Chemical Equation

It can also be observed that there are coefficients assigned to each of the symbols of the corresponding reactants and products. These coefficients of entities in a chemical equation are the exact value of the stoichiometric number for that entity.

Representing the Direction of the Chemical Reaction

The reactants and the products (for which the chemical formulae are written in chemical equations) can be separated by one of the following four symbols.

• In order to describe a net forward reaction, the symbol ‘→’ is used.

• In order to describe a state of chemical equilibrium, the symbol ‘⇌’ is used.

• To denote stoichiometric relationships, the ‘=’ symbol is used.

• In order to describe a reaction that occurs in both forward and backward directions, the symbol ‘⇄’ is used.

Multiple entities on either side of the reaction symbols describe above are separated from each other with the help of the ‘+’ symbol in a chemical equation. It can be noted that the ‘→’ symbol, when used in a chemical equation, is often read as ‘gives rise to’ or ‘yields’.

Representing the Physical States of the Reacting Entities

Apart from the stoichiometric coefficients of the reacting and the produced entities, symbols enclosed in parentheses are also written adjacent to them in order to describe their physical states over the course of the chemical reaction. These symbols may be one of the following.

• The symbol (s) describes an entity in the solid state

• The symbol (l) denotes the liquid state of an entity

• The symbol (g) implies that the entity is in the gaseous state.

• The (aq) symbol corresponding to an entity in a chemical equation denotes an aqueous solution of that entity.

In some reactions, a reactant or a product may be in the form of a precipitate which is insoluble in the solution that the reaction is taking place in. The ‘↓’ symbol is written next to the chemical formula of these entities to describe them as precipitates.

How is the Input of Energy Represented in a Chemical Equation?

Some chemical reactions require an input of energy in order to proceed. The energy requirements of these reactions are described above the arrow symbol (forward reaction) in their corresponding chemical equations with the help of the following symbols

• The Greek letter delta in its capitalized form (Δ) is used to state that an input of heat energy is required by the reaction.

• The formula ‘hv’ which describes the energy of a photon is used above the arrow symbol to state that the reaction requires an input of light to proceed.

It is important to note that the stoichiometric coefficients that are assigned to each entity in the chemical equation are used to make the entire equation obey the law of conservation of charge and the law of conservation of mass.

Ionic Chemical Equations

In ionic chemical equations, the electrolytes (substances that dissociates into ions when dissolved in polar solvents) are split up and written as separate ions. These equations are very useful in describing single displacement reactions and salt metathesis reactions (generally referred to as double displacement reactions).

Example

An example of an ionic chemical equation is provided below.

Chemical Equation: CaCl2 + 2AgNO3 → Ca(NO3)2 + 2AgCl↓

Ionic Equation: Ca2+ + 2Cl– + 2Ag+ + 2NO3– → Ca2+ + 2NO3– + 2AgCl↓

Comparing the reactants and the products of the ionic equation and the chemical equation, it can be observed that the Ca2+ (calcium ion) and the NO3– (nitrate) ions are present on both sides of the ionic equation. These ions are referred to as spectator ions because they do not participate in the chemical reaction.

The net ionic equation for the example above can be written by removing the spectator ions and writing only the reaction between the participating ions, as shown below.

2Cl– + 2Ag+ → 2AgCl↓

This ionic chemical equation can be interpreted as follows – two chloride ions originating from calcium chloride react with two silver cations originating from silver nitrate, forming a precipitate of silver chloride as the product.

Frequently Asked Questions

What are Chemical Equations?

They are equations that make use of chemical formulae and symbols to represent chemical reactions. The left-hand side of a chemical equation represents the reactants and the right-hand side represents the products. These entities are separated by a symbol that describes the direction of the reaction. Each reacting entity is also assigned its corresponding stoichiometric coefficient.

What are the Four Symbols used to Denote Physical States in these Equations?

The symbols used to denote the physical states of the reacting entities include:

• (s) for solid.

• (l) for liquid.

• (g) for gas.

The aqueous solution of a chemical is often represented with the symbol (aq).

What is an Ionic Equation?

The chemical equations in which electrolytes are represented in the form of dissociated ions are commonly referred to as ionic equations. They are often used to represent the displacement reactions that take place in aqueous mediums. In these reactions, some ions participate in the reaction and some do not. The ions that do not react are called spectator ions and are usually omitted from the net ionic equation.

List some Examples of Chemical Equations.

A few examples of chemical equations are listed in bulleted text below.

• PCl5 + 4H2O → H3PO4 + 5HCl

• SnO2 + 2H2 → 2H2O + Sn

• TiCl4 + 2H2O → TiO2 + 4HCl

• H3PO4 + 3KOH → K3PO4 + 3H2O

• Na2S + 2AgI → 2NaI + Ag2S

Thus, the fundamentals of writing chemical and ionic equations are discussed. To learn more about chemical equations and how to balance them with the correct stoichiometric coefficients, register with BYJU’S and download the mobile application on your smartphone.

Balancing Simple Chemical Equations

When a chemist encounters a new reaction, it does not usually come with a label that shows the balanced chemical equation. Instead, the chemist must identify the reactants and products and then write them in the form of a chemical equation that may or may not be balanced as first written. Consider, for example, the combustion of n-heptane (C7H16), an important component of gasoline:

C7H16(l)+O2(g)→CO2(g)+H2O(g)

The complete combustion of any hydrocarbon with sufficient oxygen always yields carbon dioxide and water.

An Example of a Combustion Reaction. The wax in a candle is a high-molecular-mass hydrocarbon, which produces gaseous carbon dioxide and water vapor in a combustion reactionThe eqution is not balanced: the numbers of each type of atom on the reactant side of the equation (7 carbon atoms, 16 hydrogen atoms, and 2 oxygen atoms) is not the same as the numbers of each type of atom on the product side (1 carbon atom, 2 hydrogen atoms, and 3 oxygen atoms). Consequently, the coefficients of the reactants and products must be adjusted to give the same numbers of atoms of each type on both sides of the equation. Because the identities of the reactants and products are fixed, the equation cannot be balanced by changing the subscripts of the reactants or the products. To do so would change the chemical identity of the species being described.

Steps in Balancing a Chemical Equation

1. Identify the most complex substance.

2. Beginning with that substance, choose an element that appears in only one reactant and one product, if possible. Adjust the coefficients to obtain the same number of atoms of this element on both sides.

3. Balance polyatomic ions (if present) as a unit.

4. Balance the remaining atoms, usually ending with the least complex substance and using fractional coefficients if necessary. If a fractional coefficient has been used, multiply both sides of the equation by the denominator to obtain whole numbers for the coefficients.

5. Check your work by counting the numbers of atoms of each kind on both sides of the equation to be sure that the chemical equation is balanced.teps in Balancing a Chemical eqution. It is sometimes convenient to use fractions instead of integers as intermediate coefficients in the process of balancing a chemical equation. When balance is achieved, all the equation’s coefficients may then be multiplied by a whole number to convert the fractional coefficients to integers without upsetting the atom balance. For example, consider the reaction of ethane (C₂H₆) with oxygen to yield H₂O and CO₂, represented by the unbalanced equation:

C2H6+O2→H2O+CO2(unbalanced)

following the usual inspection approach, one might first balance C and H atoms by changing the coefficients for the two product species, as shown:

C2H6+O2→3H2O+2CO2(unbalanced)

This results in seven O atoms on the product side of the equation, an odd number—no integer coefficient can be used with the O₂ reactant to yield an odd number, so a fractional coefficient, 7/2 is used instead to yield a provisional balanced equation: 7/2 , is used instead to yield a provisional balanced equation:

C2H6+72O2→3H2O+2CO2

A conventional balanced equation with integer-only coefficients is derived by multiplying each coefficient by 2:

2C2H6+7O2→6H2O+4CO2

Finally with regard to balanced equations, recall that convention dictates use of the smallest whole-number coefficients. Although the equation for the reaction between molecular nitrogen and molecular hydrogen to produce ammonia is, indeed, balanced,

3N2+9H2→6NH3

the coefficients are not the smallest possible integers representing the relative numbers of reactant and product molecules. Dividing each coefficient by the greatest common factor, 3, gives the preferred equation:

N2+3H2→2NH3