Solutions and Mixtures Flashcards
What is a phase?
A region with uniform chemical composition and physical properties
What is a phase diagram?
A diagram that shows the most stable phase under different thermodynamic conditions
What is the critical point in a phase diagram?
The point on the phase diagram where the liquid and vapour phases of a substance merge into a single phase
What is the supercritical region of a phase diagram?
The region of a phase diagram where the temperature and pressure is higher than the liquid-vapour phase diagram
What degree of freedom does the triple point have?
- F (Degrees of freedom) = 0
- This means that only one set of temperature and pressure values are allowed, if you change temperature and/or pressure, you’re no longer at the triple point
What degree of freedom does the boundary between two phases have?
- F = 1
- If you change T then P must change accordingly to stay on the same line
What degree of freedom does one phase region have, either liquid solid or vapour?
- F = 2
- You can change T and P independently and still be in the same area
What is the Gibbs Phase rule?
- F = C-P+2
- Where F is the degrees of freedom, C is the number of chemical components and P is the number of phases
How is the Gibbs phase rule derived?
- 2 = the maximum degrees of freedom
- CP = in each phase there are C components
- -P = Total mole fraction for each phase = 1
- -C(P-1) = Equilibrium condition for each component, subtract P-1 degrees of freedom x C components
- Add together, 2 + CP - P -C(P-1)
How do you determine the boiling point for a mixture from a phase diagram?
At the given composition of the mixture draw a line up until you hit the bottom of the l+v region
How do you estimate the composition of the vapour that boils off from a mixture of a certain composition?
Find the boiling point and draw a line from the liquid line of the l+v region to the vapour line, then draw a line down from the vapour line to get the composition of the vapour
What is an azeotropic mixture?
Mixtures of liquids where the liquid and vapour line merge into one and the mixture now has a constant boiling point
What degrees of freedom does the dew line of a phase diagram of a mixture have?
2, it has 2 phases (liquid and vapour) and 2 components assuming its a mixture of 2 components, 2-2+2 = 2
Why is the degree of freedom on a dew line 2, when the rule is that on the line degrees of freedom is 1?
- In the phase diagram of a mixture, pressure isn’t included in the graph, we can see from the graph that you can’t change temperature and mole fraction is independent so we would assume from the graph that degrees of freedom is 1
- Because the calculated value is 2, the extra degree of freedom must be accounting for something else and in this case it is accounting for the phase diagram having constant pressure
What is a liquid-liquid miscibility diagram?
- A diagram that has a curve where one side is a solvent rich phase and one side is a solute rich phase
- Between these two sides, the composition separates into two phases
What is the lever rule?
- From your starting composition, trace up to the given temperature, you will have a distance to (A) rich phase and (B) rich phase
- The (A) rich distance is the distance from your starting composition to the (B) rich phase
- The (B) rich distance is the distance from your starting composition to the (A) rich phase
- If the question asks for the ratio of (A) rich phase to (B) rich phase then the equation is: (A) rich phase / (B) rich phase
What is the upper critical solution temperature?
The temperature above which when the mixture passes, the phases combine, no matter what the starting composition of the mixture is
Why is a eutectic mixture good for solubility?
The eutectic point is the composition of the mixture where melting point is at its
What is the Clapeyron equation?
- dP/dT = Δs/Δv = ∆h / T∆v, ∆s can be replaced with ∆h/T as on the slope ∆G = 0 as it is the equilibrium between 2 phases, therefore 0 = ∆h - T∆s, T∆s = ∆h, ∆s = ∆h/T
- Where dP/dT is the gradient of the slope separating two phases
- ∆s is the molar entropy change
- ∆v is the molar volume change
- ∆h is the molar entropy change
When a dP/dT slope is negative, what does this tell us about melting point as pressure increases?
- As dP/dT is negative, we know that dT/dP is also negative
- This means that as you increase pressure, melting point also decreases
What is the Clausius-Clapeyron equation?
- dP/dT = P∆s / RT = P∆h
- The phase above the slope is liquid/solid, the phase below the slope is gas
- ∆s is the molar entropy change
- ∆v is the molar volume change
- ∆h is the molar entropy change, ∆h(gas) - ∆h(liquid)
In the Clausius-Clapeyron equation, ∆v is the molar volume change, gas volume - (liquid/solid volume). Why can we take ∆v as simply the gas volume?
The liquid/solid volume is much smaller than the gas volume, hence can be omitted and we can use the gas volume as ∆v
Why does decane have a higher ∆h than hexane?
- Decane is a larger molecule, this leads to stronger Van der Waals interactions h(liquid) of decane is therefore lower than that of hexane
- The h(gas) of both molecules is similar meaning that ∆h is larger for decane as there is larger difference in enthalpy between the liquid and vapour forms of decane
Why is it impossible for a liquid-vapour boundary phase to be negative?
- Negative slope means negative ∆h
- ∆h = h(gas) - h(liquid) < 0
- hence h(gas) < h(liquid) which is impossible because gases have more energy than liquids
