Chemical Equation Definition | How to Write It

Definition of chemical equation: In chemistry, a chemical equation is a way of representing a chemical reaction with the help of their chemical formulae and symbols. In other words, a chemical equation is a symbolic representation of a chemical change or reaction.

When a substance (i.e. molecule or compound) decompose or breaks down into another substance by reacting, it is called chemical change or chemical reaction. Substances in which the chemical change occurs are called reactants and the substances which form as the result of chemical change are called products.

The relationship between reactants and products is represented by their chemical formulae. These chemical formulae are called chemical equations. For example, when a zinc metal reacts with dilute sulphuric acid, then it forms zinc sulphate and hydrogen.

We can write the chemical reaction like this:

Zn (metal) + H₂SO₄ (dil.) ————-> ZnSO₄ + H₂

In this reaction, the chemical formulae written on the left-hand side of equality (=) or (—->) sign are reactants and the formulae written on the right-hand side are products. The plus (+) sign adds the formulae on both the sides. We know such an equation as a skeleton equation.

Notation used in Chemical Equations

There are the following notations used in chemical equations as to provide more information about chemical change:

• Write the upper arrow (↑) immediately after the gaseous product to represent it.
• Write a lower arrow (↓) immediately after the insoluble substance (such as solid) to represent it, which deposits from a solution.
• To represent the physical state of the reactants or products, write the symbols (s) for solid, (I) for liquid, and (g) for gas.
• Substances which dissolved in the water, we write a symbol (aq.) to represent it.
• Write a symbol (△) over an arrow or over an equality sign to represent heating.

How to write a Chemical Equation?

There are the following key points to write a chemical equation for a chemical reaction that you should keep in mind. They are:

• Recognize reactants and products of the chemical reaction or change.
• Write down the formulae of reactants on the left-hand side with a plus (+) sign between them if more than one reactant is present.
• Write down the formulae of products formed on the right-hand side with a plus (+) sign between them if more than one product is formed.
• To separate two sides (reactants and products), use the sign of equality (=) or an arrow (⟶) pointing towards the products.
• For a reversible reaction, use a sign ( ⇌) in the place of arrow.
• Now count the number of atoms of each element present on the both sides. If the number of atoms of each element on both sides is equal, then the equation is called balanced chemical equation.
• If the number of atoms of any one or more of the elements on both side are not equal, then make it equal by adjusting coefficients before formulae of reactants and products. The process by which the number of atoms of each element on both sides (reactant and product) is made equal is called balancing of chemical equation.
• At last, make the chemical equation molecular, if needed.

Example:

When a zinc metal reacts with dilute hydrochloric acid, then it forms zinc chloride and hydrogen. Reaction between them occurs as follows:

Zn (metal) + HCl = ZnCl₂ + H₂ (Skeleton equation)

Zn (metal) + 2HCl = ZnCl₂ + H₂ (Balanced chemical equation )

Information obtained by a Chemical Equation

A chemical equation provides the following two types of information about the reaction that are as follows:

1. Qualitative information: A chemical equation gives two qualitative information about the reaction. It tells us about
   • names of reactants which occur in the chemical reaction.
   • names of products formed in the chemical reaction.
2. Quantitative information: A chemical equation gives four quantitative information about the reaction. It tells us about,
   • the relative number of atoms or molecules of reactants and products occurring in the reaction.
   • the number of moles of each substance (i.e. molecule or compound) involved in the chemical reaction.
   • the mass of each substance involved in the chemical change.
   • the mass-mass, mass-volume, volume-volume relationships between reactants and products.
   • the relative volumes of gaseous reactants and products.

For example, consider the following reaction:

In the above reaction, zinc reacts with hydrochloric acid to form zinc chloride and hydrogen. This equation quantitatively tells the following information:

• In the above reaction, one atom of Zn reacts with two molecules of HCl to produce one molecule of ZnCl₂ and one molecule of hydrogen.
• One mole of zinc (Zn) reacts with two moles of hydrochloric acid (HCl) to form one mole of zinc chloride (ZnCl₂) and one mole of hydrogen gas (H₂).
• When one mole of zinc reacts with two moles of hydrogen chloride (HCl), then 22.4 litre of hydrogen gas is evolved at NTP.
• When the total mass of reactants (65g + 73g = 138g) is equal to the total mass of products (136g + 2g = 138g), it supports the law of indestructibility.
• In the above equation, one zinc atom is replacing two hydrogen atoms from one molecule of hydrogen chloride, so its valency is 2.
• In the above chemical equation, 1g hydrogen obtained from 32.5g of zinc. Therefore, the equivalent mass of zinc is 32.5g.
• In the above reaction, 65g zinc (Zn) reacts with 73g hydrochloric acid (HCl), and 136g zinc chloride (ZnCl₂) and 2g hydrogen gas (H₂) formed.

Limitations of a Chemical Equation

A chemical equation fails us to give the following information that are as:

• A chemical equation does not give the information about the physical state of reactants and products.
• It does not give the information about the actual concentration of reactants taken and the actual concentration of the products obtained.
• It does not give the information regarding a reaction conditions, such as temperature, pressure, and the presence of catalyst, etc.
• A chemical equation does not give any knowledge to us whether heat is absorbing or evolving in the chemical reaction.
• It does not provide us anything about the rate of reaction i.e. it does not give any information regarding the time taken for the completion of the chemical change or reaction.
• It does not tell us about whether a gas is evolved or precipitate is formed.
• A chemical equation does not give any information about the nature of reaction, i.e. whether the chemical reaction is reversible or irreversible.

How to make a Chemical Equation more informative?

You can make a chemical equation more informative by adding some additional information to the equation. They are as:

(1) Reaction conditions: If a chemical reaction occurs under some specific reaction conditions, such as temperature, pressure, and catalyst, then you must mention them above the arrow () or above the sign of equality (=). A general chemical reaction taking place at t°C and p atm pressure, and in the presence of catalyst, can be represented as follows:

In the above chemical equation, we have provided a complete information for the reaction between nitrogen and hydrogen. When nitrogen reacts with hydrogen under the conditions, such as temperature 450°C, pressure 200 – 900 atm, and in the presence of a catalyst (a mixture of iron and molybdenum), it forms ammonia (NH₃).

(2) Physical states of reactants and products: You can mention the information about the physical states of reactants and products by using letters (s), (l), (g), and (aq) for solid, liquid, gas, and a water solution, respectively at the end of the formula of the substance involved.

For example, at the room temperature, the solid sodium metal reacts with water, then it produces sodium hydroxide in water and hydrogen gas. The complete chemical equation is as:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

We can indicate the gaseous substance evolved (or given out) during the chemical reaction by an arrow pointing towards upwards (↑). A solid substance which precipitating out from the reaction mixture, we can indicate it by an arrow pointing downwards (↓).

(3) Heat absorbed or evolved: A chemical reaction takes place with the absorption or evolution of heat. The reaction in which a amount of heat is absorbed is an endothermic reaction and the reaction in which a amount of heat is given out is an exothermic reaction. This information is indicated by adding or subtracting the amount of heat on the right-hand side of the product of the chemical equation. For example,

(a) When a carbon burns in air (or oxygen), then it produces heat. We can write a chemical equation for it as follows:

C(s) + O₂(g) = CO₂(g) + Heat (393 KJ) (heat evolved)

The above reaction with + heat term on the product side is called exothermic reaction.

(b) When carbon reacts with sulphur, it produces carbon disulphide with the absorption of heat. This is an endothermic reaction. We can write the chemical equation for this reaction as follows:

C(s) + 2S(g) = 2CS2(g) – Heat (92 KJ) (heat absorbed)

The above reaction with – heat term on the product side is known as endothermic reaction. A chemical reaction in which the amount of heat is absorbed or given out (or evolved) is called as a thermochemical equation in chemistry.

(c) Concentration of reactants and products: We can indicate this information to the chemical equation by adding the term dil. (for dilute) and conc. (for concentrated) before the formulae of reactants and products.

Consider an example, we can add the term (dil.) before the formula of sulphuric acid when a reaction occurs between zinc and dilute sulphuric acid.

Zn(s) + dil. H₂SO₄(aq) = ZnSO₄(aq) + H₂(g)

(d) Rate of reaction: We generally do not write rate of reaction to the chemical equation. Sometimes, however, we may write a term fast or slow over the arrow if the reaction is fast or slow. For instance, the reaction between HCl and NaOH in solution is a fast reaction.

(e) Reversible or irreversible nature: Reversible reactions are indicated with sign of double arrow (⇌), while irreversible reaction are indicated with the sign of equality (=) or an arrow (→). For example,

N2(g) + 3H2(g) ⇌ 2NH3(g)

Types of Chemical Equations

There are two types of chemical equations. They are as:

• Molecular equations
• Ionic equations

When we represent reactants and products in the form of molecules, they are called molecular equations. For example.

BaCl2 + Na2SO4 = BaSO4 (↓) + 2NaCl

When we write the reactants and products in the ionic form, they are called ionic equations. We can write the above molecular reaction in an ionic form as:

Ba²⁺ + 2Cl^– + 2Na⁺ + SO₄^2- = Ba²⁺ + SO₄^2- (↓) + 2Na⁺ + 2Cl^–

In this tutorial, we have discussed a simple definition of chemical equation with an example. Hope that you will have understood the basic points of writing a chemical equation as well as its notation, and limitations.

Thanks for reading!!!