Thermodynamics

Chemical thermodynamics deals with the energy changes that accompany chemical and physical processes, and with predicting whether a process can occur spontaneously. It defines system, surroundings and state functions such as internal energy (U) and enthalpy (H). It explains the first law of thermodynamics, delta U = q + w, with the work done in irreversible and reversible isothermal expansion of an ideal gas. You will learn the relation delta H = delta U + (delta n) RT for gaseous reactions, extensive and intensive properties, heat capacity, and the relation Cp - Cv = R. You will study the measurement of delta U with a bomb calorimeter and of delta H at constant pressure, and write thermochemical equations. The chapter covers standard enthalpies of formation, combustion, atomisation, bond dissociation, solution and lattice enthalpy, and Hess's law of constant heat summation. Finally, it explains spontaneity, entropy (delta S = q rev / T) and the second law, Gibbs energy (delta G = delta H - T delta S), and the relation delta G zero = -RT ln K.

Detailed Worksheet: Thermodynamics Section A - Definitions (10 marks) 1. Distinguish between open, closed and isolated systems with examples. (2 marks) 2. What is a state function? Give two examples. (2 marks) 3. State the first law of thermodynamics and write its mathematical form. (2 marks) 4. State Hess's law of constant heat summation. (2 marks) 5. Define Gibbs energy. What does a negative delta G indicate? (2 marks) Section B - Calculations and Applications (15 marks) 6. For the reaction N2(g) + 3H2(g) -> 2NH3(g), delta H = -92.4 kJ at 298 K. Calculate delta U for the reaction (R = 8.314 J per mol per K). (3 marks) 7. A gas expands from 2.0 L to 10.0 L against a constant external pressure of 1.0 bar. Calculate the work done in joules (1 L bar = 100 J). What would the work be if the gas expanded into a vacuum? (3 marks) 8. Calculate the work done when 1.0 mol of an ideal gas expands isothermally and reversibly from 10 L to 20 L at 300 K (log 2 = 0.3010). (3 marks) 9. Given: C(graphite) + O2(g) -> CO2(g), delta H = -393.5 kJ/mol; CO(g) + 1/2 O2(g) -> CO2(g), delta H = -283.0 kJ/mol. Calculate the standard enthalpy of formation of CO using Hess's law. (3 marks) 10. For a reaction, delta H = +30 kJ/mol and delta S = +100 J/K per mol. Calculate delta G at 250 K and 400 K. Above what temperature does the reaction become spontaneous? (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of a bomb calorimeter. (4 marks) 12. Draw an enthalpy diagram for an exothermic and an endothermic reaction. (3 marks) 13. Draw a Born-Haber cycle for the formation of NaCl from its elements. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain why delta H and delta U differ for reactions involving gases. Under what conditions are they nearly equal? (5 marks) 15. Explain the concept of entropy and why some endothermic reactions are spontaneous. Use the dissolution of ammonium nitrate in water as an example. (5 marks) 16. Explain how the signs of delta H and delta S together decide the spontaneity of a reaction at different temperatures. Give one example for each case. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show all calculation steps with units and signs (IUPAC convention). Draw diagrams neatly with labels.