Class 8th Science Chapter 5 Chemical Reactions

Class 8th Science Chapter 5 Chemical Reactions

Class 8th Science Chapter 5 Chemical Reactions. Chemical reaction, a process in which one or more substances, the reactants, are converted into one or more different substances, the products. Substances are chemical elements or compounds. A chemical reaction rearranges the constituent atoms of reactants to create different substances as products.

Class 8th Science Chapter 5 Chemical Reactions
Blue precipitate is formed in a beaker as a result of a chemical reaction.

Chemical reactions are an integral part of technology, culture, and indeed life itself. The burning of fuels, the smelting of iron, the making of glass and ceramics, the making of beer, and the making of wine and cheese are some of the many examples of activities that incorporate chemical reactions that have been known and used for thousands of years. . Chemical reactions abound in Earth’s geology, in the atmosphere and oceans, and in a wide range of complicated processes that occur in all living systems.

Class 8th Science Chapter 5 Chemical Reactions
Class 8th Science Chapter 5 Chemical Reactions 6

Chemical reactions must be distinguished from physical changes. Physical changes include changes of state, such as ice melting into water and water evaporating into vapor. If a physical change occurs, the physical properties of a substance will change, but its chemical identity will remain the same. No matter what its physical state is, water (H2O) is the same compound, with each molecule made up of two hydrogen atoms and one oxygen atom. However, if water, such as ice, liquid, or vapor, meets metallic sodium (Na), the atoms will redistribute to give the new substances molecular hydrogen (H2) and sodium hydroxide (NaOH). By this, we know that a chemical change or reaction has occurred.

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SHORT ANSWERS

Q1. What are the characteristics of chemical reactions?

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Ans. Characteristics of chemical reactions

Evolution of gas, Change in colour, temperature or state, formation of precipitate, and formation of liquid are some of the characteristics of chemical reactions.

Q2. Define chemical reaction and its types.

Ans. Chemical reactions: The process of formation or breaking down of a chemical compound is called chemical reaction.

Types of chemical reactions: There are four types of chemical reactions.

  1. Addition reaction 2) Decomposition reaction
  2. Single displacement reaction 4) Double displacement reaction

Q3. Explain each type of chemical reaction with one example each.

Ans. the four types of reactions are the following

  1. Addition reaction: Two or more substances react to form a product.

2Na + Cl2 2NaCl

  1. Decomposition reaction: One reactant breaks up to form two or more products.

CaCO3 CaO + CO2

  1. Single displacement reaction: More reactive element displaces a less reactive element.

Zn + 2HCl H2 + ZnCl2

  1. Double displacement element: Exchange of ions occur between two compounds.

HCl + NaOH NaCl + H2O

Q4. Exothermic reactions are very important in our daily life give two reasons.

Ans. Exothermic reactions are very important in our daily life for two reasons.

  1. We take heat from the exothermic reactions taking place in the sun.
  2. We cook food from the exothermic reactions taking place during burning.

Q5. What is a balanced chemical equation? Why chemical equations need to be balanced?

Ans: A balanced chemical equation:

A chemical equation is called balanced when the number of atoms of each element is equal on both sides of the equation.

Balanced chemical equation is needed to observe law of conservation of mass.

DETAILED ANSWERS

Q1. Explain the steps of balancing chemical equation with two examples

Ans: The following steps are involved in balancing a chemical equation.

  1. Write unbalanced chemical equation with correct symbols and formulae.
  2. Count the number of elements on both sides.
  3. If the number of elements are different, write the required number as coefficients of symbols or formulae.
  4. Work with one element at a time.
  5. Always start with a relatively small number.
  6. Start with elements that appear only once.
  7. Leave the diatomic elements till the last.

Examples of balancing chemical equation

  1. CH4 + O2 CO2 + H2O

CH4+ 2O2 CO2 + 2H2O

  1. Fe +H2O Fe3O4 + H2

3Fe+4H2O Fe3O4+ 4H2

Q2. (a) Define law of conservation of mass. Explain with the help of two examples

Ans. Law of conservation of mass

According to this law, “Mass can neither be created nor destroyed during a chemical reaction.” This law was discovered by a French chemist Lavoisier in 1785.

Examples: 1). When wood burns the mass of wood and oxygen (the reactants) is equal to the mass of soot, ashes and gases (the products).

2). A piece of iron gets rusted and gains mass in the moist air. The increase in mass is just the mass of oxygen. The mass of iron and oxygen is equal to the mass of the rust.

(b) When a pile of wood is burnt, the ash left behind is less as compared to wood. How law of conservation of mass is applicable in this situation?

Ans: When a pile of wood burns, the mass of ash left behind is less as compared to wood because the remaining mass was released in the air in the form of carbon dioxide. The law of conservation of energy is applicable here because the mass of wood (the reactant) is equal to the mass of the ash and the released carbon dioxide (the products).

Q3. Define heat of reaction. Differentiate between exothermic and endothermic chemical reactions with examples.

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Ans. Heat of reaction

Heat released or absorbed during a chemical reaction is called heat of reaction.

Endothermic Reactions

Endo means inside and thermal means heat. These are the reactions in which, heat is absorbed from the surrounding.

Temperature of the surrounding drops. These reactions proceed with the addition of heat.

Exothermic Reactions

Exo means outside and thermal means heat. These are the reactions in which, heat is released to the surrounding. Temperature of the surrounding increases. These reactions proceed with the release of heat.

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