Chemical reactions and equations

Index

Chemical changesBalanced chemical equationsCombination reactionsDecomposition reactionsDisplacement reactionOxidation and reduction reactions

Chemical changes

The formation of new substances takes place with different chemical properties is called chemical changes. A chemical change can be confirmed by any or all of the following observations:•change in state•change in color•change in temperature•evolution of gas

• A chemical change is always accompanied by a chemical reaction. Reaction is the term used for depicting a change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.

Representing a reaction in a sentence form can be quite complex and laborious at times. Thus, to write a concise chemical reaction, equations of the reactions are written. These equations can be written in different ways such as word equations and chemical equations. In a word equation, reactants are written on the left hand side of a forward arrow. 

A chemical equation has reactants on the left hand side. Reactants are substances that are present at the initiation of a reaction. Hence, magnesium (Mg) and oxygen (O2) are reactants. On the other hand, new substances formed after the completion of the reaction are termed as products. Thus, magnesium oxide (MgO) which is written on the right hand side of the equation is a product. The arrow in the equation signifies the direction of change.

Thus, a chemical equation is an easier and more concise method for representing a chemical reaction. It involves writing symbols and formulae (instead of words) for all substances involved in the reaction. A chemical equation also indicates the number of atoms of each element involved in a reaction.

Balanced Chemical Equations

There are three ways of writing a chemical equation:

Word equation

The above chemical reaction between zinc and mineral acid can be represented as:Zinc + Hydrogen chloride → Zinc chloride + HydrogenIn a word equation, the reactants are written on the left hand side of a forward arrow. The products are written on the right hand side of this arrow.The arrow signifies that the reaction proceeds from the reactants towards the products.

In a chemical reaction, the total mass of the reactants should be equal to the total mass of the products. This means that the total number of atoms of each element should be equal on both sides of a chemical equation. Such an equation is called a balanced chemical equation, and the method by which it is obtained is called balancing of chemical equations.

Balancing a chemical equation

Step-by-step process of balancing a chemical

equation

Step II: List the number of atoms of the various elements present in the unbalanced equation in the form of a table.

ElementNumber of atoms

on the reactant side (L.H.S)

Number of atoms on the product side

(R.H.S)

Ba 1 1

Cl 2 3

Al 2 1

S 3 1

O 12 (4×3) 4

Step III: In the next step, select a compound which contains the maximum number of atoms. In this case, the compound will be Aluminium sulphate (it has 2 atoms of Al, 3 atoms of S, and 12 atoms of O). From this compound, select the element which has the maximum number of atoms, and which is present in only one compound on both sides i.e. oxygen in this case.

To balance the number of oxygen atoms, we can multiply barium sulphate present on the right hand side by 3 (as shown below). It should be kept in mind that coefficient ‘3’ will be written as 3BaSO4 and not as (BaSO4)3.Oxygen atoms

Number of atoms on L.H.S

Number of atoms on

R.H.S

Before balancing

12 in Al2(SO4)3 4 in BaSO4

To balance 12 3 × 4

ElementNumber of atoms on

L.H.S

Number of atoms on

R.H.SBa 1 3

Cl 2 3

Al 2 1

S 3 3

O 12 12

Step IV: As the atoms of both oxygen and sulphur are balanced, balance the atoms of Aluminium.

Aluminium atoms

Number of atoms on L.H.S

Number of atoms on

R.H.SBefore

balancing 2 in Al2(SO4)3 1 in AlCl3To balance 2 2 × 1

Element Number of atoms on L.H.S

Number of atoms on

R.H.S

Ba 3 3

Cl 6 6

Al 2 2

S 3 3

O 12 12

To make a chemical equation more informative, the physical state of the reactants and the products is mentioned along with their chemical formulae. They are written in common brackets.Solids are denoted by writing (s),Liquids are denoted by writing (l),Gases are denoted by writing (g), andSolutions in water are denoted by writing (aq).

The energy changes involved in a reaction are denoted by writing the changes involved in the equation itself.If energy is used in the reaction, then it will be written on the left hand side. If it is released in the process, then it is written on the right hand side.

Combination Reactions

Chemical reactions are primarily of five types. They are listed as follows:1.Combination reactions2.Decomposition reactions3.Displacement reactions4.Double displacement reactions5.Oxidation and reduction reactions

Combination reactions

1. Combination of two elements

2. Combination of two compounds

Decomposition Reactions

Decomposition reactions require a source of energy in the form of heat, light, or electricity to decompose the compound involved. Hence, these reactions can be classified into three types, depending on the source of energy for the reaction.a) Decomposition by heat or thermal decompositionb) Decomposition by electricity or electrolysisc) Decomposition by light or photolysis

a) Decomposition by heat or thermal energy

In this reaction, one compound i.e. calcium carbonate breaks down to form two compounds, namely calcium oxide and carbon dioxide. Hence, it is an example of decomposition reactions. Commercially, this reaction is very important as calcium oxide (obtained as a product in this reaction) is used in cement and glass industries.

b) Decomposition by electricityWhen electricity is passed through water containing a few drops of sulphuric acid, it breaks down to give its constituent elements as products i.e. hydrogen and oxygen. This is known as electrolysis of water.

c) Decomposition by lightWhen silver chloride is kept in the sun, it decomposes to form chlorine gas and silver. As the reaction proceeds, the white coloured silver chloride turns grey because of the formation of silver. Chlorine produced in the reaction escapes into the environment as it is produced in the gaseous state.

Displacement Reactions

In displacement reactions, a more reactive metal replaces a less reactive metal from the latter’s salt.Displacement reactions are of two types:1.Single Displacement Reactions2.Double Displacement Reactions

Single Displacement Reactions can be better understood with the help of the following figure. In the above figure, you have three blocks. It will be observed that while red and blue blocks are fixed in, green block is aloof. Now, if a blue block is detached from the red and fixed into the green, it will mean that the green block displaces the red block.

Thus, in a single displacement reaction, an uncombined single element replaces the other in a compound.

These blocks are detached. Then, the blue block is exchanged with the yellow block. This represents a double displacement reaction.A Double Displacement Reaction is a bimolecular process in which parts of two compounds are exchanged to give two new compounds. The general equation used to represent double displacement reactions can be written as:AB + CD → AD + BC

Double Displacement Reactions have two common features: 1.Firstly, two compounds exchange their ions resulting in the formation of new compounds.2. Secondly, one of the new products formed would be separated from the mixture in some way (commonly as a solid or gas).Double Displacement Reactions can be further classified as precipitation, gas formation, and acid-base neutralization reactions.

Oxidation and Reduction Reactions

Oxidation is defined as a process that involves a gain of oxygen or a loss of hydrogen. When a substance gains oxygen or loses hydrogen during a reaction, it is oxidized.Reduction is defined as a process that involves a gain of hydrogen or a loss of oxygen. When a substance loses oxygen or gains hydrogen during a reaction, it is reduced.

Oxidation and reduction always take place simultaneously. Therefore, reactions involving oxidation and reduction are known as Redox (‘Red’ for reduction and ‘ox’ for oxidation) reactions. In a redox reaction, one substance is oxidized, while the other is reduced.

The substances that are reduced (provide oxygen or remove hydrogen) in course of the reaction are called oxidizing agents. These substances oxidize other chemicals in the reaction and are reduced in the process. On the other hand, the substances that are oxidized (remove oxygen or provide hydrogen) are called reducing agents.

For example:

In the above reaction, CO2 gets reduced to CO and here, CO2 is the oxidizing agent. On the other hand, hydrogen gets oxidized to form water and here, H2 is the reducing agent.

Corrosion

It may be defined as a process where materials, usually metals, are deteriorated because of a chemical reaction with air, moisture, chemicals, etc. For example, corrosion causes damage to car bodies, bridges, iron railings, ships, and all objects made of metals (especially those made from iron).

Rancidity

When fats and oils are oxidized, they become rancid and their smell and taste also changes. Thus, the oxidation of fats and oils can be easily observed by a change in their taste and smell.

Oxidation of food can be prevented in many ways. Two common methods are discussed below.1. Storing food in air tight containers. By doing so, the oxygen available for oxidation becomes limited. Hence, oxidation can be prevented.2. Sometimes, antioxidants are added to food to prevent their oxidation. These antioxidants are oxidized first, which slows down the process of rancidity. These are reducing agents. Normally, vitamin C and vitamin E are added as antioxidants.

Made by-Anushka NinamaClass- 10th DON’T FORGET TO LEAVE UR COMMENTS