Acids and bases

Traps in Acids and bases

• Uses everyday or macroscopic thinking instead of particles
  Ask what the particles, ions or electrons are doing. Properties of a substance come from its particles and bonds, not from how it looks or feels.
• Takes one factor as the whole answer
  Ask whether the option is necessary or sufficient. If the stem involves two changes or two conditions, both must appear in the answer.
• Over-generalises a rule beyond where it holds
  Every rule in chemistry has conditions. Before applying one, check the conditions in the stem (electrode type, concentration, moles of gas on each side, metal position).
• Drops the minus sign in pH = −log[H⁺]
  pH = −log₁₀[H⁺]. For [H⁺] = 1 × 10⁻ⁿ, pH = n. Ordinary acids and alkalis have pH between 0 and 14.
• Confuses pH and pOH
  Use pH = −log[H⁺] when you are given [H⁺]. Only when you are given [OH⁻] do you find pOH first and then pH = 14 − pOH. Check: acids < 7, alkalis > 7.
• Acid–base definition errors
  Brønsted–Lowry: acid = proton donor, base = proton acceptor. STRONG = fully ionised; CONCENTRATED = much solute per dm³. A dilute strong acid is still strong.
• Reverses a trend, direction or effect
  State the rule with its direction before looking at the options (down group 1 reactivity INCREASES; exothermic forward reaction: heating shifts LEFT). An option that is the mirror image of the rule is the trap.
• Chooses a true but irrelevant statement
  Re-read the question word (why, which, what causes). A true fact that answers a different question is still wrong.