1. A recipe for preparing water replenishment for adults is as follows:
• 1 liter of cooked and salt-free water
• 6 teaspoons of sugar
• 0.5 teaspoons of cooking salt
a) Describe the phenomenon's of osmosis, freezing point lowering and boiling point increase.
b) What is the osmotic pressure of the above salt/sugar solution in comparison to pure water?
c) By how much is the freezing point lowered and the boiling point increased?
d) Why can the above phenomena be used for determining the molecular mass of a water soluble compound, and which of the three phenomena is most suitable for that? Why?
Assume for your calculations that one teaspoon of sugar equals 4 g and that I teaspoon of salt weighs 9 g. The molecular masses are 342.3 g/mol for sugar and 58.4 g/mol for cooking salt.
2. Why are most salts water soluble, but most non-polar molecules not? Give detailed molecular explanations that lead to statements about the enthalpy and entropy changes in the two processes.
3. During fermentation carbohydrates are broken down into ethanol and carbon dioxide. Although this is a complex reaction that involves many enzyme catalyzed reaction steps, the fermentation of glucose can be described by the following overall reaction:
C6H12O6(s) → 2C2H5OH(I) + 2CO2(g)
At 298K and I bar the following thermodynamic data have been determined for the molecules involved in this process:
|
Molecule
|
M/gmol-1
|
ΔiHo/kjmol-1
|
So/JK-1mol-8
|
|
C6H12O6(s)
|
180.2
|
-1273
|
212
|
|
C2H5OH(I)
|
46.1
|
-278
|
161
|
|
CO2(g)
|
44.0
|
-394
|
214
|
Assume that 24.0 g glucose is completely used up during the above fermentation reaction.
a) What is the general definition of a catalyst and how does it affect the Gibbs energy of activation and the Gibbs energy of the overall reaction? Explain in words and by drawing diagrams of Gibbs energy versus reaction coordinate.
b) Determine the reaction enthalpy of the fermentation reaction.
c) Determine the reaction entropy of the fermentation reaction.
d) Determine the total entropy change of the fermentation reaction and use this information to motivate if this reaction is a spontaneous process or not.
4. Biodegradable polymers are interesting systems for the controlled release of biologically active molecules. Drugs embedded in such polymers are released with the speed the respective polymer is broken down. For example, polydimethylsiloxane (PDMS) has a hydrophobic character and can thus be loaded with hydrophobic drugs in order to achieve a slow release profile. In a recent study the breakdown of PDMS was shown to follow initially a first-order reaction with a half-time of I I days. After 25 days, the reaction kinetic changed and was best described by a zero-order reaction. After 40 days all PDMS was degraded.
a) What is the meaning of the reaction order n?
b) How much of the original amount of PMDS is left after 25 days?
c) Draw a graph showing how the amount of PMDS changes as a function of reaction time from 0 hours to 40 hours. Be as quantitative as possible.