Chemistry Ch. 2

Front 1

Standard temperature and pressure (STP)

Back 1

For a gas, STP is 0 degrees C and 1 atm.

Front 2

mean free path

Back 2

-the distance traveled by a gas molecule between collisions.

Front 3

4 characteristics of ideal gas

Back 3

1. Gas molecules have zero volume
2. Gas molecules exert no forces other than repulsive forces due to collisions
3. Gas molecules make completely elastic collisions
4. The average kinetic energy of gas molecules is directly proportional to the temperature of the gas.

Front 4

Ideal Gas Law

Back 4

PV = nRT

Front 5

Characteristics of the ideal gas law

Back 5

-Both temperature and pressure are related to the KE of the molecules.
-The pressure is related to the KE per volume, and the temperature is related to the average KE per mole.

Front 6

standard molar volume of gas

Back 6

-ALL gases will have the same volume if they have the same temperature, pressure, and number of molecules (based on ideal gas law).
-At STP one mole of any gas behaving ideally is 22.4 L.

Front 7

Kinetic molecular theory

Back 7

-In this theory, gases behave ideally and lack certain characteristics no shared by a real gas.
-pressure is due to collisions between molecules.
-explains macroscopic properties of gases such as pressure, temperature, and volume.

Front 8

Partial pressure

Back 8

Pa = XaPtotal

-partial pressure is the total pressure of the gaseous mixture times the molar fraction of the particular gas.
-In a mixture of gases, each gas contributes to the pressure in the same proportion as it contributes to the number of gas molecules.
-Each gas in a mixture behaves as if it were in a container alone.

Front 9

mole fraction

Back 9

-the number of moles of gas 'a' divided by the total number of moles of gas in a sample.

Front 10

Dalton's law

Back 10

P = P1 + P2 + P3

-The total pressure exerted by a gaseous mixture is the sum of the partial pressures of each its gases

Front 11

Equation relating KE and gas temperature

Back 11

KEavg = 3/2RT
-valid for any fluid system
-This is the AVERAGE KE for a mole of gas molecules and not the energy for an individual molecule

Front 12

Graham's Law

Back 12

v1/v2 = ((m2)^1/2)/((m1)^1/2)

-The ratio of rms velocities of two gases in a homogeneous mixture.
-gives us information about both effusion and diffusion

Front 13

Effusion

Back 13

-The spreading of a gas from high pressure to very low pressure through a 'pinhole'

Front 14

Relationship between effusion rate or diffusion rate and molecular weight of gas

Back 14

-effusion rate1/effusion rate2 = ((M2)^(1/2))/((M1)^(1/2))
-M is the molecular weight of the compound
-the effusion rate is also equal to the ratio of the gas's RMS velocity

Front 15

RMS velocity

Back 15

-the square root of the average of the squares of the molecular velocities.
-slightly greater than the average speed.

Front 16

Properties of REAL gases (How they vary from ideal gases)

Back 16

-Gases deviate from ideal behavior at high pressure or low temperature (when molecules are close together)

1. volume
-since molecules of a real gas does have volume, the real volume is greater than the ideal volume.
2. Pressure
-molecules exhibit real forces on each other, tend to be attractive
-since intermolecular forces are predominantly attractive, molecules strike the container with less force than predicted by kinetic molecular theory, thus real pressure is less than ideal pressure.

Front 17

Chemical kinetics

Back 17

-The study of reaction mechanisms and rates.
-typically deals with the reaction as it moves towards equillibrium.
-tells us how fast the equillibrium is achieved.

Front 18

Collision model

Back 18

-Most collisions do not result in a reaction
-two requirements for a given collision to create new molecules:
1. the KE of the colliding molecules must reach a threshold energy called the activation energy. Deals only with KE of velocity.
2. The colliding molecules must have the proper spatial orientation.

Front 19

Activation Energy

Back 19

-The relative KE of colliding molecules must reach a threshold energy.
-Velocity in a direction away from another molecule decreases the relative KE of a collision

Front 20

Elementary reaction

Back 20

-a reaction that occurs in a single step.
-in this type of reaction, the coefficient tells you how many molecules participate in the reaction producing the collision.

Front 21

Reaction intermediates

Back 21

-species that are products of one reaction and reactants of a later reaction in a reaction chain.
-concentration of intermediates tends to be very low because they are unstable and react very quickly.

Front 22

Rate law (forward reaction)

Back 22

rate(forward) = Kf[A]^alpha[B]^beta

-true only when consider forward reactions.
-alpha and beta are the order of each respective reactant.
-the sum of alpha + beta is the overall order of the reaction.
-If the reaction is elementary, then alpha = a and beta = b.
-Kf is the rate constant for the forward reaction.

Front 23

The overall order of a reaction

Back 23

The overall order of a reaction is the sum of the orders of each reactant.

Front 24

How to determine the rate law

Back 24

-The rate law must be determined through experimentation
-Do this through manipulating the concentration of one reactant and observing the effect that this has on the reaction rate.
-once the rate law is derived from experimental data, we can solve for rate constant k.

Front 25

Rate determining step

Back 25

-The rate of the slowest elementary step determines the rate of the overall reaction
-If the first step is the slow step, the rate law can be derived directly from this step and no other.
-If the slow step is other than the first step, the steps prior to the RDS will contribute to the rate law. Steps after make no contribution.

Front 26

Catalyst

Back 26

-a substance that increases the rate of a reaction without being consumed or permanently altered.
-enhances selectivities and reduces energy consumption
-may lower the activation energy or increase the steric factor.
-works by providing an alternative reaction mechanism that competes with the uncatalyzed mechanism.
-reactions with catalysts require seperate rate constants. The overall rate is given by the sum of the rates for both the catalyzed and uncatalyzed reaction

Front 27

Heterogeneous catalyst

Back 27

-in a different phase than the reactants and products.
-reactants may physically or chemically adsorb to the surface of a solid catalyst.
-bind to the surface b/c metal atoms at surface of catalyst of unfilled valence requirements.
-binding is almost always exothermic, rate depends on strength of bond.

Front 28

Homogeneous catalyst

Back 28

-in the same phase as the reactants and products, usually in the gas or liquid phase.
-acidic or basic solutions often act as homogeneous catalysts

Front 29

Effects of solvent on rate

Back 29

-liquid molecules make more collisions than gas molecules because they are closer together.
-rate constant in a liquid is a function of the solvent as well as the temperature.
-solvents can electrically insulate reactants reducing the electrostatic forces between them.
-viscosity can also have an effect due to the 'cage' effect.
-ultimately, collisions in a liquid occur at about the same rate as in a gas.

Front 30

Chemical equillibrium

Back 30

-Condition in which the forward reaction rate equals the reverse reaction rate
-At this point there is no change in the concentration of products or reactants.

Front 31

The law of mass action

Back 31

-denotes the relationship between the equilibrium constant (K) and a chemical equation.

K = ([C]^c[D]^d)/([A]^a[B]^b) = Products^coefficients/Reactants^coefficients
-good for ALL chemical equations, including non-elementary equations.
-use the chemical coefficients as exponents of the concentrations regardless of the molecularity
-the concentration of a pure liquid or pure sold is given a value of 1 for the equillibrium expression.

Front 32

Partial pressure equillibrium constant

Back 32

-For reactions involving gases, the equilibrium constant can be written in terms of partial pressures.

Front 33

The reaction quotient

Back 33

Q = Products^coefficients/Reactants^coefficients
-Gives us information about a reaction that is not at equilibrium (equilibrium constant describes only equilibrium conditions)
-Q is not constant; it can have any positive value.
-If Q = K, then reaction is at equilibrium
-If Q > K, then there are more products than reactants and reaction will proceed towards reactants.
-If Q < K, there are more reactants than products and reaction will proceed towards products.

Front 34

Le Chetlier's Principle

Back 34

-State when a system at equilibrium is stressed, the system will shift in a direct that will reduce the stress.
-3 types of stress that usually obey the principle:
1. addition or removal of a product or reactant.
2. changing the pressure of the system
3. heating or cooling the system.

-if raise the temperature in an exothermic reaction, the reaction will be pushed towards the lower temperature side.
-if increase pressure, equillibrium will shift towards side with less molecules.