MCAT General Chemistry 2024
STP - 0°C and 1 atm
Ideal Gas Law - PV = nRT
At STP, 1 mol of gas will occupy ... - 22.4 L
Characteristics of an Ideal Gas - 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 avg. kinetic energy of gas molecules is directly proportional to the
temperature of the gas
Partial pressure of a gas (equation) - Partial pressure equals the mole fraction of
the gas (number of moles of gas "i" divided by total number of moles of gas in
sample) times the total pressure.
Average Kinetic Energy (of gas) - KEavg = (3/2)RT
(R = .08206 L atm K⁻¹ mol⁻¹ or 8.314 J K⁻¹ mol⁻¹)
Average kinetic energy applies to ALL gases, regardless of mass.
,MCAT General Chemistry 2024
Diffusion - Spreading of one gas into another gas or into empty space.
Approximated by Graham's Law.
Graham's law (diffusion) -
Effusion - Spreading of a gas from high pressure to very low pressure through a
"pinhole."
Graham's law (effusion) - Where rates are effusion rates, and Ms are the molar
masses of the gases.
When do "real" gases deviate from ideal gas behavior? - When molecules are
close together:
- volume becomes significant
- electrostatic forces become significant
- high pressure pushes molecules together
- low temperatures cause gas molecules to settle close together
How does Volume compare for real and ideal gases? - V(real) > V(ideal)
,MCAT General Chemistry 2024
How does Pressure compare for real and ideal gases? - P(real) < P(ideal)
(PV)/(RT) Graph: Real vs. Ideal gas
- What is the cause of POSITIVE deviation?
- What is the cause of NEGATIVE deviation? - - Positive deviation cause: molecular
volume
- Negative deviation cause: attractive intramolecular forces
Kinetics vs. Thermodynamics - Kinetics: deals with rate of reaction; how fast
equilibrium is achieved
Thermodynamics: deals with balance of reactants/products after equilibrium
reached; what equilibrium looks like
According to the Collision Model, two conditions must be met for a collision to
lead to a reaction: - 1. Colliding molecules must reach a threshold energy called
"activation energy."
2. Colliding molecules must have proper spatial orientation.
Effect of temperature on reaction rate - Rate increases with temperature increase
because more collisions with adequate kinetic energy occur each second.
, MCAT General Chemistry 2024
Factors affecting rate of reaction - 1. Temperature
2. Pressure
3. Concentration of certain substances
Rate Law (fwd) - Rate(fwd) = k [A]ⁿ¹[B]ⁿ²
(where n1 and n2 are the order of each respective reactant and n1 + n2 = overall
order of reaction)
How do you determine the rate law? - Experimentally - NOT using the coefficients
in the balanced chemical equation!!!
Zero order rxn graph
- what do you plot?
- shape of graph?
- what does slope equal? - Plot: concentration vs. time
Shape: straight line
Slope = -k
STP - 0°C and 1 atm
Ideal Gas Law - PV = nRT
At STP, 1 mol of gas will occupy ... - 22.4 L
Characteristics of an Ideal Gas - 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 avg. kinetic energy of gas molecules is directly proportional to the
temperature of the gas
Partial pressure of a gas (equation) - Partial pressure equals the mole fraction of
the gas (number of moles of gas "i" divided by total number of moles of gas in
sample) times the total pressure.
Average Kinetic Energy (of gas) - KEavg = (3/2)RT
(R = .08206 L atm K⁻¹ mol⁻¹ or 8.314 J K⁻¹ mol⁻¹)
Average kinetic energy applies to ALL gases, regardless of mass.
,MCAT General Chemistry 2024
Diffusion - Spreading of one gas into another gas or into empty space.
Approximated by Graham's Law.
Graham's law (diffusion) -
Effusion - Spreading of a gas from high pressure to very low pressure through a
"pinhole."
Graham's law (effusion) - Where rates are effusion rates, and Ms are the molar
masses of the gases.
When do "real" gases deviate from ideal gas behavior? - When molecules are
close together:
- volume becomes significant
- electrostatic forces become significant
- high pressure pushes molecules together
- low temperatures cause gas molecules to settle close together
How does Volume compare for real and ideal gases? - V(real) > V(ideal)
,MCAT General Chemistry 2024
How does Pressure compare for real and ideal gases? - P(real) < P(ideal)
(PV)/(RT) Graph: Real vs. Ideal gas
- What is the cause of POSITIVE deviation?
- What is the cause of NEGATIVE deviation? - - Positive deviation cause: molecular
volume
- Negative deviation cause: attractive intramolecular forces
Kinetics vs. Thermodynamics - Kinetics: deals with rate of reaction; how fast
equilibrium is achieved
Thermodynamics: deals with balance of reactants/products after equilibrium
reached; what equilibrium looks like
According to the Collision Model, two conditions must be met for a collision to
lead to a reaction: - 1. Colliding molecules must reach a threshold energy called
"activation energy."
2. Colliding molecules must have proper spatial orientation.
Effect of temperature on reaction rate - Rate increases with temperature increase
because more collisions with adequate kinetic energy occur each second.
, MCAT General Chemistry 2024
Factors affecting rate of reaction - 1. Temperature
2. Pressure
3. Concentration of certain substances
Rate Law (fwd) - Rate(fwd) = k [A]ⁿ¹[B]ⁿ²
(where n1 and n2 are the order of each respective reactant and n1 + n2 = overall
order of reaction)
How do you determine the rate law? - Experimentally - NOT using the coefficients
in the balanced chemical equation!!!
Zero order rxn graph
- what do you plot?
- shape of graph?
- what does slope equal? - Plot: concentration vs. time
Shape: straight line
Slope = -k
