Common Mistakes
Top Mistakes to Avoid
Mistake #1: Changing Subscripts to Balance
Error: Changing H2O to H2O2 to balance an equation.
Correct: Only change coefficients. Subscripts change the identity of the substance.
Mistake #2: Forgetting Diatomic Elements
Error: Writing O instead of O2 for oxygen gas.
Correct: Remember HONClBrIF - these exist as diatomic molecules.
Mistake #3: Confusing Exo- and Endothermic
Error: Thinking "exo" means absorbs because it sounds like "extra."
Correct: Exo = exits (releases heat). Endo = enters (absorbs heat).
Practice: Find the Errors
Problem 1: Student balances by changing H2O to H4O2. Correct?
Error: Changed subscripts. H4O2 is not water.
Problem 2: Student writes N + N → N2. Is this balanced?
Error: Nitrogen exists as N2, so it should be N2 → N2 (no reaction) or 2N → N2
Problem 3: Student says combustion is endothermic. Correct?
Error: Combustion releases heat, so it's exothermic.
Problem 4: For 2H2 + O2 → 2H2O, student says there are 4 hydrogen atoms on the left. Correct?
Answer: Yes, correct. 2H2 means 2 x 2 = 4 H atoms.
Problem 5: Student says catalysts are consumed in reactions. Correct?
Error: Catalysts are NOT consumed. They speed up reactions without being used up.
Problem 6: NaCl + KNO3 → NaNO3 + KCl is classified as synthesis. Correct?
Error: This is double replacement, not synthesis.
Problem 7: Student says single replacement involves two compounds. Correct?
Error: Single replacement has an element and a compound, not two compounds.
Problem 8: AB → A + B is classified as synthesis. Correct?
Error: This is decomposition. Synthesis is A + B → AB.
Problem 9: Student says all reactions need a catalyst to occur. Correct?
Error: Many reactions occur without catalysts. Catalysts just speed things up.
Problem 10: Fe + CuSO4 → FeSO4 + Cu is double replacement. Correct?
Error: This is single replacement. Fe (element) replaces Cu from the compound.