Lesson 1 - Chemical Reactions and Equations-Class 10 Science Notes

- June 30, 2021

Chemical Reactions and Equations

Chemistry:-

The theoretical and experimental study of nature, composition, preparation, isolation and uses of matter is known as Chemistry.

Chemical Reaction:-

Chemical reaction is the process by which two or more substances react with each other to form new substance with different properties.

Only a rearrangement of atoms takes place in a chemical reaction.

Example: 2H2 + O2 → 2H2O

Here H2 and O2 take part in chemical reaction and produce a product of new property H2O

Some more examples of chemical reactions:

➤Iron rod comes in contact of moisture and gets rusted.

➤Grapes get fermented

➤Food is cooked.

➤Food get digested in our body.

➤Respiration

Determination of Chemical Reaction:-

These are the following changes to determine that the chemical reaction has taken place:

Change in state
Change in colour
Evolution of gas
Change in temperature
Formation of precipitate

Characteristics of Chemical Reactions:-

1. Change in state:- The combustion reaction of candle wax is characterized by a change in state from solid to liquid and gas (candle wax is solid, water formed by the combustion of wax is liquid and carbon dioxide produced by the combustion of wax is a gas).

2. Change in color:- The chemical reaction between citric acid and purple colored Potassium Permanganate solution is characterized by a change in color from purple to colorless.

3. Evolution of gas:- The chemical reaction between Zinc and dilute Sulphuric acid is characterized by the evolution of hydrogen gas.

Zn (s) +H2SO4 (aq) → ZnSO4 (aq) + H2 (g) ↑

4. Change in temperature:- The chemical reaction between quick lime and water to form slaked lime is characterized by a change in temperature (which is a rise in temperature).

CaO (s) + H2O (l) → Ca(OH)2 (aq)

5. Formation of precipitate:- The chemical reaction between Sulphuric acid and Barium chloride solution is characterized by the formation of a white precipitate of Barium sulphate.

BaCl2 (aq) + H2SO4 (aq) → BaSO4 (ppt) + 2HCl (aq)

Where-

s → solid

l → liquid

g → gas

aq → aquas

ppt → precipitate

Chemical Equation:-

A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae, wherein the reactant entities are given on the left-hand side and the product entities on the right-hand side.

Its general form is- A + B → C + D

In this equation, A and B are called reactants and C and D are called the products. The arrow shows the direction of the chemical reaction.

Example:- When Magnesium ribbon is burnt in oxygen, it gets converted to Magnesium oxide.

Magnesium + Oxygen → Magnesium oxide

To write the equation in such manner is called as Word equation.

Mg + O2 → MgO

This is symbolic form of chemical equation.

There are two part of chemical reaction-

1. Reactants:- The substances which take part in chemical reaction are called Reactants.

Example: Mg and O2

2. Products:- The substances which formed in a chemical reaction are called Products.

Example: MgO

Types of Chemical equation:-

A chemical equation can be divided into two types-

1. Unbalanced (Skeletal) Chemical Equation:- If the number of atoms of each element in reactant is not equal to the number of atoms of each element present in the product, then the equation is called Unbalanced (Skeletal) Chemical Equation.

Example:- Mg + O2 → MgO

Fe + H2O → Fe3O4 +H2

2. Balanced Chemical Equation:- If the number of atoms of each element in reactant is equal to the number of atoms of each element present in the product, then the equation is called Balanced Chemical Equation.

Example:- 2Mg + O2 → 2MgO

Zn + H2SO4 → ZnSO4 + H2

Law of conservation of Mass:-

Mass can neither be created nor destroyed in a chemical reaction. So number of elements involved in chemical reaction should remain same at reactant and product side.

Balancing Chemical Equation:-

We use Hit and Trial method or Inspecting method to balance chemical equation. Here we take an unbalanced equation to balance Fe + H2O → Fe3O4 + H2

Step 1:- Write a chemical equation and draw boxes around each formula. Do not change anything inside the box.

Fe + H2O → Fe3O4 + H2

Step 2:- Count the number of atoms of each element on both the side of chemical equation.

Step 3:- Equalize the number of atoms of elements which has maximum number by putting a number in front of it.

3 Fe + H2O → Fe3O4 + H2

Step 4:- Try to equalize all the atoms of each elements of reactant and product side by adding coefficient in front of it.

3 Fe + 4 H2O → Fe3O4 + 4 H2

Now all the atoms of each elements are equal on both sides.

Step 5:- Write the physical state of reactants and products.

3Fe(s) +4 H2O(g) → Fe3O4 (s)+ 4H2(g)

s → solid

l → liquid

g → gas

aq → aquas

ppt → precipitate

Step 6:- Write necessary conditions of temperature, pressure or catalyst on arrow above or below.

Types of Chemical reactions:-

Chemical reactions can be classified in the following types-

1. Combination reaction
2. Decomposition reaction
➽Thermal Decomposition
➽Electrolytic Decomposition

➽Photolysis or Photo Decomposition reaction

3. Displacement reaction
4. Double Displacement reaction
➽Precipitation reaction

➽Neutralization reaction

5. Oxidation and Reduction reaction
6. Exothermic and Endothermic reaction

Types of Chemical Reactions:-

Chemical reactions can be classified in following types-

1. Combination Reaction:-

Reactions in which two or more reactants combine to form one product are called Combination Reactions. A general combination reaction can be represented as-

A + B → AB

Example:- When magnesium is burnt in the air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.

Mg(s) + O2(g) → 2MgO(s)

Magnesium + Oxygen → Magnesium Oxide

Some more examples of Combination reactions-

(i) Burning of coal:

C (s) + O2 (g) → CO2(g)

(ii) Formation of water:

2H2(g) + O2(g) → 2H2O(l)

(iii) Formation of sulphur dioxide:

S(s) + O2(g) → SO2(g)

(iv) Rusting (Formation of Ferrous oxide):

2Fe(s) + O2(g) → 2FeO(s)

(v) Quick lime to Slaked lime:

CaO(s) + H2O(l) → Ca(OH)2(aq)

** Calcium hydroxide Ca(OH)2 is used for white washing on walls. This is a precipitate reaction, when calcium hydroxide is painted on walls it reacts with CO2 present in the air to form a thin layer of calcium carbonate and also produce water which get evaporated.

2. Decomposition Reaction:-

The reaction in which a single reactant breaks down into two or more than two simpler products is known as decomposition reaction.

General equation: A → B + C

Example:-When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.

CaCO3(s) → CaO(s) + CO2(g)

When Ferric hydroxide is heated, it decomposes into ferric oxide and water

2Fe(OH)3(s) → Fe2O3(s) +2H2O(l)

Thermal Decomposition:- The decomposition of a substance on heating is known as Thermal Decomposition.

Example:- 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)

2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g)

CaCO3(s)(Lime stone) → CaO(s) (Quick lime) + CO2(g)

Calcium Oxide:- It's common name is quick lime or burnt lime. It is white in color, caustic, alkaline and crystaline. It obtained from thermal decomposition of calcium carbonate (lime stone).

Uses of Calcium oxide:-

1. Manufacturing of cement

2. Manufacturing of various types of glass.

3. In agriculture, It is used for treating acidic soils.

4. In white washing, it is used with water.

Electrolysis Decomposition:- Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as Electrolytic Decomposition. This is also known as Electrolysis.

Example:- 2H2O(l) → 2H2 (g) + O2(g)

Photolysis or Photo Decomposition Reaction:- Reactions in which a compound decomposes because of sunlight are known as Photolysis or Photo Decomposition Reaction.

Example:- When Silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.

2AgCl(s) (white) → 2Ag(s) (grey) + Cl2(g)

Photographic plate has a coat of silver chloride, which turns into grey when exposed to sunlight. It happens because of silver chloride is colorless while silver is a grey metal.

3. Displacement Reaction:-

The reactions in which the highly reactive element displaces the less reactive element from the reaction, such reaction is known as displacement reaction.

General equation: Ab + C → Cb + A

Here C is highly reactive element which displace A from the reaction and joins b.

Example:- 1. When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

2. When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.

Zn(s) + CuSO4 (aq) → ZnSO4(aq) + Cu(s)

3.* When iron nail is placed in the aqueous solution of copper sulphate iron displaces copper from the reaction or its compound and blue color of copper sulphate turns grey brown because iron is highly reactive metal than copper.

CuSO4(aq) + Fe(s) → FeSO4(aq) + Cu(s)

4. CuCl2(aq) + Pb(s) → PbCl2(aq) + Cu(s)

4. Double Displacement Reaction:-

Such a reaction in which there is an exchange of ions between the reactants are called double displacement reaction;

General equation: Ab + Cd → Ad + Cb

Example:- 1.NaOH (aq) HCl (aq) → NaCl (aq) + H2O (l)

2. Na2SO4 + BaCl2 → BaSO4 + 2NaCl

3. NaOH + H2SO4 → Na2SO4 + H2O

4. NaCl + AgNO3 → AgCl + NaNO3

5. BaCl2 + H2SO4 → BaSO4 +HCl

6. BaCl2 + K2SO4 → BaSO4 + KCl2

Precipitation reaction:- The reaction in which precipitate is formed by the mixing of the aquous solution of two salts is called precipitation reaction.

NaCl (aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)

Neutralization reaction:- The reaction in which an acid reacts with a base to form salt and water by an exchange of ions is called neutralization reaction.

NaOH (aq) HCl (aq) → NaCl (aq) + H2O (l)

5. Oxidation and Reduction:-

Oxidation:- Oxidation is the gain of oxygen or loss of hydrogen.

In other words, The addition of oxygen to a substance or the removal of hydrogen from a substance or both is called oxidation.

Example:-

Gain of Oxygen:

1. Carbon gains oxygen and gets oxidized in carbon dioxide.

2. Phosphorous gains oxygen and gets oxidized in Phosphorous pentoxide.

3. Copper gains oxygen and gets oxidized in copper oxide.

Removal of Hydrogen:

1. Sulphur gives/losses Hydrogen and gets oxidized.

2. Sulphur gives/looses Hydrogen and gets oxidized.

3. Carbon gives/looses Hydrogen and gets oxidized.

Here Carbon losses Hydrogen and gains Oxygen and gets oxidized.

Reduction:- Reduction is the loss of oxygen or gain of hydrogen.

In other words, The addition of hydrogen to a substance or removal of oxygen from a substance or both is called reduction.

Example:-

Removal of Oxygen:

1. Copper oxide gives/losses oxygen and gets reduced.

2. Zinc oxide gives/losses Oxygen and gets reduced.

Gain of Hydrogen:

3. Chlorine gains Hydrogen and gets reduced.

4. Bromine gains Hydrogen and gets reduced

−→

Redox Reaction:-

Reaction in which one reactant gets oxidized while the other gets reduced during a reaction is called redox reaction.

In other words, When oxidation and reduction both take place in same reaction, then the reaction is known as redox reaction.

Example:- Zinc losses Oxygen and gets reduced while Carbon gains Oxygen and gets oxidized.

Oxidizing Agent:-

The substance which gives oxygen for oxidation or removes hydrogen for reduction is called oxidizing agent.

Example: Copper oxide gives Oxygen for oxidation is oxidizing agent.

Reducing Agent:-

The substance which is responsible for removing oxygen or gives Hydrogen for reduction called reducing agent.

Example: Hydrogen is responsible for removing oxygen from CuO. So H2 is reducing agent.

Summary:-

(i) Oxidized substance: H2 {which gains in Oxygen}

(ii) Reduced substance: CuO {which losses Oxygen}

(iii) Oxidising agent: CuO {which provides Oxygen for oxidation}

(iv) Reducing agent: H2 {which is responsible for removal of Oxygen}

Effects of Oxidation Reaction:-

Corrosion of metals and Rancidity of foods are common examples due to oxidation reaction.

1. Corrosion:-

The process in which metals losses their surface gradually by the action of air, water and moisture is called corrosion.

In other words, The process in which metals surface get corroded by reaction with air, water and moisture is called corrosion.

Example:- Rusting of iron.

Note:- Corrosion is not a rusting while rusting causes corrosion

Rusting:- Iron when reacts with oxygen and moisture forms red substance which is called Rust.

The rusting of iron is a redox reaction.

Corrosion (rusting) weakens the iron and steel objects and structures such as railing, car bodies, bridges and ships etc. and cuts short their life.

Corrosion of Copper:- Copper objects lose their lustre and shine after some time because the surface of these objects acquires a green coating of basic copper carbonate, CuCO3.Cu(OH)2 when exposed to air.

Corrosion of Silver Metal:- The surface of silver metal gets tarnished (becomes dull) on exposure to air, due to the formation of a coating of black silver sulphide (Ag2S) on its surface by the action of H2S gas present in the air.

Preventing Corrosion:- There are following method to prevent corrosion-

(i) By galvanization

(ii) By painting metal surfaces

(iii) By oiling or greasing metal surfaces.

Galvanization:- Coating of thin layer of zinc (Zn) on metal surface by electro-plating is called galvanization.

2. Rancidity:-

The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is called Rancidity. It is caused due to the oxidation of fat and oil present in food materials.

Effect of Rancidity:-

(i) Rancidity spoils the food materials prepared in fat/oil. like, chips, Bhujiya and Sabji etc.

(ii) Oxidized food materials taste and smell are changed.

(iii) Rancid food is unfit for eating.

Preventing foods from rancidity:-

(i) Rancidity can be prevented to remove oxygen gas form container and filling Nitrogen gas during packaging.

(ii) Rancidity can be retarded by keeping food in air tight containers that helps to slow down oxidation.

(iii) Rancidity can be retarded by keeping food in a refrigerator that cause to slow down oxidation.

(iv) Rancidity can retarded by storing food items away form heat and light.

(v) Rancidity can be prevented by adding anti-oxidants to fat/oil containing foods.

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