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Types of Chemical Reactions

Fireworks, dazzling sparkles with different colours are an example of chemical change or chemical reaction. A chemical reaction involves the conversion of one substance into another substance. The involved chemical substance is known as reactants and newly formed substances are called as a product.

A chemical reaction is material changing from a beginning mass to a resulting substance. It produces new substances which have different physical and chemical properties. Such changes are usually irreversible in nature because the newly formed substances cannot easily change back into the original substances. Chemical changes can easily identify with the help of change in color, odour, energy level and physical state. Some common examples of chemical changes or chemical reactions are baking of cake, boiling of egg, burning of paper, rancidity of food etc. Any chemical change can easily represented with the help of chemical equations. The chemical equation is a symbolic representation of a chemical reaction which involves the molecular or atomic formulas of reactants and products.

The reactant molecules must be written on the left side of the equation and products will come on the right side of the equation. Both reactants and products are separated by a single headed arrow pointed towards products. On the basis of cleavage and formation of chemical bonds, chemical reactions can be classified in different types.

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What is a Chemical Reaction?

A chemical equation represents or depicts the compounds reacting in a chemical reaction and products formed in it. A chemical equation is, therefore, a very good sequential representation of a chemical reaction.

Chemical equation is said to show the number of compounds reacting, as well as, the moles of each component reacting and moles of products formed.

Writing a Chemical Reaction

A chemical reaction is written with the formula of reactants on the left side, and formula of the products formed on the right. An arrow is placed in-between the reactants and the products.

Example:1

In a reaction between Hydrochloric acid (HCl) and sodium hydroxide (NaOH), the chemical equation is written as:

HCl + NaOH H2O + NaCl

An arrow separates the reactants from the products. The arrow is usually pointed in the direction in which it is proceeding. If a reaction is reversible, the middle arrow is shown as double arrows:

( $\rightarrow$
$\leftarrow$).

Since each chemical reaction follows law of mass action, the moles of a particular element will be same throughout. If 2 moles of chlorine are reacting, 2 moles of chlorine would be present, in any combined form in the products.

Example:2

In the reaction between nitrogen and hydrogen to form ammonia, 2 moles of N2 react with 3 moles of H2 to form 2 moles of NH3. This is shown in the form of equation as:

2 N2 + 3H2 $\rightleftharpoons$ 2NH3

So, every chemical equation has to be solved or balanced for the moles of reactants reacting and moles of products formed.
A chemical reaction is written with the formula of reactants on the left side, and formula of the products formed on the right. An arrow is placed in-between the reactants and the products.

Types of Chemical Reaction and Equations

In a chemical equation, the formula of the reactants and products are used. Reactants are substance(s) that undergo the chemical reaction.
• The products are the substances produced during the chemical reaction.
• The reactants and products are connected by an arrow ().
• The arrow may be read as "to yield" or "to form" or "to give".
• The reactants are placed on the left side of the arrow and the products on the right side.
• The different reactants as well as products are connected by a plus sign (+).
Some examples of chemical reactions:

Example :1

Calcium + Hydrochloric $\rightarrow$ Calcium + Water + Carbon
Carbonate acid Chloride dioxide
CaCO3 + HCl $\rightarrow$ CaCl2 + H2O + CO2

In the above reaction, the balanced equation would be,

CaCO3 + 2HCl $\rightarrow$ CaCl2 + H2O + CO2

Calcium carbonate combines with hydrochloric acid to form three new products.

Example : 2

NaCl + AgNO3 → AgCl + NaNO3
This reaction is a double displacement reaction, where sodium and silver ions exchange the anions between them.

Example : 3

2Na +S → Na2S
Sodium combines with sulfur to form sodium sulfide. This is a simple combination or a synthesis reaction, where two elements, sodium and sulfur combines to form sodium sulfide.

Example : 4

CaCO3 → CaO + CO2
Calcium carbonate decomposes in the above reaction to give two new products, calcium oxide and carbon dioxide. The carbon dioxide is a gas. It is usually denoted with an upward arrow, to show that it has escaped out.(↑)

Example : 5

CO2 +H2→ CO + H2O
This reaction, though a combination reaction to give two new products, is also called a disproportionation reaction. A reaction, where two compounds combine and form two new compounds is called a disproportionation reaction.

5 Types of Chemical Reactions

The major types of chemical reactions are:

1. Combination or Synthesis Reaction

A combination or synthesis reaction is one, where a new product is synthesized by combination of two or three reactants.

Example

Hydrogen + Oxygen $\rightarrow$ Water
H2 + O2 $\rightarrow$ H2O

In this reaction, hydrogen and Oxygen combine to form water. So, since they are combining to form a new product, and a new compound, water is synthesized here, this reaction is said to be a synthesis reaction.

Also, it is an unbalanced equation, because, in the reactant side, there are 2 atoms of oxygen, but on the product side, there is only one atom of oxygen. An equation or a chemical reaction is valid only when the number of moles of reactants is equal to number of moles of products.

Since the participating elements atom numbers should remain proportionately constant before and after the reaction, add 2 in front of H2O, to make the number of oxygen atoms equal to 2.

H2 + O2 $\rightarrow$ 2H2O

Now there are 4 hydrogen on the right side, but only 2 on the left side. So add 2, in front of H2, to make it equal to 4, i.e.,

2H2 + O2 $\rightarrow$ 2H2O

2. Decomposition Reaction

Decomposition reaction is one, where one compound decomposes or breaks into two or more different products.

Example

Lead nitrate $\rightarrow$ Lead monoxide + Nitrogen dioxide + Oxygen
Pb(NO3 )2 $\rightarrow$ PbO + NO2 + O2

On balancing this, we will get,

2Pb(NO3)2 $\rightarrow$ 2PbO + 4NO2 + O2

Here, lead nitrate, is getting decomposed, or it breaks down to form lead oxide, nitrogen dioxide and oxygen. This is an example of decomposition reaction.

3. Displacement or Replacement Reaction

There are two types of displacement reaction.
• Single displacement reaction.
When a cation or an anion is exchanged from a compound, this is called as single displacement reaction.

XY + Z $\rightarrow$ XZ + Y

Example

Zn + H2SO4 $\rightarrow$ ZnSO4 + H2

In the above reaction, zinc replaces hydrogen from hydrogen sulphate or sulfuric acid, to form zinc sulfate. Since only cation is exchanged here, this is a single displacement reaction.

• Double displacement reaction
The anions are exchanged between two compounds, or salts. Such reactions results in different combination of cations and anions, at the end.

XY + AZ $\rightarrow$ XZ + AY

Example

BaCl2 + Na2SO4 $\rightarrow$ BaSO4 + 2NaCl

Chloride ion leaves Barium and gets attached itself to sodium. In this process, sulfate ion leaves sodium and attaches itself to Barium. Thus, there is an exchange of anions among Barium and sodium resulting in a double replacement or displacement reaction. Since both the compounds are changing, it is different from a single displacement reaction.

4. Acid Base Reactions

An acid and a base combines to give salt and water. This reaction is called as a neutralization reaction or just acid-base reaction.

Example

HBr + KOH $\rightarrow$ H2O + KBr
Acid Base water salt

HBr, an acid reacts with a base, potassium hydroxide, to form water and a salt, potassium bromide. These are very important type of reactions, occurring in biological systems too.

5. Combustion Reaction

A reaction where mostly an organic compound burns in the presence of oxygen to yield mostly carbon dioxide, water, and other products, is also a type of combination reaction. Combination of any substance with oxygen results in combustion, leading to the burning of the compounds to its elementary products.

Example

C4H10 + O2 $\rightarrow$ CO2 + H2O

Butane, an organic compound, burns in the presence of oxygen to give carbon dioxide and water.

Solving Chemical Equations

Chemical equations can be solved by balancing the moles of each component in the reactant side to that in the product side.

Example :1

Mg + O2 $\rightarrow$ MgO

To solve this chemical equation, following steps are required.
Step: 1

Count the number of moles of each element present in the product and reactant side.

 Elements Reactant side Product side Mg 1 1 O 2 1

Same moles of Magnesium is present in both reactant and product side. So, Magnesium is balanced. There are two moles of oxygen, while only one atom of oxygen is present in the product side. So, in the next step, let us balance the oxygen atom.

Step: 2

We add a mole of oxygen or an atom of oxygen to balance Oxygen

 Elements Reactant Side Product Side Mg 1 1 O 2 2 (1 x 2)

So, the equation now becomes,

Mg + O2 $\rightarrow$ 2MgO

Step: 3

On adding ‘2’ in front of magnesium oxide in the product side, the moles of magnesium also has increased from 1 to 2.

 Elements Reactant Side Product Side Mg 1 2 O 2 2 (1 x 2)

Step: 4

This unbalances Magnesium. To balance it, we need to increase the moles of Mg on the reactant side also to two.

 Elements Reactant Side Product Side Mg 2 2 O 2 2 (1 x 2)

The balanced chemical equation can be written as,

2Mg+ O2 $\rightarrow$ 2MgO

Example :2

Let us consider the chemical equation,

MnO2 + HCl $\rightarrow$ MnCl2+ Cl2 + H2O

Step: 1

Counting the moles of each element/component in the chemical equation

 Elements Reactant Products Mn 1 1 H 1 2 O 2 1 Cl 1 4

Step: 2

Except for Manganese, all the other elements are unbalanced in the reaction. Let us start with chlorine.
There are four moles of chlorine in the product side and one mole chlorine in the reactant side.

 Elements Reactant Products Mn 1 1 H 4 2 O 2 1 Cl 4 4

MnO2 + 4HCl $\rightarrow$ MnCl2+ Cl2 + H2O

Step: 3

Since the moles of Hydrogen in the reactant side have also increased to ‘4’, we need to balance Hydrogen accordingly.

 Elements Reactants Products Mn 1 1 H 4 4 O 2 2 Cl 4 4

Balancing of hydrogen also balances oxygen atom. Thus, the balanced chemical equation is:

MnO2 + 4HCl MnCl2 + Cl2 + 2H2O

Example :3

In the equation,

H2O2 + PbS $\rightarrow$ PbSO4 + H2

 Elements Reactants Products H 2 2 O 2 5 Pb 1 1 S 1 1

Except for Oxygen, all the other elements are balanced in this reaction. And, since the oxygen in this case cannot be balanced by simple multiplication, both sides have to be manipulated to get equal number of atoms.

The balanced equation would therefore be:

4H2O2 + PbS $\rightarrow$ PbSO4 +4 H2

Each element is balanced by its moles.

Limitation of a Chemical Equation

There are certain limitation of chemical equations as follows :
• Chemical equation does not clarify the state of the substances. So (s) for solid, (l) for liquid, (g) for gas and (vap) for vapor may be added.
• The reaction may or may not be conclude but the equation does not reveal it.
• Chemical equation does not give any information regarding the speed of the reaction.
• Chemical equation does not give the concentration of the substances and in some cases, the terms like diluted and concentrated are used.
• Chemical equation does not give the conditions of temperature, pressure, catalyst, etc., but this could be overcome by mentioning these above or below the arrow, e.g.,
• Chemical equation will not give any idea about color changes during the exchange, which has to be mentioned separately.
• Chemical equation will never give any indication regarding the production or absorption of heat. This is mentioned separately.

• Some chemical reactions are reversible. They are represented by:
or

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