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📝 Enthalpy H in biological systems (14 MCQs)

📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 14 questions available

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What is Enthalpy H in biological systems?

Definition:
Enthalpy ( $H$ ) in biological systems is a thermodynamic property that represents the total heat content of a system at constant pressure, defined as $H = U + PV$ , where $U$ is internal energy, $P$ is pressure, and $V$ is volume, and the change in enthalpy ( $\Delta H$ ) is the heat absorbed or released during a reaction, such as the heat produced during metabolic reactions, and it is a key factor in determining reaction favorability.

Working:
In biochemical reactions, $\Delta H$ is often negative (exothermic) for reactions that release heat, like cellular respiration, and positive (endothermic) for reactions that absorb heat, like photosynthesis; enthalpy changes are measured by calorimetry, and along with entropy, they contribute to Gibbs free energy, where $\Delta G = \Delta H - T\Delta S$ , so knowing $\Delta H$ helps predict whether a reaction is spontaneous under specific conditions.

Example:
A simple example is the hydrolysis of ATP to ADP and inorganic phosphate, which has a $\Delta H$ of about -20 kJ/mol (exothermic), meaning it releases heat, contributing to the free energy change, while the combustion of glucose has a $\Delta H$ of about -2800 kJ/mol, releasing large amounts of heat that are partially captured in ATP.

Reason:
Understanding enthalpy is important for biochemistry because it helps quantify the heat changes in metabolism, is essential for calorimetric studies, and contributes to the understanding of reaction energetics, which is crucial for studying metabolic pathways, enzyme kinetics, and drug metabolism.

Easy

Medium

Hard

📝 All Enthalpy H in biological systems MCQs

Q1. At constant pressure, a biochemical reaction changes from reactants with $H=420$ kJ to products with $H=365$ kJ. Which interpretation best explains the energy change?

A.The system absorbs 55 kJ of heat

B.The system releases 55 kJ of heat ✅

C.The system absorbs 785 kJ of heat

D.No heat transfer occurs because enthalpy is unchanged

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: At constant pressure, $\Delta H=H_{\mathrm{products}}-H_{\mathrm{reactants}}$ . Here, $\Delta H=365-420=-55$ kJ, so the system undergoes an enthalpy decrease and releases 55 kJ of heat to the surroundings.

Q2. Two reactions begin with identical reactants. Reaction A forms products at an enthalpy 80 kJ lower than the reactants, while Reaction B forms products 25 kJ higher. Which conclusion is most defensible?

A.Both reactions must absorb heat

B.Reaction A is exothermic and Reaction B is endothermic ✅

C.Reaction A has the larger activation energy

D.The products of both reactions must have identical stability

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: A negative enthalpy change indicates heat release, whereas a positive enthalpy change indicates heat absorption under constant-pressure conditions. Therefore, Reaction A is exothermic with $\Delta H=-80$ kJ and Reaction B is endothermic with $\Delta H=+25$ kJ.

Q3. A researcher measures a reaction at constant pressure and obtains $H_{\mathrm{reactants}}=150$ kJ and $H_{\mathrm{products}}=190$ kJ. A student claims that 340 kJ of heat was released. What is the main error?

A.The student subtracted products from reactants incorrectly

B.The student added the two enthalpy values instead of calculating their difference ✅

C.The student assumed constant pressure

D.The student treated enthalpy as temperature

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The enthalpy change is calculated from the difference between final and initial enthalpy, not their sum. Thus, $\Delta H=190-150=+40$ kJ, indicating heat absorption rather than release.

Q4. A cell carries out a reaction whose enthalpy change is $-75$ kJ per mole. If 0.40 mol reacts completely at constant pressure, how much heat is released?

A.18 kJ

B.30 kJ ✅

C.75 kJ

D.187.5 kJ

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The molar enthalpy change must be multiplied by the amount reacting: $q_p=n\Delta H$ . Thus, $q_p=(0.40)(-75)=-30$ kJ. The negative sign denotes heat leaving the system, so 30 kJ is released.

Q5. A metabolic reaction has $\Delta H=-120$ kJ/mol. A modified pathway changes the reaction so that its enthalpy change becomes $-70$ kJ/mol. Which statement correctly compares the two pathways?

A.The modified pathway releases 50 kJ/mol less heat ✅

B.The modified pathway absorbs 50 kJ/mol more heat

C.The modified pathway must have a faster reaction rate

D.The modified pathway has an activation energy exactly 50 kJ/mol lower

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The original reaction releases 120 kJ/mol, whereas the modified reaction releases 70 kJ/mol. Therefore, the modified pathway releases 50 kJ/mol less heat. Enthalpy change alone cannot determine reaction rate or activation energy.

Q6. A student argues: 'Because an enzyme makes a reaction proceed much faster, it must make the reaction more exothermic.' Which evaluation is best?

A.Correct, because faster reactions always release more heat

B.Correct, because enzymes increase the products' enthalpy

C.Incorrect, because reaction speed and overall enthalpy change describe different properties ✅

D.Incorrect, because enzymes prevent all heat transfer

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: Reaction rate depends on the pathway and its energy barrier, whereas overall $\Delta H$ depends on the enthalpy difference between reactants and products. An enzyme can accelerate a reaction without changing its overall enthalpy change.

Q7. An energy-profile graph shows reactants at 500 kJ and products at 440 kJ. The curve rises to 650 kJ before descending. Which quantity is represented by the 60 kJ difference between reactants and products?

A.Activation energy

B.Negative enthalpy change ✅

C.Total enthalpy of the surroundings

D.Reaction temperature

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The difference between the reactant and product energy levels determines the reaction enthalpy change. Here, $\Delta H=440-500=-60$ kJ, while the rise to 650 kJ relates to the energetic barrier rather than the overall enthalpy change.

Q8. Consider two energy profiles for the same overall reaction. Both start at 300 kJ and end at 220 kJ, but one profile reaches 500 kJ and the other reaches 380 kJ before forming products. What can be concluded?

A.They have different overall

\Delta H

values

B.They have the same overall

\Delta H

, but different activation barriers ✅

C.The higher peak indicates a more negative

\Delta H

D.The lower peak means the products have greater enthalpy

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Both profiles have identical initial and final enthalpy values, so both have $\Delta H=220-300=-80$ kJ. Their different peak heights indicate different energetic barriers, but the overall enthalpy change remains the same.

Q9. A reaction is performed twice at constant pressure. In Trial 1, 2 mol reacts with $\Delta H=-40$ kJ/mol. In Trial 2, 5 mol reacts under otherwise comparable conditions. What is the expected total heat release in Trial 2?

A.8 kJ

B.40 kJ

C.200 kJ ✅

D.400 kJ

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: For an extensive quantity such as total heat, the amount of material matters. Trial 2 releases $5\times40=200$ kJ. The negative sign in $q_p=-200$ kJ indicates release, so the magnitude of heat released is 200 kJ.

Q10. A researcher compares two measurements: Method X gives $\Delta H=-48$ kJ/mol, while Method Y gives $\Delta H=+48$ kJ/mol for the same reaction orientation. Assuming both measurements are otherwise reliable, what is the most likely issue?

A.The reaction direction was reversed in one method ✅

B.Both values must be simultaneously correct for the same reaction

C.Enthalpy cannot have a sign

D.The temperature must have become zero

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Reversing a reaction reverses the sign of its enthalpy change. Therefore, $+48$ kJ/mol and $-48$ kJ/mol can correspond to the same chemical transformation written in opposite directions, rather than indicating two unrelated magnitudes.

Q11. A biochemical system undergoes two sequential reactions. Reaction 1 has $\Delta H=-30$ kJ/mol and Reaction 2 has $\Delta H=+10$ kJ/mol. If both occur once, what is the net enthalpy change?

-40

kJ/mol

-20

kJ/mol ✅

+20

kJ/mol

+40

kJ/mol

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: For sequential reactions, the enthalpy changes combine algebraically. Therefore, $\Delta H_{\mathrm{net}}=(-30)+(+10)=-20$ kJ/mol. The negative value means the combined process releases 20 kJ per mole under the stated conditions.

Q12. A reaction has $\Delta H=+35$ kJ/mol. Another student says this proves that the reaction cannot occur spontaneously. Which response is most scientifically appropriate?

A.Correct, because all endothermic reactions are impossible

B.Correct, because positive enthalpy always means negative entropy

C.Incorrect, because spontaneity depends on more than enthalpy alone ✅

D.Incorrect, because enthalpy determines only reaction speed

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: A positive $\Delta H$ indicates that heat is absorbed under constant pressure, but it does not by itself determine spontaneity. Spontaneity can depend on both enthalpic and entropic contributions, as well as temperature.

Q13. An energy diagram shows a reactant level at 250 kJ and a product level at 310 kJ. A second diagram for the same reaction uses a catalyst but shows the same starting and ending levels. Which comparison is correct?

A.The catalyst changes

\Delta H

from +60 to 0 kJ

B.The catalyst changes the product enthalpy but not the reactant enthalpy

C.Both diagrams have

\Delta H=+60

kJ, while the catalytic pathway may have a different energy barrier ✅

D.The catalyst necessarily makes

\Delta H=-60

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: In both diagrams, $\Delta H=310-250=+60$ kJ because the reactant and product levels are unchanged. A catalyst can alter the pathway and energetic barrier without changing the overall enthalpy difference between initial and final states.

Q14. A reaction releases 90 kJ when 3 mol of substrate reacts completely at constant pressure. A second experiment uses 1.5 mol under comparable conditions. What heat transfer should be expected, assuming the same molar enthalpy change?

A.15 kJ absorbed

B.30 kJ released

C.45 kJ released ✅

D.90 kJ absorbed

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: The first experiment corresponds to $90/3=30$ kJ released per mole, or $\Delta H=-30$ kJ/mol. For 1.5 mol, $q_p=(1.5)(-30)=-45$ kJ, meaning 45 kJ of heat is released.

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