MCAT General Chemistry Exam
Complete Questions And Answers
STP CORRECT ANSWERS 0°C and 1 atm
Ideal Gas Law CORRECT ANSWERS PV = nRT
At STP, 1 mol of gas will occupy ... CORRECT ANSWERS 22.4 L
Characteristics of an Ideal Gas CORRECT ANSWERS 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) CORRECT ANSWERS 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) CORRECT ANSWERS 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.
Diffusion CORRECT ANSWERS Spreading of one gas into another gas or into empty
space. Approximated by Graham's Law.
Graham's law (diffusion) CORRECT ANSWERS
Effusion CORRECT ANSWERS Spreading of a gas from high pressure to very low
pressure through a "pinhole."
Graham's law (effusion) CORRECT ANSWERS Where rates are effusion rates, and Ms
are the molar masses of the gases.
When do "real" gases deviate from ideal gas behavior? CORRECT ANSWERS 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? CORRECT ANSWERS V(real) >
V(ideal)
,How does Pressure compare for real and ideal gases? CORRECT ANSWERS 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? CORRECT ANSWERS - Positive
deviation cause: molecular volume
- Negative deviation cause: attractive intramolecular forces
Kinetics vs. Thermodynamics CORRECT ANSWERS 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: CORRECT ANSWERS 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 CORRECT ANSWERS Rate increases with
temperature increase because more collisions with adequate kinetic energy occur each
second.
Factors affecting rate of reaction CORRECT ANSWERS 1. Temperature
2. Pressure
3. Concentration of certain substances
Rate Law (fwd) CORRECT ANSWERS 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? CORRECT ANSWERS 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? CORRECT ANSWERS Plot: concentration vs. time
Shape: straight line
Slope = -k
First order rxn graph
- what do you plot?
, - shape of graph?
- what does slope equal? CORRECT ANSWERS Plot: ln[A] vs. time
Shape: straight line
slope = -k
Second order rxn graph CORRECT ANSWERS Plot: 1/[A] vs. time
Shape: straight line
slope = +k
Third order rxn graph CORRECT ANSWERS Plot: 1/(2[A]²)
Shape: straight line
slope: +k
Catalyst CORRECT ANSWERS Lowers activation energy and thereby increases
reaction rate, but does NOT alter equilibrium constant
Heterogeneous catalyst CORRECT ANSWERS In a different phase than the reactants
and products
Homogeneous catalyst CORRECT ANSWERS In the same phase as the reactants and
products
Chemical equilibrium: definition CORRECT ANSWERS Forward and reverse reaction
rates are equal.
(Note: NET reaction rate is zero, but there is still a fwd and a rev rxn rate!)
Equilibrium constant equation
(aka the law of mass action) CORRECT ANSWERS Used to study reactions at
equilirbium.
K = equilibrium constant; C, D = products; A, B = reactants; and all lowercase letters (a,
b, c, d) = coefficients from balanced chemical equation.
Notes: K is dimensionless, and depends on T. Do NOT use pure liquids (i.e., water) or
solids in this equation!
Reaction quotient equation CORRECT ANSWERS Used to study reactions NOT at
equilibrium.
Q = reaction quotient; C, D = products; A, B = reactants; and all lowercase letters (a, b,
c, d) = coefficients from balanced chemical equation.
When Q = K ... CORRECT ANSWERS The chemical reaction is at equilibrium.
When Q > K ... CORRECT ANSWERS The ratio of product:reactant is greater than at
equilibrium, so the reaction will shift left to form more reactants.
When Q < K ... CORRECT ANSWERS The ratio of product:reactant is less than at
equilibrium, so the reaction will shift right to form more products.
Complete Questions And Answers
STP CORRECT ANSWERS 0°C and 1 atm
Ideal Gas Law CORRECT ANSWERS PV = nRT
At STP, 1 mol of gas will occupy ... CORRECT ANSWERS 22.4 L
Characteristics of an Ideal Gas CORRECT ANSWERS 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) CORRECT ANSWERS 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) CORRECT ANSWERS 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.
Diffusion CORRECT ANSWERS Spreading of one gas into another gas or into empty
space. Approximated by Graham's Law.
Graham's law (diffusion) CORRECT ANSWERS
Effusion CORRECT ANSWERS Spreading of a gas from high pressure to very low
pressure through a "pinhole."
Graham's law (effusion) CORRECT ANSWERS Where rates are effusion rates, and Ms
are the molar masses of the gases.
When do "real" gases deviate from ideal gas behavior? CORRECT ANSWERS 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? CORRECT ANSWERS V(real) >
V(ideal)
,How does Pressure compare for real and ideal gases? CORRECT ANSWERS 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? CORRECT ANSWERS - Positive
deviation cause: molecular volume
- Negative deviation cause: attractive intramolecular forces
Kinetics vs. Thermodynamics CORRECT ANSWERS 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: CORRECT ANSWERS 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 CORRECT ANSWERS Rate increases with
temperature increase because more collisions with adequate kinetic energy occur each
second.
Factors affecting rate of reaction CORRECT ANSWERS 1. Temperature
2. Pressure
3. Concentration of certain substances
Rate Law (fwd) CORRECT ANSWERS 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? CORRECT ANSWERS 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? CORRECT ANSWERS Plot: concentration vs. time
Shape: straight line
Slope = -k
First order rxn graph
- what do you plot?
, - shape of graph?
- what does slope equal? CORRECT ANSWERS Plot: ln[A] vs. time
Shape: straight line
slope = -k
Second order rxn graph CORRECT ANSWERS Plot: 1/[A] vs. time
Shape: straight line
slope = +k
Third order rxn graph CORRECT ANSWERS Plot: 1/(2[A]²)
Shape: straight line
slope: +k
Catalyst CORRECT ANSWERS Lowers activation energy and thereby increases
reaction rate, but does NOT alter equilibrium constant
Heterogeneous catalyst CORRECT ANSWERS In a different phase than the reactants
and products
Homogeneous catalyst CORRECT ANSWERS In the same phase as the reactants and
products
Chemical equilibrium: definition CORRECT ANSWERS Forward and reverse reaction
rates are equal.
(Note: NET reaction rate is zero, but there is still a fwd and a rev rxn rate!)
Equilibrium constant equation
(aka the law of mass action) CORRECT ANSWERS Used to study reactions at
equilirbium.
K = equilibrium constant; C, D = products; A, B = reactants; and all lowercase letters (a,
b, c, d) = coefficients from balanced chemical equation.
Notes: K is dimensionless, and depends on T. Do NOT use pure liquids (i.e., water) or
solids in this equation!
Reaction quotient equation CORRECT ANSWERS Used to study reactions NOT at
equilibrium.
Q = reaction quotient; C, D = products; A, B = reactants; and all lowercase letters (a, b,
c, d) = coefficients from balanced chemical equation.
When Q = K ... CORRECT ANSWERS The chemical reaction is at equilibrium.
When Q > K ... CORRECT ANSWERS The ratio of product:reactant is greater than at
equilibrium, so the reaction will shift left to form more reactants.
When Q < K ... CORRECT ANSWERS The ratio of product:reactant is less than at
equilibrium, so the reaction will shift right to form more products.
