Chemical Reactions & Equations

Chemical Reactions & Equations

Chemical - Involving changes to the structure of a substance.

Chemical Reactions - A chemical reactions is a process in which one or more substances, also called reactants, are converted to one or more different substances, known as products.

Chemical Equations :

Chemical equations make use of symbols to represent factors such as the direction of the reaction and the physical states of the reacting entities. Chemical equations were first formulated by the French chemist Jean Beguin in the year 1615.

Chemical reactions can be represented on paper with the help of chemical equations, an example for which is represented below (for the reaction between hydrogen gas and oxygen gas to form water).

2H₂ + O₂ → 2H₂O

Writing a Chemical Equations :

Certain rules have to be followed while writing a chemical equation.

1. The reactants taking part in the reaction are written in terms of their symbols or molecular formulae on the left-hand side of the equation.

2. A plus (+) sign is added between the formulae of the reactants.

3. The products of reaction are written in terms of their symbols or molecular formulae on the right-hand side of the equation.

4. A plus (+) sign is added between the formulae of the products.

5. In between the reactants and the products an arrow sign (⟶) is inserted to show which way the reaction is occurring.

A + B ⟶ C + D

In this chemical equation, A and B are the reactants, and C and D are the products. The arrow indicates that the reaction proceeds towards the formation of C and D.

Balanced Chemical Equations

A balanced equation is an equation for a chemical reaction in which the number of atoms for each element in the reaction and the total charge is the same for both the reactants and the products. In other words, the mass and the charge are balanced on both sides of the reaction.

The method used for balancing chemical equations is called Hit & Trial Method.

Types of Chemical Reactions

Chemical reactions are of many types. Here, we are covering the most common eight types of chemical reactions, which are part of the class X chemistry syllabus as well.

• Decomposition reaction

• Combination reaction

• Combustion reaction

• Neutralization reaction

• Single displacement reaction

• Double displacement reaction

• Precipitation reaction

• Redox reaction

• Exothermic reaction

• Endothermic reaction

• Decomposition Reaction – In a decomposition reaction, molecules or compounds break down into two or more than two simpler chemically new substances. For example, electrolysis of water. In the electrolysis of water, water breaks down into hydrogen and oxygen, which show completely different properties than water.

• Combination Reaction – In a combination reaction, two or more molecules are combined together chemically to form a new substance (compound). Combination and decomposition reactions are opposite of each other. For example, when we burn magnesium ribbon (or magnesium), it gives grey-black ash of magnesium oxide.

Reaction – Mg + O₂ → MgO

• Combustion Reaction – It is an exothermic reaction that releases energy, generally in the form of heat. It is a reaction between fuel and an oxidant (generally atmospheric oxygen) that produces smoke, water and heat generally. For example, when we burn methane, it gives carbon dioxide and water.

Reaction – CH₄ + 2O₂ → CO₂ + 2H₂O

• Neutralization Reaction – In these reactions, acid and base react with each other and form salt and water. For example, hydrochloric acid reacts with sodium hydroxide (base) and forms sodium chloride (salt) and water.

• Single Displacement Reaction – In these reactions, more reactive metal displaces less reactive metal from its salt. In these reactions, products can be determined through reactivity series. Reactivity series is a series in which elements are arranged in decreasing order of their reactivity. It means the elements present at the top of this reactivity series are more reactive than the elements present at the bottom.

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

Reaction – 2K + MgCl₂ → 2KCl + Mg

• Double displacement reaction – In these reactions, two aqueous ionic compounds exchange their ions (mostly cations) and produce two new compounds. For example, potassium nitrate reacts with aluminium chloride and forms aluminium nitrate and potassium chloride.

Reaction – KNO₃ + AlCl₃ ↔ Al(NO₃)₃ + KCl

• Precipitation Reaction – In these reactions, an insoluble precipitate is formed. In precipitation reactions, two soluble salts in aqueous solutions are combined and form an insoluble precipitate.

Reaction – AgNO_3(aq) + KCl_(aq) ↔ AgCl + KNO_3(aq)

• Redox Reaction - Those chemical reactions in which oxidation and reduction take place simultaneously are called redox reactions. Oxidation is the addition of oxygen, while reduction is the addition of hydrogen (or removal of oxygen). We have already discussed oxidation and reduction in a separate article with the title ‘Oxidation and Reduction’. You can refer to this article for a detailed explanation of oxidation and reduction.

The reaction of copper oxide with hydrogen is an example of a redox reaction. In this reaction, hydrogen has undergone oxidation by gaining oxygen atoms while copper oxide has undergone reduction by removing oxygen.

• Exothermic Reaction - An exothermic reaction is defined as a reaction that releases heat and has a net negative standard enthalpy change. Examples include any combustion process, rusting of iron, and freezing of water.

Reaction - CH₄ + O₂ → CO₂ + H₂O + Heat

• Endothermic Reaction - An endothermic reaction is any chemical reaction that absorbs heat from its environment. The absorbed energy provides the activation energy for the reaction to occur. A hallmark of this type of reaction is that it feels cold.

Reaction - CH₄ + H₂O + Heat → CO + 3H₂

Corrosion

It is basically defined as a natural process that causes the transformation of pure metals into undesirable substances when they react with substances like water or air. This reaction causes damage and disintegration of the metal starting from the portion of the metal exposed to the environment and spreading to the entire bulk of the metal.

Corrosion is usually an undesirable phenomenon since it negatively affects the desirable properties of the metal. For example, iron is known to have good tensile strength and rigidity (especially alloyed with a few other elements). However, when subjected to rusting, iron objects become brittle, flaky, and structurally unsound. On the other hand, corrosion is a diffusion-controlled process and that it mostly occurs on exposed surfaces. Example – Rusting Of Iron

Rancidity :

When fats and oils stay in open for long, they get oxidized and become rancid (old and stale), and their smell and taste change. This process is known as rancidity.

The term ‘rancidity’ refers to a condition in which food has become undesirable. It is often used to describe the oxygen content of oils. It’s a term used to describe the oxygen damage in food. When oxygen molecules interact with the oil and food, the normal structure of the food is damaged, resulting in a change in odour and taste, and it is not recommended for consumption.

Example: An example of rancidity is when a chips pack is exposed to atmospheric air which results in a change in taste and odour.

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_{Himanshu Sharma}