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HESI A2 Chemistry: chemical reactions practice questions

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These are original practice items written to the HESI A2 blueprint (4 options, no penalty for wrong answers), not actual exam questions. Try each one before you open the answer.

What this topic covers

Chemistry: chemical reactions sits inside the Chemistry section of the HESI A2. Our bank holds 33 verified items tagged to it, and the 4 below are a fixed sample so the page stays the same every time you open it. Each one shows the reasoning, not just the letter.

The topic tag comes from our own items rather than from a published syllabus, so treat it as a map of what we cover. For the official section breakdown see the HESI A2 page.

Chemistry: chemical reactions sample questions

Question 1

In the balanced chemical equation 2H₂ + O₂ → 2H₂O, how many hydrogen atoms appear on the product side?

  1. A 2
  2. B 1
  3. C 4
  4. D 3
Show the answer and explanation

Correct answer: C

The product side shows two water molecules (2H₂O). Each water molecule contains two hydrogen atoms, so 2 × 2 = 4 hydrogen atoms.

Why the other options are wrong:
A: 2 is the number of hydrogen atoms in a single water molecule, not in two molecules.
B: 1 is incorrect; water has two hydrogen atoms per molecule.
D: 3 does not match the stoichiometry of the balanced equation.

Question 2

When the equation __CH₄ + __O₂ → __CO₂ + __H₂O is balanced, what is the coefficient for oxygen (O₂)?

  1. A 2
  2. B 3
  3. C 1
  4. D 4
Show the answer and explanation

Correct answer: A

Balanced equation: CH₄ + 2O₂ → CO₂ + 2H₂O. Carbon and hydrogen balance first (1 C, 4 H), then oxygen: 2 O₂ gives 4 O atoms, which matches 2 in CO₂ and 2 in H₂O (total 4). Coefficient for O₂ is 2.

Why the other options are wrong:
B: With coefficient 3, oxygen atoms would be 6, too many; the equation would be unbalanced.
C: With coefficient 1, oxygen atoms would be 2, insufficient to form CO₂ (2 O) and H₂O (1 O) needing 3 O total.
D: With coefficient 4, oxygen atoms would be 8, far too many.

Question 3

In the reaction 2H₂ + O₂ → 2H₂O, if 4 moles of H₂ react with excess O₂, how many moles of H₂O are produced?

  1. A 2
  2. B 8
  3. C 4
  4. D 1
Show the answer and explanation

Correct answer: C

The mole ratio of H₂ to H₂O is 2:2 (or 1:1), so 4 moles of H₂ produce 4 moles of H₂O.

Why the other options are wrong:
A: 2 moles would be produced if the ratio were 2:1, but it is 2:2.
B: 8 moles would require 8 moles of H₂ or a different stoichiometry.
D: 1 mole would be produced from only 1 mole of H₂.

Question 4

Consider the unbalanced combustion reaction of propane: C₃H₈ + 5 O₂ → 3 CO₂ + ? H₂O. What coefficient should be placed in front of H₂O to balance the equation?

  1. A 2
  2. B 4
  3. C 3
  4. D 5
Show the answer and explanation

Correct answer: B

Propane has 8 hydrogen atoms, and each H₂O holds 2, so 8 ÷ 2 = 4 water molecules are needed. Oxygen then checks out: 3 CO₂ (6 O) + 4 H₂O (4 O) = 10 O, matching 5 O₂ on the left.

Why the other options are wrong:
A: A coefficient of 2 gives only 4 hydrogen atoms and 8 oxygen atoms, leaving both elements unbalanced.
C: A coefficient of 3 gives 6 hydrogen atoms instead of 8; it likely copies the 3 from the carbon count.
D: A coefficient of 5 gives 10 hydrogen atoms and 11 oxygen atoms, more than the reactants supply.

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