HESI A2 Chemistry 2025/2026 – Verified &
Updated Questions & Answers, Full Practice Set
| Evidence-Based Rationales
Topic 1: Chemical Reactions (Questions 1–9)
Question 1: In the combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O), what type of reaction is this, and
what is the role of oxygen?
A. Synthesis; electron donor.
B. Decomposition; catalyst.
C. Exothermic oxidation; oxidizing agent.
D. Displacement; reducing agent.
Rationale: Combustion is an exothermic redox reaction where methane is oxidized by oxygen (O₂), the
oxidizing agent accepting electrons to form CO₂ and H₂O. This follows the law of conservation of mass
(balanced equation) and applies to cellular respiration analogs in metabolism, where incomplete
combustion risks CO toxicity (ACS, 2025).
Question 2: For the reaction N₂ + 3H₂ ⇌ 2NH₃ (Haber process), increasing pressure shifts equilibrium
how, per Le Chatelier's principle?
A. Left, favoring reactants.
B. Right, favoring products.
C. No shift.
D. Toward endothermic.
Rationale: Le Chatelier's principle predicts that increasing pressure favors the side with fewer gas moles
(4 → 2), shifting right to form more NH₃. This equilibrium dynamic is key in industrial fertilizer
production and nitrogen balance in nutrition (NIH, 2025).
Question 3: In acid-base neutralization (HCl + NaOH → NaCl + H₂O), what drives the reaction to
completion?
A. Gas formation.
B. Water formation and salt precipitation.
C. Color change only.
D. Temperature drop.
, Rationale: Neutralization is a double-displacement reaction forming ionic salt (NaCl) and covalent water,
with high K_w (10⁻¹⁴) driving completion. In healthcare, this buffers blood pH at 7.4, preventing acidosis
(ABG interpretation, ACSM, 2025).
Question 4: A single replacement reaction like Zn + 2HCl → ZnCl₂ + H₂ indicates Zn is more reactive
than:
A. Cl.
B. H.
C. Zn²⁺.
D. Na.
Rationale: Reactivity series places Zn above H, so Zn displaces H⁺ from acid. This follows
electrochemical potential (E° Zn = -0.76 V > H = 0 V), relevant to galvanic cells and zinc
supplementation in wound healing (NIH, 2025).
Question 5: In photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂), what is the primary energy source, and
is it endothermic?
A. Heat; exothermic.
B. Light; endothermic.
C. Glucose; endothermic.
D. O₂; exothermic.
Rationale: Light provides ΔH > 0 energy to break bonds in CO₂/H₂O, making it endothermic (ΔG
negative via chlorophyll catalysis). This mirrors ATP synthesis in mitochondria, essential for glucose
homeostasis (Biochem, ACS, 2025).
Question 6: Balancing the equation Fe + O₂ → Fe₂O₃ requires coefficients:
A. 2, 3, 1.
B. 4, 3, 2.
C. 1, 2, 3.
D. 3, 2, 4.
Rationale: Conservation of mass requires 4Fe and 3O₂ to yield 2Fe₂O₃ (8 Fe, 12 O atoms). Rust
formation exemplifies oxidation, with implications for iron overload in hemochromatosis (NIH, 2025).
Question 7: In a decomposition reaction like 2HgO → 2Hg + O₂, heat acts as:
A. Reactant.
B. Activator energy.
C. Product.
D. Inhibitor.
Updated Questions & Answers, Full Practice Set
| Evidence-Based Rationales
Topic 1: Chemical Reactions (Questions 1–9)
Question 1: In the combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O), what type of reaction is this, and
what is the role of oxygen?
A. Synthesis; electron donor.
B. Decomposition; catalyst.
C. Exothermic oxidation; oxidizing agent.
D. Displacement; reducing agent.
Rationale: Combustion is an exothermic redox reaction where methane is oxidized by oxygen (O₂), the
oxidizing agent accepting electrons to form CO₂ and H₂O. This follows the law of conservation of mass
(balanced equation) and applies to cellular respiration analogs in metabolism, where incomplete
combustion risks CO toxicity (ACS, 2025).
Question 2: For the reaction N₂ + 3H₂ ⇌ 2NH₃ (Haber process), increasing pressure shifts equilibrium
how, per Le Chatelier's principle?
A. Left, favoring reactants.
B. Right, favoring products.
C. No shift.
D. Toward endothermic.
Rationale: Le Chatelier's principle predicts that increasing pressure favors the side with fewer gas moles
(4 → 2), shifting right to form more NH₃. This equilibrium dynamic is key in industrial fertilizer
production and nitrogen balance in nutrition (NIH, 2025).
Question 3: In acid-base neutralization (HCl + NaOH → NaCl + H₂O), what drives the reaction to
completion?
A. Gas formation.
B. Water formation and salt precipitation.
C. Color change only.
D. Temperature drop.
, Rationale: Neutralization is a double-displacement reaction forming ionic salt (NaCl) and covalent water,
with high K_w (10⁻¹⁴) driving completion. In healthcare, this buffers blood pH at 7.4, preventing acidosis
(ABG interpretation, ACSM, 2025).
Question 4: A single replacement reaction like Zn + 2HCl → ZnCl₂ + H₂ indicates Zn is more reactive
than:
A. Cl.
B. H.
C. Zn²⁺.
D. Na.
Rationale: Reactivity series places Zn above H, so Zn displaces H⁺ from acid. This follows
electrochemical potential (E° Zn = -0.76 V > H = 0 V), relevant to galvanic cells and zinc
supplementation in wound healing (NIH, 2025).
Question 5: In photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂), what is the primary energy source, and
is it endothermic?
A. Heat; exothermic.
B. Light; endothermic.
C. Glucose; endothermic.
D. O₂; exothermic.
Rationale: Light provides ΔH > 0 energy to break bonds in CO₂/H₂O, making it endothermic (ΔG
negative via chlorophyll catalysis). This mirrors ATP synthesis in mitochondria, essential for glucose
homeostasis (Biochem, ACS, 2025).
Question 6: Balancing the equation Fe + O₂ → Fe₂O₃ requires coefficients:
A. 2, 3, 1.
B. 4, 3, 2.
C. 1, 2, 3.
D. 3, 2, 4.
Rationale: Conservation of mass requires 4Fe and 3O₂ to yield 2Fe₂O₃ (8 Fe, 12 O atoms). Rust
formation exemplifies oxidation, with implications for iron overload in hemochromatosis (NIH, 2025).
Question 7: In a decomposition reaction like 2HgO → 2Hg + O₂, heat acts as:
A. Reactant.
B. Activator energy.
C. Product.
D. Inhibitor.
