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Chemical Equations
Definition: A chemical equation is a symbolic representation of a chemical reaction using chemical formulae and symbols.
Components:
• Reactants: Substances that react (left side of arrow)
• Products: Substances formed (right side of arrow)
• Arrow (→): Indicates direction of reaction
Example: Mg + O₂ → MgO
Word equation: Magnesium + Oxygen → Magnesium oxide
Types:
• Skeletal equation: Unbalanced equation showing only formulae
• Balanced equation: Equal atoms on both sides
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Law of Conservation of Mass
Statement: "Mass can neither be created nor destroyed in a chemical reaction."
Implications:
• Total mass of reactants = Total mass of products
• Number of atoms of each element must be equal on both sides
• Chemical equations must be balanced
Historical Context: First stated by Antoine Lavoisier (1774)
Example: 2H₂ + O₂ → 2H₂O
Mass: 4g + 32g = 36g (balanced)
Atoms: 4H + 2O = 4H + 2O (balanced)
Why balance? To satisfy the law and represent actual chemical changes accurately.
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Balancing Chemical Equations
Hit-and-Trial Method Steps:
Step 1: Write the skeletal equation
Step 2: Draw boxes around each formula (don't change inside)
Step 3: List all atoms present
Step 4: Start with compound having maximum atoms
Step 5: Balance atoms one by one using coefficients
Step 6: Verify balance by counting all atoms
Example: Fe + H₂O → Fe₃O₄ + H₂
Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Rules:
• Use smallest whole number coefficients
• Never change subscripts in formulae
• Balance polyatomic ions as single units
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Physical States & Reaction Conditions
Physical State Symbols:
• (s): Solid state
• (l): Liquid state
• (g): Gaseous state
• (aq): Aqueous solution (dissolved in water)
Example: 2Mg(s) + O₂(g) → 2MgO(s)
Additional Information:
• Temperature: Δ (heat), 300°C
• Pressure: 2 atm
• Catalyst: MnO₂
• Conditions: hv (light), electrolysis
Complete Example:
2H₂O(l) → 2H₂(g) + O₂(g) [electrolysis]
Importance: Helps understand reaction conditions and practical applications.