants are written on the left hand side while the products on the right hand side. An arrow heading towards the products is drawn in between them. This arrow indicates
the direction of the reaction.

(b) If the reactants or products are two or more, they are linked with a plus sign (+) in between them.

CuSO4 + Zn −−→> ZnSO4 + Cu

(c) To make the chemical equation more informative the physical states of the reactants are indicated in the equation. Their gaseous, liquid and solid states are indicated by writing the letters (g), (l) and (s), respectively in the brackets. Moreover, if the product is gaseous, instead of (g) it can be indicated by an arrow ↑ pointing upwards . If
the product formed is insoluble solid, in the form of a precipitate, then instead of (s) it can be indicated by an arrow ↓ pointing downwards. When reactants and products are in the form of solution in water, they are said to be present in aqueous solution state. This state is indicated by putting the letters aq in brackets after their formula.

CuSO (aq) + Zn (s) −−→> ZnSO (aq) + Cu (s)

(d) When heat is to be given from outside to bring about a reaction, it is indicated by the sign ∆ written above the arrow that indicates the direction of the reaction. For example, the reaction in which slaked lime is formed on heating lime stone is written as follows.

Similarly, the fact that heat is released during the reaction between the aqueous solution of copper sulphate and zinc dust is indicated as follows.

CuSO (aq) + Zn (s) −−→ ZnSO (aq) + Cu (s) + Heat

(e) It is necessary to fulfill certain conditions like specific temperature, pressure, catalyst, etc. to bring about some reactions. These conditions are indicated below or above the arrow indicating the direction of the reaction.

Example: The reaction of a vegetable oil takes place at the temperature of 60◦ C with hydrogen gas in presence of the Ni catalyst, written :

(f) Special information or names of reactants/ products are written below their formulae.

Example : Copper on reaction with concentrated nitric acid gives reddish coloured poisonous nitrogen dioxide gas.

However, on reaction with dilute nitric acid, the product formed is nitric oxide gas.

Q7. What is a balanced equation ?

Ans. When the number of atoms of the elements in the reactants in this equation is same as the number of atoms of those elements in the products. the equation is called a balanced equation.

Q8. Write the steps for balancing a chemical equation ?

Ans. A chemical equation is balanced step by step. A trial and error method is used for this purpose.

(a) Write the chemical equation from the given word equation.

(b) Check whether the equation is balanced or not by comparing the number of atoms of Element Number of atoms Number of atoms the
various elements present on the two sides of the equation.

(c) It is convenient to start balancing an equation from the compound which contains the maximum number of atoms. Moreover it is convenient to first consider that element in this compound, which has unequal number of atoms on the two sides.

(d) Write down the final balanced equation again.

Q9. What is a Combination Reaction ?

Ans. When two or more reactants combine in a reaction to form a single product, it is called a combination reaction.

Combination reactions

Q10. What is Decomposition Reaction ?

Ans. The chemical reaction in which two or more products are formed from a single reactant is called decomposition reaction.

Decomposition reaction

Q11. What is Displacement Reaction ?

Ans. The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction.

Displacement Reaction

Q12. What is a Double displacement reaction ?

Ans. The reaction in which the ions in the reactants are exchanged to form a precipitate are called double displacement reactions.

Double displacement reaction

Q13. What are endothermic processes ?

Ans. When heat from outside is absorbed during some physical changes, these processes are called as Endothermic processes.
Example : (i) melting of ice
(ii) dissolution of potassium nitrate in water.

Q14. What are exothermic processes ?

Ans. When heat is given away during some physical changes, these processes are called as Exothermic processes.

Examples : (i) formation of ice from water
(ii) dissolution of sodium hydroxide in water.

Q15. Why only a small quantity of concentrated sulphuric acid at a time is added to water with stirring ?

Ans. (i) In the process of dilution of concentrated sulphuric acid with water, very large amount of heat is liberated.
(ii) As a result, water gets evaporated instantaneously, if it is poured in to the concentrated sulphuric acid, which may cause an accident.
(iii) To avoid this, required amount of water is taken in a glass container and small quantity of concentrated sulphuric acid at a time is added with stirring.
(iv) Therefore, only a small amount of heat is liberated at a time.

Q16. Give an example of endothermic and exothermic reactions ?

Ans. The examples of endothermic and exothermic reactions are :

Endothermic and exothermic reactions

Coming soon : Part 2

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