Chemical Equation: Definition, Methods, Direction and Representation

Chemical Equations are common interpretations used in chemistry. Chemical equations are the interpretation of chemical reactions that encompasses numbers and symbols of various compounds and elements. In a chemical reaction, the reactants are on the left side of the chemical equation while the products or results are on the right side. The arrow in the equation represents the direction of the reaction i.e., the reaction can be unidirectional or bidirectional (or reversible). 

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What is Chemical Equation?

Chemical Equations are worded or symbolic representations of the reaction that will take place when two or more elements will be combined. Chemical Equations need to be balanced in order to satisfy all the conditions including the law of conservation of matter. 

Also read: Precipitation Titration

The number of elements on the left side of an arrow is the reactant elements and elements on the right side are product (or resultants). A balanced equation happens when the number of atoms of an element on the reactant side is equal to the number of atoms of elements on the product side. 

What are Chemical Equations?

An example of a chemical reaction is:

An aqueous solution of sodium chloride (NaCl[aq]) and another aqueous solution of silver nitrate (AgNO₃[aq]) are mixed together forming sodium nitrate (NaNO₃[aq]) and silver chloride (AgCl[s]). 

The symbolic representation will be:

NaCl(aq) + AgNO₃(aq) → NaNO₃(aq) + AgCl(s)

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Methods of Balancing Chemical Equations

There are two methods of balancing a chemical equation. One is by balancing by inspection and the other is balancing by Numerical method. Balancing by inspection is one of the basic and simple methods to balance the chemical equation. This method is used for simpler problems. In the Balancing by Numerical method a reaction is balanced and the coefficients can be seen. Hence, the Balancing by Numerical method is mostly used as it describes if the equation is balanced or not. Balancing by inspection does not represent if the equation is balanced or not. 

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Direction of Chemical Reaction

Every chemical equation can be unique and can be housed with different elements giving different results. The product obtained can be by different reactions. There are four different representations of chemical equations that are used in chemical equations. 

• The symbol ‘→’ represents net forward reaction. 
• \(\rightleftharpoons\)’ is used to represent the chemical equilibrium state. 
• ‘=’ symbol is used to denote stoichiometric relationships
• ‘⇔’ represents reactions occurring in both forward and backward reactions.

Different entities in a chemical reaction are separated by the ‘+’ symbol and it should be noted that ‘→’ is read as ‘gives rise to’ or ‘yields to’.

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Representation of Reacting Entities

Physical State Representation of Reacting Entities

All the elements are of different forms of matter i.e., Solid, liquid, and gas. The elements are represented as: 

• ‘(s)’ represents Solid State. 
• ‘(l)’ represents Liquid State.
• ‘(g)’ represents the Gaseous State.
• ‘(aq)’ represents an Aqueous solution.
• ‘↓’ symbol describes the elements as precipitates. 

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Chemical Representations of Reacting Entities

Some chemical reactions require energy from an external source or yield energy during the reaction. The chemical representation of the energies are as follows:

• ‘Δ’ represents the requirement of heat energy during the chemical reaction. 
• ‘Hv’ represents the photon energy (or light energy) to yield a result. 

These coefficients are assigned to obey the law of conservation of charge and the law of conservation of mass.

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Ionic Equation

When electrolytes are written as dissociated ions the equation so formed is called the Ionic equation. Ionic equations are used to represent single and double displacement reactions that occur in an aqueous solution.

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Enthalpy Formula

Example

The ionic representation of CaCl₂ + 2AgNO₃ → Ca(NO₃)₂ + 2AgCl↓ is 

Ca²⁺ + 2Cl– + 2Ag⁺ + 2NO₃^– → Ca²⁺ + 2NO₃^– + 2AgCl↓.

From the above ionic representation, it can observe that Ca²⁺ and NO₃^- are present as separate entities on both sides of the reaction. The elements that are present on both sides of the reaction are the elements that do not take part in the chemical reaction and are called spectator ions. hence the above ionic equation without the spectator ions can be written as ‘2Cl^– + 2Ag⁺ → 2AgCl↓’ or ‘Ag⁺ + Cl^- →AgCl’.

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Things to Remember

• Chemical Equations are worded or symbolic representations of the reaction that will take place when two or more elements will be combined.
• There are two methods to balance a chemical equation. These are Balancing by Inspection and Balancing by Numerical method.
• When electrolytes are written as dissociated ions the equation so formed is called the Ionic equation. 

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