Lecture 2: Gases, Kinetics, and Chemical Equilibrium Flashcards
Volume of molecules in gas / Total volume
0.1%
Definition of a gas
Loose collection of weakly attracted atoms or molecules moving rapidly in random directions
STP
Standard temperature and pressure: 0 C and 1 atm
Mean free path
Distance traveled by a gas molecule between collisions
Ideal Gas Law
PV = nRT
Characteristics of an ideal gas
- Zero volume
- Exert no forces other than repulsive forces due to collisions
- Completely elastic collisions
- Average kinetic energy of gas molecules is directly proportional to the temperature of the gas
Universal gas constant R
0.08206 L•atm / K•mol
Standard molar volume
Volume of 1 mole of any ideal gas at STP: 22.4 liters
In a mixture of ideal gases, each behaves as if…
…it is alone
Partial pressure
Pressure of a gas in gaseous mixture x Mole fraction of that gas
Dalton’s law
Total pressure exerted by gaseous mixture = sum of partial pressures of each of its gases
Average kinetic energy
= (3/2)RT —ie determined by temperature, not the kind of gas
Graham’s law
v1/v2 = sqrt(m2)/sqrt(m1)
v = velocity m = mass
Effusion
Spreading of a gas from high pressure to low pressure through a pinhole
Diffusion
Spreading of one gas into another gas or into empty space
Effusion rate equation
Effusion rate1 / Effusion rate2 = sqrt(M2) / sqrt(M2)
M = molecular weight
When do real gases deviate from ideal gases?
When molecules are close together:
- At high pressure, or
- At low temperature
Volume of real vs. ideal gas
V real > V ideal
Pressure exerted by real vs. ideal gas
P real < P ideal
Chemical kinetics
Study of reaction mechanisms and rates (ie how quickly equilibrium is achieved)
Collision theory
Molecules must collide to react, but most collisions do not result in a reaction
2 requirements for a collision to create a reaction
- Relative kinetic energies of colliding molecules must reach threshold energy (activation energy)
- Molecules must have proper spatial orientation
Relative kinetic energy
Kinetic energy due to relative velocity only; direction matters
Why does rate of reaction increase with temperature?
Because more collisions with sufficient kinetic energy occur each second
Intermediate
Product of one reaction, reactant of later reaction in reaction chain; present at very low concentration because unstable, reacts quickly
Unit for rate of reaction
Molarity per second
mol / L•s
What affects rate of reaction?
Temperature
Pressure
Concentration of substances
Meaning of stoichiometry coefficients in elementary (single step) reaction
How many molecules collide to make a reaction
Rate law
Rate forward = kf[A]^alpha[B]^beta
kf = rate constant for forward reaction
alpha and beta = order of each reactant
alpha+beta = overall order of reaction
Rate law by experiment:
2x concentration of reactant A = 2x reaction rate.
What is the order of reactant A?
First order
Rate law by experiment:
2x concentration of B = 4x reaction rate.
What is the order of reactant B?
2nd order
Rate law by experiment:
2x concentration of C = 1x
What is the order of reactant C?
0 order
Rate determining step
In a complex reaction, slowest elementary step that determines a reaction’s overall rate
If the slow step in a reaction comes first…
Slow step determines rate
If slow step in a reaction is after the fast step…
Slow step still determines rate, but fast step determines the concentration of one or more reactants in the slow step
Catalyst
Substance that increases the reaction rate without being consumed or permanently altered
How does catalyst increase reaction rate?
- Decreases activation energy, or
- Increases steric factor
Provides alternative reaction mechanism that competes with uncatalyzed reaction
Heterogenous catalyst
Catalyst in different phase from reactants and products (usually solid catalyst for liquid/gas reactants and products)
What is the rate law for elementary reaction aA + bB —> cC + dD?
Rate forward = kf[A]^a[B]^b
kf = rate constant for forward reaction
a, b = orders of reactants (= stoichiometric coefficients for an elementary reaction)
a+b = overall order of reaction
2A + B + C —> 2D
Rate orders:
A: 1st order
B: 2nd order
C: 0 order
What is the rate law for this reaction?
rate forward = kf[A]^1[B]^2[C]^0
= kf[A][B]^2
Steric factor (p)
The fraction of collisions in a reaction having the effective spatial orientation to lead to a reaction
Homogenous catalyst
Catalyst in the same phase as reactants or products (usually gas or liquid)
If concentration of catalyst compared to the concentration of reactants and products is SMALL, increasing the concentration of the catalyst…
Increases the rate of reaction
If concentration of catalyst compared to the concentration of reactants and products is LARGE, increasing the concentration of the catalyst…
Does not affect or only slightly increases the the rate of the reaction
Total rate of a catalyzed reaction
Rate = sum of original reaction rate law + catalyzed reaction rate law
(Usually, rate of original uncatalyzed reaction is negligible)
How do reactants adsorb to the surface of enzymes?
- Physically, via van der Waals forces
2. Chemically, usually via covalent bonds
Why do molecules bond to the surface of a metal catalyst?
Metal atoms at the surface, unlike interior metal atoms, have unfulfilled valence requirements
Auto catalysis
Reaction generates the catalyst as a product
Do ideal gases exhibit condensation?
No, because condensation is due to intermolecular attractions, which are neglected for ideal gases
Condensation
Phase change from gas to liquid
How does adding a catalyst change reaction equilibrium?
Adding a catalyst does NOT change reaction equilibrium
