Chemical Reactions – Chapter 5 Comprehensive Study Notes

Clues That a Chemical Reaction Has Occurred

  • Chemical change is sometimes obvious (flame, explosion) but can also be subtle or invisible; chemists therefore rely on diagnostic clues.
    • Visual signals are the first line of evidence but must be confirmed by a balanced equation or further tests.
  • Core experimental clues (Table 6.1):
    • Color change
    • Formation of a solid (precipitate)
    • Formation of gas bubbles (effervescence)
    • Energy change
    • Heat and/or flame produced (exothermic)
    • Heat absorbed (endothermic – surroundings feel cold)
  • Significance
    • Each clue indicates that the original bonding pattern has been disrupted and new substances with different properties have formed.
    • Absence of a clue does NOT prove that no reaction occurred; microscopic or slow reactions may show no immediate macroscopic sign.

Classification of Chemical Reactions (Five Major Types)

  • Synthesis (Combination)
    • Two or more reactants → single product.
    • Generic form: A + B \rightarrow AB
    • Example: \text{Na} + \text{Cl}_2 \rightarrow \text{NaCl}
  • Decomposition
    • Single reactant → two or more products.
    • Generic form: AB \rightarrow A + B
    • Example: \text{NH}4\text{OH} \rightarrow \text{NH}3 + \text{H}_2\text{O}
  • Combustion
    • Rapid combination with \text{O}2 producing heat, light; for hydrocarbons usually → \text{CO}2 and \text{H}_2\text{O}.
    • Example: \text{CH}4 + 2\text{O}2 \rightarrow \text{CO}2 + 2\text{H}2\text{O} (strongly exothermic)
  • Single-Replacement (Displacement)
    • One element replaces a similar element in a compound.
    • Generic form: A + BC \rightarrow AC + B
    • Metal displaces metal OR non-metal displaces non-metal.
    • Examples:
    • \text{K} + \text{NaBr} \rightarrow \text{Na} + \text{KBr} (metal for metal)
    • \text{NaBr} + \text{Cl}2 \rightarrow \text{NaCl} + \text{Br}2 (halogen for halogen)
  • Double-Replacement (Metathesis)
    • Two ionic compounds swap partners.
    • Generic form: AB + CD \rightarrow AD + CB
    • Analogy: “double date gone wrong (or right?)” → two couples exchange partners.
    • Example: \text{NaCl} + \text{AgNO}3 \rightarrow \text{AgCl} + \text{NaNO}3 (precipitate \text{AgCl} forms)

Examples & Classification Practice

  • Identify the reaction type for each equation (answers in parentheses):
    • \text{C}2\text{H}6(g) + \text{O}2(g) \rightarrow \text{CO}2(g) + \text{H}_2\text{O}(g) (Combustion)
    • \text{Fe}(s) + \text{O}2(g) \rightarrow \text{Fe}2\text{O}_3(s) (Synthesis)
    • \text{Mg}(s) + \text{H}2\text{S}(g) \rightarrow \text{MgS}(s) + \text{H}2(g) (Single-replacement)
    • \text{H}2\text{CO}3(aq) \rightarrow \text{H}2\text{O}(l) + \text{CO}2(g) (Decomposition)
    • \text{FeS}(s) + \text{HCl}(aq) \rightarrow \text{FeCl}2 + \text{H}2\text{S}(g) (Double-replacement)

Chemical Equations—Fundamentals

  • A chemical equation is a symbolic sentence describing a chemical reaction.
    • Reactants: left of the arrow; Products: right of the arrow.
  • Law-level principle: atoms are neither created nor destroyed → total atoms of each element must be equal on both sides.
  • Subscripts vs. coefficients
    • Subscript = part of chemical identity → NEVER change while balancing.
    • Coefficient = number of formula units/molecules → adjust these ONLY.
  • States of matter are routinely included for clarity: (s), (l), (g), (aq).
    • Example (reaction of potassium with water):
    • \text{K}(s) + \text{H}2\text{O}(l) \rightarrow \text{KOH}(aq) + \tfrac12\,\text{H}2(g) (hydrogen collected as gas)

Law of Conservation of Mass

  • Scientific law = concise statement summarizing extensive experimental evidence.
  • Conservation of matter: total mass in a closed system remains constant.
    • Expressed qualitatively as “matter can be neither created nor destroyed by a chemical process.”
  • Chemical‐equation balancing is the bookkeeping tool that enforces this fundamental law at the symbolic level.

Strategy for Balancing Chemical Equations

  • Steps (Inspection Method)
    1. Write correct formulas for all reactants and products from the reaction description.
    2. Begin balancing with the most complex formula (largest number of different atoms).
    3. Leave free elements (e.g., \text{O}2, \text{H}2) for last.
    4. Adjust coefficients one element at a time; never alter subscripts.
    5. If fractional coefficients arise, multiply entire equation by the least common denominator to obtain whole numbers.
    6. Final check: same number of each atom on both sides; coefficients are the smallest whole integers.
  • Example—Combustion of Methane
    • Unbalanced: \text{CH}4 + \text{O}2 \rightarrow \text{CO}2 + \text{H}2\text{O}
    • Balance C (already 1), balance H (4 → need 2 \text{H}2\text{O}), balance O (4 on right → need 2 \text{O}2).
    • Balanced: \text{CH}4 + 2\text{O}2 \rightarrow \text{CO}2 + 2\text{H}2\text{O}

Guided Example—Hydrogen + Oxygen → Water

  • Step 1 (formulas & states): \text{H}2(g) + \text{O}2(g) \rightarrow \text{H}_2\text{O}(l)
  • Step 2 (inspection):
    • H: 2 left, 2 right (OK).
    • O: 2 left, 1 right (NOT ok) → place coefficient 2 in front of \text{H}_2\text{O}.
    • Now H: 2 left, 4 right → put coefficient 2 before \text{H}_2.
  • Balanced: 2\text{H}2(g) + \text{O}2(g) \rightarrow 2\text{H}_2\text{O}(l)

Practice Balancing (Problems from Slides)

  • Balance each equation (solutions shown):
    • 4\text{FeO}(s) + \text{O}2(g) \rightarrow 2\text{Fe}2\text{O}_3(s)
    • 2\text{CaO} + 3\text{C} \rightarrow \text{CaC}2 + \text{CO}2
    • 2\text{C}2\text{H}6(g) + 7\text{O}2(g) \rightarrow 4\text{CO}2(g) + 6\text{H}_2\text{O}(g)
    • \text{FeS}(s) + 2\text{HCl}(aq) \rightarrow \text{FeCl}2 + \text{H}2\text{S}(g)
    • 4\text{Fe}(s) + 3\text{O}2(g) \rightarrow 2\text{Fe}2\text{O}_3(s)

Unbalanced-Equation Exercise (Blue-Light Photolysis)

  • Problem statement: Blue light causes a mixture of hydrogen and chlorine gases to form gaseous hydrochloric acid explosively.
  • Correct unbalanced skeleton: \text{H}2(g) + \text{Cl}2(g) \rightarrow \text{HCl}(g)
    • (Option d in the slide)

Balancing Recap—Key Takeaways

  • Atom conservation is mandatory; a balanced equation embodies the Law of Conservation of Mass.
  • Never tamper with subscripts; only change coefficients.
  • Coefficients reveal the stoichiometric ratio of molecules/ions participating.
  • Fractional coefficients are acceptable intermediates but final answers are usually in the lowest whole-number ratio.