Free Ultimate AP Chemistry Exam Question and Answers — Questions and Answers
Question 1: Helium gas weighs 0.40 g in a 2.0 L flask. What will happen to the helium's entropy if it is evacuated into a bigger container while the temperature is kept constant?
- As the gas becomes more organized in the larger flask, it will decrease.
- As long as there are the same number of helium molecules, it will not alter.
- As the gas molecules get more spread in the bigger flask, it will increase. (Correct answer)
- It will fall because there will be fewer collisions between the molecules and the edges of the larger flask than there were in the smaller flask.
Correct answer: As the gas molecules get more spread in the bigger flask, it will increase.
Entropy is a measure of the disorder or the number of possible microstates a system can occupy. When helium gas is evacuated into a larger container, the gas molecules have a greater volume to occupy, allowing for more possible positions and arrangements. This increased dispersal of the gas molecules in the larger flask directly corresponds to an increase in the system's entropy.
Question 2: What is occurring to the forward reaction's rate at t = 60 s?
- It is zero
- It is remain constant (Correct answer)
- It is decreasing
- It is increasing
Correct answer: It is remain constant
At t = 60 s, the system is likely at equilibrium, as indicated by the phrasing of the question in a context implying a reaction reaching a steady state. At equilibrium, the rate of the forward reaction becomes equal to the rate of the reverse reaction. Therefore, the net change in concentrations is zero, and the forward reaction's rate remains constant, balanced by the reverse reaction's rate.
Question 3: Estimate the vapor pressures of each chemical in liquid form at the same temperature, going from highest to lowest based on the strength of the intermolecular forces in each substance.
- ethanal > methanol > ethene > propane
- methanol > ethanal > propane > ethene
- propane > ethanal > ethene > methanol
- ethene > propane > ethanal > methanol (Correct answer)
Correct answer: ethene > propane > ethanal > methanol
Vapor pressure is inversely related to the strength of intermolecular forces (IMFs); stronger IMFs lead to lower vapor pressure. Ethene and propane are nonpolar, with only London Dispersion Forces (LDFs), but ethene is smaller, so it has weaker LDFs than propane. Ethanal is polar with dipole-dipole forces and LDFs. Methanol has hydrogen bonding, the strongest IMF among these. Thus, IMFs strength: Methanol > Ethanal > Propane > Ethene. Therefore, vapor pressure (highest to lowest) is Ethene > Propane > Ethanal > Methanol.
Question 4: Which two compounds are most likely to mix with water when in a liquid state?
- methanol and propane
- ethene and ethanal
- methanol and ethanal (Correct answer)
- propane and ethene
Correct answer: methanol and ethanal
The principle 'like dissolves like' dictates that polar substances tend to mix with other polar substances, and nonpolar with nonpolar. Water is a highly polar molecule capable of hydrogen bonding. Methanol (CH3OH) is polar and can form hydrogen bonds with water. Ethanal (CH3CHO) is also polar and can form dipole-dipole interactions and hydrogen bonds (via its oxygen) with water. Propane and ethene are nonpolar hydrocarbons and will not readily mix with water.
Question 5: Which will most likely have a greater boiling point, propane or ethene, and why?
- propane, because it has a more polarizable electron cloud (Correct answer)
- ethene, because it is smaller in size
- propane, because it has a greater molar mass
- ethene, because of the double bond
Correct answer: propane, because it has a more polarizable electron cloud
Both propane (C3H8) and ethene (C2H4) are nonpolar molecules, so their primary intermolecular forces are London Dispersion Forces (LDFs). LDFs increase with the number of electrons and the polarizability of the electron cloud. Propane has more electrons (26) and a larger, more polarizable electron cloud than ethene (16 electrons). Therefore, propane exhibits stronger LDFs, requiring more energy to overcome, resulting in a higher boiling point than ethene.
Question 6: High-energy photons hit chlorine neutral atoms, which causes the ejection of electrons from the variably populated subshells. Which subshell's initial electrons would be ejected with the highest velocity?
- 2p
- 1s
- 3d
- 3p (Correct answer)
Correct answer: 3p
When high-energy photons eject electrons (photoelectric effect), the kinetic energy of the ejected electron is the photon's energy minus the electron's binding energy. Electrons in the outermost subshell (valence electrons) have the lowest binding energy because they are furthest from the nucleus and experience the most shielding. For chlorine, the valence electrons are in the 3p subshell. With the lowest binding energy, these 3p electrons will be ejected with the highest kinetic energy and thus the highest velocity.
Question 7: One mole of carbon atoms has an average mass in grams of
- the ratio of the number of carbon atoms to the mass of a single carbon atom
- the mass, in grams, of the most abundant isotope of carbon
- the average mass of a single carbon atom, measured in amu (Correct answer)
- the number of carbon atoms in one amu of carbon
Correct answer: the average mass of a single carbon atom, measured in amu
The molar mass of an element, expressed in grams per mole, is numerically equivalent to the average atomic mass of that element, expressed in atomic mass units (amu). This relationship arises from the definition of a mole, where Avogadro's number of amu is exactly 1 gram. Therefore, one mole of carbon atoms has an average mass in grams that is numerically equal to the average mass of a single carbon atom in amu.
Free Ultimate AP Chemistry Exam Question and Answers
Helium gas weighs 0.40 g in a 2.0 L flask.
What will happen to the helium's entropy if it is evacuated into a bigger container while the temperature is kept constant?