lecture 7

# lecture 7 - Calculate the change in entropy of water...

This preview shows pages 1–5. Sign up to view the full content.

1 Water – Ice Problem Calculate the change in entropy of water, S system , when 2 mol of water freezes at 0 o C (273 K). For ice, H fusion = 6.01 kJ/mol S = S final – S initial = q/T A) + 44.0 J/K B) – 44.0 J/K C) + 0.022 J/K D) – 0.022 J/K Clausius figured out that you can calculate the change in entropy by dividing the energy transferred, q, by the Kelvin temperature, T. Rudolf Clausius German Physicist 1822 - 1888 2 S = q/T and q = -n H fusion n = 2 mol H fusion = 6.01 kJ/mol T = 273 K q H2O is negative since energy flows out of the system. q H2O = 2 x 6.01 x 10 3 J = 1.202 x 10 4 J S H2O = q H2O /T H2O = 1.202x10 4 / 273 = 44.0 J K –1

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
2 4 When 2 mol of water freezes, S water = –44 J/K. The entropy decreases! Makes sense liquid solid. Water freezes spontaneously, but we said S > 0 for a spontaneous process. What gives here? Need to consider the total entropy not just the entropy of the system. 5 Total Entropy Change When water freezes, energy is transferred to the surroundings. Which means that the energy is dispersed over more states in the surroundings. More states in the surroundings are accessible. So the entropy of the surroundings increases. S total = S system + S surroundings S total also is called S universe
3 6 Calculate the change in entropy of the freezer (surroundings), S surroundings , when 2 mol of water freezes at 0 o C (273 K). The temperature of the freezer is –15 o C (258 K) For ice, H fusion = 6.01 kJ/mol S = S final – S initial = q/T A) + 46.6 J/K B) – 46.6 J/K C) + 44.0 J/K D) – 44.0 J/K 7 Freezer (the surroundings) S freezer = q freezer /T freezer The freezer absorbs energy at its temp. ( 15°C) so q freezer > 0. q freezer = q H2O so q freezer = 2mol (6.01 kJ/mol) q freezer = 1.202 x 10 4 J S freezer = q freezer /T freezer = 1.202 x 10 4 / 258 = 46.6 J K -1

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
4 8 The Universe (total) S universe = S system + S surroundings S universe = 44.0 J/K + 46.6 J/K = + 2.6 J/K
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern