Chemical Reactions and Equations NCERT Class 10 Ch 1 Notes for CBSE

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Chemical Equation

A chemical equation is the symbolic representation of a chemical change.

Reactants and Products

The substances, in which the chemical change is brought, are called reactants and the substances that are formed as a result of the chemical reaction are called products.

Chemical Reaction

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

We can observe or recognize a chemical reaction by observing

  • Change in state
  • Change in colour
  • Evolution of a gas
  • Change in temperature

Word Equation

A word-equation shows the change of reactants to products through an arrow placed between them. The reactants are written on the left-hand side (LHS) with a plus sign (+) between them. Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them. The arrowhead points towards the products and shows the direction of the reaction.

Example: Magnesium + Oxygen → Magnesium oxide

Skeletal Chemical Reaction

A chemical reaction in which, the reactants and products are identified with their chemical formula but their quantity and proportion are not identified.

Example: Mg + O2 → MgO

The physical state of reactants and the products are mentioned to make the chemical reaction more informative. e.g. we use (g) for gas, (l) for liquid, (s) for solid and (aq) for aqueous.

Precipitate

The insoluble substance formed in a chemical reaction is called precipitate.

  • The formation of precipitate in a chemical reaction is represented by the Downward Facing arrow (), immediately after the precipitate.
  • Precipitation Reaction: Any reaction that produces a precipitate can be called a precipitation reaction.

Evolution of a gas in a chemical reaction is represented by an Upward Facing Arrow (), immediately after the chemical formula of that gas.

Balanced Equation

A balanced equation is the one in which the number of atoms on the reactant and product sides is equal.

Balancing a chemical equation by Trial and Error method

Most chemical reactions can be balanced by trial and error, using three simple principles.

  1. The number of atoms of each element must be the same on each side of the reaction arrow.
  2. You can manipulate only the coefficients and not the subscripts.
  3. The final set of coefficients should be whole numbers and should be the smallest whole numbers that will do the job.

Example: Balancing a given chemical equation (Ferric Oxide with Carbon)

Step 1. Write the word-equation

Ferric Oxide + Carbon → Carbon Dioxide + Iron

Step 2. Represent the word-equation as a Chemical Equation

Fe2O3 + C → CO2 + Fe (Unbalanced)

Step 3. List the number of atoms of different elements present in the unbalanced equation.

Element Number of atoms in reactants (LHS) Number of atoms in products (RHS)
Fe 2 1
O 3 2
C 1 1

Step 4. Let’s begin with balancing CO2 molecule (as in the reaction, the maximum number of atoms is of Oxygen)
Here we observe that there are 3 atoms of oxygen in LHS and 2 atoms in the RHS. So, to make them equal we’ll multiply both CO2 and Fe2O3 with a number so that the resultant number must be the LCM of 2 and 3 i.e. 6, thus we’ll multiply Fe2O3 with 2 and CO2 with 3, hence

2Fe2O3 + C → 3CO2 + Fe (Partially balanced)

Step 5. Now the number of atoms of Oxygen in both RHS and LHS is equal. Now, we’ll work on iron. We observe that there are 4 atoms of Fe in LHS and 1 in RHS. To equate them, we just have to multiply Fe with 4 in RHS (as LCM of 1 and 4 is 4). Thus, we get

2Fe2O3 + C → 3CO2 + 4Fe (Partially balanced)

Step 6. After Fe, now we’ll work on Carbon. We observe that there is just 1 carbon atom in LHS, whereas there are 2 Carbon atoms in RHS. So, we need to multiply Carbon in LHS with 2 (LCM of 1 and 3 is 3), and thus we get the final balanced chemical equation in which all the elements are balanced (LHS = RHS).

2Fe2O3 + 3C → 3CO2 + 4Fe (Balanced)

Step 7. The last step would be to write the symbols of the physical states of the compounds. Here, as we know that Ferric Oxide, Carbon and Iron are solids, whereas CO2 is a gas. Thus, we get our final chemical equation

2Fe2O3(s) + 3C(s) → 3CO2(g) + 4Fe(s)

Types of Reaction (Based on Absorption/Desorption of Heat)

1. Exothermic Reaction

Reactions in which heat energy is released along with the formation of products are called exothermic chemical reactions.

Examples of Exothermic Reaction:

  1. Reaction of Quick Lime (Calcium oxide) with Water: CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat
  2. Reaction of Methane with Oxygen: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + Heat
  3. Respiration (Reaction of Glucose with Oxygen): C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + Heat
  4. The decomposition of vegetable matter into compost.

2. Endothermic Reaction

Reactions which require or absorb heat energy from its surroundings are called exothermic chemical reactions.

Examples of Endothermic Reaction:

Ba(OH)2 + 2NH4Cl + Heat → BaCl2 + 2NH3 + 2H2O

Types of Reaction (Based on the behaviour of the reactants)

I. Combination Reaction

The reaction in which two or more substances (elements or compounds) combine to form a new single substance.

A + B → AB

Examples of Combination Reaction:

  1. Reaction of Quick Lime and Water: CaO(s) + H2O(l) → Ca(OH)2(aq)
  2. Burning of Coal: C(s) + O2(g) → CO2 (g)
  3. Formation of water from the combination of Hydrogen and Oxygen: 2H2(g) + O2(g) → 2H2O(l)

II. Decomposition Reaction

The reaction in which a single reactant decomposes to give two or more products.

AB → A + B

Decomposition reactions can be divided into three types:

Thermal Decomposition

When a decomposition reaction is carried out by heating.

  1. Thermal Decomposition of Ferrous Sulphate: 2FeSO4(s) + Heat → Fe2O3(s) + SO2(g) + SO3(g)
  2. Thermal Decomposition of Limestone: CaCO3(s) + Heat → CaO(s) + CO2(g)
  3. Thermal Decomposition of Lead Nitrate: 2Pb(NO3)2(s) + Heat → 2PbO(s) + 4NO2(g) + O2(g)

Electrolytic Decomposition

When a decomposition reaction is carried out by electric current.

Examples:

  1. Electrolytic Decomposition of Sodium Chloride: 2NaCl (Electrolysis) → 2Na (at Cathode)+ Cl2 (at anode)
  2. Electrolytic Decomposition of Aluminium Chloride: 2AlCl3 (Electrolysis) → 2Al (at Cathode) + 3 Cl2 (at anode)

Photo Decomposition

When a decomposition reaction is carried out by sunlight.

Examples:

  1. Photo Decomposition of Silver Chloride: 2AgCl(s) + Sunlight → 2Ag(s) + Cl2(g)
  2. Photo Decomposition of Silver Bromide: 2AgBr(s) + Sunlight → 2Ag(s) + Br2(g)

III. Displacement Reaction

The chemical reaction in which an element displaces another element from its solution.

A + BC → AC + B

Examples:

  1. A reaction of Iron and Copper Sulphate: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
  2. A reaction of Zinc and Copper Sulphate: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
  3. A reaction of Lead and Copper Chloride: Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)

Note: In the above reactions, Iron, Zinc and Lead are more reactive than Copper. Thus they displace copper from its compounds.

IV. Double Displacement Reaction

The reaction in which two different atoms or group of atoms is mutually exchanged.

AB + CD → AD + CB

Examples:

  1. Reaction of Sodium Sulphate and Barium Chloride: Na2SO4(aq) + BaCl2(aq) → BaSO4(s)↓ + 2NaCl(aq)

V. Redox Reaction

The reaction in which one reactant gets oxidised while the other gets reduced.

AO + B → A + BO
  • Oxidation: Oxidation is the gain of oxygen or loss of hydrogen.
  • Reduction: Reduction is the loss of oxygen or gain of hydrogen.

Example:

  1. The endothermic Reaction of Copper Oxide and Hydrogen: CuO + H2 → Cu + H2O
  2. The reaction between Zinc Oxide and Carbon: ZnO + C → Zn + CO
  3. The reaction between Magnesium Oxide and Hydrogen Chloride: MnO2 + 4HCl → MnCl2 + 2H2O + Cl2

The effects of Oxidation Reactions in everyday life

Corrosion

When metallic substances come in contact with acid or moisture then the substances corrode and the process is called corrosion.

  • Rusting is the term used for corrosion of iron.
  • Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, especially those of iron.

Rancidity

When the food materials containing fats or oils are left for a long time, they are oxidised i.e., they become rancid and their smell and taste changes. The phenomenon is known as rancidity.

  • Usually, substances which prevent oxidation (antioxidants) are added to foods containing fats and oil.
  • Keeping food in airtight containers helps to slow down oxidation.
  • Chips manufacturers usually flush bags of chips with a gas such as nitrogen to prevent the chips from getting oxidised.

Important Chemical Compounds Name and Formula

Chemical Name Formula Chemical Name Formula
Magnesium oxide MgO Sulphuric acid H2SO4
Hydrochloric acid HCl Nitric acid HNO3
Potassium sulphate K2SO4 Sodium sulphate Na2SO4
Zinc sulphate ZnSO4 Barium sulphate BaSO4
Copper sulphate CuSO4 Ferrous sulphate FeSO4
Ferrous oxide Fe2O3 Ferric oxide Fe3O4
Aluminium oxide Al2O3 Lead oxide PbO
Copper oxide CuO Lead nitrate Pb(NO3)2
Calcium nitrate Ca(NO3)2 Sodium nitrate NaNO3
Water H2O Glucose C6H12O6
Methanol CH3OH Quick lime CaO
Slaked lime Ca(OH2) Limestone CaCO3
Aluminium chloride AlCl3 Barium chloride BaCl2
Lead chloride PbCl2 Copper chloride CuCl2
Manganese chloride MnCl2 Magnesium chloride MgCl2
Silver chloride AgCl Silver bromide AgBr
Barium bromide BaBr2 Sodium Chloride NaCl

Special Chemical Reactions

Photosynthesis

Plants and other organisms (that have chlorophyll) takes carbon dioxide and water to produce food (glucose) for themselves and oxygen as a byproduct. The reaction can only take place under sunlight which is tapped by the green colour pigments (chlorophyll).

6CO2 + 6H2O → C6H12O6 + 6O2

Respiration

Aerobic respiration involves taking oxygen from the atmosphere to break glucose molecules for the production of energy along with byproducts (water and carbon dioxide).

6O2 + C6H12O6 → 6H2O + 6CO2 + Energy

Reactions of Lime

Exothermic reaction of Quick Lime with Water

The quick lime (CaO) reacts with water (H2O) to produce slaked lime (Ca(OH2)) and large amount of heat.

CaO + H2O → Ca(OH2) + Heat

Reaction of Slaked Lime with Carbon dioxide

The slaked lime (Ca(OH2)) reacts with carbon dioxide (CO2) to produce limestone and water.

Ca(OH2) + CO2 → CaCO3 + H2O

Endothermic reaction of Limestone

The Limestone on heating produces quick lime and carbon dioxide.

CaCO3 + Heat → CaO + CO2

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