Thermodynamics

Helium gas expands from 125 kPa, 350 K and 0.25 m3 to 100 kPa in a polytropic process with n = 1.667. Is the work positive, negative or zero?

How can you change s of a substance going through a reversible process?

For a given reaction, if ∆H°  rxn is (negative/positive/either) and ∆S° rxn is (negative/positive/either), then the value of ∆G° rxn will always decrease as you raise the temperature.

1. either, positive

2. positive, either 

3. either, negative 

4. negative, either 

5. positive, negative 

6. negative, positive

A steel cylinder of mass 2 kg contains 4 L of liquid water at 25°C at 200 kPa. Find the total mass and volume of the system. List two extensive and three intensive properties of the water 

Make a control volume that includes the steam flow around in the main turbine loop in the nuclear propulsion system in Identify mass flows (hot or cold) and energy transfers that enter or leave the C.V.

Find the internal energy of 1 kg of superheated steam at a pressure of 10 bar and 280ºC. If this steam is expanded to a pressure of 1.6 bar and 0.8 dry, determine the change in internal energy. Assume specific heat of superheated steam as 2.1 kJ/kg-K.

Consider the formation reactions below and pick the most stable species from the answer choices.

2C_graphite(s) + 3H₂(g) → C₂H₆(g) 

∆G◦ _f = −7.86 kcal · mol⁻¹

3Cgraphite(s) + 4H₂(g) → C₃H₈(g)

∆G◦ f = −5.614 kcal · mol⁻¹

8Cgraphite(s) + 9H₂(g) → C₈H₁₈(g)

∆G◦ _f = 4.14 kcal · mol⁻¹

10Cgraphite(s) + 11H₂(g) → C₁₀H₂₂(g)

∆G◦ _f = 8.23 kcal · mol⁻¹

1. The change in entropy is large. 

2. The change in entropy is small. 

3. The reaction is endothermic. 

4. The reaction is exothermic

You observe that carbon dioxide sublimes. Which of the following statements about the signs of this process is/are true? 

(I) Work (w) is positive. 

(II) Heat (q) is negative. 

(III) Change in Gibbs free energy (∆G) is positive. 

(IV) Change in entropy (∆S) is positive. 

1. II and III 

2. I only 

3. I and II 

4. I, II and III 

5. IV only 

6. III and IV

Electrical appliances (TV, stereo) use electric power as input. What happens to the power? Are those heat engines? What does the second law say about those devices?