Types of Chemical Reactions

All chemical reactions may be divided into five categories. Knowing how to identify the products of unknown reactions requires a detailed grasp of these sorts of reactions. Combination, decomposition, single-replacement, double-replacement, and combustion are the five fundamental kinds of chemical processes.

What is a Chemical Reaction?

A chemical reaction, a process in which the bonds are broken within reactant molecules, and in order to form a new substance new bonds are formed within product molecules. Let us first clear physical and chemical changes.

• Physical change: When a usually reversible change in the physical properties of a substance, as size or shape takes place. Freezing a liquid, melting of candle, crushing a can, shredding papers all are examples of physical change.
• Chemical change: When a chemical property of matter changes it is a type of chemical change such properties are density change, change in temperature, energy change etc, It is commonly called a chemical reaction. Combustion, rusting, fermentation, etc. are examples of chemical change.

Properties of a chemical reaction

• A chemical reaction occurs when two or more molecules interact and forms a new product(s).
• Compounds that interact to form a new compound called Reactant. and the compounds that formed after the interaction of reactant are known as Product.
• In a chemical reaction, the chemical change must occur within the physical change in the reaction example heat evolution, colour change etc.
• A reaction can take place between two atoms or ions or molecules of reactants, and they form a new bond by breaking old bonds and no atom is destroyed or created but a new product is formed.
• The rate of reaction depends on various factors like pressure, temperature, the concentration of reactants.

Chemical Equations

A chemical equation is a mathematical statement that shows the product formation from reactants while stating certain conditions for which how the reaction has been conducted.

A + B ⇢ X + Y

The reactants are on the left-hand side Of the equation whereas products formed on the right-hand side of the equation are connected by one-headed or two-headed arrows. In chemistry, reactant and products are chemical species. Take an example of the reaction between Zn and HCl:

Chemical Reaction Representation

• Exothermic reaction: Chemical reactions in which heat energy is released are known as exothermic reactions. The burning of methane gas in the presence of air produces heat energy. All the combustion reactions are exothermic in nature.

CH₄ (g) + 2O₂ (g) → CO₂(g) +  2H₂O (l) + heat

• Endothermic reaction: Chemical reactions that involve absorption of energy are known as endothermic reactions. Example: Formation of nitric oxide during the reaction of nitrogen and oxygen.

N₂ (g) + O₂ (g) + Heat → 2NO

Types of Chemical Reactions

On the basis of the formation of different kinds of products or change in the condition of reactants, different types of chemical reactions are:

1. Combustion reaction
2. Neutralization reaction
3. Decomposition reaction
4. Redox Reaction
5. Double-Displacement Reaction or Precipitation

Combustion Reaction

A combustion reaction is a type of chemical reaction between a fuel and an oxidant that produces a product called ad oxidized product. an oxidizer is a chemical fuel required to burn such as oxygen. Generally, it’s a reaction between hydrocarbon and oxygen to yield carbon dioxide, water and heat.

Hydrocarbon + Oxygen -> Carbon dioxide + Water + Heat

Examples:

• CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g) + Heat
• 2Mg + O₂ → 2MgO + Heat

Neutralization Reaction

A Neutralization reaction is a reaction between an acid and a base to give salt and water as the products. The formation of the water molecules is by the combination of OH– ions and H+ ions. The overall pH of the products when a strong base and a strong acid undergo a neutralization reaction must be 7. Consider the example of the neutralization reaction between Hydrochloric acid (HCl) and Sodium Hydroxide (NaOH) giving out sodium chloride (Common Salt) and water.

• HCl (l) + NaOH (aq) -> NaCl (aq) + H2O (l)

Neutralization Reaction

Decomposition Reaction

In the Decomposition reaction, a single component breaks down into two or more (multiple) products. a certain amount of energy is required to break the bond between reactants such as heat, electricity, light etc.

Examples of decomposition reactions are as:

• Decomposition of calcium carbonate (CaCo3) giving out CaO (Quick Lime) which is a major component of cement.

                                                                   CaCO₃(s)    ^heat >   CaO(s)   +    CO₂(g)

• Calcium hydroxide (Slaked lime) on decomposition gives Quick lime (CaO).

                                                                   Ca(OH)₂ ^heat>   CaO  +  H₂O

• The decomposition of salt (NaCl) gives Na and Cl as a product.

                                                                   NaCl    ->   Na  +  Cl

Redox Reaction

A Reduction-oxidation reaction in which reduction(gain of the electron) and oxidation (loss of electron) takes place, so it is the process of transfer of electronic species. Consider an example of an electron-chemical cell-like redox reaction between Zinc and Hydrogen.

                                                                                    Zn + 2H⁺ →  Zn²⁺ + H₂

Redox Reaction

In the above reaction, a Zinc atom reacts with two positively charged hydrogen to which electrons get transferred from the zinc atom and hydrogen becomes a stable molecule and Zinc ion as the product.

• Oxidation – Oxidation is a loss of electrons and the oxidation state of given reactant increases.
• Reduction – Reduction is a gain of electrons and the reactant state of given reactant decreases.

Single displacement reaction 

In this type of reaction more reactive metal displaces less reactive metal from its salt. In these reactions, through reactivity series, products can be determined. Reactivity series is a kind of series in which elements are arranged in decreasing order of their reactivity. In this series, the elements present at the top of the series are more reactive than the elements present at the bottom of the series.  

• 2K + MgCl2  -> 2KCl + Mg

The reaction of potassium with magnesium chloride (MgCl₂)is an example of a single displacement reaction. In this reaction, potassium displaces magnesium from its salt because of the more reactivity of potassium than magnesium. Potassium is present at the top of the reactivity series and hence, it is the most reactive element.

Precipitation or Double-Displacement Reaction

It is a kind of displacement reaction in which two chemical species react and consequently, their ions i.e cations and anions switch places forming two new products. now look at the reaction between silver nitrate and sodium chloride. The products of the reaction will be silver chloride and sodium nitrate after the double-displacement reaction.

Double decomposition reaction

Example:

• AgNO₃  +  NaCl  →  AgCl  +  NaNO₃

Sample Problems

Question 1: Guess the type of reaction given below: CH₄ + 2O₂  ⇢ CO₂ + 2H₂O + energy.

Answer:

The above two reactions are combustion reaction.

Question 2: Define exothermic and endothermic reactions.

Answer:

An exothermic reaction is a reaction in which there is an release of energy which can be in the form of light or heat. Few examples are nuclear fission, rusting of iron.

On the other hand, an endothermic reaction is the reaction in which the system absorbs heat from the surrounding in the form of heat. Few examples are melting of ice, cooking etc.

Question 3: Define the term Reactant and Product.

Answer:

Reactant- The molecules or chemical species that react to form a product is known as reactant.

Product- The resultant species that formed after the reaction is known as Product.

Question 4: In the given reaction, identify the substance which get oxidized and which get reduced: MnO₂ + 4HCl → 2H₂O + Cl₂

Answer:

In the above equation HCl is oxidized to Cl₂ and MnO₂ is reduced to H₂O, hence HCl is oxidized and MnO₂ is reduced in the above reaction.

Question 5: Find the oxidizing agent for the following reaction: H₂S + I₂ > 2HI + S.

Answer:

Addition of hydrogen is known as reduction whereas the reactant which get reduced is known as oxidizing agent. It is clear that I₂ is reduced and hence it is an oxidizing agent.

Question 6: Why does the colour of copper sulfate (CuSo₄) solution change when an iron nail is dipped in it?

Answer:

When an iron nail is dipped in copper sulfate (CuSO₄) solution, the displacement reaction takes place. The color of copper sulfate solution disappears due to the formation of light green solution of iron sulfate.

                                                                       Fe(S) + CuSO₄(aq) -> FeSO₄(aq) + Cu(S)

Question 7: Which reactions is called the opposite of combination reactions? Write equations for these reactions.

Answer:

Decomposition reaction, is known as the opposite of combination reactions, In this reaction single compound breaks down to produce two or more simpler substances.

For example:

2H₂O + electricity->  2H₂  +  O₂

While, combination reaction, is when two or more substances simply combine to form a new substance.

For example:

2H  +  O₂  ->  2H₂O