Kaplan MCAT General Chemistry Chapter 8: The Gas Phase Edited

matter can exist in three physical forms called phases or states

solid liquid gas

like liquids, gases are classified as

fluids

because they

because they can flow and take on the shapes of their containers.

however, they differ from liquids because they

move rapidly and are far apart from each other

what exists between gas molecules and what does this cause

very weak intermolecular forces which allows them to easily expand to fill any volume and to be very easily compressible

what variables do we use to define the gaseous state

PVTn

SI unit for gas pressure and conversion

1 atm= 760 mmHg= 760 torr= 101.325 kPa (pascal)

units of pressure relationships

1 atm= 760 mmHg= 760 torr= 101.325 kPa

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.

describe how a mercury barometer works

Atmospheric pressure creates a downward force on the pool of mercury at the base of the barometer

the mercury in the column exerts an opposing force (its weight) based on its density.

The weight of the mercury creates a vacuum in the top of the tube.

When the external air exerts a higher force than the weight of the mercury in the column, the column rises.

When the external air exerts a lower force than the weight of the mercury, the column falls.

Thus, a reading can be obtained by measuring the height of the mercury column (in mm), which will be directly proportional to the atmospheric pressure being applied.

is atmospheric pressure the only thing that can exert this force

no

For instance, a clinical blood pressure cuff creates a force that is opposed by the person’s systolic and diastolic arterial blood pressure.

processes involving gas take place under

STP

STP

273 K (0 celcius) and 1 atm

what to note about STP

not the same as standard state conditions

STP is for gas law calculations, the other is for standard enthalpy, entropy, free energy changes, and electrochemical cell voltage

standard state conditions are: 298k, 1 ATM, 1M concentration

ideal gas

no intermolecular forces and occupies no volume

When do real gases deviate from ideal behavior?

Under high pressure, low volume and low temperature conditions.

low temp because they move slow allowing imfs to develop

Combined gas law

equation for density using PVRT and practice deriving with knowledge that moles (n) = given mass/ molar mass ( find this by doing a simple mole conversion)

state alternative method for calculating density

find molar mass from this alternative method

density equals: PM/RT

A different approach could start with the fact that a mole of an ideal gas at STP occupies 22.4 L. We can then calculate the effect of changes in pressure and temperature when they differ from STP conditions, predicting the volume of the gas. Finally, we’ll calculate the density by dividing the mass by the predicted volume. The following equation, the combined gas law, is an amalgam of some of the special cases we will discuss in the following section. It can be used to relate changes in temperature, volume, and pressure of a gas: m/V2

The molar mass can then be calculated as the product of the gas’s density at STP and the STP volume of one mole of gas, 22.4

Avogrado's principle eqtn

relate moles to volume

directly proportionality

Boyle's law

relate pressure to volume

inversely proportional

Charle's law

relate volume to temp

direct proportionality

Gay-Lussac's law

relate pressure and temp

directly propotional

Dalton's law

(total pressure from partial pressure)

Dalton's law (partial pressure from total pressure)

Henry's law equation and concept

: Applied pressure is what directly increases gas solubility

vapor pressure

pressure exerted by evaporated particles above the surface of a liquid

Average kinetic energy of a gas particle

Root-mean squared speed

the speed of an individual gas molecule is nearly impossible to define. Therefore, the speeds of gases are defined in terms of their average molecular speed. One way to define an average speed is to determine the average kinetic energy per particle and then calculate the

speed to which this corresponds

Graham's law equation

diffusion and effusion

Diffusion—When gases mix with one another.

Effusion—When a gas moves through a small hole under pressure.

Both will be slower for larger molecules.

Both conditions use the same equation.

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— adding to it bc ideal gas law assumes pressure is lower than it is

b is for big particles— subtracting bc ideal gas law assumes volume is higher than it is

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)

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)

Gas Pressure (units?)

Expressed in:

1 ATM = 760 mmHg (equv. to torr) = 100 kPa

The SI unit for pressure is the pascal (Pa)

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

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).

Ideal gas

A hypothetical gas that has no intermolecular forces and occupies no volume.

Although real gases deviate from this idea at high pressure or low volume low temp, it is still a close estimate

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)

Ideal gas constant

Use 0.08 when using atm in numerator!

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.

Density of a gas

use PM/RT for gas; regular eq is mass/volume

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.

moles/v=moles/v

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.

Boyle's law graph

as pressure increases, the volume decreases and vice versa

Charles law

States that at constant pressure, the volume of a gas is proportional to its absolute temperature.

v/t=v/t

Charles law graph

volume and temp are directly proportional: when one increases, the other increases in direct proportion

Gay-Lussac's Law

Relates temperature and pressure

p/t=p/t

Ideal gas law

(related to other laws)

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.

mole frac times total pressure= partial pressure of gas

Partial pressure

the contribution each gas in a mixture of gases makes to the total pressure

mole fraction times total pressure = partial pressure of gas

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.

equatiom

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

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

Collisions between any two gas particles (or between particles and

the container walls) are elastic, meaning that there is conservation of

both momentum and kinetic energy..

The higher the temp, the _________ the molecules move

The larger the molecules, the ________ they move

KE equat

root m square speed eq

diff between the two

The higher the temp, the __faster__ the molecules move

The larger the molecules, the __slower__ they move

KE IS THE SAME WHEM TEMP IS THE SAME REGARDLESS OF MASS BUT SPEED CHAMGES

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.

Diffusion

The spending out of particles from high concentration to low concentration

Effusion

The net movement of gas under pressure from one compartment to another through a small opening

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 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

at extremely low temperatures,

gases will again occupy more space than predicted because particles cannot be compressed to zero volume

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)

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

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