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

Signs of Chemical Reactions

There are four main signs that indicate a chemical reaction has taken place:

change in color change in state Evolution of gas change in temperature

liquid

solid

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.

CHEMICAL EQUATIONS

• When a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide.

• This description of a chemical reaction in a sentence form is quite long. It can be written in a shorter form. The simplest way to do this is to write it in the form of a word-equation.

• The word-equation for the above burning of magnesium ribbon reaction would be –

Magnesium + Oxygen → Magnesium oxide

(Reactants) (Product)

CHEMICAL EQUATIONS

• Reactants – the substances that exist before a chemical change (or reaction) takes place.

• Products – the new substance(s) that are formed during the chemical changes.

• CHEMICAL EQUATION indicates the reactants and products of a reaction.

REACTANTS PRODUCTS

A word-equation shows change of reactants to products through an arrow placed between them.

The reactants are written on the left-hand side (LHS) with a plus sign (+) between them. Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them.

The arrowhead points towards the products, and shows the direction of the reaction.

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.

Cl

Cl

ClH

H

H

ClClCl

ClHH

H

H

H2 + Cl2 HCl H2 + Cl2 2 HCl

reactants products

H

Cl

reactants products

H

Cl

2

2

2 2

2 2

1

1

(unbalanced) (balanced)

Unbalanced and Balanced 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.

Let us try to balance the following chemical equation

–

Step I: To balance a chemical equation, first draw boxes around each formula. Do not change anything inside the boxes while balancing theequation.

Step II: List the number of atoms of different elements present in the unbalanced equation

Step III: It is often convenient to start balancing with the

compoundthat contains the maximum number of atoms. It may be a reactant or aproduct. In that compound, select

the element which has the maximum

number of atoms. Using these criteria, we select Fe3O4 and the

element oxygen in it. There are four oxygen atoms on the RHS and only

one on the LHS.

(partly balanced equation)

Step IV: Fe and H atoms are still not balanced. Pick any of these elementsto proceed further. Let us balance hydrogen atoms in the partly balanced equation.To equalise the number of H atoms, make the number of molecules of hydrogen as four on the RHS.

(partly balanced equation)Step V: Examine the above equation and pick up the third element which is not balanced. You find that only one element is left to be balanced, that is, iron.

To equalise Fe, we take three atoms of Fe on the LHS.

Step VI: Finally, to check the correctness of the balanced

equation, we count atoms of each element on both sides of the

equation.

(balanced equation)

The numbers of atoms of elements on both sides are equal. This equation is now balanced. This method of balancing chemical equations is called hit-and-trial method as we make trials to balancethe equation by using the smallest whole number coefficient.

The balanced Eq. becomes

q1. Why should a magnesium ribbon be cleaned before burning in air?

q2. Write the balanced equation for the following chemical reactions.

(i) Hydrogen + Chlorine → Hydrogen chloride

(ii) Barium chloride + Aluminium sulphate → Barium sulphate +

Aluminium chloride(iii) Sodium + Water → Sodium hydroxide

+ Hydrogen

q3. Write a balanced chemical equation with state symbols for the following reactions.

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium

sulphate and the solution of sodium chloride.(ii) Sodium hydroxide solution (in water) reacts

with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

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

Definition-A reaction in which a single product is formed from two or more reactants is known as a combination reaction.

• For ex-Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

In this reaction, calcium oxide and water combine to form a single product, calcium hydroxide.

combination reaction (Synthesis)

General form: A + B AB

element or element or compoundcompound compound

Na

ClNa

Cl

2 Na + Cl2 2 NaCl

ClNa

Na

Cl

Example of combination Reactions:-

CO2 + H2O C6H12O6 + O2

H2 + O2 H2O

Na + Cl2 NaCl

6 6 6

Photosynthesis

Formation of water

2 2

2 2

Formation of salt

A + B C

General Form

Let us discuss some more examples of combination reactions.

(i) Burning of coal

(ii) Formation of water from H2(g) and O2(g)

In simple language we can say that when two or more substances

(elements or compounds) combine to form a single product, the reactions are called combination reactions.

Reactions in which heat isreleased along with the formation of products are called exothermic

chemical reactions. Other examples of exothermic reactions

are –(i) Burning of natural gas

(ii) Do you know that respiration is an exothermic process?

We all know that we need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with

oxygen in the cells of our body and provides energy. The special name of this reaction is

respiration.

iii) The decomposition of vegetable matter into compost is also an example of an exothermic reaction.

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.

q1. A solution of a substance ‘X’ is used for white washing.(i) Name the substance ‘X’ and write its formula.(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Definition-A reaction in which a one element displaced or replaced or removed another element, is known as a displacement reaction.

• For example-The reaction between iron & copper sulphate.

In this reaction, iron has displaced or removed another element,copper, from copper sulphate solution.

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.

Other examples of displacement reactions are

Zinc and lead are more reactive elements than copper. They displace copper from its compounds.

Definition-A reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

• For example-Formation of barium sulphate and sodium chloride from sodium sulphate and barium chloride.

Any reaction that produces a precipitate can be called a

precipitation reaction.

Single and Double displacement Reactions

Double-replacement reaction

CaCO3 + 2 HCl CaCl2 + H2CO3

General form: AB + CD AD + CB

Single-replacement reaction

Mg + CuSO4 MgSO4 + Cu

General form: A + BC AC + B

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.

The surface of copper powder becomes coated with black copper(II) oxide. Why has this black substance formed? This is because oxygen is added to copper and copper oxide is formed.

If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes placeand copper is obtained.

If a substance gains oxygen during a reaction, it is said to be oxidised. If a

substance loses oxygen during a reaction, it is said to be reduced. During this

reaction,the copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidised.In other words, one reactant gets oxidised while the other gets reduced during a

reaction. Such reactions are called oxidation-reduction reactions

or redox reactions.

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.

Definition-When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. The black coating on silver and the green coating on copper are other examples of corrosion.•AFFECTS-Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron. Corrosion of iron is a serious problem. Every year an enormous amount of money is spent to replace damaged iron.

Definition-When fats and oils are oxidised, they become rancid and their smell and taste change. Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation. Do you know that chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidised ?

q1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

q2. Give an example of a double displacement reaction other than the one

given in Activity 1.10.

q3. Identify the substances that are oxidised and the substances that are reduced in the

following reactions.(i) 4Na(s) + O2(g) → 2Na2O(s)

(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)