Overview
This lecture introduces chemical reactions and equations, explaining their types, symbolic representation, and real-life applications such as corrosion and rancidity.
Chemical Reactions: Daily Life Examples
- Chemical changes occur when the initial substances change their nature and identity.
- Examples: souring of milk, rusting of iron, fermentation of grapes, cooking, digestion, respiration.
Observing Chemical Reactions
- Indicators of a chemical reaction: change in state, color, evolution of gas, change in temperature.
Chemical Equations and Their Representation
- Chemical reactions can be written as word equations (e.g., Magnesium + Oxygen → Magnesium oxide).
- Reactants are on the left; products are on the right, separated by an arrow.
- Chemical equations can be further simplified using chemical formulae.
Balancing Chemical Equations
- Equations must be balanced to obey the law of conservation of mass.
- Balancing involves making the number of atoms of each element equal on both sides.
- Physical states are denoted: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
Types of Chemical Reactions
Combination Reactions
- Two or more substances combine to form a single product (e.g., CaO + H2O → Ca(OH)2).
- These reactions are often exothermic (release heat).
Decomposition Reactions
- A single substance breaks down into two or more products (e.g., CaCO3 → CaO + CO2).
- Can require heat (thermal), light (photolytic), or electricity (electrolytic).
- Endothermic if energy is absorbed.
Displacement Reactions
- A more reactive element displaces another from its compound (e.g., Fe + CuSO4 → FeSO4 + Cu).
Double Displacement Reactions
- Exchange of ions between two compounds produces new compounds (e.g., Na2SO4 + BaCl2 → BaSO4 + 2NaCl).
- Precipitation reactions form insoluble solids.
Oxidation and Reduction (Redox Reactions)
- Oxidation: substance gains oxygen or loses hydrogen.
- Reduction: substance loses oxygen or gains hydrogen.
- Both processes can occur simultaneously in a redox reaction.
Real-life Effects: Corrosion and Rancidity
- Corrosion: metals react with substances like moisture, resulting in damage (e.g., rusting of iron).
- Rancidity: oxidation of fats/oils spoils food; prevented by antioxidants or airtight storage.
Key Terms & Definitions
- Reactant — substance that undergoes change in a reaction.
- Product — new substance formed in a reaction.
- Balanced Equation — equation with equal atoms of each element on both sides.
- Combination Reaction — two or more reactants form a single product.
- Decomposition Reaction — one reactant breaks into two or more products.
- Displacement Reaction — one element replaces another in a compound.
- Double Displacement Reaction — exchange of ions between two compounds.
- Oxidation — gain of oxygen or loss of hydrogen.
- Reduction — loss of oxygen or gain of hydrogen.
- Corrosion — destruction of metals by chemical or electrochemical reactions.
- Rancidity — spoilage of fats/oils due to oxidation.
Action Items / Next Steps
- Practice balancing chemical equations and include state symbols.
- Identify reaction types in given equations for homework or exercises.
- Read about real-life examples of corrosion and rancidity.