Chemistry: Lecture 2: Gases, Kinetics, and Chemical Equilibrium
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- Standard Temperature and Pressure (STP)
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-0 degrees Celcius and 1 atm
-one mole of any gas (behaving ideally) occupies 22.4L - Mean Free Path
- -the distance traveled by a gas molecule between collisions
- Unlike liquids, all gases are __________ with each other, regardless of polarity differences
- miscible
- Why does hot air rise?
- Because it is less dense than cold air.
- Ideal gases have four characteristics not shared by a real gas:
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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 (KEavg = (3/2)RT. - Ideal Gas law
- PV = nRT
- R (Universal Gas Constant)
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0.08206 L atm K^-1 mol^-1
OR
8.314 J K^-1 mol^-1 - It is possible to cool a gas by increasing the ________.
- Volume
- Partial Pressure
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-Total pressure of the gaseous mixture times the mole fraction of the particular gas.
P = X*Ptot - Dalton's Law
- -states that the total pressure exerted by a gaseous mixture is the sum of the partial pressures of each of its gases.
- Graham's Law
- v1/v2 = sqrt(m2/m1)
- Real Gases
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-Deviate from ideal behavior when their molecules are close together. So high P, low T.
-Vreal > Videal
-Preal < Pideal
-If PV/RT is greater than one for one mole of gas, then the deviation due to the molecular volume must be greater than that caused by intermolecular forces. - Chemical Kinetics
- -Tells us HOW FAST equilibrium is achieved, while thermodynamics tells us what equilibrium looks like.
- The Collision Model
- -In order for a chemical reaction to occur, the molecules must collide
- The Collision Theory
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There are two requirements for a given collision to create new molecules in a reaction:
1) The relative kinetic energies of the colliding molecules must reach a threshold energy (activation energy)
2) The colliding molecules must have the proper spatial orientation - The Arrhenius Equation
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k = zpe^(-Ea/RT)
The value of the rate constant depends upon pressure, catalysts, and temperature. - The rate of a reaction ALWAYS increases with __________.
- temperature! (more collisions with sufficient relative kinetic energy occur each second)
- Factors affecting the RATE of a reaction
- temperature, pressure, and concentration of certain substances in the reacting system
- Elementary Reaction
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-A reaction that occurs in a single step
-The stoichiometric coefficients of an elementary equation give the molecularity of the reaction. - Rate Law
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rate(forward) = k[A]^a*[B]^b
-a and b are the order of each respective reactant, and a+b is the overall order of the rxn
-determined by EXPERIMENT - Zero Order Reaction
- t vs [A] has slope -kf
- First Order Reaction
- t vs LN[A] has slope -kf
- Second Order Reaction
- t vs 1/[A] has slope kf
- Third Order Reaction
- t vs 1/(2[A]^2) has slope kf
- Rate Determining Step
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-The rate of the slowest elementary step determines the rate of the overall reaction.
-If the slow step is first, then the rate law can be derived directly from this step and no other. - Heterogeneous Catalyst
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-In a different phase than the reactants and products.
-A reactant may physically adsorb (via van der Waals forces), or, more often, chemically adsorb (via covalent forces) to the surface of the solid catalyst.
-Reaction rates can be increased by increasing the surface area of the catalyst. - Homogeneous Catalyst
- -In the same phase as the reactants and products
- Most of the collisions in a liquid are with the ________.
- Solvent (no reaction). However, collisions in a liquid occur at about the same rate as in a gas.
- The rate constant in a liquid is a function of what?
- -The solvent as well as the temperature
- Chemical Equilibrium
- -Forward reaction rate equals the reverse reaction rate. There is no change in the concentration of the products or reactants. (Point of greatest entropy)
- The Law of Mass Action
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K = Products^coefficients / Reactants^coefficients
-This is good for ALL chemical equations
-Don't use solids or PURE liquids in this equation (concentration is usually 1) - The Equilibrium Constant (K) ONLY depends upon:
- Temperature.
- The Partial Pressure Equilibrium Constant
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Kp = Kc(RT)^(dn)
Kp = partial pressure equilibrium constant
n = the sum of the coefficients of the products minus the sum of the coefficients of the reactants
Kc = concentration equilibrium constant - The Reaction Quotient
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Q = Products^coefficients / Reactants^coefficients
Q = K --> reaction in eq.
Q > K --> left shift
Q < K --> right shift - Three types of stress that usually obey Le Chatelier's principle:
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1) addition or removal of a product/reactant
2) changing the pressure of the system
3) heating or cooling the system - If total pressure increases at constant pressure, and total n is greater on the right side, where does it shift?
- To the left.