Chemical Reactions: Types and Indicators

• A chemical reaction converts reactants into products by breaking and forming chemical bonds.
• Indicators of a chemical reaction include color change, gas production, temperature change, and precipitate formation.
• According to the Law of Conservation of Mass, the mass of reactants equals the mass of products.
• Atoms are rearranged rather than created or destroyed in chemical reactions.

Types of Chemical Reactions

• Combustion is a reaction with oxygen that releases heat and light.
• Complete combustion of a hydrocarbon produces carbon dioxide and water if sufficient oxygen is present, for instance: CH₄ + 2O₂ → CO₂ + 2H₂O.
• Incomplete combustion occurs with limited oxygen and produces carbon monoxide (CO) or carbon (soot) instead of CO₂; for example, CH₄ + 1½O₂ → CO + 2H₂O.
• Thermal decomposition involves a compound breaking down when heated, such as CuCO₃ → CuO + CO₂.
• Neutralization is the reaction between an acid and a base to form a salt and water, like HCl + NaOH → NaCl + H₂O.
• Displacement reactions involve a more reactive element displacing a less reactive one from its compound, e.g., Zn + CuSO₄ → ZnSO₄ + Cu.
• Oxidation involves a gain of oxygen or loss of electrons, while reduction involves a loss of oxygen or gain of electrons.
• Example of oxidation: 2Mg + O₂ → 2MgO.
• Example of reduction: CuO + H₂ → Cu + H₂O.

Energy Changes in Reactions

• Exothermic reactions release energy, increasing the temperature of the surroundings (e.g., combustion, respiration, neutralization).
• Endothermic reactions absorb energy, decreasing the temperature of the surroundings (e.g., photosynthesis, thermal decomposition, melting).
• Breaking bonds is endothermic, requiring energy, while making bonds is exothermic, releasing energy.
• In an exothermic reaction, reactants have more energy than products and energy is released.
• In an endothermic reaction, products have more energy than reactants and energy is absorbed.
• Overall energy change equals the energy required to break bonds minus the energy released when bonds form.
• Combustion of Methane example: Energy to break bonds: 2640 kJ/mol; Energy released forming bonds: 3316 kJ/mol; Overall energy change: -676 kJ/mol (Exothermic).

Chemical Tests for Identifying Substances

• Oxygen (O₂): A glowing splint relights.
• Hydrogen (H₂): A lit splint produces a 'squeaky pop' sound.
• Carbon Dioxide (CO₂): Limewater turns cloudy.
• Chlorine (Cl₂): Damp blue litmus paper turns white (bleaches).
• Ammonia (NH₃): Damp red litmus paper turns blue.
• Lithium (Li⁺): Crimson flame.
• Sodium (Na⁺): Yellow flame.
• Potassium (K⁺): Lilac flame.
• Calcium (Ca²⁺): Orange-red flame.
• Copper (Cu²⁺): Green flame.
• Sulfates (SO₄²⁻): A white precipitate forms after adding barium chloride solution followed by hydrochloric acid.
• Halides (Cl⁻, Br⁻, I⁻): Add silver nitrate solution followed by nitric acid.
• Chloride (Cl⁻): White precipitate.
• Bromide (Br⁻): Cream precipitate.
• Iodide (I⁻): Yellow precipitate.
• Carbonates (CO₃²⁻): Effervescence occurs with dilute acid, and the gas turns limewater cloudy.

Chemical Equations and Balancing

• Chemical equations must have equal numbers of atoms of each element on both sides.
• Balance equations by adjusting coefficients to ensure the same number of atoms for each element on both sides.
• Balanced equation example: CH₄ + 2O₂ → CO₂ + 2H₂O.

Common Reaction Equations

• Combustion of Propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
• Thermal Decomposition of Calcium Carbonate: CaCO₃ → CaO + CO₂.
• Neutralization Reaction: Hydrochloric Acid + Sodium Hydroxide → Sodium Chloride + Water

Types of Reactions

• Combustion is the reaction of a fuel with oxygen, releasing heat and light energy.
• Complete combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O.
• Complete combustion of propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
• Complete combustion of butane: C₄H₁₀ + 6.5O₂ → 4CO₂ + 5H₂O.
• Incomplete combustion of methane: CH₄ + 1.5O₂ → CO + 2H₂O.
• Incomplete combustion of propane: C₃H₈ + 2O₂ → 3C + 4H₂O.
• Thermal decomposition: A compound breaks down into simpler substances when heated.
• Calcium Carbonate Decomposition: CaCO₃ → CaO + CO₂.
• Copper(II) Carbonate Decomposition: CuCO₃ → CuO + CO₂.
• Neutralization reactions involve an acid reacting with a base to form a salt and water.
• Hydrochloric Acid + Sodium Hydroxide: HCl + NaOH → NaCl + H₂O.
• Sulfuric Acid + Potassium Hydroxide: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O.
• Displacement reactions: A more reactive element displaces a less reactive element from its compound.
• Zinc + Copper(II) Sulfate: Zn + CuSO₄ → ZnSO₄ + Cu.
• Iron + Copper(II) Sulfate: Fe + CuSO₄ → FeSO₄ + Cu.
• Redox reactions: Oxidation = Gain of oxygen / Loss of electrons, Reduction = Loss of oxygen / Gain of electrons.
• Magnesium Oxidation: 2Mg + O₂ → 2MgO.
• Copper(II) Oxide Reduction by Hydrogen: CuO + H₂ → Cu + H₂O.

Gas Tests

• Oxygen test: Insert a glowing splint into the gas, it relights.
• Hydrogen test: Hold a lit splint in the gas, produces a ‘squeaky pop’ sound.
• Carbon dioxide test: Bubble the gas through limewater, turns it cloudy.