Enthalpy and Enthalpy Change - UNSOLVED PRACTICE SET
Chapter: Thermodynamics | Topic: Enthalpy and Enthalpy Change
ENTHALPY AND ENTHALPY CHANGE - UNSOLVED PRACTICE SET
Topic: Enthalpy and Enthalpy Change
Multiple Choice Questions
Q1. Enthalpy (H) is defined as:
- H = U + PV
- H = U – PV
- H = U/PV
- H = q + w
Q2. For an exothermic reaction at constant pressure:
- ΔH is positive
- ΔH is negative
- ΔH is zero
- ΔH cannot be determined
Q3. The enthalpy change (ΔH) for a reaction carried out at constant pressure is equal to:
- The heat absorbed at constant volume
- The heat absorbed or released at constant pressure
- The work done by the system
- The change in internal energy only
Q4. For the reaction: 2H₂(g) + O₂(g) → 2H₂O(l), ΔH = –572 kJ. This means:
- 572 kJ of heat is absorbed when 2 moles of water are formed
- 572 kJ of heat is released when 1 mole of water is formed
- 572 kJ of heat is released when 2 moles of water are formed
- 572 kJ of heat is absorbed when 1 mole of oxygen reacts
Q5. The relationship between ΔH and ΔU for a reaction involving only solids and liquids is:
- ΔH = ΔU + ΔnRT
- ΔH = ΔU
- ΔH = ΔU – ΔnRT
- ΔH = ΔU + RT
Q6. Which of the following processes is endothermic?
- Combustion of methane
- Neutralisation of HCl and NaOH
- Dissolution of ammonium chloride in water
- Formation of water from hydrogen and oxygen
Short Answer Questions
Q7. Define enthalpy (H) and explain why it is a state function. Why is enthalpy change more useful than internal energy change for reactions at constant pressure?
Q8. Differentiate between exothermic and endothermic reactions with one example each. How does the sign of ΔH help you identify them?
Q9. Derive the relationship between ΔH and ΔU for a reaction involving gases. What is the significance of Δn in this relationship?
Q10. Explain why the enthalpy of neutralisation of a strong acid by a strong base is approximately constant (about –57.1 kJ/mol).
Q11. A reaction has ΔH = +120 kJ/mol. Is this reaction exothermic or endothermic? Explain what happens to the temperature of the surroundings during this reaction.
Q12. Why is the enthalpy change of a reaction independent of the path taken? Explain with reference to the state function nature of enthalpy.
Long Answer Questions
Q13. (a) Define enthalpy and derive the expression H = U + PV.
(b) Show that at constant pressure, ΔH = qp (heat exchanged at constant pressure).
(c) A student performs a reaction in a bomb calorimeter. Would you calculate ΔH or ΔU directly? Explain why.
Q14. (a) Explain the difference between ΔH and ΔU with a suitable example.
(b) For the reaction: N₂(g) + 3H₂(g) → 2NH₃(g), calculate the difference between ΔH and ΔU at 298 K.
(c) When would ΔH be equal to ΔU for a gaseous reaction? Give an example.
Q15. (a) Define standard enthalpy of reaction (ΔH°r). What conditions must be satisfied for a reaction to be considered under standard conditions?
(b) Explain why enthalpy is an extensive property with a suitable example.
(c) A reaction has ΔH = –250 kJ for 2 moles of product. What would be ΔH for the formation of 0.5 moles of the same product? Explain.
Numerical / Application-Based Problems
Q16. For the combustion of propane: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)
Given: ΔH = –2220 kJ/mol
(a) Calculate ΔU for this reaction at 298 K.
(b) Is the difference between ΔH and ΔU significant? Explain why or why not.
(R = 8.314 J/mol·K)
Q17. A student performs the following reaction in the laboratory:
2NO(g) + O₂(g) → 2NO₂(g)
Given: ΔU = –114.2 kJ at 298 K
(a) Calculate ΔH for this reaction.
(b) Is this reaction exothermic or endothermic? Explain.
(c) Predict whether the temperature of the reaction mixture would increase or decrease.
(R = 8.314 J/mol·K)
Q18. In a school chemistry lab, a student mixes 50 mL of 1.0 M HCl with 50 mL of 1.0 M NaOH in a coffee cup calorimeter. The temperature of the solution rises from 25.0°C to 31.5°C.
(a) Calculate the enthalpy change of neutralisation per mole of water formed.
(b) Compare your result with the standard value of –57.1 kJ/mol and suggest possible reasons for any difference.
(Given: Density of solution = 1 g/mL, Specific heat capacity = 4.18 J/g·°C)