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CHEMICAL REACTIONS AND EQUATIONS

 CHEMICAL REACTIONS AND EQUATIONS

 

A. CHEMICAL REACTIONS

a.       Chemical change – one or more new substances with new physical and chemical properties are formed.

When copper sulphate reacts with iron, two new substances, i.e., ferrous sulphate and copper, are formed.

 b.      Physical change – change in colour or state occurs, but no new substance is formed.

Example: Water changes to steam on boiling, but no new substance is formed. This involves only a change in state (liquid to vapour).

Chemical Reaction: The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc.

Observations that Help Determine a Chemical Reaction: A chemical reaction can be determined with the help of any of the following observations.

·       Evolution of a gas

·       Change in temperature

·       Formation of a precipitate

·       Change in colour

·       Change of state

Writing Chemical Equations: Representation of a chemical reaction in terms of symbols and chemical formulae of the reactants and products is known as a chemical equation.

B. BALANCING CHEMICAL EQUATIONS

QUE. Why do we need to balance a chemical reaction?

Ans. Law of Conservation of Mass: According to the Law of Conservation of Mass, no atoms can be created or destroyed in a chemical reaction, so the number of atoms for each element on the reactants side has to balance the number of atoms that are present on the products side.
In other words, the total mass of the products formed in a chemical reaction is equal to the total mass of the reactants participating in a chemical reaction.

Balanced chemical equation: The chemical equation in which the number of atoms of each element on the reactants side is equal to that of the products side is called a balanced chemical equation.

Steps for Balancing Chemical Equations

Hit and trial method: While balancing the equation, change the coefficients (the numbers in front of the compound or molecule) so that the number of atoms of each element is the same on each side of the chemical equation.

abc method for Balancing a Chemical Equation

Example:              aCaCO3 + bH3PO4 → cCa3(PO4)2 + dH2CO3

Set up a series of simultaneous equations, one for each element.

Ca: a=3c

C: a=d

O: 3a+4b=8c+3d

H: 3b=2d

P: b=2c

Let’s set c=1 (you can assume any variable =1)

Then a=3 and

d = a = 3

b = 2c = 2

So, a=3; b=2; c=1; d=3

The balanced equation is

3CaCO3 + 2H3PO4 → Ca3 (PO4)2 + 3H2CO3

C. TYPES OF CHEMICAL REACTIONS

Taking into consideration different factors, chemical reactions are grouped into multiple categories.
A few examples are:

1.      Combination

2.      Decomposition

3.      Single Displacement

4.      Double displacement

Combination Reaction: In a combination reaction, two elements or one element and one compound or two compounds combine to give one single product.

Decomposition: A reaction in which a single reactant decomposes/breaks down to form multiple simpler products is called decomposition reaction.

Types of decomposition reactions:

a.       Decomposition reactions which require heat-thermolytic decomposition or thermolysis.

CaCO3 → CaO + CO2

2 KClO3 (s) → 2 KCl (s) + 3 O2 (g)

2 Fe(OH)3 → Fe2O3 + 3 H2O

H2SO4 ⟶ H2O + SO3


2HgO ⟶ O2 + 2Hg

b. Decomposition reactions which require light-photolytic decomposition or photolysis.

2 AgCl (s) → 2 Ag (s) + Cl2 (g)

2 AgBr (s) →  2 Ag (s) + Br2 (g)

2 H2O2 (l) →  2 H2O (l)  + O2 (g)

c. Decomposition reactions which require electricity – electrolytic decomposition or electrolysis.

2 H2O (l) →  2 H2 (g) + O2 (g)

2 NaCl → 2 Na + Cl2

MgCl2 ⟶ Mg + Cl2 

Displacement Reaction: A more reactive element displaces a less reactive element from its compound or solution.

Double Displacement Reaction or Precipitation Reaction: An exchange of ions between the reactants takes place to give new products.

An insoluble compound called precipitate forms when two solutions containing soluble salts are combined.

One of the best examples of precipitation reactions is the chemical reaction between potassium chloride and silver nitrate, in which solid silver chloride is precipitated out. This is the insoluble salt formed as a product of the precipitation reaction. The chemical equation for this precipitation reaction is provided below.

AgNO3(aqueous) + KCl(aqueous) AgCl(precipitate) + KNO3(aqueous)

D. OTHER SUBTYPES OF CHEMICAL REACTIONS

1.     Redox Reaction: A redox reaction occurs when the oxidation states of the substrate change. The loss of electrons or an increase in the oxidation state of a chemical or its atoms is referred to as oxidation. The gain of electrons or a decrease in the oxidation state of a chemical or its atoms is referred to as reduction.

Oxidation and reduction take place simultaneously.

Oxidation: Substance loses electrons or gains oxygen or loses hydrogen.

Reduction: Substance gains electrons or loses oxygen or gains hydrogen.

Oxidising agent – a substance that oxidises another substance and self-gets reduced.

Reducing agent – a substance that reduces another substance and self-gets oxidised.

Chemical Reactions and Equations-11

2.   Exothermic Reaction – heat is evolved during a reaction. Most of the combination reactions are exothermic.

Al + Fe2O3 → Al2O3 + Fe + heat

CH4 + 2O2 → CO2 + 2H2O + heat

3.      Endothermic Reaction – Heat is required to carry out the reaction.

6CO2 + 6H2O + Sunlight → C6H12O6 + 6O2

Most of the decomposition reactions are endothermic.

 Corrosion: Gradual deterioration of a material, usually a metal, by the action of moisture, air or chemicals in the surrounding environment.

Rusting: 4Fe(s) + 3O2(from air) + xH2O(moisture) → 2Fe2O3.xH2O(rust)

Corrosion of copper: Cu(s) + H2O (moisture) + CO2 (from air) → CuCO3.Cu(OH)2(green)

Corrosion of silver:  Ag(s) + H2S (from air) → Ag2S(black) + H2(g)

Rancidity: It refers to the oxidation of fats and oils in food that is kept for a long time. It gives foul smell and bad taste to food. Rancid food causes stomach infections during consumption.

Prevention:

(i) Use of air-tight containers

(ii) Packaging with nitrogen

(iii) Refrigeration

(iv) Addition of antioxidants or preservatives

 

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