Gases, Kinetics, and Chemical Equilibrium Flashcards
Standard Temperature and Pressure (STP)
0 Celsius, and 1 atm
mean free path
distance traveled by a gas molecule between collisions
polarity differences and gas…
polarity differences do not cause gases to separate as liquids do
temperature differences and gases
temp differences –> gases separate
Kinetic Molecular Theory
model of ideal gas
- gas molecules have zero volume
- molecules exert no forces other than repulsive forces due to collisions
- molecules make completely elastic collisions
- average KE of gas molecules is directly proportional to temp of gas
idea gas law
PV = nRT
at STP one mole of any ideal gas will occupy…
the standard molar volume of 22.4 liters
in mixture of gases, each gas contributes to pressure…
each gas contributes to pressure in same proportion as # of molecules in gas
partial pressure
Pa = X*Ptot.
total pressure of mixture * mole fraction
mole fraction = moles of gas a/total moles
Dalton’s Law
total pressure exerted by gaseous mixture is sum of partial pressures
average translational kinetic energy (KE) for 1 mol of ideal gas =
KE = 3/2RT
Graham’s Law
V1/V2 = sqrt(m2)/sqrt(m1)
average speed of molecules of pure gas is inversely proportional to square root of mass of gas molecules
effusion
spreading of gas from high pressure to very low pressure through a pinhole
effusion rate 1/effusion rate 2 = sqrt(M2)/sqrt(M1)
diffusion
spreading of one gas into another gas or into empty space
- much slower than rms velocity of molecules
(same equation as effusion)
Real gases
deviate from ideal behavior when molecules are close together
-from high pressure and cold temps
V real > V ideal
P real < P ideal
chemical kinetics vs. thermodynamics
kinetics: how fast equilibrium is achieved
thermodynamics: what equilibrium looks like
collision model of reactions
method for visualizing chemical reactions
in order for reaction to occur, reacting molecules must collide
-rate of given reaction is much lower than the frequency of collisions –> all collisions do not result in a reaction
2 requirements for a given collision to create a new molecule in a reaction
relative kinetic energies of the colliding molecules must reach the activation energy, colliding molecules must have proper spatial orientation
rate of a reaction and temperature
the rate of a reaction increases with temperature
-more collisions with sufficient relative KE occur each second
activation energy and temperature
Ea is independent of temperature, activation energy does not change with temperature
intermediates
species that are products of one reaction and reactants of a later reaction in a chain
rate law for aA + bB –> cC + dD
rate(forward) = k [A]^alpha [B]^beta
k = rate constant
alpha and beta = order of each respective reactant
order of each respective reactant
alpha and beta
for elementary reaction: alpha = a and beta = b
overall order of reaction
sum or order of each respective reactant
determine rate law by experiment
see pages 38-39
zero order rate law
plot [A] with respect to time –> straight line with slope of -k
first order rate law
[A] decreases exponentially
graph of ln[A] with respect to time –> straight line with slope of -k
rate determining step
rate of the slowest elementary step determines overall reaction rate
if first step is slow step…
rate law derived directly from this step
if slow step is not first….
use equilibrium approximation… page 42
catalyst
increases the rate of a reaction without being consumed or permanently altered
creates a new reaction pathway that typically includes an intermediate
-increases rate of forward and reverse reactions
-does not change equilibrium conditions, just gets it there more quickly
-most lower activation energy
a catalyst cannot alter the
equilibrium constant of a reaction
heterogeneous catalyst
in a different phase than the reactant and products. usually solid while the reactants/products liquids or gases
homogenous catalyst
same phase as reactants and products (often aq. acid or base solutions)
chemical equilibrium
when forward reaction rate equal reverse reaction rate
- no change in concentration of products and reactants
- net reaction rate is 0, but there is a forward and reverse reaction rate
equilibrium is point of greatest ___
greatest entropy
*nature wants to increase entropy
Law of Mass Action
for aA + bB –> cC + dD
equilibrium constant K = [C]^c [D]^d / [A]^a [B]^b = products^coefficients/reactants ^coefficients
*good for all equations, not must elementary
equilibrium constant, K, depends on..
temperature only
don’t use ___ in law of mass action
don’t use solids or pure liquids (water) in law of mass action
Q, reaction quotient
same formula as K, equilibrium constant
used to predict direction in which reaction will proceed when not in equilibrium
Q > K
more products than reactants, reaction shifts left (increases reactants)
reverse reaction rate greater than forward rate
Q < K
more reactant than products, reaction shifts right (increase products)
forward reaction rate greater
Le Chatelier’s Principle
when a system at equilibrium is stressed, the system will shift in a direction to reduce that stress
