Questions on Enthalpy
Multiple-Choice Questions – Enthalpy
1. What does enthalpy (H) represent in a chemical system?
A) The speed of a reaction
B) The total internal energy of a system
C) The amount of disorder in the system
D) The pressure of a gas in the system
E) The change in temperature of the system
2. What is the SI unit of enthalpy?
A) Kelvin (K)
B) Joule (J)
C) Mole (mol)
D) Pascal (Pa)
E) Watt (W)
3. Enthalpy change (ΔH) for an exothermic reaction is:
A) Always positive
B) Always negative
C) Always zero
D) Sometimes negative, sometimes positive
E) Greater than the activation energy
4. Enthalpy change (ΔH) for an endothermic reaction is:
A) Always positive
B) Always negative
C) Always zero
D) Equal to entropy
E) Undefined
5. Which of the following reactions has a negative ΔH?
A) Photosynthesis
B) Evaporation of water
C) Ice melting
D) Combustion of methane
E) Boiling ethanol
6. What type of enthalpy change occurs when bonds are formed?
A) Positive
B) Negative
C) Zero
D) Infinite
E) Always positive at high temperatures
7. In thermochemical equations, the enthalpy change is typically written:
A) Above the arrow
B) As a coefficient
C) Beside the chemical formulas
D) After the reaction with ΔH
E) As a temperature value
8. Which statement is true about enthalpy and heat at constant pressure?
A) Enthalpy is unrelated to heat
B) Heat equals the enthalpy change
C) Enthalpy only applies to gases
D) Heat is always greater than enthalpy
E) They are the same at any temperature
9. What is the symbol for standard enthalpy change?
A) ΔH
B) ΔH°
C) H*
D) ΔG
E) Q
10. Which process involves a positive enthalpy change?
A) Condensation of steam
B) Freezing of water
C) Dissolving NaOH in water
D) Boiling of water
E) Combustion of propane
11. Enthalpy is a type of:
A) Kinetic energy
B) Potential energy
C) Chemical potential
D) State function
E) Pressure-volume work
12. A chemical reaction absorbs heat from the surroundings. This means:
A) ΔH < 0
B) The reaction is exothermic
C) ΔH = 0
D) ΔH > 0
E) Temperature must decrease
13. Enthalpy of formation refers to:
A) Heat released during combustion
B) Heat required to break a compound
C) Enthalpy change to form 1 mole of compound from elements
D) Energy lost when freezing a liquid
E) Energy stored in ionic bonds
14. Which of the following has ΔHf = 0?
A) H₂O(l)
B) CO₂(g)
C) O₂(g)
D) CH₄(g)
E) NH₃(g)
15. Hess's Law is used to:
A) Measure temperature
B) Calculate bond enthalpies
C) Determine reaction rate
D) Calculate ΔH using multiple steps
E) Determine entropy
16. If the enthalpy of the products is less than the reactants, the reaction is:
A) Endothermic
B) Reversible
C) Exothermic
D) Non-spontaneous
E) Catalyzed
17. Which process is not associated with a change in enthalpy?
A) Boiling
B) Melting
C) Phase change
D) Isothermal compression
E) Chemical reaction
18. A reaction with ΔH = +125 kJ/mol is:
A) Spontaneous
B) Endothermic
C) Exothermic
D) Impossible
E) Neutral
19. Why is enthalpy a state function?
A) It depends on the path
B) It changes with pressure
C) It depends only on initial and final states
D) It is a kinetic property
E) It changes with catalysts
20. When a reaction occurs at constant pressure, enthalpy change equals:
A) Internal energy
B) Heat absorbed or released
C) Temperature
D) Entropy
E) Work done
- Questions about Exothermic Reactions
- Questions about Endothermic Reactions
- Questions about Exothermic and Endothermic Processes
Answers with Explanations
1. B – Enthalpy represents the total internal energy plus pressure-volume work of a system.
2. B – The SI unit for enthalpy is the Joule (J).
3. B – Exothermic reactions release heat, so ΔH is negative.
4. A – Endothermic reactions absorb heat, so ΔH is positive.
5. D – Combustion releases energy, so it has a negative ΔH.
6. B – Bond formation releases energy (ΔH < 0).
7. D – ΔH is usually written after the equation, e.g., ΔH = -500 kJ.
8. B – At constant pressure, the heat exchanged equals ΔH.
9. B – ΔH° symbolizes the standard enthalpy change (at 1 atm and 25°C).
10. D – Boiling is endothermic (requires heat), so ΔH > 0.
11. D – Enthalpy is a state function; it depends only on state, not path.
12. D – If heat is absorbed, ΔH > 0 (endothermic).
13. C – Enthalpy of formation is the enthalpy change to form 1 mol of a compound from its elements.
14. C – Elements in their standard state, like O₂(g), have ΔHf = 0.
15. D – Hess’s Law adds multiple ΔH values to find overall ΔH.
16. C – Less enthalpy in products = exothermic (energy released).
17. D – Isothermal compression doesn’t change enthalpy as temperature stays constant.
18. B – Positive ΔH indicates heat absorbed → endothermic.
19. C – State functions depend only on initial and final states.
20. B – At constant pressure, ΔH equals the heat exchanged with surroundings.
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