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Chemical Equations

The chemical changes are associated with the change in chemical composition of chemical substances. On the contrary, physical changes are involved in the change in physical properties only but the chemical composition of substances remains same. For example; when the ice melts, it converts into liquid state but the chemical composition remains same before and after melting that is $H_{2}O$. All inter-conversions of the physical state of matter are examples of physical changes.

Physical changes are usually reversible in nature. Chemical changes alter the chemical composition and lead to the formation of new chemical substances that are known as products. The chemical and physical properties of products are different from the reactants. For example, burning or combustion of hydrocarbon forms carbon dioxide and water with large amount of energy. It changes the chemical composition and properties of methane completely.

Any chemical change or chemical reaction can be represented with the help of chemical equations which are written with the help of molecular formula of reactants and products. Let’s discuss different types of chemical equations with various examples. Try to relate these examples with your everyday life.

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What is a Chemical Equation?

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

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Steps followed to write a chemical equation are

  1. The reactants and products of the reaction are identified and written down.
  2. The formula or symbols of the reactants are written on the left hand side with a '+' sign in between them.
  3. The formula or symbols of the products are written on the right hand side with a '+' sign in between them.
  4. The two sides (reactants and products) are separated either by a sign of equality (=) or that of an arrow () pointing towards the products.
  5. For a reversible reaction, this sign () is used in place of an arrow.
  6. The number of atoms of each element are counted on both the sides. If they are equal, then it is termed as a balanced chemical equation. If they are not equal, the balancing of the equation is done by adjusting the coefficients before the symbols and formula of the reactants and products.
In qualitative terms, a chemical equation conveys
  • The names of various reactants and the products.
In quantitative terms, it represents
  • The relative number of atoms and molecules (reactant and product species) taking part in the reaction.
  • The relative number of moles of the reactants and products.
  • The relative masses of the reactants and products.
  • The relative volumes of gaseous reactants and products.

Thus, the chemical equation gives us the quantitative relationship between the reactants and products or the stoichiometry of the 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

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 ( ⇔ ).

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

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

2N2 + 3H2 2NH3

So, every chemical equation has to be solved or balanced for the moles of reactants reacting and moles of products formed.


Four basic types of chemical reactions are :

1. Synthesis or Combination Reaction

Two molecules or atoms or compounds combine to form a new compound. These type of chemical equations result in synthesis of a new compound.

Example-1: 2H2 + O2 2H2O

Example-2: 2Mg + O2 2MgO


2. Decomposition Reaction

One compound disintegrates or breaks into two or three new compounds or molecules or one compound and one molecule.

Example-1

CaCO3
CaO + CO2
In this case, one compound, calcium carbonate breaks into two new compounds, calcium oxide and calcium carbonate.

Example-2


2HgO 2Hg + O2
In example 2, one compound, HgO breaks down into its elements, mercury and Oxygen.


3. Displacement or Replacement Reactions

Displacement reactions are of two types, Single displacement reaction and double displacement reaction.

  • Single replacement reaction

In a single replacement or displacement reaction, one element replaces the other in a compound.

Example:
2KI + Cl2 2KCl + I2

  • Double replacement reaction

In a double displacement reaction, there is an exchange of ions between two ionic compounds, resulting in two new compounds.

Example: AgNO3 + NaCl NaNO3 + AgCl


4. Acid Base Reaction or Neutralization Reaction


An acid and a base combine to form salt and water in an acid base reaction. This reaction can also be called as a neutralization reaction.

Example-1

NaOH + HCl NaCl + H2O
In this reaction, sodium hydroxide is a base and hydrochloric acid is an acid. Both combine to form the salt sodium chloride.

Example-2

KOH + HBr KBr + H2O
Potassium hydroxide is a base and it reacts with an acid, hydrogen bromide to form potassium bromide.
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Solving Chemical Equations

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Chemical equations can be solved by balancing the moles of each component on the reactant side with those in the products side.

Solve Mg + O2 MgO

To solve this chemical equation above, the 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 are present in both the reactant and product sides. 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 (1x2)

So, the equation now becomes:
Mg + O2 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 1 2
O 2
2 (1x2)

Step 4 : This unbalances Magnesium. To balance it, we need to increase the moles of Mg on the reactant side also to two.

Elements
Reactants side
Product side
Mg 1 2 1 2
O 2 2 (1x2)

The balanced chemical equation can be written as:

2 Mg + O2 2 MgO


2. Solve
MnO2 + HCl MnCl2 + Cl2 + H2O

Step 1 : Counting the moles of each element/component in the chemical equation

Elements
Reactant side
Product side
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 side Product side
Mn 1 1
H
1 4 2
O
2
1
Cl
1 4 4

Now the equation becomes

MnO2 + 4HCl 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 Reactant side
Product side
Mn 1 1
H
1 4 2 4
O
2 1 2
Cl
1 4 4

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

MnO2 + 4 HCl MnCl2 + Cl2 + 2 H2O

3. Solve H2O2 + PbS PbSO4 + H2O

Elements
Reactant side
Product side
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, therefore, would be

4 H2O2 + PbS PbSO4 + 4 H2O

Note that each element is balanced by its moles.

Essentials of a Chemical Equation

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A chemical equation should satisfy the following conditions :

  1. It should represent a true chemical reaction.
  2. It should be molecular in nature. The ionic reactions must be represented by ionic equations.
  3. It should be a balanced equation i.e. the number of atoms of each element on both sides should be equal.

Parts of a Chemical Equation

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  • The physical states of the reactants and products are specified. We use 's' for solids, 'l' for liquids and 'g' for gases. Sometimes, 'aq' is used for aqueous - to represent that the given substance has been dissolved in excess of water.
Zn(s) + 2HCl(aq) $\rightarrow$ ZnCl2(aq) + H2(g)
  • The strength of acid or base used in the reaction is prefixed by the words 'conc'. for concentrated and 'dil'. for diluted.
Zn(s) + dil.2HCl(aq) $\rightarrow$ ZnCl2(aq) + H2(g)
  • The conditions of the reaction such as catalyst, temperature, pressure etc. are written on the arrow between the reactants and products. The above equation indicates that the reaction has been carried out in the presence of a catalyst Fe/Mo at 723 K at 456000 mm of Hg pressure.
  • Heat changes (absorption and evolution) in the reaction may be expressed in the equation as,
N2(g) + O2(g) $\rightarrow$ 2NO(g) - 180.5 kJ (heat is absorbed)
C(s) + O2(g) $\rightarrow$ CO2(g) + 394 kJ (heat is evolved)

  • The distinction between slow and fast reactions can be made by writing the words slow and fast on the arrow head.
  • The evolution of a gas in the reaction can be indicated by an arrow pointing upward (symbol used to denote evolution of a gas) while the formation of a precipitate can be indicated by an arrow pointing downwards (symbol used for formation of a precipitate) or by writing the word 'ppt'.
  • The reversible nature of the reaction is indicated by a double headed arrow, indicating that the reaction occurs in the forward as well as backward direction.
N2(g) + 3H2(g) $\rightleftharpoons$ 2NH3(g)
More topics in Chemical Equations
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