Chemical Reaction And Equation

Change is the law of nature.

There are so many situations of daily life, where we can observe various changes. Like,

(i) Conversion of water into vapours from a cup of hot tea.

(ii)Corrosion of iron articles (rusting) if exposed to a humid atmosphere.

(iii) Cooking of food.

(iv) Digestion of food in our body.

(v) Breaking of any article like glass. 

(vi) Combustion of fuel in our vehicle.

Scientists classify these changes as

(1) Physical changes (2) Chemical changes

1.0Physical Changes

A change in which physical properties of a substance changes but the chemical composition does not change.

For example, freezing, melting, boiling, condensation, etc.

Characteristic Features of Physical Changes 

(1) The identity of the substance is maintained.

(2) The change is generally temporary.

(3) Heat change may or may not take place.

(4) Only the physical state or some of the physical properties of the substances are changed.

2.0Chemical Changes

A change in which one or more substances change into new substances with a different chemical composition.

For example, burning of a candle, rusting of iron, combustion of fuel, etc.

Characteristic Features of Chemical Changes 

(1) The identity of original substance is completely lost.

(2) The change is generally permanent.

(3) The change is generally accompanied by energy change.

(4) The change cannot be reversed generally.

3.0Chemical Reaction And Its Characteristics

The process in which a substance or substances undergo a chemical change to produce new substances, with entirely new properties are known as chemical reactions. 

Characteristics of Chemical Reactions

(i) Change in state: The physical state of the substances normally changes.

For example, 

Formation of solid MgO from solid Mg and gaseous O₂.

2Mg(s) + O₂(g) = 2MgO(s)

Magnesium Oxygen Magnesium oxide

        (from air) (White powder)

(ii) Change in colour: In some of the chemical reactions, change in colour can be observed.

For example,

(a) Formation of reddish-brown rust on grey iron nails.

(b) Formation of yellow ppt. of lead iodide from colourless solution of Pb(NO₃)₂ and KI.

(iii) Evolution of a gas: In some cases, a gas may be evolved.

For example,

Evolution of H2 gas, in the reaction between Zn and dil HCl.

Zn + 2HCl ⎯→ ZnCl₂ + H₂(g)↑

(iv) Change in temperature: Most of the reactions are accompanied by temperature change i.e. increase or decrease in temperature. 

For example,

(a) In the reaction between Zn and H₂SO₄, the flask was found to be warm. Thus, a rise in temperature has taken place (Exothermic).

(b) If a reaction between barium hydroxide (Ba(OH)₂) and ammonium chloride (NH₄Cl) is carried out in a test tube, it is observed that the bottom of the test tube becomes cooler (Endothermic).

Memory map

4.0Writing A Chemical Equation

Reaction between magnesium and oxygen can be described as – when a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide. This description of a chemical reaction in sentence form is quite long. It can be written in a shorter form. So, some short-hand representation of chemical reaction is followed.

It can be done in two ways:

Word Equation

A chemical equation which represents a chemical reaction briefly in words is called the word equation.

For example, the word equation can be written as:

Magnesium + Oxygen ⎯→  Magnesium oxide

Chemical Equation 

A chemical equation is a short-hand method that describes a chemical reaction in terms of symbols and formulae of different reactants and products.

For example, the chemical equation can be written as

Mg + O₂ → MgO

Rules For Writing A Chemical Equation

(i) The symbols and formulae of the reactants are always written on the L.H.S. (left hand side) of the arrow and a plus (+) sign is put between them.

(ii) The symbols and formulae of the products are always written on the R.H.S. (right hand side) of the arrow and a plus (+) sign is put between them.

(iii) An arrow (⎯→) sign is put between the reactants and the products, pointing from reactants towards products.

For example,

Na + H₂O → NaOH + H₂

A chemical equation expressed in symbols and formulae, such that the number of atoms of different elements towards the side of the reactants is not equal to the number of atoms of different elements towards the side of the products, is called skeletal equation or unbalanced equation. To make this equation meaningful, this equation is balanced according to the law of conservation of mass, then it is called a balanced chemical equation.

5.0How to balance an unbalanced chemical equation

According to the law of conservation of mass, the total mass of products must be equal to the total mass of the reactants (as mass can neither be created nor destroyed). This is possible only if the number of atoms of each element is equal on the two sides of the equation.

Balancing a chemical equation means making the number of atoms of each element equal on both sides of the equation.

(i) To understand this, let us consider the following word equation for

Zinc + Sulphuric acid ⎯→ Zinc sulphate + Hydrogen

Chemical equation for the above word equation will be,

Zn + H2SO4⎯→ ZnSO4 + H2

Let us examine the number of atoms of different elements on both the sides of the arrow.

As the number of atoms of each element is the same on both sides of the arrow, the equation can be said to be a balanced chemical equation.

(ii) Now consider another chemical equation.

Fe + O2 + H2O ⎯→ Fe2O3.H2O

On counting the number of different atoms on both the sides of the arrow, we will find that this equation is not balanced.

Let us learn about balancing a chemical equation step by step.

Step-I : Write the word equation for the given chemical reaction.

Step-II : Convert the formed word equation in the chemical equation (Skeletal chemical equation).

Step-III : Formula of each reactant and product has to be enclosed in boxes, so that during balancing, formula of substances cannot be changed.

Step-IV : Listing of the number of reactants and products is done.

Step-V: Balancing should be started with compounds which have the biggest formula.

Step-VI : Then, balancing of different elements is done one by one.

Step-VII : Finally, the equation is checked.

(iii) Let us balance some of the chemical reactions by following the above steps.

Magnesium metal reacts with hydrochloric acid to form magnesium chloride and hydrogen.

Law of conservation of mass

Step-I : Word equation

Magnesium + Hydrochloric acid ⎯→ Magnesium chloride + Hydrogen

Step-II : Chemical equation,

Mg + HCl ⎯→ MgCl₂ + H₂

Step-III : Enclose all formulae into boxes.

Step-IV: Count the number of atoms for all elements.

We can see that the number of Mg is the same on both sides but Cl and H atoms differ on both sides.

Step-V: As the number of atoms is deficient at the reactant side, let’s begin from here. On the reactant side HCl is the bigger formula, so we will start with it.

Step-VI: Put coefficient 2 before HCl to make chlorine equal to reactant side.

Mg + 2HCl → MgCl2 + H2

Here, we can see that H, automatically gets balanced.

Step-VII : Now, check the number of atoms of different elements on both sides of the equation. These are equal. This means that the equation is balanced.

6.0Balancing of Equations

Steam is passed over red hot iron to form Iron (II, III) oxide and hydrogen in the presence of air.

Step-I: Iron   + Steam ⎯→ Iron (II, III) Oxide + Hydrogen

Step-II : Fe       + H2O  ⎯→    Fe3O4 + H2

Step-III:

  Step-IV : Formula selected is Fe₃O₄ to start balancing oxygen,

(i) To balance O-atoms, multiply H₂O in LHS by 4.

(ii) Now balance Fe atoms.

(iii) Balance H atoms.

Note: Mixture of ferrous oxide (FeO) and ferric oxide (Fe₂O₃) is also known as magnetic oxide of iron.

Step-V : On checking the number of all elements, we found that the equation is balanced now.

3Fe       +     4H₂O ⎯⎯→     Fe₃O₄         +          4H₂ 

7.0Making Chemical Equations More Informative

On examining a balanced chemical equation, we observe that it does not give any information about the physical state of the reactant and product.

Let’s make balanced chemical equations more informative by following instructions.

(1) Writing symbols for the physical state of reactants and the products, 

(s) for solid state

() for liquid state

(g) for gaseous state

(aq) for aqueous solution i.e. solution prepared in water.

(2) Sometimes a gas evolved in a reaction is shown by the symbol (↑) i.e. by an arrow pointing upwards.

Similarly, the precipitate if formed during the reaction is indicated by the symbol (↓) i.e. by an arrow pointing downwards.

The abbreviation ‘ppt’ is also used to represent the precipitate if formed.

(3) The conditions of temperature, pressure and the presence of catalyst, if any, may be represented by writing these conditions above and / or below the arrow (⎯→) between the reactants and the products.

Few examples,

CO(g) +2H₂ (g)340atm→CH₃OH (g)

6CO₂(g) + 6H₂O(g)⎯→Chlorophyll Sunlight C₆CH₁₂O₆(aq) + 6O₂(g)

2Na(s) + 2H₂ O(g) ⎯→ 2NaOH(aq) +H₂ (g) or H₂

Ca(OH)₂(aq) + CO₂(g) ⎯→ CaCO₃(↓) (or ppt) +H₂ O(g)

C(s) + O₂(g) ⎯→ CO₂(g) + heat

N₂(g) + O₂(g) + heat ⎯→ 2NO(g)

8.0Also Read