Chemical Reaction

Chemical Reactions Examples

A chemical reaction in chemistry is a process that transforms one or more substances or reactants to form new types of substances with entirely different properties. Combination, decomposition, displacement, redox (oxidation-reduction reaction), and neutralization types of chemical reactions are common examples of exothermic and endothermic reactions that continuously happen in our surroundings. Therefore, all chemical changes that occur in our daily life are accompanied by chemical reactions and these are represented by chemical equations. The reactants and products that participate in a chemical reaction are elements, molecules, or ions. The reacting species (molecule, atom, ion) are called reactants and the new species formed are called products. For example, the reactants sodium and water react to form the new products sodium hydroxide and hydrogen.

Commonly, a chemical reaction is a process where two or more reactants interact to form new products. Sometimes one reactant molecule interacts to form two or more products. The rate of chemical reaction always depends on pressure, temperature, and the concentration of reactants. We also use chemical catalysts to carry out the reaction fast.

In our daily lives, chemical reactions are continuously happening in our bodies and surroundings. For example, photosynthesis, rusting of iron, digestion of food, combustion, fermentation, etc. The chemical change in a chemical reaction is generally observed by some physical changes such as precipitation, heat production, color change, etc.

In our daily lives, we come across a variety of changes which may be physical or chemical changes. According to the law of conservation of mass, no atom is destroyed or created during the chemical reaction. Therefore, a chemical reaction in chemistry commonly occurs by breaking one or more chemical bonds between atoms of reactant molecules and the formation of one or more new chemical bonds between atoms of product molecules.

Chemical Equations

The symbolic representation of the chemical reaction in the form of symbols and formulas is called the chemical equation. Therefore, symbols and formulas are used for the conversion of chemical equations from chemical reactions. For example, the burning of methane gas can be written in words as follows,

Methane + Oxygen → Carbon dioxide + Water

Such an equation is called a word equation. The word equation can be changed into a chemical equation by writing symbols and formulas of reactants and products.

Writing a Chemical Equation

A chemical equation can be written by symbols and formulas of reactants to products through an arrow placed between them. On the left-hand side (LHS) of the arrow, reactants are written with a plus (+) sign between them. Similarly, right-hand side (RHS) of the arrow, products are written with a plus (+) sign between them.

The burning of magnesium in the presence of oxygen can be written by the following equation,

Mg + O₂ → MgO

In the above chemical equation, the number of oxygen atoms on both sides is not the same or unbalanced. Therefore, it is called an unbalanced chemical equation. The balancing chemical equation for such a reaction is:

2Mg + O₂ → 2MgO

Make Chemical Reaction More Informative

We can add various other facts in writing a chemical equation to make the reaction more informative. These facts are:

• Physical states: The physical states of reactants and products can make the chemical equation more informative. The symbol (g) is used for gas, the symbol (s) is used for sold (s), the symbol (l) is used for liquid, and (aq) is used for aqueous solution.
• Reaction condition: The specific conditions such as temperature, pressure, and used chemical catalyst are also written above or below the chemical equation.
• Emission or Absorption of Heat: It can provide information about exothermic and endothermic chemical equations. When the heat sign is waiting on the right-hand side, heat is emitted and the reaction is exothermic. Similarly, if the heat sign waiting on the left-hand side, the reaction is endothermic.
• We can also use the symbol (↓) for precipitation and (↑) for emission of gas from the reaction vessel.

C (s) + O₂ (g) → CO₂ (g) + Heat

From the above chemical equation, solid carbon combines with gaseous oxygen to form gaseous carbon dioxide and the reaction is exothermic.

NH₄Cl (s) + Heat → NH₃ (g) + HCl (g)

Similarly, when solid ammonium chloride decomposes by heat, it produces ammonia gas and hydrogen chloride gas. It is an endothermic reaction because heat is required to carry out such a chemical process.

Balanced Chemical Equation

A balanced chemical equation is an equation where the total number of atoms of each element is equal on both sides of the equation. Balancing chemical equations is necessary because the number of atoms of each element remains the same before and after the reaction. We can use the following methods for balancing a chemical equation:

• Hit and trial method
• Oxidation number method
• Ion electron method

The balancing reaction that occurs between magnesium and oxygen for the formation of magnesium oxide (MgO) is written by the following chemical equation,

2Mg + O₂ → 2MgO

Types of Chemical Reactions

Chemical reactions are classified into different classes depending on the type of chemical changes that occur during the reactions. These reactions are classified into the following types:

• Combination reaction
• Decomposition reaction
• Exothermic chemical reaction
• Endothermic chemical reaction
• Displacement reaction
• Neutralization Reaction
• Oxidation and reduction reaction

Combination Reaction

A chemical reaction where two or more reactants combine to form a single product is called a combination reaction. It is also called a synthesis reaction. A combination reaction is an exothermic chemical process because energy always evolves when combining two or more reactants.

Calcium oxide (quick lime) combines vigorously with water to form calcium hydroxide (slaked lime). The reaction is highly exothermic because a lot of heat is produced during the chemical combination.

CaO (s) + H₂O (l) → Ca(OH)₂ (aq) + Heat

During burning coal, carbon combines with oxygen to form carbon dioxide.

C (s) + O₂ (g) → CO₂ (g)

Water is formed during combination of hydrogen gas and oxygen gas.

2H₂ (g) + O₂ (g) → 2H₂O (l)

Decomposition Reaction

Decomposition reaction is a chemical reaction where a single reactant breaks down to form two or more products. Such chemical transformation is the opposite of a combination reaction. Therefore, energy is required to carry out such types of chemical reactions and they are called endothermic reactions.

Thermal Decomposition

Thermal decomposition is a type of chemical reaction where thermal energy has been use for the decomposition of a reactant. The decomposition of calcium carbonate and ferrous sulfate are examples of thermal decomposition.

When heating, calcium carbonate decomposes to form calcium oxide and carbon dioxide.

CaCO₃ (s) → CaO (s) + CO₂ (g)

Ferrous sulfate or green vitriol can lose crystallization water after heating to form dehydrated ferrous sulfate. On further heating, it decomposes to form ferric oxide, sulfur dioxide, and sulfur trioxide.

2FeSO₄ (s) → Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)

Similarly, during the heating of lead nitrate, thermal decomposition occurs and it produces yellow lead monoxide (PbO), nitrogen dioxide (NO₂), and oxygen gas.

2Pb(NO₃)₂ (s) → 2PbO (s) + 4NO₂ (g) + O₂ (g)

Electrolysis Reaction

In an electrolysis reaction, electrical energy is used for the decomposition of the reactant molecules. The decomposition of water and molten sodium chloride by electrical energy are examples of electrolysis reactions.

When a direct current (DC) is passed through acidulated water, an electrolysis reaction occurs to give oxygen to the anode and hydrogen to the cathode.

2H₂O (l) + Electric current → 2H₂ (g) + O₂ (g)

Similarly, during the electrolysis of molten sodium chloride, sodium metal is obtained at the cathode, and chlorine gas is obtained at the anode.

2NaCl (l) + Electric current → 2Na (s) + Cl₂ (g)

Photochemical Decomposition Reaction

Photochemical decomposition is a chemical reaction where light energy is used for the decomposition of the reactant molecules.

When light-sensitive solid silver chloride (AgCl) is exposed to sunlight, it decomposes to give silver metal and chlorine gas.

2AgCl (s) + Sunlight → 2Ag (s) + Cl₂ (g)

Similarly, silver bromide can decompose to form silver metal and bromine gas in the presence of light.

2AgBr (s) + Sunlight → 2Ag (s) + Br₂ (g)

Above types of decomposition reactions of silver halides are used in black and white photography.

Exothermic Chemical Reaction

Exothermic reaction is a chemical reaction in which energy is released in the form of heat or light during the formation of products. Respiration, burning of natural gas, burning of magnesium ribbon, and decomposition of vegetable matter are examples of exothermic chemical reactions or combination reactions.

Energy in the form of heat is released during the burning of natural gas or methane gas. Therefore, the burning of natural gas is an example of an exothermic chemical reaction.

CH₄ (g) + 2O₂ (g) → CO₂ (g) + H₂O (g) + Heat

Similarly, the burning of magnesium robin in the presence of oxygen is an example of an exothermic chemical reaction.

2Mg (s) + O₂ (g) → 2MgO (s) + Heat

Endothermic Chemical Reaction

The chemical reaction that occurs in the absorption of energy in the form of heat, light, or electricity is called endothermic reaction. All decomposition reactions are endothermic because energy is required to carry out such types of chemical processes. Photosynthesis and decomposition of mercuric oxide or ammonium chloride by heat are examples of endothermic chemical reactions.

Photosynthesis is a common example of an endothermic chemical reaction that occurs in plants. During photosynthesis carbon dioxide, water, and light energy are used for the production of glucose.

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6CO₂ + 6H₂O

Mercury and oxygen are formed when mercuric oxide decomposes by heat. Therefore, it is an endothermic chemical reaction because heat is required for such decomposition.

2HgO (s) + Heat → 2Hg (l) + O₂ (g)

Similarly, ammonium chloride decomposes by heat to form ammonia and hydrogen chloride. It is an endothermic reaction because heat is required to carry out such a chemical process.

NH₄Cl (s) + Heat → NH₃ (g) + HCl (g)

Displacement Reaction

A displacement reaction is a type of reaction in which an atom of the more reactive chemical element displaces an atom of the less reactive element from its compound. Such chemical processes can be divided into two types,

• Single displacement reaction
• Double displacement reaction

Single Displacement Reaction

A single displacement reaction is a type of chemical reaction where a reactive element displaces a less reactive element from its compound.

Zinc is more reactive than copper metal. Therefore, when zinc is added to copper sulfate solution, zinc displaces the copper atom from copper sulfate to form zinc sulfate and copper metal.

Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu

Similarly, iron can displace copper from an aqueous copper sulfate (CuSO₄) solution to form ferrous sulfate (FeSO₄) because iron is more reactive than copper.

Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu

A white-colored lead chloride is formed when lead displaces copper from its chloride solution.

Pb (s) + CuCl₂ (aq) → PbCl₂ (aq) + Cu

Double displacement reaction

A double displacement reaction is a type of chemical process where two different ions or groups of atoms in the reactant molecules are displaced by each other to form new products. It is also called the precipitation reaction because a precipitate is always produced in such a chemical process.

When sodium sulfate is added to barium chloride, a curdy white precipitate of barium sulfate is formed by a double displacement reaction. Sodium chloride also forms during this double displacement.

Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ ↓ + 2NaCl (aq)

When silver nitrate is added to sodium bromide, a yellow precipitate of silver bromide is formed by a double displacement reaction. Sodium nitrate also forms during this double displacement.

AgNO₃ (aq) + NaBr (aq) → AgBr ↓ + NaNO₃ (aq)

Neutralization Reaction

A neutralization reaction is a type of chemical reaction in which acid or acidic oxide reacts with base or basic oxide to form salt and water. For example, when sodium or potassium hydroxide reacts with sulfuric acid, a salt sodium or potassium sulfate, and water is formed.

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

2KOH + H₂SO₄ → K₂SO₄ + 2H₂O

A neutralization reaction is a very important class of chemical reaction where the activity of an acid is neutralized by a base.

Oxidation and Reduction Reactions

In the electronic concept, oxidation-reduction reactions or redox reactions are chemical reactions that involve the transfer of electrons between species. These types of chemical reactions are also defined in many other ways.

Oxidation Reaction

The process of oxidation can be defined in the following way:

• Oxidation is the process in which oxygen is added to a substance.
  2Cu + O₂ → 2CuO
• Oxidation is a chemical process where hydrogen is removed from a chemical substance or entity.
  2H₂S + O₂ → 2S + 2H₂O
• The process in which a substance loses one or more electrons or electrons is called oxidation.
  Zn → Zn⁺² + 2e⁻
  Cl⁻ → Cl + e⁻

Reduction Reaction

Similarly, the process of reduction can be defined in the following way:

• Reduction is the chemical process in which oxygen is removed from a substance.
  2KClO₃ → 2KCl + 3O₂
• The process in which hydrogen is added to a substance is called reduction.
  2Na + H₂ → 2NaH
• The chemical process in which a substance loses one or more electrons or electrons is called a reduction reaction.
  Zn⁺² + 2e⁻ → Zn
  Cl + e → Cl⁻

Equilibrium in Chemical Reaction

Many chemical reactions in our daily lives do not go to completion even if favorable external conditions are maintained. Therefore, it is a common fact for many chemical reactions. Sometimes, a portion of reactants in a chemical reaction are unaltered in the system.

A chemical reaction proceeds to some extent and then they come to a stop. A stage is reached where no further chemical changes occur. Therefore, chemical equilibrium is a state or stage in a chemical reaction where the concentrations of reactants and products no longer alter over time.

If some hydrogen and iodine vapor can kept in a closed vessel at a constant temperature, only a portion is converted to product, and then the reaction stops.

H₂ + I₂ ⇌ 2HI

• A mixture of hydrogen, iodine, and hydrogen iodide will remain in the vessel for an unlimited time when the temperature is kept constant.
• Similarly, if some hydrogen iodide is kept in a vessel at the same temperature, it decomposes to form a mixture of hydrogen, iodine, and hydrogen iodide.

In both experiments, the proportion of hydrogen, iodine, and hydrogen iodide will be the same. When a stage is reached, no further changes in the chemical reaction are apparent and such a stage is called the equilibrium of a chemical reaction.

For more details, you can see chemical equilibrium.

Kinetics of Chemical Reaction

Chemical kinetics is a branch of chemistry that deals with the study of the speed of a chemical reaction. Therefore, it enables us to understand the mechanism by which chemical reactions are performed.

In chemical equilibrium, only initial and final states are considered and energy relations between reactants and products are obtained by thermodynamics. Therefore, time and intermediate steps are not considered in chemical equilibrium.

In chemical kinetics, the rate of a reaction or time variables is introduced. Commonly, the rate of a reaction depends on the concentration of reactants and the temperature at which the reaction occurs. The reaction rates are also influenced by the presence of foreign substances or catalysts. In photochemical reactions, the rates are accelerated by the absorption of light that has a specific wavelength.

For many chemical reactions, the rate measurement is quite difficult because they occur very fast or very slow. For example, ionic reactions and some explosive reactions occur almost instantly while rusting of iron occurs very slowly. Therefore, the speed between the very fast and very slow can be measured very easily. Examples of such types of chemical reactions are, the decomposition of hydrogen iodide (HI) or hydrogen peroxide (H₂O₂), hydrolysis of ester, mutarotation of sugar, etc.

For more details, you can see chemical kinetics.

Frequently Asked Questions

What is a Chemical Reaction in Chemistry?

A chemical reaction in chemistry is a process that transforms one or more substances or reactants to form new types of substances with entirely different properties. All chemical changes in our daily life are accompanied by chemical reactions and these are represented by chemical equations.

How a Chemical Reaction Occurs?

A chemical reaction in chemistry commonly occurs by breaking one or more chemical bonds between atoms of reactant molecules and the formation of one or more new chemical bonds between atoms of product molecules.  Therefore, no atom is destroyed or created when a chemical reaction occurs.

What are Exothermic Reactions?

A chemical reaction in which energy is released in the form of heat or light during the formation of products is called exothermic reaction. The common examples of exothermic chemical reactions or combination reactions are respiration, burning of natural gas, burning of magnesium ribbon, decomposition of vegetable matter, etc.

What is Rate of Reaction in Chemical Kinetics?

The rate or velocity of a reaction in kinetics is the amount of chemical change occurring per unit time. The rate is generally expressed by the decrease in the concentration of a reactant molecule or increasing in the concentration of a product molecule per unit time.

What is Precipitation in Chemical Reaction?

Precipitation is a process in a chemical reaction that occurs when two or more different salt solutions are combined to form an insoluble solid salt that precipitates out from the solution. It is also called the double displacement reaction because two different ions or groups of atoms in the reactant molecules are displaced by each other to form an insoluble precipitate.