Hesss Law - UNSOLVED PRACTICE SET

Q7. State Hess's Law of constant heat summation. Why is it called a 'law of constant heat summation'?

Q8. Explain how Hess's Law can be used to calculate the enthalpy of formation of a compound that cannot be prepared directly from its elements.

Q9. Why is Hess's Law considered an application of the First Law of Thermodynamics? Explain briefly.

Q10. Given the following data:

(i) A → B; ΔH = +50 kJ

(ii) B → C; ΔH = –30 kJ

Calculate ΔH for the reaction A → C using Hess's Law.

Q11. A student wants to calculate the enthalpy of formation of methane. Why can't this be done directly in the laboratory? How does Hess's Law help overcome this problem?

Q12. Draw an enthalpy level diagram to illustrate Hess's Law for the formation of CO(g) from C(s) and O₂(g) via two different paths.

Q13. (a) State Hess's Law and explain its thermodynamic basis.

(b) Describe how you would use Hess's Law to calculate the enthalpy of formation of ethanol (C₂H₅OH) using combustion data.

(c) Why is Hess's Law particularly useful in thermochemistry? Give two reasons.

Q14. (a) Explain the Born-Haber cycle as an application of Hess's Law for calculating lattice enthalpy.

(b) Draw a schematic diagram of the Born-Haber cycle for NaCl and label all the steps.

(c) How does the Born-Haber cycle demonstrate the power of Hess's Law in solving complex thermochemical problems?

Q15. (a) Given the following thermochemical equations:

(i) N₂(g) + O₂(g) → 2NO(g); ΔH = +180.5 kJ

(ii) 2NO(g) + O₂(g) → 2NO₂(g); ΔH = –114.1 kJ

(iii) N₂(g) + 2O₂(g) → 2NO₂(g); ΔH = ?

Calculate ΔH for reaction (iii) using Hess's Law and explain your steps clearly.

(b) Draw an enthalpy diagram representing the above reactions.

(c) A student claims that if reaction (i) is reversed, the sign of ΔH should also be reversed. Is the student correct? Justify your answer using Hess's Law.

Q16. Calculate the enthalpy of formation of methanol (CH₃OH) using the following data:

C(s) + O₂(g) → CO₂(g); ΔH = –393.5 kJ/mol

H₂(g) + ½O₂(g) → H₂O(l); ΔH = –285.8 kJ/mol

CH₃OH(l) + ³⁄₂O₂(g) → CO₂(g) + 2H₂O(l); ΔH = –726.5 kJ/mol

Q17. Given the following thermochemical data:

(i) C(s) + 2H₂(g) → CH₄(g); ΔH = ?

(ii) C(s) + O₂(g) → CO₂(g); ΔH = –393.5 kJ

(iii) H₂(g) + ½O₂(g) → H₂O(l); ΔH = –285.8 kJ

(iv) CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l); ΔH = –890.3 kJ

(a) Calculate the enthalpy of formation of methane (CH₄) using Hess's Law.

(b) Draw an enthalpy cycle diagram for this calculation.

(c) Verify your answer by checking if the sum of enthalpies around the cycle equals zero.

Q18. In India, many villages use biogas (mainly methane) for cooking. A student wants to compare the energy efficiency of biogas with that of wood.

Given:

Enthalpy of combustion of CH₄(g) = –890.3 kJ/mol

Enthalpy of combustion of carbon (wood) = –393.5 kJ/mol

Enthalpy of combustion of hydrogen = –285.8 kJ/mol

(a) Use Hess's Law to calculate the enthalpy of formation of methane.

(b) A typical biogas plant produces 2 m³ of methane per day (at STP, 1 mol = 22.4 L). Calculate the total heat energy available per day.

(c) If the same amount of energy were to be obtained from wood (carbon), what mass of wood would be needed?

(d) Discuss two advantages of using biogas over wood from both thermodynamic and environmental perspectives.

(Molar mass of carbon = 12 g/mol)