Questions: Chemical Equations: Writing and Balancing Reactions

Question 1

A student needs to balance the equation: H₂ + O₂ → H₂O. They write: H₂ + O₂ → H₂O₂. What is the critical error?

• A. The student added the wrong number of oxygen atoms to the right side
• B. The student changed a subscript, converting water into hydrogen peroxide — a different compound
• C. The student should have added a coefficient of 2 in front of O₂ instead
• D. The equation does not need balancing because both sides already have 2 hydrogen atoms

Answer: (B)

Changing H₂O to H₂O₂ changes the subscript, which changes the identity of the substance — H₂O is water; H₂O₂ is hydrogen peroxide, a completely different compound with different properties. The rule is absolute: subscripts define what the substance is and must never be changed to balance an equation. The correct balanced form is 2H₂ + O₂ → 2H₂O, using coefficients only.

Question 2

What is the correct coefficient for O₂ in the balanced combustion equation for propane: C₃H₈ + _O₂ → 3CO₂ + 4H₂O?

• A. 3
• B. 4
• C. 5
• D. 8

Answer: (C)

Count oxygen atoms on the right: 3 CO₂ contributes 6 oxygen atoms; 4 H₂O contributes 4 more = 10 total. Each O₂ molecule supplies 2 oxygen atoms, so you need 10 ÷ 2 = 5 O₂. The fully balanced equation is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, which conserves 3 carbons, 8 hydrogens, and 10 oxygens on each side.

Question 3

To balance a chemical equation, you may change the subscripts in a formula if the coefficients alone cannot produce equal atom counts on both sides.

• T. True
• F. False

Answer: False

Subscripts are never changed to balance an equation. A subscript defines the chemical identity of a substance — H₂O and H₂O₂ are fundamentally different compounds. Changing a subscript does not balance the equation; it changes what the equation describes. Only coefficients (the numbers in front of complete formulas) are adjusted. If atom counts cannot be balanced without changing subscripts, the equation itself is incorrectly written.

Question 4

In a complete combustion reaction of any hydrocarbon (compound containing only carbon and hydrogen), the products are always carbon dioxide and water.

• T. True
• F. False

Answer: True

Complete combustion of a hydrocarbon by definition means fully oxidizing all carbon to CO₂ and all hydrogen to H₂O. The pattern holds regardless of the hydrocarbon's size or structure: CH₄ → CO₂ + H₂O; C₈H₁₈ → CO₂ + H₂O; C₃H₈ → CO₂ + H₂O. Recognizing this reaction type lets you predict products before balancing, which is one of the practical payoffs of learning reaction classifications.

Question 5

Why is it incorrect to change subscripts when balancing a chemical equation, even if doing so would make the atom counts equal?

Model answer: Subscripts define the molecular formula — they specify the exact ratio of atoms bonded together in that compound. Changing a subscript creates a different chemical substance with different properties, bonds, and behavior. Balancing requires accounting for the same substances on both sides; substituting a different compound violates this requirement and misrepresents the chemistry. Coefficients, by contrast, simply state how many formula units participate — they scale quantities without altering identities.

A classic example: changing H₂O to H₂O₂ might seem to 'fix' the oxygen count, but it converts water to hydrogen peroxide — which has a peroxide bond, different reactivity, and different density. The resulting equation no longer describes the reaction that actually occurs. Conservation of mass requires the same substances before and after, just in different proportions — which is exactly what coefficients (not subscript changes) accomplish.