CHAPTER-1 Chemical Reaction and Equation / What is a Chemical Reaction Class 10
The change of one chemical substance into another chemical substance is known as a chemical reaction. Like- rusting of iron, formation of curd of milk, digestion of food, respiration etc.
The chemical physical properties of the new substance formed in a chemical reaction are completely different from the properties of the original substance, so we can say that a chemical change takes place in a chemical reaction.
👉There is only rearrangement of atoms in a chemical reaction.
👉The substances which take part in a chemical reaction are called Reactants.
👉The new substances produced as a result of a chemical reaction are called Products.
The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.
2Mg(s) + O2(g) △→ 2MgO(s)
Facts
👉 Before burning in the air, the magnesium ribbon is cleaned by rubbing it with sandpaper.
👉This is done to remove the protective layer of basic magnesium carbonate from the surface of the magnesium ribbon.
👉The substances that take part in a chemical reaction are called reactants. Thus Mg and O2 are the reactants in this reaction.
👉The new substance formed after a chemical reaction is called a product. So MgO product in this reaction
Characteristics of Chemical Reactions:
(i) Evolution of gas:- Hydrogen gas is evolved by chemical reaction between zinc and dilute sulfuric acid.
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) ↑
(ii) Change in Colour:- Due to the chemical reaction between citric acid and violet colored potassium permanganate solution, the color of this solution changes from purple to colourless.
👉A chemical reaction between sulfur dioxide gas and an acidic potassium dichromate solution results a color change from orange to green.
(iii) Change in state of substance: - In the reaction of candle combustion, wax changes from solid to liquid and gaseous state.
(iv) Change in temperature: - Chemical reaction between slaked lime water to make slaked lime results a change in temperature i.e., increase in temperature.
👉The chemical reaction between zinc granules and dilute sulfuric acid also shows a change in temperature.
(v) Formation of precipitate: - In the chemical reaction between sulfuric acid and barium chloride solution, a white precipitate of barium sulphate is formed.
BaCl2(aq) + H2SO4(aq) → BaSO4(s) (ppt) + 2HCl(aq)
Chemical Equation (What is a chemical Equation Class 10)
A chemical equation is a way of presenting a chemical reaction in a concise and informative way.
👉The symbolic representation of a chemical reaction using symbols and formulas of substances is called a chemical equation.
👉When hydrogen reacts with oxygen it gives water. This reaction can be represented by the following chemical equation:
Hydrogen + oxygen → water
H2 + O2 → H2O
Two types of Chemical Equations
A chemical equation can be divided into two types: -
1. Balanced Chemical Equation and
2. Unbalanced Chemical Equation
👉Balanced chemical equation: In a balanced chemical equation, the number of atoms of each element is equal on both the sides. As-
Zn + H2SO4 → ZnSO4 + H2
This equation has equal number of zinc, hydrogen and sulfate on both sides, so it is a balanced chemical equation.
According to the Law of Conservation of Mass - Mass can neither be created nor destroyed in a chemical reaction.
👉to obey this rule, the total mass of the elements present in the reactants must equal the total mass of the elements present in the products.
👉Unbalanced Chemical Equation: If the number of atoms of each element in the reactants is not equal to the number of atoms of each element present in the products, then the chemical equation is called unbalanced chemical equation. -
Fe + H2O → Fe3O4 + H2
How to make Equations More Informative
👉Writing the physical states of matter makes a chemical equation more informative.
The gaseous state is represented by the symbol (g).
The liquid state is represented by the symbol (l).
The solid state is written by the symbol (s).
Aqueous solutions are written with the symbol (aq).
👉Above and/or below the arrow of a chemical equation, the position in which the reaction occurs is written.
What are the types of a chemical reaction Class 10
Chemical reactions can be classified into the following types:
(i) Combination Reaction
(ii) Decomposition Reaction
(iii) Displacement Reaction
(iv) Double Displacement Reaction
(v) Oxidation and Reduction Reactions-
(i)Combination Reaction - Those reactions in which two or more reactants combine to form a single product are called combination reactions. eg- A + B → AB
👉When magnesium is burnt in air (oxygen), magnesium oxide is formed.
Mg(s) + O2(g) → 2MgO(s)
magnesium + oxygen → magnesium oxide
👉When carbon is burnt in oxygen (air), carbon dioxide is formed.
C(s) + O2(g) → CO2(g)
carbon + oxygen → carbon dioxide
(ii)Decomposition Reaction - Those reactions in which a compound decomposes into two or more compounds or elements are called decomposition reactions. A decomposition reaction is just the opposite of a combination reaction. eg -ab → a + b
👉When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.
CaCO3(s) heat−→− CaO(s) + CO2(g)
calcium carbonate → calcium oxide + carbon dioxide
👉When ferric hydroxide is heated, it decomposes into ferric oxide and water.
2Fe(OH)3(s) △→ Fe2O3(s) + 3H2O(l)
Energy is essential for the chemical decomposition of a substance in decomposition reactions, depending on the source of this energy,
Decomposition reactions can be divided into three types-
👉Thermal Decomposition – In this, a substance decomposes on heating.
2Pb(NO3)2(s) heat−→− 2PbO(s) + 4NO2(g) + O2(g)
👉Electrolytic Decomposition - Those reactions in which compounds are decomposed into simpler compounds due to the passage of electricity.
👉When electricity is passed through water, it decomposes into hydrogen and oxygen.
2H2O(l) undefined control sequence → 2H2(g) + O2(g)
👉Photolysis or Photo Decomposition Reaction - Those reactions in which a compound is decomposed due to sunlight
👉 When silver chloride is kept in sunlight, it decomposes into silver metal and chlorine gas.
2AgCl(s) (white) Sunlight−→ 2Ag(s) (grey) + Cl2(g)
(iii)Displacement Reaction - Those chemical reactions in which a more reactive element displaces a less reactive element from a compound.
👉A typical displacement reaction can be represented using a chemical equation as follows: a + bc → ac + b
👉Displacement reaction takes place only when 'A' is more reactive than 'B'. If 'B' is more reactive than 'A', then 'A' will not displace 'C' from 'BC' and the reaction will not occur.
👉When zinc reacts with hydrochloric acid, it changes into hydrogen gas and zinc chloride.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
👉When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
(iv) Double Displacement Reaction: Those reactions in which there is an exchange of ions between two reactants to form new compounds are called double displacement reactions.
AB + CD → AC + BD
👉When a solution of barium chloride reacts with a solution of sodium sulphate, a white precipitate of barium sulphate is formed along with sodium chloride.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) (precipitate) + 2NaCl(aq)
👉When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
The two major forms of double displacement reaction are precipitation reactions and neutralization reactions-
👉Precipitation reaction - The reaction in which a precipitate is formed from a mixture of aqueous solution of two salts is called precipitation reaction. Example-
AgNO3(aq) + KCl(aq) —– AgCl(precipitate) + KNO3(aq)
👉Neutralization Reaction - The reaction in which an acid reacts with a base to form salt and water by exchange of ions is called neutralization reaction. Example-
HCl + NaOH → NaCl + H2O
(v) Oxidation and Reduction Reactions
👉Oxidation - Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is called oxidation.
👉Reduction Reactions - Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction.
👉Oxidizing agent - The substance which gives oxygen for oxidation or the substance which removes hydrogen is called oxidizing agent.
👉Reducing agent - The substance which gives hydrogen for reduction or the substance which removes oxygen is called reducing agent.
The reaction in which both oxidation and reduction take place simultaneously is called Redox reaction.
👉When copper oxide is heated with hydrogen, copper metal and hydrogen are formed. As-
CuO + H2 → Cu + H2O
(vi) Exothermic and Endothermic Reactions
👉Exothermic Reactions - The reaction which produces energy is called exothermic reaction.
👉Most decomposition reactions are exothermic.
👉Respiration is a decomposition reaction in which energy is released.
6CO2(g)+6H2O (l)+energy→C6H12O6(s)+6O2(g)
CaCO3(s)→CaO (s)+CO2(g)
👉Endothermic Reactions - A chemical reaction in which heat energy is absorbed.
C6H12O6(s)+6O2(g)→6CO2(g)+6H2O(l)+Energy
Effects of Oxidation Reactions in Everyday life-
1. Corrosion
2. Rancidity
Corrosion: -The process of slow conversion of metals into their undesirable compounds due to reaction with oxygen, water, acids, gases etc. present in the atmosphere is called corrosion. Example: Rusting of iron.
👉 When iron reacts with oxygen and moisture, it forms a red substance called rust.
👉Rusting of iron is a redox reaction.
2FeO(OH)⇌ Fe2O3 + H2O
Methods to Prevent Rusting
1. By painting.
2. By lubricating and oiling.
3. By Galvanization.
👉Corrosion of Copper- Copper articles lose their luster over time because the surface of these articles gets covered with a green layer of native copper carbonate, CuCO3.Cu(OH)2, when it exposed to air.
👉 Corrosion of Silver - The surface of silver metal gets tarnished when it comes in contact with air, because a layer of black silver sulphide (Ag2S) is formed on its surface due to the action of H2S gas present in the air.
Rancidity- The taste and smell of foods containing fats and oils change when they are exposed to air for a long time. This is called rancidity.
👉It is caused by the oxidation of fats and oils present in foods.
Methods to prevent rancidity
1. By adding antioxidants.
2. Vacuum packing.
3. Replacement of air by nitrogen.
4. Cooling of foods.
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