Gas & Liquid Properties

Question 1

What is the Ideal Gas Law?

Answer 1

P V = n R T

Pressure (atm) * Volume = number moles * gas constant * Temperature (Kelvin)

R=.082

Question 2

What is the Kinetic Molecular Theory?

Answer 2

KE = (1/2)(mass)(velocity)²

• At the same Temp, all gases have the same average KE.
  
  • Speed increases with temperature, and decreases with mass.
  • Heavier gases are going to move slower but will have the same KE at that temperature.
• Sidenote: At the same temperature, pressure, and volume, there will also be the same number of molecules. PV=nRT

Question 3

Direct and Inverse relationships:

Answer 3

Just look at the PV = nRT fomula.

Inverse if both in numerator on same side of equal sign.

Question 4

How do you relate pressure and volume, if everything else is constant?

PV = nRT

Answer 4

P₁V₁ = P₂V₂

Question 5

How do you relate volume and temperature, if everything else is contant?

PV = nRT

Answer 5

V₁/T₁=V₂/T₂

Question 6

How do you relate Pressure and Temperature if everything else is constant?

(PV=nRT)

Answer 6

P₁/T₁=P₂/T₂

Question 7

What is Standard Molar Volume?

Answer 7

• Equal volumes of gases at the same T and P have the same number of molecules.
• The volume for 1mol of gas at Standard Temperature (273 K) and Pressure (1.00 atm) is 22.4 Liters.

Question 8

Convert from Celsius to Kelvin

Answer 8

Add 273

T = ^oC + 273 = K

Question 9

Convert from mm Hg to atm

Answer 9

Divide by 760

1 atm=760 mm Hg

Question 10

What are the equations to calculate density?

Answer 10

d = m/v

d = PM/RT

Question 11

Steps for gas stoichiometry problems

Answer 11

Question 12

What is diffusion?

Answer 12

Diffusion is the gradual mixing of molecules of different gases.

• The rate of diffusion (or of effusion) of a gas is directly proportional to temperature and inversely proportional to its molar mass (technically the square root of it)

Question 13

What is effusion?

Answer 13

EFFUSION is the movement of molecules through a small hole into an empty container.

• Ex: air escaping knotted balloon through pores
• The rate of effusion (and diffusion) is directly proportional to Temperature, and inversely proportional to Molar mass.

Question 14

What is Dalton’s Law of Partial Pressures,

and how could you calculate the partial pressure of each component?

Answer 14

• P_total in gas mixture = P_A + P_B + …
• Partial pressure of component A = the moles of A divided by total moles (aka the mole fraction of A) X Total Pressure

Question 15

What is the combined gas law?

Answer 15

PV/T is a constant

Question 16

Describe the deviations from the ideal gas law, and the constants in the van der Waals equation

Answer 16

• The real Pressure is smaller than the ideal Pressure as a result of attractive forces between real gas molecules. They don’t go directly where they’re headed so hit wall less frequently.
  
  • a is for attraction. Larger a value means stronger attractive forces. ADD to P to get ideal.
• The real Volume is larger than the ideal Volume because real molecules do take up some space.
  
  • b is for bigness. Larger b value means larger molecular sizes. Subtract from V to get ideal.

Question 17

What are the 5 types of intermolecular forces, in order of their typical strength?

Answer 17

1. Ion-dipole attractions
2. Hydrogen bonding (type of dipole-dipole attraction)
3. Other Dipole-dipole attractions
4. Dipole-induced dipole attractions
5. Induced dipole-induced dipole attractions. (aka London Dispersion Forces, Van der Waal’s Forces)

Question 18

What are ion-dipole attractions?

Answer 18

• You have to have ions. The charged end of the ion interacts with a polar molecule (like water).
  
  • Look for polar molecule that dissociates and polar solvent (water).
  • Acids and bases in water will have ion dipole forces.
• Similar to ion-ion except you have one ion and a partial ion which is a dipole.
• The force of attraction depends on size of the molecule (how close can the nuclei get) and the size of the charge (Coulomb’s law)
• The greater the force of attraction, the more exothermic the hydration enthalpy

Question 19

What are dipole-dipole attractions?

Answer 19

• Two permanently polar molecules will be attracted to each other. (Whether the same or different substances.)
• Hydrogen bonding is the strongest type of dipole-dipole.
  
  • Both molecules need to have a Hydrogen bond between H directly with FON. So both have to have either H-F H-O or H-N.
• You can compare the strength for dipole dipole interactions if MM is similar - strength depends on how electronegative the central atom is.

Question 20

What is hydrogen bonding?

Answer 20

• Hydrogen bonding is the strongest type of dipole-dipole attraction.
• Special because nitrogen, oxygen and fluorine are very electronegative and often accompanied by lone pairs in structure. The hydrogen is attracted to their lone pair.
• Both molecules need to have a Hydrogen bond between H and either F,O,N.
  
  • Tip: having fon
  • Look for H-O H-F H-N

Question 21

What are induced-dipole dipole attractions?

Answer 21

• Occurs if there are polar and nonpolar molecules in a sample.
• One of the molecules is permanently polar, and induces a nonpolar molecule to have a temporary dipole.
  • The larger a molecule is, the more polarizable it is.
  • If it’s in water, you’re going to see induced dipole

Question 22

What are Induced dipole-induced dipole attractions?

aka London Dispersion Forces

aka Van der Waal’s Forces

Answer 22

• Everything will have them. They are the only attractive forces that exist between nonpolar molecules. Polar molecules have them too.
• If it so happens that the electrons are all on the other side temporarily, the molecule can induce another temporary dipole moment.
• Weak force but higher in molecules with larger molar mass (larger electron cloud, and ↑ polarizability)

Question 23

What is surface tension?

Answer 23

• Molecules at the surface experience net inwards force of attraction.
  
  • Water molecules at the surface are pulled inwards by its neighbors below (hydrogen bonding)

Question 24

What is wetting?

Answer 24

• The spreading of a liquid across a surface to form a thin film
• The IMF between the liquid and surface are about the same strengh as the IMF within the liquid
• So the surface must be hydrophillic
  
  • If the surface is hydrophobic, it does not have strong IMF forces with water. So the water molecules pull inwards, so only the fewest possible water molecules touch the surface.

Question 25

What is viscosity?

Answer 25

• Resistance to change in form (less flowy)
• Influenced by intermolecular attractions and by molecular shape and size
  
  • For molecules of similar size, as the strength of the IMF increase, so does viscosity

Question 26

What is capillary action?

Answer 26

• The movement of a liquid through or along the surface of another material in spite of other forces, such as gravity
• Capillary action occurs when adhesive forces (attraction to diff particles) outweigh cohesive forces.
• Ex: meniscus in glass, absorbance with paper towel

Question 27

What is vapor pressure and how does it relate to boiling point?

Answer 27

• Vapor pressure is the pressure exerted by vapor over liquid in a closed container.
• When vapor pressure = atmospheric pressure, that’s the boiling point.
• Very strong IMF’s make it harder for molecules to vaporize. So they have a lower overall vapor pressure, and require more heat, higher boiling point.

Question 28

heat of fusion vs. heat of vaporization?

Answer 28

• Heat of fusion: heat required to melt a solid
• Heat of vaporization: heat required to vaporize a liquid
  
  • Both are determined by the strength of the Intermolecular Forces. Substances with high IMF will have higher melting and boiling points. It will require more energy to break the IMF.

Question 29

What is enthalpy of hydration?

Answer 29

• The energy that is released when an ion is surrounded by water
  
  • How much more stable the ion is when hydrated.
• Negative enthalpy, because an ion surrounded by water is more stable.
• The force will be of greater magnitude if the ion is smaller (Coulomb’s law)
  
  • ​Can also think that with larger ions, the H2O can’t get as close, and there is a weaker ion-dipole interaction.

Question 30

Relationship between boiling point and polarity?

Answer 30

The more polar, the higher the boiling point.

(requires more energy to break greater IMFs.)

Question 31

What are the six phase changes and their associated energies?

Answer 31

• Freezing: liquid to solid. Exothermic.
• Melting: solid to liquid. Endothermic.
• Boiling (vaporization): liquid to gas. Endothermic.
• Condensation: gas to liquid. Exothermic.
• Sublimation: solid to gas. Endothermic. (It’s the most endothermic.)
• Deposition: gas to solid. Exothermic.

Hint: when you’re going into a higher phase, you have to use up energy to break the IMF.

Question 32

When is the boiling point higher?

Answer 32

• Generally the boiling point is higher with heavier molecules
• However, small molecules can have higher boiling point if there are strong IMF’s, like hydrogen bonding!
  
  • BP of H₂O, HF, and NH₃ are higher

Question 33

What are the consequences of hydrogen bonding in water and ammonia?

Answer 33

• It’s harder to boil
• It’s less dense when a solid because of the length of the bond (ice floats)

Question 34

What is polarizability?

Answer 34

• The degree to which electron cloud can be distorted
• The larger the molecule, the more loosely held the electrons, and the more easily it is polarized
• More easily polarized –> more soluble
• The bigger a molecule is, the easier it is to push the electrons around, polarize it, and dissolve it.

Question 35

What is an isothermal process?

Answer 35

• It means that the process is happening at the same temperature throughout
• Phase changes are isothermal.
  
  • Heat is being used to break the intermolecular forces, not increase temp

Question 36

What is volatility, and what is it related to?

Answer 36

• Volatility is the tendency of a substance to vaporize.
• Directly related to the vapor pressure. Indirectly related to boiling point.
• A more volatile substance has weaker intermolecular forces.
  
  • Ex: nail polish remover has many molecules escaping into gas phase, more volatile than water.

Question 37

What is the difference between a phase diagram for water (or ammonia) and any other substance?

Answer 37

Whereas most substances will have a slightly positive solid-liquid curve, water and ammonia have slightly negative curves because their solids are less dense than their liquids.

Question 38

How can you tell which gas exhibits the most ideal behavior at STP?

Answer 38

• Look for weak IMF’s… because that’s what can throw it off. Within weak IMF, look for which gas is less polarizable
• OR look for the lowest boiling point, since this also means weakest IMF

Question 39

What is the link between Pressure and # of moles (n)?

Answer 39

• More moles, higher pressure
  
  • The pressure of a gas results from collisions between the gas particles and the walls of the container. An increase in the number of gas particles in the container increases the frequency of collisions with the walls and therefore the pressure of the gas.

Question 40

Describe the trend for solubility of molecular compounds.

Answer 40

• We learned the solubility rules for ionic compounds.
• Totally different for molecular compounds. They are more soluble (dissolve) if they are more polarizable (bigger).
  
  • Ex: Can dissolve more O2 than N2 in water.
  • They will dissolve, not dissociate.

Question 41

What is Beer’s Law?

(Liquids’ absorbance)

Answer 41

• There is a linear relationship between concentration and absorbance
• Absorbance = E (molar absorptivity constant) * length of prism * concentration
• Standard curve: Plot absorbance as a function of concentration.
  
  • And you can see the absorption spectrum by using a spectrometer and plotting absorbance as a function of wavelength

Question 42

What is Avogadro’s hypothesis?

(for gases)

Answer 42

• Equal Volumes of gas at the same T and P have the same number of molecules
  
  • Goes along with PV=nRT

Question 43

How do you read a manometer?

Answer 43

Look at the height difference of the mercury.

If the gas has pushed the mercury up the tube, it has a higher pressure, equal to atmospheric pressure + the pressure of the mercury.