PC-Praktikum
PC-Praktikum 5. Semester an der Universität Basel (2014)
PC-Praktikum 5. Semester an der Universität Basel (2014)
Kartei Details
| Karten | 100 |
|---|---|
| Sprache | Deutsch |
| Kategorie | Chemie |
| Stufe | Universität |
| Erstellt / Aktualisiert | 23.01.2015 / 25.01.2015 |
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How do you calculate the \(\Delta H\) of a reaction from the \(\Delta H_{formation}\)
\(\Delta H=\Delta H_{f}(Products)-\Delta H_{f}{Educts}\)
\(E°(cell)=...\)
\(E°(cell)=E°(red.)-E°(ox.)\)
Give the Born-Haber Cycle for Ag2O. How do you determine the lattive Energy of Ag2O?
Bild.
What chemical properties can we get from a DSC measurement?
- melting point
- formation enthalpy
How is a first order phase transition classified and what is the physical consequence?
How is a seconds order phase transition classified?
A first-order phase transition is defined for transitions where the first derivative of the chemical potential with respect to temperature is discontinuous. As a consequence at a certain temp. (melting temp.) there is a heat uptake without a change in temp.
second order: first derivative is continuous, but 2. derivative is discont.
a) phase transition of first order
b) second order transitions
What properties has a liquid crystal?
- flows like a liquid
- molecules are oriented like in a crystal
Def. Isotropie
Unabhängigkeit einer Eigenschaft von der Richtung.
Def. spez. Wärmekapazität.
Energie, die Notwendig ist, um 1 kg eines Stoffes um 1 °C zu erwärmen.
\(c = {Q \over m*\Delta T}\), Einheit: J/(kg*K)
How do we find the melting point with a DSC measurement?
Several measurements of a sample need to be taken, where the sample is heated at different Heating Rates. By plotting the Onset Temp. of the melting vs. the heating rates, the melting point is found as the y-axis (where we have an infinitely slow heating rate).
Why do we need to measure the melting temperature of Indium at the beginning of a DSC experiment?
In order to calibrate the instrument and therefore get rid of systematic errors. We get a scaling factor, which we need to use for our measurements.
\(K_T={T_{lit.} \over T_{exp}}\)
A thermotropic liquid crystal is a liquid crystal, which undergoes phase transitions upon changing the temperature. Going from solid to liquid they undergo 3 phase transitions. Describe the phases in terms of their isotropy.
solid state: conventional crystal, which is anisotropic (properties depend upon the direction)
smectic phase: well defined layers, which can slide over another (soap properties), still anisotropic
nematic phase: no layer structure, but the molecules still point in the same direction --> anisotropy
liquid phase: fully isotropic
Define Viscosity
The viscosity of a fluid is a measure of its resistance to gradual deformation by shear or tensile stress. It can be regarded as an internal force of resistance, that delays deformation of a liquid (ex. pouring). It is a result of intermolecular interactions.
When can viscosimetry not be used?
Insoluble polymers due to cross-linking.
What is the number average molecular weight and with what method is it determined?
It is the ordinary arithmetic mean of the molecular masses. It is calculated as: total weight of sample / number of molecules in sample
Determined by VPO or Viscosimetry,
What is the weight average molecular weight and with what method can it be determined?
Bild.
This quantitiy depends on the number and the size of the molecules. A larger molecule will have a larger contribution than a smaller one.
It is determined by light scattering of polymer solutions, where the intensity of the scattered light depends upon the number and the weight of the molecules.
What is the difference between number average and weight average molecular weight.
Can MN and MW be the same?
Number average mol. weight is not influenced by the molecular weight of the molecules and only depends on the number of molecules, whereas weight average mol. weight depends on both.
They can only be the same for a monodispers polymer solution.
Define reaction rate
It is a measure for the change in concentration of the reactant or the change in concentration of the products per unit of time.
Give an example for an anode/cathode combination of a battery
Ag2O/ Zn, Pb / PbO
name three attributes, that a fluorescence quencher must have.
- accept the energy from the excited fluorophore (emission of fluorophore and absorption of quencher have to overlap)
- be close enough to transfer the energy
- quencher must return to the ground state without emitting light. (transform into heat)
Define a zero-order reaction.
The reaction rate is independent of the concentration of the reactant.
