Chapter 6
Chemical Reactions
Introduction
A chemical reaction is a modification in two or more chemical structures and properties when they react to create a new material. We witness several chemical reactions in our daily lives. The process of combustion or burning, photosynthesis, and even baking happens in the form of a chemical reaction. The substances reacting with one another are known as reactants, while the compound thus produced is known as a product.
Let us take a look at the types of chemical reactions.
A combination reaction is one where two or more reactants interact to create a single new product. It involves a reaction of two or more elements or compounds, and ultimately, a new product is synthesized; hence a combination reaction is also known as a synthesis reaction.
It is represented as Y + Z → YZ
An example of a combination reaction is
4Fe + 3O2 → 2Fe2O3
Here, Iron reacts with Oxygen to form Iron Oxide. Thus it is a combination reaction.
A single compound disintegrates into two or more elements or compounds, known as a decomposition reaction. It includes breaking down a compound into simpler substances. Hence decomposition reaction is the opposite of combination reaction.
It is represented as YZ → Y + Z
An example of a decomposition reaction is
2H2O → 2H2 + O2
Here, two water molecules decompose into two Hydrogen molecules and one oxygen molecule.
In some cases, Decomposition reactions create oxides as a by-product. Let us consider such instances:
- When a metal carbonate decomposes, it results in metal oxide formation and a product of carbon dioxide gas.
- CaCO3(s)→CaO(s)+CO2(g)
- An important example is the decomposition of calcium carbonate into calcium oxide and carbon dioxide gas.
- When a metal hydroxide is heated, it decomposes into metal oxide and water.
- 2NaOH(s)→Na2O(s)+H2O(g)
- The above example shows the decomposition of sodium hydroxide, resulting in sodium oxide and water formation as a by-product.
- When certain unstable acids are present at room temperature, they decompose into non-metal oxides and water.
- H2CO3(aq)→CO2(g)+H2O(l)
- Here, the decomposition reaction of carbonic acid results in the formation of carbon dioxide and water.
A chemical reaction where a more reactive element displaces a less reactive element from its aqueous salt solution is known as a displacement reaction. Since a reactive element substitutes another element in this reaction, the reaction is also known as a substitution reaction.
It is represented as M + NP → MP + N
An example of a displacement reaction is
Mg + 2H2O → Mg(OH)2 + H2
Magnesium reacts with water in this reaction, and Magnesium replaces Hydrogen to form Magnesium hydroxide and Hydrogen gas.
A chemical reaction where specific ions transfer between two reactants forming new compounds, is a double displacement reaction. In a double replacement reaction, the anions and cations of two compounds swap places and create two completely different compounds.
It is represented as XY + AB → XB + YA.
An example of a displacement reaction is
Pb(NO3)2 + 2KI → PbI2 + 2KN3
In this reaction, Lead Nitrate reacts with Potassium Iodide. As a result, Lead Iodide and Potassium Nitrate are formed due to double displacement.
A chemical reaction where the reactants require energy, usually in heat, from the environment or surroundings to create a product is known as an endothermic reaction. Such reactions decrease the temperature of the nearby area, producing a cooling effect. Endothermic reactions typically constitute the creation of chemical bonds via the intake of heat from the environment.
It is represented as Energy + A+ B → C+ D or A+ B → C+ D – Heat.
An example of an endothermic reaction is
C6H12O6 + 6 H2O → 12 H2 + 6 CO2
This reaction represents the process of photosynthesis. Here, plants incorporate the heat energy from the Sun to create products.
An exothermic reaction is a reaction that releases energy as light or heat. Thus, energy is transferred to the environment in an exothermic reaction.
It is represented as AB → A+ B + energy
An example of an Exothermic reaction is
4Fe + 3O2 → 2Fe2O3
In this reaction, the oxidation of iron results in the creation of iron oxide. This reaction also releases heat energy and is thus an exothermic reaction. The heat produced as a result of the oxidation of Iron is actively used in hand dryers.
A chemical reaction where an acid and a base react to form salt and water as products is a neutralization reaction.
It is represented as MH + N → M + NH.
An example of an Exothermic reaction is
HCl + NaOH → NaCl + H2O
In the above chemical reaction, Hydrochloric acid reacts with Sodium hydroxide, which is a base, to form Sodium chloride(salt) and water as by-products.
The reactivity series of metals lists the metals in descending order of their Reactivity. This list also indicates if a metal can displace another metal during a displacement reaction and predict metals’ Reactivity with acids and water.
MetalReactivityPotassiumMost reactiveSodium↓Decreasing order of Reactivity
CalciumMagnesiumAluminiumZincIronLeadHydrogenCopperMercurySilverGoldPlatinumLeast Reactive
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