1. Thermodynamics and Chemical Equilibrium Flashcards
1
Q
What characterises a solid? What forms may solids take?
A
Particles packed tightly together and are not able to vibrate around fixed positions.
May be crystalline (arranged in regular ordered patterns) or amorphous (not arranged in regular patterns)
2
Q
What are some defects in the crystalline structure of solids (4)?
A
Vacancies: Atom/molecule is missing from its place in regular pattern
Interstitial defects: extra atom/molecule in position between the usual locations of the pattern
Line defects: plane or regular pattern stops abruptly in middle of crystal
Planar defects: complete misalignment between crystal structure on either side of a plane through solid
3
Q
What are the amorphous and crystalline forms of silicon dioxide?
A
Amorphous: glass
Crystalline: quartz
4
Q
What characterises a liquid?
A
Particles free to move relative to each other, may be locally ordered but no long range order. Liquids are incompressible.
5
Q
What characterises a gas?
A
Fluid (like a liquid) but much less dense, particles are in constant random motion. Collision between particles may lead to formation of a dimer.
6
Q
What is the equation for pressure?
A
Pressure = Force / Area
Pressure of a gas is related to the force exerted by the gas molecules every time they collide with the sides of the container
7
Q
What is equal to the force exerted by a molecule during collision with a wall? What does this mean?
A
The rate of change of momentum of the molecule.
Therefore the greater the component of the molecules velocity in the direction of the wall (before the collision) the greater the force exerted by the molecule
8
Q
Which factors affect gas pressure?
A
number of molecules, temperature and volume of the container (n, T, V)
As n increases, P increases
As T increases, P increases
As V increases, P decreases
9
Q
Define temperature
A
The average speed of the atoms/molecules in the gas
10
Q
What is the ideal gas equation?
A
pV = nRT (R=8.314 J/mol/K
11
Q
What is the fourth state of matter?
A
Plasma, a gas in which a portion of the molecules have been ionised. Removed electrons remain part of the plasma so it is electrically neutral. Constituents of plasma interact strongly via electromagnetic fields - so plasma behaves very differently to a gas
12
Q
Give examples of some of the other, less common states of matter
A
Glasses, liquid crystals, superfluids, quark-gluon plasma
13
Q
What is the name of the process converting solid to gas?
A
Sublimation
14
Q
What is the definition of a liquid solution?
A
A homogenous mixture of two or more distinct chemical species in the liquid phase
Can also have solid solutions
15
Q
Is a pure liquid a solution?
A
No
16
Q
Is milk a solution?
A
No, complete homogeneity is required for a solution and milk has two fluids which are immiscible and form an emulsion
17
Q
What is relative permittivity (epsilon r)?
A
Dielectric constant -a measure of how well the solvent is able to store electrical energy by concentrating lines of electrical flux - it is a measure of polarity
18
Q
What is viscosity?
A
A measure of the resistance to flow. Important when considering mobility of solute molecules (in the solvent).
19
Q
What solvent trait may increase solubility?
A
ability to form hydrogen bonds
20
Q
What is the name of an ionic solute?
A
an ELECTROLYTE ZAP ZAP
21
Q
What do the properties of the solute molecule depend on?
A
The nature of the solvent and the amount of the solute
22
Q
What is the shortening used for solute and solvent?
A
Solvent = A Solute = B
23
Q
What is an ideal solution?
A
A solution where the solute molecules interact with the solvent molecules in the exact same way they interact with each other.
24
Q
How would you ensure that the greatest dissolution occurred?
A
Make sure the solute and solvent behaved as similarly as possible (most similar interactions) - ‘like dissolves like’ - the closer to ideal solution the better
25
What are the different types of interaction that may occur between molecules?
Ionic (strong, long range), covalent (strong, short range), Van der waals (weak)
26
Describe the three Van Der Waals forces
Keesom forces: between molecules with permanent dipole moments (Asymmetric charge distributions)
Debye forces: between one molecule with permanent dipole moment & one with transient dipole moment
London forces: between two molecules, neither of which has a permanent dipole, interaction is as a result of mutual instantaneous induced dipoles
27
If dissolving a highly polar/ionic substance what type of solvent would be a good call?
A solvent with a high relative permittivity - e.g. water 78.54
28
What are the relative permittivity relative permittivity of water, ethanol and cyclohexane?
Water - 78.54
Ethanol - 24.30
Cyclohexane - 2.01 (not polar)
29
What is the difference between molarity and molality? Why is molality used?
Molarity: moles per volume of solution
Molality: moles per mass of solvent
Molarity depends on temperature (because volume depends on temperature), molality avoids this
30
What is the equation for mole fraction?
mole fraction of solute B = xB = nB/(nA + nB)
Number of moles of the solute over the total number of moles of a solution
31
What is the vapour pressure?
This is the pressure of the gas inside a container if liquid is inserted into a sealed container which previously held a vacuum. Over time molecules will evaporate and condensate from the liquid leading to a lack of vacuum, it is this pressure that is measured.
32
What is the partial pressure?
This is the vapour pressure of the solvent or solute when a solution is added to a sealed container holding a vacuum. The total pressure is the sum of the partial pressures of the solute (B) and solvent (A)
33
What is Raoult's law? Which solutions is Raoult's law most true for?
PA = PA* . xA
```
PA = partial pressure of solvent A
PA* = vapour pressure of pure A
xA = mole fraction of A
```
Most true for ideal solutions
34
How would an ideal solution be identified?
Plotting a graph of paRTIAL PRESSURE AGAINST MOLE FRACTION, if ideal then the lines will be straight.
P = PA + PB
35
Do non-ideal solutions obey Raoult's law?
nope - if you plot partial pressure against mole fraction the lines would be curvy
36
At what point do non-ideal solutions follow Raoult's Law?
When the mole fraction of the solute (B) is small - aka when the solution is almost pure solute. In this case, most of the acetone molecules can only see other acetone molecules so solution behaves ideally.
37
What type of behaviour is described by Henry's law? What is the equation?
The linear variation of solute partial pressure at low mole fractions (of solute)
PB = KB . xB
```
PB = partial pressure of solute B
KB = experimentally determined parameter
xB = mole fraction of solute B
```
38
What is the difference between Henry's and Raoult's law?
Henry's law does not use the pure liquid (solvent A) vapour pressure as the constant, instead it uses an experimentally determined quantity
39
What is an ideal-dilute solution?
Very dilute solution in which solvent (A) obeys Raoult's and solute (B) obeys Henry's law.
40
What is the impact of Henry's law on biology?
Important in determining implications for respiration under abnormal pressures (e.g. diving, climbing) - as need to know how gases are exchanged between blood/air in alveoli
41
At what concentration of solute may electrolyte solutions show some form of ideality?
0.001 moldm-3
42
How may an electrolyte solution be formed?
Dissolution of an ionic species such as NaCl
43
Why do macromolecule solutions exhibit strongly non-ideal characteristics?
Due to their bulk - large numbers of solvent molecules are stuck on the surfaces of the macromolecules which affect the entropy of the solvent (among other thermodynamic properties). The solvent molecules solvate onto the outside also increased the drag of the macromolecules increasing the viscosity.
44
What is enthalpy and what is the equation for it?
Enthalpy is the heat content
Enthalpy change is the heat flow into a system at constant pressure
H = U + pV
```
H = enthalpy
U = internal energy
p = pressure
V = volume
```
45
What is entropy and what is the equation for it?
A measure of how evenly energy is distributed in a system (state of disorder of a system)
S = k.lnW
```
S = entropy
k = Boltzmann constant
W = multiplicity -
```
46
What is the second law of thermodynamics?
A system with constant volume and internal energy will adopt a configuration that maximises it's entropy
47
What does entropy require?
A particular direction for time (arrow of time)
48
What is Gibbs free energy?
The maximum amount of energy available in a system to do non-expansion work under conditions of constant temperature and pressure
aka in a biological system this tells us how much energy is available to do biological work
49
Define temperature
A measure of how much the internal energy changes when the entropy is varied and volume held constant BUT thats a bit intense so
amount of kinetic energy of the molecules in the system
50
What is the heat capacity (Cv/Cp)? What are the equations for this?
A measure of how much internal energy/enthalpy changes as the temperature is varied. Equations are approximately equal not completely.
Cv (constant volume) = deltaU/deltaT
Cp (constant pressure) = deltaH/deltaT
```
U = internal energy
T = temperature
H = enthalpy
```
51
What is another equation for the heat capacity Cp?
C = q/deltaT
q = heat supplied to the system
52
Why are the heat capacity equations only approximately equal?
There are assumptions made about the nature of heat transfers and unchanging nature of C with T
53
What is an extensive property? What is an intensive property?
One which depends on the amount of substance in the sample
The opposite
54
What is another definition for Cp (heat capacity at constant pressure)?
Energy required to raise the temperature of the sample by 1K at constant pressure.
55
What is cp?
Hint: it is different to Cp
cp is the specific heat capacity (intensive property)
cp = Cp/m
m=mass
56
What is the molar heat capacity Cp,m?
The heat required to raise the temperature of one mole of the sample by 1K at constant pressure
57
What is the equation for Gibbs free energy?
deltaG = deltaH - TdeltaS
At constant temperature
58
What is the equation for heat capacity (w/ constant pressure)?
Cp = deltaH/deltaT
OR
deltaH = Cp.deltaT
59
What is the equation for the standard gibbs free energy (deltaG0)?
deltaG0 = -RT.ln(K0)
K0 = standard equilibrium constant (this also depends on the amounts of different species present)
60
How would you show that a quantity has changed in an equation?
Subscript 'r'
61
Translate: delta r G 0
The standard gibbs free energy of a reaction
62
What is the standard state of a pure - one component - system?
The state of the pure system at standard pressure
63
What is the standard state of a solute?What is the standard state of a solvent?
Taken as the hypothetical state of the solute at standard molality (1mol/kg), pressure and exhibiting ideal-dilute behaviour
Equivalent to the state of the pure solvent at standard pressure
64
How does the pH of chemical and biological standard states vary?
Chemists use pH0 as standard, whereas biologists use pH7 or molality of 1x10^-7 mol/kg H+
65
What gibbs free energy will a system at equilibrium adopt?
The lowest possible gibbs free energy
66
What is the gibbs free energy a function of (4)?
Temperature
Pressure
number of moles of solvent A
number of moles of solute B
67
How does a system minimise the gibbs free energy?
By undergoing a chemical change/reaction which converts one set of chemical species into an another
68
When does a thermodynamic driving force occur?
When a reaction is able to lower the gibbs free energy of a system - a thermodynamic driving force will favour the reaction
69
What is the equation of chemical potential?
muB = deltaG/deltanB
nB = number of moles of solute
70
What is the equation for the total chemical potential of a reaction?
deltaG/deltanB = muB - muA
At equilibrium muA = muB
71
What is true about chemical potential at equilibrium?
The chemical potentials of all components of a mixture are equal across all phases at equilibrium
72
What is the equation for the activity of species in solution?
muX = muX0 + RT.ln(aX)
```
muX = chemical potential of species X
muX0 = standard chemical potential of species x
aX = relative activity of X
```
73
How do you derive the equation for K (equilibrium constant)?
For reaction AB:
1. Know that dnA = -dnB
2. Assume that infinitesimal amount (dxi) of reactant A changes into B, so change in moles of A (dnA) = -dxi and dnB = dxi
3. Know that rate of change of G = (deltaG/deltanA).dnA + (deltaG/deltanB).dnB
4. Rate of change of G = chemical potential so can rearrange, dG = muAdnA + muBdnB
5. As dnA = -dxi and dna = dxi you can rearrange to: dG = (muB-muA).dxi
6. so dG/dxi = muB-muA
7. dG/dxi = deltaG
8. muB = muB0 + RTln(aB) and same for A
9. So can combine to form: deltaG = (muB0 + RTln(aB))-(muA0 + RTln(aA))
10. This can be arranged to deltaG = (muB0-muA0) + RTln(aB/aA) then as deltaG0 = muB0-muA0 --> deltaG = deltaG0 + RTln(aB/aA)
11. We know that at equilibrium deltaG = 0
12. so deltaG0 = -RTln(aB/aA)
13. As deltaG0 = -RTln(K0)
14. K0 = aB/aA
15. And so K0 is found
74
What do thermodynamic tables typically consist of?
Standard molar enthalpy of formation, standard molar gibbs free energy of formation, standard molar entropies, molar heat capacities
75
How is the sum of deltarH0 calculated?
sigma.deltafH0 (products) - sigma.deltafH0 (reactants)
This process is the same for deltaG0 and deltaCp,m
76
Are we able to convert data values for H, G, K and S at a certain temperature into values at another temperature?
Yes - look at Module 1 2b slide 4 for the equations cause I am not writing out those fuckers
77
What is a colligative property?
Properties which depend on the collective effect of a number of solute particles
78
What physical properties of a solution differ from those of a pure solvent?
Vapour pressure, boiling point, freezing point, osmotic pressure
79
What do colligative properties depend on?
The NATURE of solvent A and the AMOUNT of solute B
80
What is assumed about B?
Assumed that B is non-volatile and that it does not dissolve in SOLID A
81
How do the following properties change when solute is added to pure solvent: vapour pressure, boiling point, freezing point and osmotic pressure?
Vapour pressure: lowers
Boiling point: increases
Freezing point: decreases
Osmotic pressure: increases
82
What is the equation for vapour pressure change due to solute addition?
deltaP = xB.PA*
```
deltaP = change in vapour pressure
xB = mole fraction of solute B
PA* = vapour pressure of pure A
```
83
What is the equation for boiling point change due to solute addition?
deltaTb = Kb.bB
```
deltaTb = change in boiling point temp.
Kb = empirical boiling point constant
bB = molality of B in solution
```
84
What is the equation for freezing point change due to solute addition?
deltaTf = Kf.bB
```
deltaTf = change in freezing point temp.
Kf = empirical freezing point constant
bB = molality of B in solution
```
85
What is the equation for osmotic pressure change due to solute addition?
pi = RT.CB
```
pi = osmotic pressure
RT = universal gas constant and temperature
CB = molarity of B in solution
```
86
If one mole of NaCl dissolved into a solvent, how many moles of Na+ and Cl- would there be?
1 mole of Na+
| 1 mole of Cl-
87
What is the cryoscopic effect? What ay it be used for?
The depression of the freezing point by addition of solute to a solvent
Cyroscopy
88
What two things may cryoscopy be used to find?
- Determine the molar mass of an unknown solute
| - determine the extent of dissociation of a solute in solution
89
How is cryoscopy used to determine the molar mass of an unknown solute?
If you know: mass of solute, mass of solvent, freezing point change and Kf
Then you can put into the following equation:
deltaTf = Kf.bB
This enables you to find the molality of the solute from where you can find the molar mass using bB = mB/(mA.MB)
```
m = mass (g)
M = molar mass (g/mol)
```
90
How is cryoscopy used to determine the extent of dissociation of a solute in solution?
BONUS POINTS!!!! work out the dissociation constant
If you know: molality of solvent, mass of solvent, freezing point change and Kf
1. Calculate molality after dissociation of solute algebraically (of products & reactant - use alpha for unknown dissociation fraction)
2. Calculate total molality by adding all the algebraically by adding modalities together
3. Calculate the molality change using bB = deltaT/Kf
4. 2=3 and solve for alpha --> molality
BONUS!!!!
5. Input alpha value into K equation (products/reactants) to find K
91
In what situation would it be impractical to determine the extent of dissociation of a solute cryoscopically?
Solutions containing macromolecules as the freezing point depression is small
92
How does the chemical potential of pure solvent at pressure p compete to the chemical potential of solvent A in solution of pressure p+pi?
They are equal
93
What are the five assumptions made when calculating osmotic pressure?
1. Assume that osmotic pressure is small - thus finite difference can be approximated as derivative
2. Assume that solution is ideal-dilute (aA = xA)
3. Assume that nA>>>nB
4. Use ln(1-x) ~= -x
5. Assume that total volume equals volume of the solvent
94
What is the Van 'T Hoff equation?
CB = nB/V
95
What is pi in the osmotic stuff?
Pi is the change in pressure (I think)
96
How may the Van 'T Hoff equation be rewritten?
pi/RT = cB = c'B/Mr
c'B = mass concentration in g/dm^3
97
What are the physical origins of colligative properties?
Caused by the reduction in chemical potential of the solvent as a result of the presence of the solute
98
What is the equation for the chemical potential of the pure solvent?
muA = muA* + RTln(xA)
```
muA* = chemical potential of pure solvent A
xA = mole fraction of A
```
99
Assuming B is non-volatile, what is the boiling temperature of a solution?
The temperature at which the chemical potential of the solvent in solution is equal to the chemical potential of the pure solvent VAPOUR (this is assuming that B is non-volatile)
100
How does chemical potential affect stability?
The lower the chemical potential the more stable a species is
101
Why does the boiling point increase and freezing point decrease in a solution compared to the pure solvent?
The chemical potential of the solution is decreased by the addition of the solute. The lowering of the chemical potential of the solution causes it to become more stable over a wider range of temperatures and thus the boiling point increases and the freezing point decreases
102
How may the decrease of vapour potential be proved by chemical potential?
The solute stabilises the liquid by decreasing the chemical potential, therefore the optimum vapour/liquid composition shifts a little more in favour of the liquid. The system can attain a lower gibbs free energy by allowing some molecules to condense from the vapour phase. Fewer vapour molecules mean a lower vapour pressure.
103
How may you explain the lowering of the vapour pressure in a molecular way?
Whilst the entropy of the vapour remains the same, the entropy of the solution is higher than the entropy of the pure solvent. This is because there are more ways to arrange the molecules in the solution. With a constant temperature, G=H-TS states that the G will decrease if S increases Therefore as the value of G the solvent already has a lower G than a pure liquid the driving force to evaporate from solution is lower and vapour pressure decreases (fewer molecules evaporate)
104
Colligative properties are purely entropic in origin
Yup
105
Do macromolecules repsond to their environment?
Yes, they will change shape radically in response to environmental changes in order to maximise favourable/minimise unfavourable interactions with solvent
106
What are the 4 levels of macromolecular structure?
Primary, secondary, tertiary, quaternary
107
What is the relative molar mass (RMM, MR)?
g/mol (molar mass/atomic mass constant)
108
What are the two categories of macromolecular solutions?
Mono disperse - all the solute molecules have the same Mr
Polydisperse - solute does not have a uniquely defined Mr - an effective molecular mass is determined here
109
What is the effective molecular mass?
The molar mass of the solute molecules in a polydisperse solution
110
Which three ways may the effective molecular mass be calculated?
1. Number weighted average - Mn - determined experimentally from measurements of colligative properties
2. Mass weighted average - Mm - determined with light scattering measurements
3. Double mass weighted average - Mz - sedimentations measurements
111
What is the equation for the number weighted average effective molecular mass calculation?
Mn = Sum of (ni.mi) / sum of ni
ni = number of molecules with mass mi
mi = the mass of a molecule
112
What is the equation for the mass weighted average effective molecular mass calculation?
Mm = Sum of (ni.mi)^2 / sum of (ni.mi)
ni = number of molecules with mass mi
mi = the mass of a molecule
113
What is the equation for the double mass weighted average effective molecular mass calculation?
Mz = Sum of (ni.mi)^3 / sum of (ni.mi)^2
ni = number of molecules with mass mi
mi = the mass of a molecule
114
What is the heterogeneity index?
The ratio of Mm/Mn - for a mono disperse solution should be below 1.1
115
Why do we need averages for the weight of macromolecules?
Because they are not well behaved, they are polymers but it is not defined how many monomers make up the polymer so their weight varies considerably
116
How do different types of contaminant affect the Mn and Mm?
Low molecular weight contaminant: Makes Mn significantly lower but has small effect on Mm
High molecular weight: Raises Mm value but has no effect on Mn
117
What is the most accurate technique for determining molecular mass?
Mass spectrometry - particularly MALDI-TOF mass spectrometry
118
Which colligative properties are used to determine the effective molecular mass? Why are others not used?
Osmometry mainly as it gives a measurable effect even for large molecules
Crysoscopy, boiling point have comparatively very small changes (unmeasurable)
119
What are some of the issues with using osmometry to measure effective molecular mass?
Low mass impurities, dissociation of ionisable groups, non-ideality of macromolecule solutions
120
How do you convert the osmotic equation: pi/C'B = RT/Mr to be plotted?
Assuming [J] is c'B/Mr
pi = [J].R.T(1+B[J]).
So gradient = R.T.B/Mr^2
Intercept = RT/Mr
```
pi = osmotic pressure
c'B = mass concentration in g/dm^3
Mr = relative molecular mass (g/mol)
B = virial coefficient
```
121
What is the equation for the viral coefficient? What is it?
Virial coefficient is an indication of how much the solution deviated from ideal behaviour.
B = 4Na.v
```
B = virial coefficient
4 = 4
Na = Avogadro's number
v = average volume occupied by a single molecule
```
122
How do you use a graph to find B and v?
Plot a graph of pi/c'B (Y) over c'B (X).
pi/C'B = (RT/Mr) + RT(B/Mr^2)C'B
So gradient = R.T.B/(Mr^2)
Intercept = RT/Mr
Rearrange to find Mr --> RT/intercept
Find B from gradient (B = gradient x Mr^2/RT
Use B to find v
B = 4Na.v
123
What problem do polyelectrolyte solutions cause when measuring the molar mass of the macromolecule (using the number average)?
When macromolecules ionise the number of ions in solution can greatly outnumber the number of macromolecules. As colligative properties only depend on the number of solute molecules (and there are loads of macromolecule ions) it gives an inaccurate molar mass of the molecule (when using number average).
Measurements of osmotic pressure cannot distinguish between macromolecular ions and their counter-ions
124
How is the polyelectrolyte number average macromolecule problem solved? What is the name of the equilibrium set up?
Use a swamping electrolyte which provides a large excess of ionise that ionise from the macromolecules. Set up an osmotic membrane which allows small ions and solvent to move across - but not the macromolecules.
A difference in osmotic pressure forms which is governed by the concentration of the macromolecules
Donna equilibrium
125
Describe the process of ultracentrifugal sedimentation for separation of macromolecules. How is the molecular mass calculated?
Macromolecules placed in test tube, spun. The gravitational sinking competes with the thermal motion of the liquid and causes bands of different mass to form.
Mn = S.R.T/b.D
```
Mn = number weighted average molecule mass
S = sedimentation constant (related to drift speed)
RT = universal gas.temperature
b = buoyancy of the solvent (density & specific volume)
D = diffusion constant (related to frictional force)
```
126
During sedimentation, how may you remove the need to obtain accurate diffusion coefficients (D)?
Equilibrium sedimentation
Measure the relative concentration of the macromolecule at two different radii in the sample
127
What methods may be used to separate macromolecules?
1. Sedimentation (ultracentrifugation)
2. Electrophoresis
3. Gel permeation chromatography
4. Viscosity measurements
5. Light scattering
6. End group labelling
128
How may measuring the viscosity of a solution allow you to determine the macromolecular mass?
Determine viscosity by how long it takes solution to pass through capillary tube (ostwald viscometer)
Compare the time taken for the solution (at different concentrations) and time taken for a pure solvent can determine intrinsic viscosity ([eta] -which you can treat like the virial coefficient).
Mark Houwink equation: [eta] = K(Mv)^a
K, a are empirical constants determined for similar molecules with known mass.
There is an equation for viscosity averaged mass = Mv
129
How does the process of light scattering allow you to determine macromolecule mass? What is the equation used to determine this?
Macromolecules usually the right size to scatter visible light, making the solution cloudy. This can be quantified.
I/I0 = K.Mm.c'X
```
I = ratio of intensity of light scattered at a particular angle to the intensity of the incident light
I0 = incident light
K = constant for the instrument
Mm = mass weighted molar mass
c'X = mass concentration of macromolecule x
```
130
Describe the process of end group labelling
Chemically adding a known group Y* to a known total mass of macromolecule. Using a large excess of Y* (to ensure as many macromolecules are 'doped' as possible). Take the doped molecules and dissolve in solvent then probe the sample to determine Y* concentration in solution. As there is one Y* for each macromolecule you can find the number of macromolecules in the solution. From there you can find the effective RMM. Useful for DNA and other repeating structures.
