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MCAT GC WORKING > Phases and Equilibria > Flashcards

Phases and Equilibria Flashcards

1
Q
A
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2
Q

Define and give the three examples of:

phase

A

A phase is a physically distinct form of a substance that can be separated from another form. Generally phase is used interchangeably with “state of matter”.

Solid, liquid, and gas are the three phases of matter that the MCAT tests.

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3
Q

What two state functions can be used to actively change the phase of a a substance?

A

Temperature (Heat, Enthalpy) can be added or removed. In general, an increase in temperature drives the phase change solid⇒liquid⇒gas.

Pressure (Volume) can be added or removed. In general, an increase in Pressure (or decrease in Volume) drives the phase change gas⇒liquid⇒solid.

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4
Q

What name is associated with a phase change from

  1. solid to liquid?
  2. liquid to solid?
A
  1. solid⇒liquid is fusion (melting)
  2. liquid⇒solid is freezing (solidification)
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5
Q

What name is associated with a phase change from

  1. gas to liquid?
  2. liquid to gas?
A
  1. gas⇒liquid is condensation
  2. liquid⇒gas is vaporization (boiling)
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6
Q

What name is associated with a phase change from

  1. gas to solid?
  2. solid to gas?
A
  1. gas⇒solid is deposition
  2. solid⇒gas is sublimation
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7
Q

What do the letters A, B, and C represent in this diagram?

A
  • A represents the Solid phase region.
  • B represents the Liquid phase region.
  • C represents the Gas phase region.
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8
Q

What phase conversions are being shown with arrows A and B?

A
  • A is Fusion (or Melting)
  • B is Freezing (or Solidification)
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9
Q

What phase conversions are being shown with arrows A and B?

A
  • A is Vaporization (or boiling)
  • B is Condensation
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10
Q

What phase conversions are being shown with arrows A and B?

A
  • A is Sublimation
  • B is Deposition
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11
Q

What points are arrows A and B pointing to?

A
  • A is the Triple Point.
  • B is the Critical Point.

Note: though the concept of Plasma exists, the MCAT does not explicitly test this as a phase.

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12
Q

Explain the significance of the critical point.

A

The critical temperature and critical pressure combine to be the critical point:

  • the critical temperature is the temperature above which a distinct liquid-to-gas vaporization can no longer be accurately deteremined.
  • the critical pressure is the pressure above which a distinct gas-to-liquid condensation can no longer be accurately determined.
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13
Q

Explain the significance of the triple point.

A

The triple point is the point at which a substance can exist in equilibrium in all three states (solid, liquid, and gas).

This means that instantaneously, a substance at its triple point can interconvert between any phase.
Ex: water at its triple point would exist as ice, liquid water, and steam - all at one temperature and pressure.

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14
Q

How does the phase diagram of water differ from the one below?

A

Water has a slightly negative sloped Solid/Liquid boundary.

This is due to water’s Solid phase being less dense than its Liquid phase.

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15
Q

Different materials require different applications of heat in order to transition into a new phase. Why is this?

A

The difference is due to intermolecular forces.

The attraction of molecules to each other determines at which temperature mixtures will have phase changes, and subsequently the amount of heat necessary to make that transition.

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16
Q

What types of intermolecular forces exist?

A

From stongest (1) to weakest (4), they are:

  1. Ionic Forces
  2. Hydrogen Bonds
  3. Dipole-Dipole
  4. London Dispersion Forces
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17
Q

Define:

dipole forces

A

Dipole forces occur when a molecule with polar bonds has the geometry to become overall polar. This results in partially positive and negative charges, and these opposite charges attract other charged molecules in the mixture.

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18
Q

Define:

hydrogen bonds

A

Hydrogen bonds are a product of H being covalently bonded to either O, N, or F. This is an extreme form of the dipole-dipole force.

The high electronegativity difference between these atoms and hydrogen creates strong dipoles, which consequently result in the strongest dipole-dipole interactions between molecules.

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19
Q

Define:

London Dispersion forces

A

Dispersion forces (also called Van der Waals forces) are an instantaneous polarization of molecules that would otherwise be non-polar.

This process is also referred to as an induced dipole because there is an instantaneous reorganization of the electron cloud leading to partial polarization. The more valence electrons present in either a molecule or in a mixture, the more possible repulsion of electron clouds, and the more induced dipoles.

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20
Q

Define:

ionic forces

A

Ionic forces occur when two oppositely charged ions (cations and anions) attract each other.

In general, ions attract to a polar molecule first, then attract each other - hence this is often called Ion-Dipole force. (Two Ions attracting each other would otherwise prefer to Ionic Bond, which is an intramolecular force, not an intermolecular force.)

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21
Q

Describe how the boiling point of a substance changes depending on the strength of the intramolecular forces present?

A

The stronger the intramolecular forces present, the higher the boiling point. This is due to the molecules being more tightly attracted to each other is the liquid phase.

In order of highest (1) to lowest (4) boiling points:

  1. Ionic Forces
  2. Hydrogen Bonds
  3. Dipole-Dipole
  4. Dispersion Forces
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22
Q

List forces from strongest to weakest:

Dipole-Dipole
Hydrogen Bonds
London Dispersion Forces
Ionic Forces

A
  1. Ionic Forces
  2. Hydrogen Bonds
  3. Dipole-Dipole
  4. London Dispersion Forces
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23
Q
A
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24
Q

Define:

Hv

A

Hv is the heat of vaporization. It represents the energy necessary to convert a liquid to a gas.

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25
Q

Define:

Hf

A

Hf stands for the heat of fusion. This is the amount of energy that is necessary to convert a solid to a liquid.

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26
Q

What processes are associated with the plateaus A and B, for the general heating curve below?

A
  • At A, fusion is occurring.
    Hf is used to calculate the heat needed for plateau A.
  • At B, vaporization is occurring.
    Hv is used to calculate the heat needed for plateau B.
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27
Q

Why are the slopes zero for plateaus A and B?

A

Zero slope means no temperature change. This occurs at these times because any heat input is dedicated to breaking bonds between molecules, instead of increasing temperature.

Ex: during the vaporization of water, heat is needed to break the hydrogen bonds between liquid water molecules and convert it to gaseous steam.

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28
Q

What equation determines the heat added in the sections where the temperature is rising in the below diagram?

A

q = mcΔT
This formula calculates heat required to raise a certain mass of a substance by a change in temperature ΔT.

where:

  • q = heat required in J
  • m = the sample’s mass in g
  • c = the substance’s specific heat in J/g*K
  • ΔT = temperature change in K
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29
Q

What is the value of specific heat (c) for water?

A

Water’s specific heat is c = 1 cal/gºK.

In SI units, c ≈ 4.2 J/gºK.

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30
Q

What is the equation to calculate heat required when at the plateaus?

A

q = mHL
This formula calculates heat required to change a certain mass of a substance from one phase to another. HL would be Hf for the first plateau and Hv for the second plateau.

where:

  • q = heat required in J
  • m = mass in g
  • HL = latent heat of phase change
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31
Q

How much heat would be required to increase the temperature of 20g water by 5 degrees, starting at 20 ºC?

A

100 cal.

At 20 ºC, water is a liquid and has specfic heat c = 1 cal/gºC.

Since temperature is changing and phase is not changing within the temperature range 20-25, use the formula q = mcΔT.

hence: q = 20(1)(5) = 100cal.

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32
Q
A
33
Q

Define:

A colligative property

A

A colligative property of a mixture is one that only depends on the number of molecules of solute dissolved in the solvent, not the solute’s chemical nature.

Ex: A solvent’s vapor pressure, boiling point, freezing point, and osmotic pressure are all colligative properties.

34
Q

Explain the relationship between boiling point and vapor pressure.

A

Boiling occurs when the vapor pressure above a liquid equals the surrounding pressure of the environment.

35
Q

Define and give the equation for:

mole fraction

A

Mole fraction is the ratio of moles of a particular substance present to the total moles of all substances present.

xA = nA / ntotal

where:

  • xA = mole fraction of A
  • nA = number of moles of A
  • ntotal = total number of moles of all substances present
36
Q

Define:

Raoult’s Law

A

Raoult’s Law: Ptotal = PAxA + PBxB + …

  1. The partial pressure of a liquid in a mixture is equal to the vapor pressure of the pure liquid times its mole fraction in the mixture.
  2. The total pressure of the mixture is the sum of all the partial pressures.
37
Q

If given a 50/50 mixture of water and acetone, at STP, what is the vapor pressure from acetone above the fluid?

A

5 atm.

Raoult’s Law: Ptotal = PAxA + PBxB + …

The partial pressure of a liquid in a mixture is equal to the vapor pressure of the pure liquid times its mole fraction in the mixture. In a 50/50 mixture of water and acetone, each one will only contribute 1/2 the total vapor pressure; hence acetone will have 1/2 the vapor pressure of pure acetone at STP.

38
Q

Define:

the van’t Hoff factor

A

The van’t Hoff factor, i, is the number of particles that an ionic solute yields upon dissociation.

Ex: NaCl dissociates into Na+ and Cl- ions, a total of 2 per equivalent of NaCl. The van’t Hoff factor for NaCl is i=2.

39
Q

What are the van’t Hoff values for:

  1. KOH
  2. C6H12O6
  3. H2SO4
  4. CaCl2
  5. H3[CuNH3Cl5]
A
  • KOH: i = 2 (K+ and OH-)
  • C6H12O6: i = 1 (glucose doesn’t dissociate)
  • H2SO4: i = 3 (2H+ and SO42-)
  • CaCl2: i = 3 (2Cl- and Ca2+)
  • H3[CuNH3Cl5]: i = 4 (3H+ and [CuNH3Cl5]3-)
    note: on the MCAT you are expected to know that a complex ion (in brackets) will not further dissociate.
40
Q

define:

Molality (m)

A

Molality is defined as the number of moles of solute dissolved into a certain mass of solvent, with units mol/kg.

Molality uses the lowercase letter m in equations, be careful not to confuse this with mass.

41
Q

Give the equation:

for boiling point elevation of a mixture.

A

ΔTb = Kbmi

where:

  • Kb= is the solvent’s ebullioscopic constant in kg*K/mol
  • m = the mixture’s molality in mol/kg
  • i = the solute’s van’t Hoff factor
  • ΔTb = boiling point temperature change in K
42
Q

Which will have a higher boiling point: a 1m NaCl solution or a 1m glucose solution?

A

The NaCl solution will have the higher boiling point.

ΔTb = Kbmi. i(NaCl) = 2, while i(glucose) = 1, so the NaCl solution will have twice the change in boiling point.

43
Q

When can the boiling point elevation equation not be used?

A

The boiling point elevation equation cannot be used when:

  1. the mixture is volatile
  2. the solute does not fully dissolve or dissociate
  3. there is a significant density difference between the mixture components such that one always sits on top of the other
44
Q

How does addition of a solute affect the freezing point of a liquid?

A

The freezing point of a liquid will be lowered (colder), as the solute hinders proper crystalization of the solvent molecules.

45
Q

Identify the equation for:

freezing point depression of a mixture.

A

ΔTf = Kfmi

where:

  • Kf= is the solvent’s cryoscopic constant in kg*K/mol
  • m = solution molality in mol/kg
  • i = solute’s van’t Hoff factor
  • ΔTf = change in freezing point temperature in K
46
Q

Which will have a lower freezing point, 1m CaCl2 solution, or a 2m CaCl2 solution?

A

The freezing point of the 2m solution will be lower.

Based on the equation ΔTf = Kfmi the 2m solution will have twice the drop in freezing point of the 1m solution.

47
Q

A chemist wants to melt the ice on her driveway on a very cold day, and has equal amounts of each of the following substances to add to the ice. Based on colligative properties, which would work the best?

  1. H3PO4
  2. MgCl2
  3. C12H22O11
A

H3PO4 will melt the ice the most effectively.

Assuming equimolal addition, H3PO4 will yield the largest decrease in the water’s freezing point, due to its having the highest van’t Hoff factor.

  • i(H3PO4) = 4 (phosphoric acid)
  • i(MgCl2) = 3 (magnesium chloride)
  • i(C12H22O11) = 1 (sucrose)
48
Q

Which of the following will have the lowest freezing point?

  1. 1M BrCl (aq)
  2. .5M Na2SO3 (aq)
  3. 1.5M KCl (aq)
A

1.5M KCl (aq) has the lowest freezing point.
Freezing point depression is depedent on the number of solute atoms per volume:

  • BrCl = 1M x 2ions = 2
  • Na2SO3 = .5M x 3ions = 1.5
  • KCl = 1.5M x 2ions = 3
49
Q

Describe the differences between:

  • Hypertonic
  • Isotonic
  • Hypotonic
A

When comparing two solutions relative to each other, one is usually in a cell or semipermeable container and is the “standard” that we compare the other solution against.

  • Hypertonic: higher concentration of solute than the standard
    a cell hypertonic to its environment will shrink or crenate
  • Isotonic: same osmolarity as the standard
    equal water into/out-of, no change in volumes
  • Hypotonic: lower concentration of solute than the standard
    a cell hypotonic to its environment will bloat or rupture
50
Q

Define:

osmotic pressure

A

Osmotic pressure is the pressure that results from osmosis.

As water molecules move across a semi-permeable membrane to create isotonicity on both sides, the pressure of both sides will change; this pressure is the osmotic pressure.

Ex: If red blood cells are placed in a hypo/hypertonic environment, they will expand/contract in response.

51
Q

Give the equation for:

osmotic pressure

A

π = nRTi
V

where:

  • n = number of moles of solute
  • R = ideal gas constant in J/K*mol
  • T = temperature in K
  • i = solute’s van’t Hoff factor
  • V = volume in L
52
Q

Define:

colloid

A

A Colloid is a system where one substance is microscopically dispersed evenly throughout another substance.

Colloidal particles will not eventually settle to the bottom due to gravity and time, and can occur in any of the three phases of matter.

Ex: air is a gaseous colloid of many gases, milk is a liquid colloid of liquid butterfat in aqueous solution, colored glass is a solid colloid of metal oxides in a silica matrix.

53
Q

What are the common characteristics of colloids?

A
  • Colloids have a continuous phase (dispersion medium) and an internal phase (dispersed medium).
  • The internal phase particles are too small to easily extract physically (though it is possible to extract them chemically).
  • Liquid and solid colloids scatter light (opaque or translucent); only gas colloids can be colorless.
54
Q

Define:

the Tyndall effect

A

The Tyndall effect is demonstrated when light hits a colloid, and is scattered by the suspended particles.

Ex: common baking flour suspended in water will look slightly blue, because blue light scatters back more strongly than red light.

55
Q

Define and give the equation for:

Henry’s Law

A

Henry’s Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid.

Assuming constant Temperature, the equation is:
Pv = c*kH

where:

  • Pv = partial pressure of the soluble gas above the solution in atm
  • c = concentration of the soluble gas in mol/L
  • kH = Henry’s Law constant of the liquid in L*atm/mol
56
Q

Explain why a can of soda will “hiss” when opened, but eventually go “flat”, based on Henry’s Law?

A

The soda is carbonated under high pressure of CO2. The CO2 is kept in solution by a high pressure of CO2 in the can, causing the “hiss” when the CO2 escapes quickly.

Once the CO2 dissipates, however, the partial pressure of CO2 above the soda drops to atmospheric levels, near zero. Henry’s Law predicts that this will lead to the amount of CO2 dissolved in the solution decreasing over time.

57
Q
A
58
Q

Define:

Standard Temperature and Pressure (STP)

A

STP is 0º C and 1 atm

The MCAT occasionally refers to Standard Ambient Temperature and Pressure (SATP), which you should know is 25ºC and 1 atm.

59
Q

What are the assumptions of:

the kinetic molecular theory of gases

A

The kinetic molecular theory assumes an idealized version of gas, which makes calculating relationships easier. There are four assumptions:

  1. A gas molecule has no volume (point molecule).
  2. The collisions between gas molecules are completely elastic.
  3. There are no dissipative forces due to collisions.
  4. The average kinetic energy of gas molecules is directly proportional to the temperature of the gas.
60
Q

Give the relationship and equation for:

Charles’s Law

A

Charles’ Law states that the volume of a gas is directly proportional to temperature while at a constant pressure.

Equation:

61
Q

If the pressure is held constant of an ideal gas system, but the temperature is doubled, what does Charles’s Law predict will happen?

A

Volume will double also.

Charles’s Law indicates that at a constant pressure, the temperature and volume of a gas are directly proportional.

62
Q

Give the relationship and equation for:

Boyle’s Law

A

Boyle’s Law states that the volume of a gas is inversely proportional to its pressure while at a constant temperature.

Equation:

63
Q

If the temperature is held constant of an ideal gas system, but the pressure is reduced by 1/2, what does Boyle’s Law predict will happen?

A

Volume will double.

Boyle’s Law indicates that at a constant temperature, the pressure and volume of a gas are inversely proportional.

64
Q

Give the relationship and equation for:

Avogadro’s Law

A

Avogadro’s Law states that the volume of a gas is directly proportional to the number of moles at a constant temperature and pressure.

V / n = k

where:

  • V = volume in L
  • n = number of moles
  • k = proportionality constant of the specific gas
65
Q

If the pressure and temperature are held constant for an ideal gas system, but the number of moles is reduced to 1/3 of the original value, what does Avogadro’s Law predict will happen?

A

Volume will decrease to 1/3 also.

Avogadro’s Law indicates that at a constant pressure and temperature, the number of moles and volume of a gas are directly proportional.

66
Q

Give the equation for:

Ideal Gas Law

A

The Ideal Gas Law combines Charles’, Boyle’s, and Avogadro’s Laws into one:

PV = nRT

where:

  • P = Pressure in atm or Pa
  • V = Volume in L
  • n = Number of moles
  • R = Ideal gas constant .082 L(atm)/mol(K) or 8.31 J/mol(K)
  • T = Temperature in K
67
Q

What is the volume of 1 mole of gas molecules at STP?

A

At STP, one mole of gas has a volume of 22.4 L

This is called the standard molar volume, and is true for a mole of any ideal gas at STP.

This value can be calculated using the Ideal Gas Law, but is worth memorizing for the MCAT.

68
Q

Give the equation for:

calculating the partial pressure of a gas

A

P<span>A</span> = x<span>A</span>Ptotal

where:

  • PA = pressure from gas A
  • xA = mole fraction of A
  • Ptotal = total pressure of the sytem from all moles of gas
69
Q

Give the equation for:

Dalton’s Law

A

Ptotal = PA + PB + PC + …

Dalton’s Law states that the total pressure of a system of ideal gases can be thought of as the sum of all of the partial pressures that each gas exerts.

70
Q

Give the equation:

for internal energy (average kinetic energy) of a gas

A

U = KEavg = (3/2)nRT

where:

  • U = potential internal energy in J
  • KEavg = average kinetic energy in J
  • n = number of moles of gas
  • R = Ideal gas constant .082 L(atm)/mol(K) or 8.31 J/mol(K)
  • T = Temperature in K
71
Q

When does a gas deviate from its ideal state?

A

Gases deviate from ideal at:

  1. Low temperatures
  2. High pressures ( or very low volume)
72
Q

Why does low temperature and high pressure cause a gas deviate from its ideal state?

A

Gases deviate from ideal at low temperatures, because the molecules have very low kinetic energy (hence low velocity) and will start to attract based on the intermolecular forces.

Gases deviate from ideal at high pressures or very low volumes, because the molecules have very little actual space to move in and will start to exhibit characteristics more like a fluid than a gas.

73
Q

Give the equation:

Van Der Waals Equation

A

(P + a(n/V)2)(V - nb) = nRT

where:

  • P = Pressure in atm or Pa
  • V = Volume in L
  • n = number of moles
  • R = Ideal gas constant .082 L(atm)/mol(K) or 8.31 J/mol(K)
  • T = Temperature in K
  • a = attraction between molecules due to intermolecular forces
  • b = actual volume taken up by gas molecules
74
Q

Explain whether a polar or nonpolar real gas will deviate more from ideal?

A

Polar gases will deviate more from ideal.

(P + a(n/V)2)(V - nb) = nRT

Recall: ‘a’ is the attractiveness between molecules. When ‘a’ is big (polar=attractive) the Pressure term will be larger. Effectively, the gas will experience more pressure holding it together than the ideal gas law predicts.

75
Q

Explain whether a small or large molecule size real gas will deviate more from ideal?

A

Larger molecule gases will deviate more from ideal.

(P + a(n/V)2)(V - nb) = nRT

Recall: ‘b’ is the bigness of the actual molecules. When ‘b’ is big (larger size=big) the Volume term will be smaller. Effectively, the gas will have less space to move in than the ideal gas law predicts.

76
Q

Define:

absolute temperature

A

The absolute temperature is any temperature that is given in Kelvin (K).

0 K is the temperature at which all molecules are assumed to cease moving completely. To convert Kelvin to Celcius:
K = C + 273.15

77
Q

Define:

entropy

A

Entropy (S) is the measure of the disorganization of a system

The entropy of the universe will always increase for spontaneous reactions. It is possible for entropy to have negative values, but energy would need to be applied.

78
Q

A plate smashes on the floor, explain what happens to the entropy of the plate-floor system?

A

Entropy increases.

Since entropy is the disorder of the system, and the molecules of the plate are now inherently less ordered, entropy must have increased.

79
Q

Which law addresses the solubility of gases in water?

A

Henry’s Law, which addresses how pressure on gases affects their solubility

MCAT GC WORKING (11 decks)

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