Kaplan MCAT General Chemistry Chapter 8: The Gas Phase Edited
Dalton's law
(total pressure from partial pressure)
units of pressure relationships
1 atm= 760 mmHg= 760 torr= 101.325 kPa
Boltzmann Constant
1.38*10^-23 J/K
At STP, a mole of gas occupies
22.4 L
STP
273 K (0 celcius) and 1 atm
Convert 380 mmHg to atm
380 (1 atm / 760 mmHg) = 0.5 atm
Ideal gas
A hypothetical gas that has no intermolecular forces and occupies no volume. *Although real gases deviate from this idea at high temp or low volume, it is still a close estimate*
Avogadro's Principle
All gases at a constant pressure and temperature occupy volumes that are directly proportional to the number of moles of gas present.
Boyle's Law (Isothermal Compression) graph and interp
As P increases, V decreases
Gay-Lussac's (Isovolumetric Heating) graph and interp
As T increases, P increases too
Charles's Law (Isobaric Expansion) graph and interp
As T increases, V increases
Kinetic molecular theory
Attempts to explain the behavior of gas particles. It makes a number of assumptions about them: 1) Gas particles have negligible volume 2) No intermolecular forces 3) Undergo random collisions with them + walls of container 4) Avg. kinetic energy of gas particle is directly proportional to temp.
an 8.00 g sample of NH4NO3 is placed into an evacuated 10L flask and heated to 227 degrees celsius. After the NH4NO3 completely decomposes, what is the approximate pressure in the flask? NH4NO3 --> N2O + H2O
BALANCE THE EQUATION, NH4NO3 --> N2O + 2H2O mass given in 8g so molar mass is .1mol because mass of NH4NO3 is 80 g/mol. so when .1 mol decomposes you get .1 mol N2O and .2 mol water based on stoic so this gives .3 mols gas product. use ideal gas caution to get pressure P=.3 x .0821 x 500/ 10 = 1.2 atm
Kinetic Energy of Gas
Boltzmann constant, kB = 1.38 x 10^-23 J/K.
Graham's law
Describes the behavior of a gas diffusion or effusion, stating that gases with lower molar masses will diffuse or effuse faster than gases with higher molar masses at the same temperature.
Real gases
Deviate from ideal behavior under high pressure (low volume) and low temperature conditions. 1) At moderately high pressure, low volume, or low temperature, real gases will occupy more volume than predicted by the ideal gas law because the actual particles occupy physical space. 2) At extremely high pressure, low temperature, real gases will occupy more volume than predicted by the ideal gas law because particles occupy physical space.
Gas Pressure (units?)
Expressed in: 1 ATM = 760 mmHg (equv. to torr) = 100 kPa The SI unit for pressure is the pascal (Pa)
Boyle's Law
For a given gaseous sample held at constant temperature (isothermal conditions), the volume of the gas is inversely proportional to the pressure.
Ideal gases: I. have no volume II. have particles with no attractive forces between them III. have mo mass
II only, (gases have volume, individual particles do not)
The solubility of a gas will _____________________ with increasing partial pressure of the gas
INCREASE
Van der Waals equation of state
Is used to correct the ideal gas law for intermolecular attractions and molecular volume. a is for attractive forces b is for big particles if a and b are both 0, then the van der Waals equation of state reduces to the ideal gas law (don't need to memorize this equation)
Gay-Lussac's Law equation
P1/T1 = P2/T2
Boyle's Law and interpretation
P1V1=P2V2 Pressure and Volume are INVERSELY related (as one increases, the other decreases)
Dalton's law (partial pressure from total pressure)
Pa = partial pressure Xa = mole fraction Pt = total pressure
Root-mean-square speed
R = ideal gas constant T = temperature (K) M = molar mass
Gay-Lussac's Law
Relates temperature and pressure
STP vs Standard State Conditions
STP = gas law calculations (1 atm + 273K) Standard state conditions = enthalpy, entropy, free energy changes, and electrochemical voltage ( 298 K and 1M concentrations).
Diffusion
The spending out of particles from high concentration to low concentration
Dalton's Law of Partial Pressures
Total pressure of a gaseous mixture is equal to the sum of the partial pressures of the individual components
When do real gases deviate from ideal behavior?
Under high pressure, low volume and temperature conditions.
Ideal gas constant
Use 0.08 when using atm in numerator!
Charles's Law equation
V1/T1 = V2/T2
Gaseous state defined by 4 variables: (variables)
We can define the state of a gaseous sample by four variables: 1) Pressure (P) 2) Volume (V) 3) Temperature (T) 4) # of Moles (n)
Dalton's Law (partial pressure from total pressure)
Xa = mols of gas A / total mols of gas
van der Waals Equation of State
a = term for ATTRACTIVE forces b = term for BIG particles
sphygmomanometers
an instrument used to measure blood pressure (mmHg) sphygmo-: relating to the pulse or pulsation. -manometers: an instrument for measuring the pressure acting on a column of fluid, especially one with a U-shaped tube of liquid in which a difference in the pressures acting in the two arms of the tube causes the liquid to reach different heights in the two arms.
Boyle's law graph
as pressure increases, the volume decreases and vice versa
experimenters notice that the molar concentration of dissolved oxygen in an enclosed water tank has decreased to one-half its original value. in an attempt to counter this decrease, they quadruple the partial pressure of oxygen in the container. what is the final concentration of the gas?
c. double the original solubility directly proportional to pressure so times 4 is times 4 but we have .5 so its .5 times 4 is double
atmospheric pressure effect on mercury
creates a downward force on the pool of mercury at the base of a barometer while the mercury in the column exerts an opposing force (its weight) based on density
at extremely low temperatures,
gases will again occupy more space than predicted because particles cannot be compressed to zero volume
height of mercury related to pressure
height of column directly proportional to atmospheric pressure being applied
gas near boiling point
intermolecular attraction causes the gas to have a smaller volume than that which would be predicted by the ideal gas law loser it gets to boiling point, less ideally it acts
Avogadro's Principle Equation and interpretation
n1/V1 = n2/V2 As the # of moles of gas increases, the volume increases in a direct proportion
ideal gas
no intermolecular forces and occupies no volume
SI unit for pressure
pascal
vapor pressure
pressure exerted by evaporated particles above the surface of a liquid
Graham's Law
r1 and r2 = diffusion rates of gas 1 and gas2 m1 and m2 are MM of gases 1 and 2
Partial pressure
the contribution each gas in a mixture of gases makes to the total pressure
Henry's Law
the mount of gas dissolved in solution is directly proportional to the partial pressure of that gas at the surface of a solution
standard state conditions and what are they used for
used for standard enthalpy, entropy, free energy changes, and electrochemical cell voltage H,S,G,Emf, His Great Elephant Socks or EFEE 298K (25 celcius), 1 atm, 1 M concentrations
Charles law graph
volume and temp are directly proportional: when one increases, the other increases in direct proportion
Ideal gas law
Shows the relationship among four variables that define a sample of gas: PV=nRT R is the ideal gas constant, which is 8.21x10^-2 and has units of ( L x atm / mol x K) or may also be seen as 8.314 ( J / K x mol)
Charles law
States that at constant pressure, the volume of a gas is proportional to its absolute temperature.
Henry's law
States that the amount of gas dissolved in solution is directly proportional to the partial pressure of that gas at the surface of solution.
Dalton's law
States that the total pressure of a gaseous mixture is equal to the sum of the partial pressures of the individual components.
Temperature at standard state vs STP
Temp @ STP: 0°C (273 K) Temp @ standard state: 25°C (298 K)
Standard temperature and pressure (STP)
The conditions under which the volume of a gas is usually measured: 1) 273 K or 0 C. 2) 1 ATM or 101 kPa 1 mole ideal gas = 22.4 L space
The higher the temp, the _________ the molecules move The larger the molecules, the ________ they move
The higher the temp, the __faster__ the molecules move The larger the molecules, the __slower__ they move
When to use the Ideal gas law?
The ideal gas law can be used to determine the missing term when given all the others. It can also be used to calculate a change in the term holding two of the others constant.
Effusion
The net movement of gas under pressure from one compartment to another through a small opening
The solubility of a gas will increase with ____________ partial pressure of the gas
The solubility of a gas will increase with __increasing__ partial pressure of the gas
