AP Chemistry UNIT 3 PPC

Which particle diagram shown above best represents the strongest intermolecular force between two ethanol, C2H6O, molecules?

Diagram 2, because it shows the formation of a hydrogen bond between an H atom bonded to an O atom with an O atom from another molecule.; Hydrogen bonding is the strongest attractive force between ethanol molecules. It requires an H atom to be bonded to a small atom with a high electronegativity, like N, O, or F. The hydrogen bond forms between atoms with partial positive and partial negative charges. In the O−H bond, the H atom has a partial positive charge, and the O atom has a partial negative charge. The hydrogen bond is formed between O−H---O−H bonds from different molecules, as shown.

At 10.°C, 20.g of oxygen gas exerts a pressure of 2.1atm in a rigid, 7.0L cylinder. Assuming ideal behavior, if the temperature of the gas was raised to 40.°C, which statement indicates the new pressure and explains why?

2.3atm, because the pressure P increases by the proportion 313/283; Rearranging the ideal gas law equation yields the equation P=nRTV. Since the number of moles of oxygen (n), the gas constant (R), and the volume (V) are all constant in this case, the relationship reduces to P=kT, where T is the absolute temperature (in kelvins). Therefore, increasing the temperature to 313 K from 283 K results in a pressure that is larger by the factor 313/283

A gaseous air‑fuel mixture in a sealed car engine cylinder has an initial volume of 600.mL at 1.0atm. To prepare for ignition of the fuel, a piston moves within the cylinder, reducing the volume of the air‑fuel mixture to 50.mL at constant temperature. Assuming ideal behavior, what is the new pressure of the air‑fuel mixture?

About 12atm, because the volume of the gas mixture decreased by a factor of 12.; For a gas or gas mixture that behaves ideally, pressure and volume are inversely proportional. Therefore, since n and T remained constant, a 12-fold decrease in volume should result in a 12-fold increase in pressure.

The diagrams above represent two allotropes of solid phosphorus. Which of the following correctly identifies the allotrope with the higher melting point and explains why?

Allotrope II, because it has covalent bonds between the phosphorous atoms that are stronger than the dispersion forces between the P4 molecules in allotrope I.; Allotrope II has a high melting point because its atoms are covalently bonded in a network solid. The covalent bonds are stronger than the dispersion forces between P4 molecules in allotrope I.

The structures of two allotropes of carbon are represented above. Which of the following statements best helps explain why diamond is much harder than graphite?

Carbon atoms in diamond have four covalent bonds, whereas graphite is made of layers that are held together by relatively weak dispersion forces.; Diamond is a covalent network solid with each carbon atom bonded to four other carbon atoms. Graphite has a layered structure with each carbon atom bonded to three other carbon atoms. Dispersion forces hold the layers together, but the forces are relatively weak, and the layers can slide past each other.

The crystal structure of NaBr is represented in the diagram above. Which statement correctly compares crystalline NaBr(s) to molten NaBr(l) in terms of electrical conductivity?

Crystalline NaBr contains no freely moving electrons to conduct electricity, but molten NaBr is composed of freely moving Na+ and Br− ions, which allows it to be a good conductor of electricity.; In crystalline NaBr, there are no free electrons (or any other charged particles) that can move to carry an electrical current. But when molten, NaBr has freely moving ions that can carry an electric current.

The two gas samples represented in the graph above are at the same temperature. Which of the following statements about the gases is correct?

Gas Z has a smaller molar mass than gas X.; Gases at the same temperature have the same average kinetic energy of their molecules. Since KE=1/2mXv2X=12mZv2Z, the gas with the higher average molecular speed has the lower molar mass (mX/mZ=v2Z/v2X)(�X/�Z=�Z2/�X2). The curve for gas ZZ shows that the molecules have a higher average speed than the molecules in gas XX, so gas ZZ has a smaller molar mass.

The diagram above represents four cations, all shown to the same scale. Which cation would be predicted by Coulomb's law to have the strongest ion-dipole attraction to water, and why?

Mg2+, because it has the largest charge-to-size ratio.; Because water is polar, the O atom has a partial negative charge that can interact with cations through coulombic forces. Coulomb's law predicts that the attractive force between the partial negative charge in the O atom and the cation will be directly proportional to the charge of the ion and inversely proportional to the size or ionic radius. Mg2+ is a relatively small ion and has the most positive charge among the cations shown.

A solid compound of a group 1 (alkali) metal and a group 17 (halogen) element dissolves in water. The diagram above represents one type of solute particle present in the solution. Which of the following identifies the solute particle and best helps explain how the solute particle interacts with water molecules?

The particle is a positive ion, and the interactions are ion-dipole attractions.; The water molecules are all oriented the same way with respect to the solute particle, with the negative ends of the water molecule dipoles directed toward the solute particle. This can only be the case if the solute particle has a positive charge. The major attractive forces between the polar water molecules and the positive ion are characterized as ion-dipole attractions.

The diagrams above use arrows to represent the speed of a gas particle. Which of the diagrams best represents the speed of the particles of a gas at a fixed temperature, and why?

Diagram 2, because the particles have a variety of different speeds.; Based on kinetic molecular theory, gas particles have a distribution of speeds that is dependent on the temperature, which is represented in diagram 2.

Two sealed, rigid 5.0L containers each contain a gas at the same temperature but at a different pressure, as shown above. Also shown are the results of transferring the entire contents of container 1 to container 2. No gases escape during the transfer. Assuming ideal behavior, which statement is correct regarding the total pressure of the gases after they are combined?

The total pressure of the gases in the mixture is the sum of the initial pressures of oxygen gas and nitrogen gas because pressure only depends on the total amount of gas when volume and temperature are held constant.; Under conditions of constant T and V, the pressure of a gas or a mixture of gases only depends on the total number of moles of gas, regardless of the identity of the gas. From the ideal gas law, P=nRTV=(RTV)n=k×n, thus P is directly proportional to n, since R, T, and V are constant. Since the total amount of gas in the mixture is the sum of the original amounts of each gas, Ptotal=(RT/5)(noxygen+nnitrogen)=(RT/5)(noxygen)+(RT/5)(nnitrogen), meaning that the total pressure of the gas mixture is simply the sum of the initial pressures of each gas.