Chemical reactions are processes in which substances, known as reactants, undergo chemical changes to form new substances, called products. These reactions involve the breaking and forming of chemical bonds, resulting in the transformation of substances with different chemical properties. The representation of these chemical reactions using symbols and formulas is known as chemical equations.

Types of Chemical Reactions

1. Combination (Synthesis) Reactions

   • Two or more reactants combine to form a single product.
   • Example: 2H2+O2→2H2O2H2​+O2​→2H2​O

2. Decomposition Reactions

   • A single compound breaks down into two or more simpler substances.
   • Example: 2H2O2→2H2O+O22H2​O2​→2H2​O+O2​

3. Single Displacement (Replacement) Reactions

   • One element replaces another element in a compound.
   • Example: Zn+2HCl→ZnCl2+H2Zn+2HCl→ZnCl2​+H2​

4. Double Displacement (Metathesis) Reactions

   • The ions of two compounds exchange places in an aqueous solution to form two new compounds.
   • Example: AgNO3+NaCl→AgCl+NaNO3AgNO3​+NaCl→AgCl+NaNO3​

5. Combustion Reactions

   • A substance combines with oxygen, releasing energy in the form of light and heat.
   • Example: CH4+2O2→CO2+2H2OCH4​+2O2​→CO2​+2H2​O

6. Redox (Oxidation-Reduction) Reactions

   • Involves the transfer of electrons between two species, leading to changes in their oxidation states.
   • Example: 2Na+Cl2→2NaCl2Na+Cl2​→2NaCl

Chemical Equations

Chemical equations use chemical symbols and formulas to represent reactants and products in a chemical reaction. A balanced chemical equation has the same number of atoms of each element on both sides, ensuring the law of conservation of mass is obeyed.

Parts of a Chemical Equation:

• Reactants: Substances that undergo change (left side of the equation).
• Products: New substances formed (right side of the equation).
• Coefficients: Numbers placed before formulas to balance the equation.
• Arrow (→): Indicates the direction of the reaction, from reactants to products.

Example of a Balanced Equation: N2+3H2→2NH3N2​+3H2​→2NH3​

Steps to Balance a Chemical Equation:

1. Write the unbalanced equation. Fe+O2→Fe2O3Fe+O2​→Fe2​O3​

2. Count the number of atoms of each element on both sides.

   • Reactants: 1 Fe, 2 O
   • Products: 2 Fe, 3 O

3. Use coefficients to balance each element.

   • Balance Fe: 4Fe+3O2→2Fe2O34Fe+3O2​→2Fe2​O3​

4. Check to ensure the equation is balanced.

   • Reactants: 4 Fe, 6 O
   • Products: 4 Fe, 6 O

Types of Equations

1. Word Equations

   • Describes the reactants and products in words.
   • Example: Iron+Oxygen→Iron(III) oxideIron+Oxygen→Iron(III) oxide

2. Formula Equations

   • Uses chemical symbols and formulas.
   • Example: 4Fe+3O2→2Fe2O34Fe+3O2​→2Fe2​O3​

3. Ionic Equations

   • Shows the ions involved in the reaction, often used for reactions in aqueous solutions.
   • Example: Ag++Cl−→AgCl(s)Ag++Cl−→AgCl(s)

4. Net Ionic Equations

   • Shows only the species that actually change during the reaction.
   • Example: Ag+(????????)+Cl−(????????)→AgCl(s)Ag+(aq)+Cl−(aq)→AgCl(s)

Important Concepts

1. Law of Conservation of Mass: Mass is neither created nor destroyed in a chemical reaction.

2. Stoichiometry: The calculation of reactants and products in chemical reactions.

3. Reaction Conditions: Factors such as temperature, pressure, and catalysts that affect the rate and outcome of a reaction.