Concise Notes: Chemical Reactions and Equations

Physical and Chemical Changes

• Change: Transformation in physical or chemical properties.
• Physical Change:
  • Identity remains the same.
  • Alters physical properties (shape, size, state).
  • Example: Melting ice, boiling water, breaking glass.
• Chemical Change:
  • Formation of new substance(s) with different properties.
  • Example: Rusting iron, burning paper.

Chemical Reactions and Equations

• Chemical Reaction: Transformation resulting in new substances with different chemical properties.
  • Example: Burning wood (wood + oxygen -> ash + carbon dioxide + heat).
• Chemical Equation: Uses symbols and formulas to represent reactants and products.
  • Reactants → Products
  • Example: $H<em>2 + O</em>2 → H_2O$

Characteristics of Chemical Reactions

• Change in color.
• Change in temperature.
  • Endothermic: Absorbs energy.
  • Exothermic: Releases energy.
• Change in state.
• Evolution of gas.
• Formation of precipitate.

Key Terms

• Precipitate: Insoluble solid that settles after a chemical reaction.
• Exothermic Reactions: Release heat energy.
• Endothermic Reactions: Absorb heat energy.
• Catalyst: Speeds up a reaction without being consumed.

Representing Chemical Reactions

• Reactants: Starting materials.
• Products: Substances produced.
• Word Reaction: Magnesium + Oxygen → Magnesium Oxide
• Chemical Equation: Mg + O → MgO
• Skeletal Equation: Unbalanced equation.

Making Equations Informative

• Mentioning Physical States: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous.
  • Example: $3Fe(s) + 4H<em>2O(g) → Fe</em>3O<em>4(s) + 4H</em>2(g)$
• Reaction Conditions:
  • Temperature, Pressure, Catalyst (e.g., $MnO_2$).
  • Example: $CO(g) + 2H<em>2(g) \xrightarrow{340 atm} CH</em>3OH(l)$
• Heat Transfer:
  • Exothermic: + Heat on product side.
  • Endothermic: Heat on reactant side.

Balancing Chemical Equations

• Law of Conservation of Mass: Atoms of each element must be the same on both sides.
• Steps:
  • Write the chemical reaction in word form.
  • Write the chemical symbols in the form of a skeletal chemical equation
  • Enclose the chemical symbol(s) and formulae in boxes
  • List the number of atoms of different elements
  • Start balancing the compound that contains the maximum number of atoms
• Example: $Fe + H<em>2O → Fe</em>3O<em>4 + H</em>2$

Limitations of Chemical Equations & Their Removal

• Lack of Physical States: Add state symbols (s, l, g, aq).
• Concentration of Acids: Indicate dilute (dil.) or concentrated (conc.).

Types of Chemical Reactions

• Combination Reaction: Two or more reactants combine to form one product. $A + B → AB$
• Decomposition Reaction: Single compound breaks down into two or more substances. $AB → A + B$
• Displacement Reaction: More reactive element replaces a less reactive element. $A + BC → AC + B$
• Double-Displacement Reaction: Exchange of ions between two compounds. $AB + CD → AD + CB$

Combination Reaction

• Two elements form a compound. $2Mg(s) + O_2(g) → 2MgO(s)$
• Two compounds form a compound.$CaO(s) + H<em>2O(l) → Ca(OH)</em>2(aq) + Heat$
• Element and a compound form a compound. $2SO<em>2(g) + O</em>2(g) → 2SO_3(g)$

Decomposition Reaction

• Thermolysis (Heat):
  • $2Pb(NO<em>3)</em>2(s) → 2PbO(s) + 4NO<em>2(g) + O</em>2(g)$
• Electrolysis (Electricity):
  • $2H<em>2O(l) → 2H</em>2(g) + O_2(g)$
• Photolysis (Sunlight):
  • $2AgCl(s) → 2Ag(g) + Cl_2(g)$

Displacement Reaction

• More active element displaces less active element.
• Example: $Fe(s) + CuSO<em>4(aq) → FeSO</em>4(aq) + Cu(s)$

Double Displacement Reaction

• Ions are exchanged between two compounds
• Precipitate Reaction: Insoluble solid forms.
• Example: $BaCl<em>2(aq) + Na</em>2SO<em>4(aq) → BaSO</em>4(s) + 2NaCl(aq)$

Redox Reaction

• Oxidation and reduction occur simultaneously.
• Oxidation: Loss of electrons, gain of oxygen, or loss of hydrogen.
• Reduction: Gain of electrons, loss of oxygen, or gain of hydrogen.
• Oxidizing Agent: Substance that causes oxidation and is reduced.
• Reducing Agent: Substance that causes reduction and is oxidized.

Corrosion

• Attack on a metal by reaction with its environment.
• Example: Rusting of iron ($Fe + O<em>2 + H</em>2O → Fe<em>2O</em>3.xH_2O$).
• Prevention: Painting, galvanization.

Rancidity

• Oxidation of fats and oils, causing bad taste and smell.
• Prevention: Antioxidants, airtight containers, nitrogen flushing.