Chemical Reactions

Last Updated : 05 Dec, 2023

Chemical Reaction is the disruption of existing bonds within reactant molecules and the creation of fresh bonds in product molecules, culminating in the formation of a new substance. These reactions are all around us, spanning from the biochemical processes in our bodies that extract energy from food to the source of sunlight, where chemical reactions are used to produce the light we receive.

In this article, we will understand the meaning and characteristics of Chemical Reactions. This article will cover the various types of Chemical Reactions: Combination or Synthesis Reaction, Decomposition Reaction, Single Displacement Reaction, and Double Displacement Reaction. We also discuss factors affecting the rate of Reaction and practice questions to understand this article.

Chemical-Reaction

Table of Content

What is a Chemical Reaction?
Characteristics of a Chemical Reaction
How would we know when a Reaction has taken place?
Chemical Equation
Balancing a Chemical Equation
Types of Chemical Reaction

What is a Chemical Reaction?

A Chemical Reaction is the transformation that takes place when a substance, or a combination of substances, comes into contact and yields new substances. The initial substances engaged in the Chemical Reaction are termed reactants or reagents, while the substances generated at the end of the reaction are referred to as products. An arrow is used to symbolize the direction of the Chemical Reaction, showing the conversion from reactants to products. For example, consider respiration, where we breathe in oxygen that reacts with glucose, resulting in the production of carbon dioxide, water, and the release of energy.

Chemical Reaction Definition

A chemical reaction is a transformative process where the initial substances, known as reactants, undergo a rearrangement of their constituent atoms to give one or more different substances, referred to as products.

These substances can be either chemical elements or compounds. Essentially, a chemical reaction results in the conversion of reactants into different chemical substances as products.

Chemical Reaction Example

Example of Chemical Reaction includes breaking down of glucose to give energy. The chemical reaction for the same is stated below:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Characteristics of a Chemical Reaction

Chemical Reactions show one or more than one of the following characteristics.

Evolution of Gas: When zinc reacts with hydrochloric acid, it produces hydrogen gas, forming zinc chloride: Zn + 2HCl → ZnCl2 + H2.

Change in Colour: Reaction between colorless lead nitrate and potassium iodide, resulting in the formation of a yellow lead iodide precipitate and colorless potassium nitrate:

Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

Change in Temperature: Some reactions are exothermic, releasing energy, while others are endothermic, absorbing energy.

Change in Energy: Reactions can either release energy, as seen in combustion reactions, or absorb energy.

Formation of Precipitate: When barium chloride reacts with sodium sulfate, it generates sodium chloride and a precipitate of barium sulfate:

BaCl₂ + Na₂SO₄ → BaSO₄+ NaCl (Precipitate)

Change in State: The reaction between ammonia gas and hydrogen chloride gas results in the formation of solid ammonium chloride crystals:

NH₃(g) + HCl(g) → NH₄Cl(s)

What Happens During a Chemical Reaction?

During a Chemical Reaction, the fundamental units, which are the atoms of the participating molecules or elements, undergo rearrangement to create new molecules. This rearrangement involves the rupture of existing chemical bonds between atoms and this process absorbs energy, and the establishment of new chemical bonds, which results in the release of energy.

Physical Changes During a Chemical Reactions

A Physical Change is the change in the physical properties of a substance. These changes are frequently observed in the course of chemical reactions, yet they do not transform the fundamental nature of the substances involved. Typically, physical changes manifest in modifications such as change in colour, the development of odours, or the release of gases. It’s important to note that the occurrence of physical changes does not necessarily entail the presence of chemical reactions.

How would we know when a Reaction has taken place?

There are five key indicators to detect a Reaction:

Change in colour
Precipitate Formation or solid substance formation and separates from a solution
Generation of Gas or Emergence of Bubble and odour
Change in Temperature
Change in pH

Chemical Equation

A Chemical Equation is a representation of Chemical Reactions. These equations are balanced to ensure that the same number of atoms of each element is present on both sides. It’s important to understand that in chemical reactions, no new matter is created, and atoms retain their identity.

Chemical Equations are distinct from numerical equations. In a typical chemical equation, the left-hand side signifies the reactants, while the right-hand side signifies the products. To accurately represent a Chemical Reaction, two important criteria must be met:

Each reactant and product must be denoted by their chemical formulas (e.g., H₂).
The number of atoms of each element must be equal on both sides of the equation. Coefficients, written in front of chemical formulas, are employed to balance the quantity of atoms (e.g., 2Mg + O₂ → 2MgO).

Furthermore, Chemical Equations utilize symbols like (s) for solid state, (aq) to denote substances dissolved in water, (l) for liquid state, and (g) for gaseous state.

Balancing a Chemical Equation

If we write the chemical reaction in the form of equation then originally it is unbalanced. Hence we need to balance the chemical reaction to make the number of atoms equal on reactant and product side. The balancing of chemical reaction is done in following steps.

Write the Initial Equation: Begin by writing the skeleton equation, which depicts the reactants and products. For example, in the reaction Li(s) + H₂O(l) → H₂(g) + LiOH(aq).
Count Atoms in Reactants: Calculate the number of atoms of each element in the reactants. In this case, there is 1 lithium (Li), 2 hydrogen (H), and 1 oxygen (O) atom in the reactants.
Count Atoms in Products: Determine the number of atoms of each element in the products. In this instance, there are 1 lithium (Li), 3 hydrogen (H), and 1 oxygen (O) atom in the products.
Balance with Coefficients: Adjust the coefficients to ensure that the quantity of each element’s atoms is the same on both sides of the arrow. In this example, we would modify it to 2Li(s) + 2H₂O(l) → H₂(g) + 2LiOH(aq) to achieve this balance.
Simplify the Coefficients: Write the coefficients in their simplest ratio, if possible. In this case, the coefficients are already in their simplest form.
Double-Check Your Work: Always review our equation to ensure it’s correctly balanced and that the number of atoms of each element is equal on both sides of the arrow.

Types of Chemical Reaction

The basis for different types of reactions is the product formed, the changes that occur, the reactants involved and so on. Different types of reactions are

Decomposition Reaction
Combination Reaction or Synthesis Reaction
Combustion Reaction
Single Displacement Reaction
Double Displacement Reaction
Precipitation Reaction
Neutralization Reaction or Acid-Base Reaction
Redox Reaction or Oxidation-Reduction Reaction
Disproportionation Reaction
Exothermic Reaction
Endothermic Reaction

Decomposition Reaction

In a Decomposition Reaction, complex molecules or compounds disintegrate into simpler new substances. This type of reaction is characterized by the breakdown of a reactant into simpler products. An example is the decomposition of hydrogen peroxide into oxygen and hydrogen gas:

H₂O₂(l) → H₂(g) + O₂(g)

Combination Reaction or Synthesis Reaction

A Combination Reaction or Synthesis Reaction unfolds when one or more compounds merge to form a more complex compound. Combination and decomposition reactions are opposites to each other. An example of Combination reaction or Synthesis Reaction is the combustion of hydrogen gas in the presence of oxygen to produce water:

2H₂(g) + O₂(g) → 2H₂O(l)

Combustion Reaction

Combustion Reactions are redox reactions where a fuel combines with an oxidizing agent, liberating heat energy. These reactions are exothermic, emitting energy as heat. An exemplary combustion reaction involves a hydrocarbon as the fuel and oxygen as the oxidizing agent, resulting in the formation of carbon dioxide and water:

C₆H₁₂O₆(s) + 6O₂(g) → 6CO₂(g) + 6H₂O(g)

Single Displacement Reaction

This type of reaction exists when an element interacts with a compound. The element replaces one of the components in the compound, generating a new compound and an elemental product. An example is the reaction between iron and copper sulfate:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Double Displacement Reaction

In a Double Displacement Reaction, two compounds react, and their ions are exchanged. This type of reaction is also known as a ‘Metathesis Reaction.’ Such reactions occur when ions with similar charges are swapped. For a double displacement reaction to occur, one of the products must be a precipitate or water.

ZnS(s) + 2Hcl(aq) → Zncl₂(aq) + H₂S(g)

There are two main categories of double displacement reactions:

Precipitation Reactions

Precipitation Reactions is a chemical reaction in which two aqueous solutions of compounds combine to form an insoluble compound (precipitate) and a soluble compound. For example, the formation of calcium sulfate as a white precipitate when sulfuric acid reacts with calcium carbonate:

CaCO₃(aq) + H₂SO₄(aq) → CaSO₄(s)↓ + H₂O(l) + CO₂(g)

Neutralization Reactions or Acid-Base Reaction

This type of reaction takes place when an acid and a base react to produce a salt and water. This reaction is named ‘Neutralization Reaction‘ since the acid and base neutralize each other. An example is the reaction between hydrochloric acid and sodium hydroxide

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Redox Reaction or Oxidation-Reduction Reaction

Redox Reactions involve changes in the oxidation states of the atoms participating in the reaction. Oxidation refers to the increase in an atom’s oxidation number, while reduction involves a decrease in the oxidation number. When the oxidation states of elements change, it signifies a redox reaction. The atom undergoing oxidation is called the reducing agent, while the one undergoing reduction is termed the oxidizing agent. An example is the reaction between hydrogen gas and fluorine gas:

H₂(g) + F₂(g) → 2HF(g)

A real-life example of Redox Reaction is “Rusting of Iron”

Disproportionation Reaction

Disproportionation Reaction is a type of redox reaction in which there is simultaneous oxidation and reduction of atom of the same element. This means that the atom of a particular element undergoes change from its existing oxidation state to two different oxidation states. Example of Disproportionation Reaction is mentioned below:

Cl₂ + H₂O → HCl + HOCl

Here, the oxidation number of Cl in Cl2 is 0 on the reactant side and the oxidation number of Cl in HCl is -1 and HOCl is +1. Thus Cl get both oxidised from 0 to +1 and also get reduced from 0 to -1.

Chemical Reaction on the Basis of Heat Evolved

On the basis of Heat Evolved, the chemical reactions are classified as follows:

Exothermic Reaction
Endothermic Reaction

Exothermic Reaction

Exothermic Reaction is a chemical reaction in which the heat energy get released after the reaction has happened. In case of exothermic reaction, the change in enthalpy is negative ΔH = -ve. Combustion Reaction is an example of exothermic reaction. Following equation represents combustion of methane

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

In above example, the ethanlpy change is ΔH = -890 kJ/mole

Endothermic Reaction

Endothermic Reaction are the chemical reactions which absorbs heat to proceed the reaction and form the product. Endothermic Reaction is just opposite of Exothermic Reaction. In case of Endothermic Reactions, the enthalpy change is positive which is given as ΔH = +ve. Sublimation, Hydrolysis, Thermal Cracking etc. are few examples of Endothermic Chemical Reaction. An example of Endothermic Reaction is mentioned below:

C₆H₁₂O₆ + 6 H₂O → 12H₂ + 6 CO₂

Here, in the above example the change in enthalpy is ΔH = +627 kJ/mol

Chemical Reaction on the Basis of Order

The order of a chemical reaction is determined by the sum of the exponents on the concentrations of the reactants in the rate law expression. This order can take on various values, such as first order, second order, or even fractional order, depending on the concentrations of the reactants. The order of a reaction is an experimental quantity, meaning it is determined through experimental observations, and it can involve fractional values.

Consider a reaction with the chemical equation aA + bB→P,

And its corresponding rate law r = k[A]^x[B]^y,

where x and y are the partial reaction orders for reactants A and B, and k is the rate constant.

The order of the reaction is then given by x+y.

Zero Order Reaction

In the case of zero-order reactions, the rate of the reaction remains constant and is independent of the concentrations of the reactants. Changes in the concentrations do not impact the reaction rate, and intermediate states are not considered, only the initial and final states are considerd.

2NH₃(g)⟶N₂(g)+3H₂(g)

Rate = k[NH₃]⁰

First Order Reaction

For first-order reactions, the rate of the reaction depends on the concentration of one reactant, and the order is 1. Even if there are multiple reactants, only one of them influences the rate, while the others have zero-order concentrations.

2H₂O₂⟶ 2H₂O+O₂

Rate = k[H₂O₂]¹

Second Order Reaction

In second-order reactions, the rate depends on the concentrations of two separate reactants or the square of the concentration of one reactant. The rate equation is expressed as

r = k[A]² or r = k[A][B], reflecting the dependence on either two separate reactants or the square of one reactant’s concentration.

2NO₂⟶2NO+O₂

Rate = k[NO₂]²

Difference Between Reversible and Irreversible Reactions

Chemical Reactions may be reversible or irreversible depending on the various situations and the equilibrium they achieve. The below table presents the comparison between Reversible and Irreversible Reaction.

Reversible Reactions	Irreversible Reactions
It can be reversed under suitable conditions.	Reversing this process is not possible
Both the forward and reverse reactions occur concurrently.	It proceeds in only one direction, without any reversal.
A state of equilibrium is reached during this process.	Equilibrium is not reached during this process.
Complete conversion of reactants into products is not achievable.	Complete conversion of reactants into products is feasible.
The pace of this process is generally moderate.	This process occurs rapidly.

Factors Affecting the Rate of Chemical Reaction

There are various factors affecting the rate of any chemical reaction, some of these factors are:

Nature of the Reactants: The nature of the substances participating in a reaction impacts its rate. For example, the reaction of sodium with hydrochloric acid occurs more faster than its reaction with acetic acid. This is because hydrochloric acid is a stronger acid than acetic acid, rendering it more reactive.

Concentration of the Reactants: Altering the quantity of reactants also influences the rate of a reaction. Concentration denotes the amount of substance present in a specific volume of a solution. Higher concentrations imply a greater number of particles per unit volume, leading to a faster reaction. For example, granulated zinc exhibits a more rapid reaction with 2M hydrochloric acid compared to 1M hydrochloric acid.

Temperature: Higher temperatures impact the majority of reactions. The application of heat to the reactants provides the necessary energy to break chemical bonds, accelerating the reaction rate

Pressure: When gaseous reactants are involved, increasing the pressure impacts the reaction rate. This results from the fact that heightened pressure causes the reacting particles to come into closer proximity, leading to more frequent collisions.

Catalyst: A catalyst is a substance that enhances the rate of a reaction without itself being consumed in the process. For example, when potassium chlorate decomposes into potassium chloride and oxygen gas, it does so at a slow rate. However, the inclusion of manganese dioxide as a catalyst significantly accelerates the reaction.

Surface Area of Reactants: When solid reactants are involved in a reaction, their finely powdered form reacts more readily than larger chunks or particles. This is due to the increased surface area provided by powdering the reactants, allowing for more frequent collisions and the availability of additional energy upon particle collision. Consequently, the reaction rate is heightened.

Also, Check

Rate of Reaction

Acid, Base and Salt

Balancing Redox Reaction

Solved Examples on Chemical Reaction

Example 1: In the given statement “sodium metal reacts with water to produce sodium hydroxide and hydrogen,” identify the reactants and products.

Solution:

Reactants: sodium metal and water; Products: sodium hydroxide and hydrogen.

Example 2: Balance the chemical equation: _Al + _O2 → _Al₂O₃ .

Solution:

Balanced equation: 4Al+3O₂→2Al₂O₃.

Example 3: Predict the products and balance the chemical equations for the given double-replacement reactions:

Zn(NO₃)₂+NaOH→?
HCl+Na₂S→?

Solution:

Zn(NO₃)₂+2NaOH→Zn(OH)₂+ 2NaNO₃

2HCl + Na₂S→2NaCl + H₂S

Example 4: Complete and balance the combustion equations:

Combustion of propane, C₃H₈
Combustion of ammonia, NH₃

Solution:

C₃H₈+ 5O₂→3CO2 + 4H₂O

4NH₃ + 3O₂→2N₂ + 6H₂O

Example 5: Assign oxidation numbers to the atoms in each substance:

Br₂
SiO₂

Solution:

For Br₂, each bromine atom has an oxidation number of 0.

For SiO₂, following the rules, oxygen is assigned an oxidation number of -2. To balance the sum of oxidation numbers, the silicon atom is assigned an oxidation number of +4.

Practice Questions on Chemical Reaction

Q1. Determine the type of reaction for the equation C₃H₆O₃ + O₂ → CO₂ (g) + H₂O (g).

Q2. Identify the oxidizing and reducing agents in the equation Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s).

Q3. Classify the species involved in the reaction 2HNO₃ (aq) + Sr(OH)₂ (aq) → Sr(NO₃)₂ (aq) + 2H₂O (l) as acid, base, or salt.

Q4. Categorize the reactions for the following equations:

P₄ (s) + Cl₂ (g) → PCl₃ (l)
Ca (s) + 2H₂O (l) → Ca(OH)₂ (aq) + H₂ (g)