11C - Phase changes and Diagrams

Question 1

Def: What are the six state transitions.

Answer 1

1/2. Melting and freezing btw solid and liquid
3/4. Vaporization and condensation btw liquid and gas
5. Deposition (Gas to solid)
6. Sublimation (Solid to gas)

Question 2

Def: What is an endothermic change?

Answer 2

A process that requires an input of energy.

Question 3

Def: What is the enthalpy of vaporization? Formula?

Answer 3

Heat added to a liquid to transform it into a gas by breaking attractive intermolecular forces.
H of vap = -H of con = H of vap - H of liq

Question 4

Theory: What three things lead to evaporation?

Answer 4

1. Increased temp
2. Increased surface area of liquid
3. Decreased strength of intermolecular forces

Question 5

Def: What is an exothermic change?

Answer 5

Process that releases energy.

Question 6

Def: What is the enthalpy of fusion? Formula?

Answer 6

Heat added to a solid to transform it into a liquid by reducing attractive intermolecular forces.
H of fus = -H of solidification = H of liq - H of solid

Question 7

Theory: Compare enthalpy of vaporization and enthalpy of fusion. What do the magnitudes of these enthalpies increase with?

Answer 7

Vap > Fus
They increase with IMF’s.

Question 8

Def: What is the formula for the enthalpy of sublimation. What thermochem property does it use?

Answer 8

deltaH-sub = deltaH-vap + deltaH-fus
Uses Hess’ law.

Question 9

Theory: Within a phase, heat flow is accompanied by what? What formulas can be therefore be used for heat change?

Answer 9

A change in temp or ave KE.
q = mCt of q =nCt

Question 10

Theory: What unit(s) can heat capacity be?

Answer 10

J/gdeg-C or J/moldeg-C depending on mass or moles used in q formula

Question 11

Theory: During a phase change, heat flow occurs at what? What is changing in a phase change? What formulas can be used?

Answer 11

Constant temp.
Ave dist btw particles changes.
q = ndelta-H-phase-change
q = mdelta-H-phase-change

Question 12

Def: What is dynamic eqm, in a solution? When does it occur?

Answer 12

Where molecules are leaving and entering liquid at the same rate.
Rate-vap = Rate-cond
In a closed system.

Question 13

Def: What is vapor pressure? In a liquid, how would the vapor pressure vs time graph look? What phase is it unique to?

Answer 13

Pressure exerted by vapor on a liquid.
Vapor pressure would increase until eqm, then stay constant.
Unique to liquids only.

Question 14

Def: What is the formal definition of vapor pressure?

Answer 14

Partial pressure of molecules in the gas state at a given temp.

Question 15

Theory: What is rate-vap and rate-cond dependent on? What is the relationship between them in general?

Answer 15

Vap: Temp, IF’s and SA of liquid
Cond: Vapor pressure
The two are independent of each other until eqm is reached.

Question 16

Def: What is a volatile substance. What is true about their IF’s? What is the relationship between volatility and vap-pressure.

Answer 16

Has a vapor pressure at ordinary temperature.
Weak IF’s.
More volatile = higher vapor pressure

Question 17

Def: What is related by the Clausius - Clapeyron eqx? What is the equation?

Answer 17

Vapor pressure, enthalpy and temp.
ln(P) = - (delta-H/R)(1/T)

Question 18

Theory: What are the 4 steps in assessing the IF’s of a molecule? What is the relationship btw IF’s and vapor pressure.

Answer 18

1. Molecular Structure
2. Polar Bonds
   - 0 - 0.5 is N-P
   - 0.5 - 1.7 is P
   - 1.7+ is Ionic
3. Determine Polarity from steps 1 and 2
4. Look at IMF’s
   More IF’s means lower VP

Question 19

Theory: What does VP increase with? C-C eqx for change in vap pressure. (what is it?)

Answer 19

Temperature.
ln(P2/P1) = - (delta-H-vap/R)(1/T2 + 1/T1)

Question 20

Def: What is bp? What is normal bp? What is the relationship between external pressure and bp?

Answer 20

bp: Temp at which vap pressure equals external pressure
nbp: Observed at STP or 760 torr.
bp is proportional to external pressure,

Question 21

Def: What is sublimation? Formula(s)? What is the heat of sublimation?

Answer 21

Phase trans from solid to gas
delta-H-sub = delta-H-fus + delta-H-vap
delta-H-sub = delta-H-dep

Heat required to bring one mole of substance from solid to gas.

Question 22

Def: If we can control the temp and pressure, we can control the ___?

Answer 22

Phase

Question 23

Def: What are phase diagrams? What are boundaries of phase diagrams? What is the triple point? What is the critical point, where is it, and what is beyond it?

Answer 23

Stable phases that a substance occupies at varying pressures and temps.

Temps and pressures where two phases are in eqm.

The temp and pressure where three states exist in eqm.

The point of high T and high P, beyond gas and liquid phases, forming supercritical fluids.

Question 24

Theory: What are the general areas of the phase diagram in terms of P and T.

Answer 24

Low/High P, and Low T = solid (left)
Low P and low/High T = Gas (bottom)
High T and high P = Liquid (Top)
Very high T and P = SC Fluid (top-Right corner)