Chemical Reactions and Equations

Introduction to Chemical Reactions

A chemical reaction is a process where one or more substances, called reactants, are transformed into one or more different substances, called products. Chemical reactions involve the breaking and forming of chemical bonds between atoms.

Chemical equations are symbolic representations of chemical reactions, where the reactants are shown on the left side and the products on the right side, separated by an arrow (→) indicating the direction of the reaction.

Example:
2H₂ + O₂ → 2H₂O
(Hydrogen + Oxygen → Water)

Visual representation of a chemical reaction process

Types of Chemical Reactions

Combination Reaction

Two or more substances combine to form a single product. A + B → AB

2H₂ + O₂ → 2H₂O

Decomposition Reaction

A single compound breaks down into two or more simpler substances. AB → A + B

2HgO → 2Hg + O₂

Displacement Reaction

More reactive element displaces less reactive element from its compound. A + BC → AC + B

Zn + CuSO₄ → ZnSO₄ + Cu

Double Displacement

Ions are exchanged between two compounds. AB + CD → AD + CB

AgNO₃ + NaCl → AgCl + NaNO₃

Oxidation-Reduction (Redox) Reactions

Reactions involving transfer of electrons between species. Oxidation is loss of electrons, reduction is gain of electrons.

Oxidation

• Loss of electrons
• Increase in oxidation number
• Substance is oxidized (reducing agent)

Reduction

• Gain of electrons
• Decrease in oxidation number
• Substance is reduced (oxidizing agent)

2Na + Cl₂ → 2NaCl
(Na oxidized, Cl reduced)

Characteristics of Chemical Reactions

Evidence of Chemical Reactions

• Evolution of Gas: Formation of bubbles indicates gas production

  Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑

• Formation of Precipitate: Appearance of solid in solution

  Example: AgNO₃ + NaCl → AgCl↓ + NaNO₃

• Change in Temperature: Heat is absorbed or released

  Exothermic: C + O₂ → CO₂ + heat
  Endothermic: N₂ + O₂ + heat → 2NO

• Change in Color: Indicator of new substance formation

  Example: Blue copper sulfate solution turns green when iron is added

Reaction Conditions

1 Temperature

Increasing temperature generally increases reaction rate by providing activation energy

2 Concentration

Higher concentrations increase collision frequency between particles

3 Pressure (for gases)

Increased pressure increases reaction rate by reducing volume

4 Catalysts

Substances that increase rate without being consumed by providing alternative pathway

Reaction Rate Graph

Balancing Chemical Equations

Law of Conservation of Mass

Chemical equations must be balanced so that the number of atoms of each element is the same on both sides of the equation. This follows the Law of Conservation of Mass which states that matter cannot be created or destroyed in a chemical reaction.

Steps to Balance Equations:

1. Write the unbalanced equation with correct formulas
2. Count atoms of each element on both sides
3. Start balancing with elements that appear once on each side
4. Use coefficients to balance atoms (never change subscripts)
5. Double-check all atoms are balanced

Example Problem

Balance the equation: C₃H₈ + O₂ → CO₂ + H₂O

Solution:
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Interactive Equation Balancer

Reactants

Products

Balanced Equation:

Practice Problems

1. Balance: Fe + Cl₂ → FeCl₃

2Fe + 3Cl₂ → 2FeCl₃

2. Balance: Al + O₂ → Al₂O₃

4Al + 3O₂ → 2Al₂O₃

3. Balance: C₂H₆ + O₂ → CO₂ + H₂O

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

Real-World Applications

Industrial Applications

• Haber Process: N₂ + 3H₂ → 2NH₃ (Ammonia production)

• Contact Process: 2SO₂ + O₂ → 2SO₃ (Sulfuric acid production)

• Battery Technology: Zn + 2MnO₂ → ZnO + Mn₂O₃ (Alkaline batteries)

Environmental Impact

• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (Carbon fixation)

• Acid Rain: SO₂ + H₂O → H₂SO₃ (Environmental concern)

• Ozone Depletion: CFCl₃ + UV → CFCl₂ + Cl (Chlorofluorocarbon breakdown)

Chemistry in Daily Life

Cooking

Maillard Reaction: Amino acids + sugars → Flavor compounds (browning of food)

Medicine

Neutralization: HCl + NaOH → NaCl + H₂O (Antacid tablets)

Cleaning

Oxidation: 2H₂O₂ → 2H₂O + O₂ (Bleaching action)

Interactive Quiz

Test Your Knowledge

Answer these 5 questions to test your understanding of chemical reactions and equations. Each question has one correct answer.

1. What type of reaction is: 2H₂ + O₂ → 2H₂O

A) Decomposition

B) Combination

C) Displacement

D) Double displacement

2. Which law does a balanced chemical equation satisfy?

A) Law of Multiple Proportions

B) Law of Conservation of Mass

C) Boyle's Law

D) Charles' Law

3. Which substance is oxidized in: Zn + CuSO₄ → ZnSO₄ + Cu

A) Zn

B) CuSO₄

C) ZnSO₄

D) Cu

4. What indicates that a chemical reaction has occurred?

A) Change in temperature

B) Formation of a precipitate

C) Evolution of gas

D) All of the above

5. What is the balanced form of: Al + HCl → AlCl₃ + H₂

A) 2Al + 6HCl → 2AlCl₃ + 3H₂

B) Al + 3HCl → AlCl₃ + H₂

C) Al + HCl → AlCl₃ + H₂

D) 2Al + HCl → 2AlCl₃ + H₂

Your Results

Score: 0/5 (0%)

Summary and Key Takeaways

Key Concepts

• Chemical reactions involve rearrangement of atoms to form new substances
• Equations must be balanced to satisfy the law of conservation of mass
• Five main types: combination, decomposition, displacement, double displacement, redox
• Evidence of reactions includes temperature change, gas evolution, color change, precipitate
• Rate affected by temperature, concentration, pressure, surface area, catalysts

Important Formulas

Reaction Type	Example
Combination	2H2 + O2 → 2H2O
Decomposition	2HgO → 2Hg + O2
Displacement	Zn + CuSO4 → ZnSO4 + Cu
Double Displacement	AgNO3 + NaCl → AgCl + NaNO3
Redox	2Na + Cl2 → 2NaCl

Practice Tips

1

Balance equations by adjusting coefficients, never change subscripts

2

Start balancing with elements that appear in only one compound on each side

3

For redox reactions, track oxidation numbers to identify what's oxidized/reduced

4

Always double-check atom counts on both sides of the equation

5

Practice writing equations for real-world reactions to understand applications

Resources and Further Reading

Books

• Chemistry: The Central Science
• Conceptual Chemistry
• NCERT Chemistry Textbook Class 10

Web Resources

• Khan Academy - Chemistry
• Chemistry LibreTexts
• Phet Interactive Simulations

Videos

• Crash Course Chemistry
• TED-Ed Chemistry Lessons
• MIT OpenCourseWare