Gas & Liquid Properties Flashcards
For final
What is the Ideal Gas Law?
P V = n R T
Pressure (atm) * Volume = number moles * gas constant * Temperature (Kelvin)
R=.082
What is the Kinetic Molecular Theory?
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
Direct and Inverse relationships:
Just look at the PV = nRT fomula.
Inverse if both in numerator on same side of equal sign.
How do you relate pressure and volume, if everything else is constant?
PV = nRT
P1V1 = P2V2
How do you relate volume and temperature, if everything else is contant?
PV = nRT
V1/T1=V2/T2
How do you relate Pressure and Temperature if everything else is constant?
(PV=nRT)
P1/T1=P2/T2
What is Standard Molar Volume?
- 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.
Convert from Celsius to Kelvin
Add 273
T = oC + 273 = K
Convert from mm Hg to atm
Divide by 760
1 atm=760 mm Hg
What are the equations to calculate density?
d = m/v
d = PM/RT
Steps for gas stoichiometry problems
What is diffusion?
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)
What is effusion?
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.
What is Dalton’s Law of Partial Pressures,
and how could you calculate the partial pressure of each component?
- Ptotal in gas mixture = PA + PB + …
- Partial pressure of component A = the moles of A divided by total moles (aka the mole fraction of A) X Total Pressure
What is the combined gas law?
PV/T is a constant
Describe the deviations from the ideal gas law, and the constants in the van der Waals equation
- 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.
What are the 5 types of intermolecular forces, in order of their typical strength?
- Ion-dipole attractions
- Hydrogen bonding (type of dipole-dipole attraction)
- Other Dipole-dipole attractions
- Dipole-induced dipole attractions
- Induced dipole-induced dipole attractions. (aka London Dispersion Forces, Van der Waal’s Forces)
What are ion-dipole attractions?
- 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
What are dipole-dipole attractions?
- 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.
What is hydrogen bonding?
- 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
What are induced-dipole dipole attractions?
- 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
What are Induced dipole-induced dipole attractions?
aka London Dispersion Forces
aka Van der Waal’s Forces
- 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)
What is surface tension?
- Molecules at the surface experience net inwards force of attraction.
- Water molecules at the surface are pulled inwards by its neighbors below (hydrogen bonding)
What is wetting?
- 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.
What is viscosity?
- 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
What is capillary action?
- 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
What is vapor pressure and how does it relate to boiling point?
- 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.
heat of fusion vs. heat of vaporization?
- 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.
What is enthalpy of hydration?
- 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.
Relationship between boiling point and polarity?
The more polar, the higher the boiling point.
(requires more energy to break greater IMFs.)
What are the six phase changes and their associated energies?
- 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.
When is the boiling point higher?
- 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
What are the consequences of hydrogen bonding in water and ammonia?
- It’s harder to boil
- It’s less dense when a solid because of the length of the bond (ice floats)
What is polarizability?
- 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.
What is an isothermal process?
- 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
What is volatility, and what is it related to?
- 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.
What is the difference between a phase diagram for water (or ammonia) and any other substance?
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.
How can you tell which gas exhibits the most ideal behavior at STP?
- 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
What is the link between Pressure and # of moles (n)?
- 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.
Describe the trend for solubility of molecular compounds.
- 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.
What is Beer’s Law?
(Liquids’ absorbance)
- 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
What is Avogadro’s hypothesis?
(for gases)
- Equal Volumes of gas at the same T and P have the same number of molecules
- Goes along with PV=nRT
How do you read a manometer?
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.
