Chemical Reactions and Equations

Class 10 Science • Chapter 1 • RBSE/NCERT Board

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei. This chapter introduces the fundamental concepts of chemical reactions, how to represent them through equations, and the various types of reactions that occur in nature and daily life.

Understanding chemical reactions is fundamental to chemistry and has wide applications in industries, medicine, agriculture, and environmental science. From the rusting of iron to the digestion of food, chemical reactions are happening all around us constantly.

1. Introduction to Chemical Reactions

A chemical reaction is a process in which one or more substances, the reactants, are converted to one or more different substances, the products. During a chemical reaction, the atoms of reactants are rearranged to form products with different properties.

1.1 Signs of Chemical Reactions

Chemical reactions can be identified by observing certain characteristic changes. The following table lists the common signs that indicate a chemical reaction has occurred:

Table 1.1: Signs of Chemical Reactions

Sign	Description	Example	Observation
1. Change in Color	The color of reactants changes to a different color	Copper carbonate heating	Green → Black
2. Evolution of Gas	Bubbles or effervescence observed	Zinc + Dilute HCl	Hydrogen gas bubbles
3. Change in Temperature	Heat is released or absorbed	Quicklime + Water	Vessel becomes hot
4. Formation of Precipitate	Insoluble solid forms in solution	AgNO₃ + NaCl	White curdy precipitate
5. Change in State	Physical state changes during reaction	Burning of candle	Solid wax → Gases

1.2 Chemical Change vs Physical Change

It is important to distinguish between chemical and physical changes. In a physical change, the composition of matter remains the same, only the physical properties change. In a chemical change, new substances with different compositions are formed.

Table 1.2: Comparison of Physical and Chemical Changes

Property	Physical Change	Chemical Change
New Substance	No new substance formed	New substance formed
Reversibility	Usually reversible	Usually irreversible
Energy Change	Small or no energy change	Energy is absorbed or released
Mass Change	No change in mass	Mass conserved (may appear to change)
Examples	Melting of ice, Dissolving sugar	Burning of paper, Rusting of iron

2. Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction in the form of symbols and chemical formulae. It provides a concise way to represent the reactants, products, and their relative quantities in a reaction.

2.1 Writing Chemical Equations

Chemical equations evolved from word equations to symbolic representations. Consider the reaction of magnesium with oxygen:

2.2 Symbols and Notations in Chemical Equations

Chemical equations use various symbols to convey additional information about the reaction conditions and the states of reactants and products.

Table 2.1: Symbols Used in Chemical Equations

Symbol	Meaning	Usage Example
+	"Reacts with" or "and"	H₂ + Cl₂
→	"Yields" or "produces"	→ 2HCl
⇌	Reversible reaction	N₂ + 3H₂ ⇌ 2NH₃
(s)	Solid state	Fe(s)
(l)	Liquid state	H₂O(l)
(g)	Gaseous state	CO₂(g)
(aq)	Aqueous solution (dissolved in water)	NaCl(aq)
↑	Gas evolved (escapes)	H₂↑
↓	Precipitate formed	AgCl↓
Δ	Heat applied	Written above arrow

3. Balancing Chemical Equations

The Law of Conservation of Mass states that matter can neither be created nor destroyed in a chemical reaction. Therefore, a balanced chemical equation must have the same number of atoms of each element on both sides of the equation.

3.1 Hit and Trial Method (Step-by-Step)

The most common method to balance equations is the hit and trial method. Let's balance the reaction of iron with steam:

3.2 Tips for Balancing Equations

Table 3.1: Practice Balancing Equations

S.No.	Unbalanced Equation	Balanced Equation
1	H₂ + O₂ → H₂O	2H₂ + O₂ → 2H₂O
2	N₂ + H₂ → NH₃	N₂ + 3H₂ → 2NH₃
3	Fe + O₂ → Fe₂O₃	4Fe + 3O₂ → 2Fe₂O₃
4	Al + HCl → AlCl₃ + H₂	2Al + 6HCl → 2AlCl₃ + 3H₂
5	Pb(NO₃)₂ → PbO + NO₂ + O₂	2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

4. Types of Chemical Reactions

Chemical reactions can be classified into several types based on how reactants transform into products. Understanding these types helps predict the products of reactions.

4.1 Combination Reaction

A combination reaction (also called synthesis reaction) is one in which two or more substances combine to form a single product.

Examples of Combination Reactions:

Reaction	Chemical Equation	Observation
Burning of magnesium	2Mg + O₂ → 2MgO	Bright white flame, white ash
Formation of water	2H₂ + O₂ → 2H₂O	Explosive reaction
Slaking of lime	CaO + H₂O → Ca(OH)₂	Heat released, hissing sound
Burning of coal	C + O₂ → CO₂	Heat and light produced
Formation of ammonia	N₂ + 3H₂ → 2NH₃	Requires high pressure, catalyst

4.2 Decomposition Reaction

A decomposition reaction is one in which a single compound breaks down into two or more simpler substances. It is the opposite of a combination reaction.

Decomposition reactions require energy input and can be classified based on the source of energy:

4.3 Displacement Reaction

A displacement reaction (also called replacement reaction) occurs when a more reactive element displaces a less reactive element from its compound.

Reactivity Series

The reactivity series is a list of metals arranged in order of their decreasing reactivity. A metal higher in the series can displace a metal lower in the series from its compound.

Table 4.1: Reactivity Series of Metals

Metal	Symbol	Reactivity
Potassium	K	Most Reactive ↑
Sodium	Na
Calcium	Ca
Magnesium	Mg
Aluminium	Al
Zinc	Zn	Medium
Iron	Fe
Lead	Pb
Hydrogen	H	Reference
Copper	Cu	Least Reactive ↓
Silver	Ag
Gold	Au

4.4 Double Displacement Reaction

A double displacement reaction (also called metathesis) occurs when two compounds exchange their ions to form two new compounds.

Examples of Double Displacement Reactions:

Reaction	Chemical Equation	Precipitate
Silver nitrate + Sodium chloride	AgNO₃ + NaCl → AgCl↓ + NaNO₃	White (AgCl)
Barium chloride + Sodium sulphate	BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl	White (BaSO₄)
Lead nitrate + Potassium iodide	Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃	Yellow (PbI₂)

5. Energy Changes in Chemical Reactions

Every chemical reaction involves energy changes. Based on whether energy is released or absorbed, reactions are classified as exothermic or endothermic.

Table 5.1: Comparison of Exothermic and Endothermic Reactions

Property	Exothermic	Endothermic
Energy	Released to surroundings	Absorbed from surroundings
Temperature	Increases	Decreases
ΔH (Enthalpy)	Negative (-ve)	Positive (+ve)
Product Energy	Lower than reactants	Higher than reactants
Examples	Burning of fuels, Respiration, Neutralization	Photosynthesis, Decomposition of CaCO₃, Dissolving NH₄Cl

6. Oxidation and Reduction Reactions

Oxidation and reduction are complementary processes that always occur together in what are called redox reactions.

6.1 Definitions

Process	Classical Definition	Modern Definition
OXIDATION	Gain of oxygen OR Loss of hydrogen	Loss of electrons
REDUCTION	Loss of oxygen OR Gain of hydrogen	Gain of electrons

7. Corrosion and Rancidity

7.1 Corrosion

Corrosion is the slow destruction of metals due to their reaction with oxygen, water, and other chemicals in the environment. The most common example is the rusting of iron.

Prevention of Corrosion

Table 7.1: Methods to Prevent Corrosion

Method	Description	Application
Painting	Creates a barrier preventing contact with air and moisture	Bridges, gates, railings
Oiling/Greasing	Prevents direct contact with oxygen and water	Machinery, bicycle chains
Galvanization	Coating iron with a layer of zinc	Iron sheets, buckets, pipes
Electroplating	Coating with chromium, nickel, or other metals	Bathroom fittings, jewelry
Alloying	Mixing iron with chromium and nickel to make stainless steel	Utensils, surgical instruments

7.2 Rancidity

Rancidity is the development of unpleasant smell and taste in fats and oils due to oxidation.

Table 7.2: Methods to Prevent Rancidity

Method	How it Works
Airtight containers	Prevents contact with oxygen
Refrigeration	Slows down the rate of oxidation
Adding antioxidants	BHA, BHT prevent oxidation
Nitrogen flushing	Replaces air with unreactive nitrogen

8. Chapter Summary

9. References