AP Chemistry Unit 2 Possible Test Questions

4 NH3(g) + 3 O2(g) —> 2 N2(g) + 6 H2O(g) If the standard molar heats of formation of ammonia, NH3(g), and gaseous water, H2O(g), are -46 kJ/mol and -242 kJ/mol, respectively, what is the value of ΔH for the reaction represented above?

(A) -190 kJ/mol (B) -290 kJ/mol (C) -580 kJ/mol *(D) -1,270 kJ/mol* (E) -1,640 kJ/mol

1/2 H2(g) + 1/2 I2(s) —> HI(g) ΔH = 26 kJ/mol 1/2 H2(g) + 1/2 I2(g) —> HI(g) ΔH = -5.0 kJ/mol Based on the information above, what is the enthalpy change for the sublimation of iodine, represented below? I2(s) → I2(g)

(A) 15 kJ/mol (B) 21 kJ/mol (C) 31 kJ/mol (D) 42 kJ/mol *(E) 62 kJ/mol*

K(s) + 1/2 Cl2(g) —> KCl(s) ∆H° = −437 kJ/mol How much heat is released or absorbed when 0.050 mol of Cl2(g) is formed from KCl(s)?

(A) 87.4 kJ is released (B) 43.7 kJ is released *(C) 43.7 kJ is absorbed* (D) 87.4 kJ is absorbed

A molecular solid coexists with its liquid phase at its melting point. The solid-liquid mixture is heated, but the temperature does not change while the solid is melting. The best explanation for this phenomenon is that the heat absorbed by the mixture

is used in overcoming the intermolecular attractions in the solid

The photoelectron spectra of the 1s electrons of two isoelectronic species, Ca2+ and Ar, are shown above. Which of the following correctly identifies the species associated with peak X and provides a valid justification?

(A) Ar, because it has completely filled energy levels (B) Ar, because its radius is smaller than the radius of Ca2+ (C) Ca2+, because its nuclear mass is greater than that of Ar *(D) Ca2+, because its nucleus has two more protons than the nucleus of Ar has*

The table above shows the structural formulas and molar masses for three different compounds. Which of the following is a list of the compounds in order of increasing boiling points?

(A) Butane < 1-propanol < acetone *(B) Butane < acetone < 1-propanol* (C) 1-propanol < acetone < butane (D) Acetone = butane < 1-propanol

Which of the following Lewis electron-dot diagrams represents the molecule that contains the smallest bond angle?

(A) CF4 *(B) NF3* (C) SO3 (D) SO2

Which of the molecules represented below contains carbon with sp2 hybridization?

(A) CH4 (B) CH2Cl2 (C) C2H6 *(D) C2H2Cl2* (E) C2H4Cl2

Resonance is most commonly used to describe the bonding in molecules of which of the following?

(A) CO2 *(B) O3* (C) H2O (D) CH4 (E) SF6

In which of the following liquids do the intermolecular forces include dipole-dipole forces?

(A) F2(l) (B) CH4(l) (C) CF4(l) *(D) CH2F2(l)*

Based on Coulomb's law and the information in the table above, which of the following cations is most likely to have the weakest interaction with an adjacent water molecule in an aqueous solution?

(A) Li+ *(B) Na+* (C) Ca2+ (D) In3+

Which of the following molecules is nonpolar but has polar covalent bonds?

(A) N2 (B) H2O2 (C) H2O *(D) CCl4* (E) CH2Cl2

The lattice energy of a salt is related to the energy required to separate the ions. For which of the following pairs of ions is the energy that is required to separate the ions largest? (Assume that the distance between the ions in each pair is equal to the sum of the ionic radii.)

(A) Na+(g) and Cl−(g) (B) Cs+(g) and Br−(g) *(C) Mg2+(g) and O2−(g)* (D) Ca2+(g) and O2−(g)

Which of the following compounds is LEAST likely to exist?

(A) PCl5 (B) PBr3 (C) NF3 *(D) NI5* (E) SbF5

Which of the following compounds contains both ionic and covalent bonds?

(A) SO3 (B) C2H5OH (C) MgF2 (D) H2S *(E) NH4Cl*

At 27°C, five identical rigid 2.0 L vessels are filled with N2(g) and sealed. Four of the five vessels also contain a 0.050 mol sample of NaHCO3(s), NaBr(s), Cu(s), or I2(s), as shown in the diagram above. The volume taken up by the solids is negligible, and the initial pressure of N2(g) in each vessel is 720 mm Hg. All four vessels are heated to 127°C and allowed to reach a constant pressure. At 127°C, the entire sample of I2 is observed to have vaporized. How does the mass of vessel 5 at 127°C compare to its mass at 27°C?

(A) The mass is less, since the I2 is in the vapor phase. *(B) The mass is the same, since the number of each type of atom in the vessel is constant.* (C) The mass is greater, since the I2 will react with N2 to form NI3 , which has a greater molar mass. (D) The mass is greater, since the pressure is greater and the particles have a higher average kinetic energy

In a paper chromatography experiment, a sample of a pigment is separated into two components, X and Y, as shown in the figure above. The surface of the paper is moderately polar. What can be concluded about X and Y based on the experimental results?

(A) X has a larger molar mass than Y does. (B) Y has a larger molar mass than X does. (C) X is more polar than Y. *(D) Y is more polar than X.*

The mass spectrum of element X is presented in the diagram above. Based on the spectrum, which of the following can be concluded about element X?

(A) X is a transition metal, and each peak represents an oxidation state of the metal. (B) X contains five electron sublevels. (C) The atomic mass of X is 90. *(D) The atomic mass of X is between 90 and 92.*

Based on the ionization energies of element X given in the table above, which of the following is most likely the empirical formula of an oxide of element X?

(A) XO2 (B) X2O *(C) X2O3* (D) X2O5

Lewis electron-dot diagrams for CO2 and SO2 are given above. The molecular geometry and polarity of the two substances are

(A) the same because the molecular formulas are similar (B) the same because C and S have similar electronegativity values (C) different because the lone pair of electrons on the S atom make it the negative end of a dipole *(D) different because S has a greater number of electron domains (regions of electron density) surrounding it than C has*

2 NH3(g) → 3 H2(g) + N2(g) ∆H

(A) −92 kJ/mol *(B) −46 kJ/mol* (C) 46 kJ/mol (D) 92 kJ/mol (E) 184 kJ/mol

Which of the following aqueous solutions has the lowest freezing point?

(A)0.2 m NaCl (B) 0.2 m CaCl2 (C) 0.2 m H2SO4 (D) 0.2 m NH3 *(E) 0.2 m Al(NO3)3*

At 27°C, five identical rigid 2.0 L vessels are filled with N2(g) and sealed. Four of the five vessels also contain a 0.050 mol sample of NaHCO3(s), NaBr(s), Cu(s), or I2(s), as shown in the diagram above. The volume taken up by the solids is negligible, and the initial pressure of N2(g) in each vessel is 720 mm Hg. All four vessels are heated to 127°C and allowed to reach a constant pressure. The gas particles in vessel 3 at 27°C are represented in the diagram above. The lengths of the arrows represent the speeds of the particles. Which of the following diagrams best represents the particles when vessel 3 is heated to 127°C?

*(A) 4 diatomic molecules with 3 having long arrows and 1 having a short arrow*

Which of the following molecules contains exactly three sigma (σ) bonds and two pi (π) bonds?

*(A) C2H2* (B) CO2 (C) HCN (D) SO3 (E) N2

Element - Electronegativity H - 2.1 C - 2.5 S - 2.5 F - 4.0 Cl - 3.0 Si - 1.8 On the basis of the information above, which of the following arranges the binary compounds in order of increasing bond polarity?

*(A) CH4 < SiCl4 < SF4* (B) CH4 < SF4 < SiCl4 (C) SF4 < CH4 < SiCl4 (D) SiCl4 < SF4 < CH4

Which of the following arranges the molecules N2 , O2 , and F2 in order of their bond enthalpies, from least to greatest?

*(A) F2<O2<N2* (B) O2<N2<F2 (C) N2<O2<F2 (D) N2 < F2 < O2

A 0.5 mol sample of He(g) and a 0.5 mol sample of Ne(g) are placed separately in two 10.0 L rigid containers at 25°C. Each container has a pinhole opening. Which of the gases, He(g) or Ne(g), will escape faster through the pinhole and why?

*(A) He(g) will escape faster because the He(g) atoms are moving at a higher average speed than the Ne(g) atoms.* (B) Ne(g) will escape faster because its initial pressure in the container is higher. (C) Ne(g) will escape faster because the Ne(g) atoms have a higher average kinetic energy than the He(g) atoms. (D) Both gases will escape at the same rate because the atoms of both gases have the same average kinetic energy.

A sample of a hard, solid binary compound at room temperature did not conduct electricity as a pure solid but became highly conductive when dissolved in water. Which of the following types of interactions is most likely found between the particles in the substance?

*(A) Ionic bonds* (B) Metallic bonds (C) Covalent bonds (D) Hydrogen bonds

Which of the following molecules is least soluble in water?

*(B) CCl4, tetrahedral geometry*

(A) H2O (B) NH3 (C) BH3 (D) CH4 (E) SiH4 4. Has two lone pairs of electrons 5. Has a central atom with less than an octet of electrons 6. Is predicted to have the largest bond angle 7. Has a trigonal-pyramidal molecular geometry

4. A 5. C 6. C 7. B

When 6.0 L of He(g) and 10. L of N2(g), both at 0°C and 1.0 atm, are pumped into an evacuated 4.0 L rigid container, the final pressure in the container at 0°C is

4.0 atm

A chemical supply company sells a concentrated solution of aqueous H2SO4 (molar mass 98 g mol−1) that is 50. percent H2SO4 by mass. At 25°C, the density of the solution is 1.4 g mL−1. What is the molarity of the H2SO4 solution at 25°C?

7.1 M

Which of the following best helps to explain why the value of ΔH° for the dissolving of CaF2 in water is positive?

Ca2+ ions have very strong ion-ion interactions with F− ions in the crystal lattice

The figure above shows that in solid hydrogen fluoride there are two different distances between H atoms and F atoms (100 pm and 150 pm). Which of the following best accounts for the two different distances?

Difference in strength between covalent bonds and intermolecular attractions

The Lewis electron-dot diagrams of the HClO3 molecule and the HClO2 molecule are shown above at the left and right, respectively. Which of the following statements identifies the stronger acid and correctly identifies a factor that contributes to its being the stronger acid?

HClO3(aq) is the stronger acid because the additional electronegative oxygen atom on the chlorine atom stabilizes the conjugate base

The diagram above shows molecules of Br2 and I2 drawn to the same scale (I2 is larger). Which of the following is the best explanation for the difference in the boiling points of liquid Br2 and I2, which are 59°C and 184°C, respectively?

I2 molecules have electron clouds that are more polarizable than those of Br2 molecules, thus London dispersion forces are stronger in liquid I2

At room temperature I2(s) is a molecular solid. Which of the following provides a characteristic of I2(s) with a correct explanation?

It is not a good conductor of electricity because its valence electrons are localized in bonding and nonbonding pairs

The potential energy of a system of two atoms as a function of their internuclear distance is shown in the diagram above. Which of the following is true regarding the forces between the atoms when their internuclear distance is x ?

The attractive and repulsive forces are balanced, so the atoms will maintain an average internuclear distance x

Benzene, C6H6 , has the structure shown above. Considering the observation that benzene is only sparingly soluble in water, which of the following best describes the intermolecular forces of attraction between water and benzene?

There are dipole-induced dipole and London dispersion interactions between water and benzene

The BF3 molecule is nonpolar, whereas the NF3 molecule is polar. Which of the following statements accounts for the difference in polarity of the two molecules?

Unlike BF3, NF3 has a nonplanar geometry due to an unshared pair of electrons on the N atom

A certain crystalline substance that has a low melting point does not conduct electricity in solution or when melted. This substance is likely to be

a molecular solid

Of the following, the best explanation for the fact that most gases are easily compressed is that the molecules in a gas

are relatively far apart

At 298 K and 1 atm, bromine is a liquid with a high vapor pressure, whereas chlorine is a gas. This provides evidence that, under these conditions, the

forces among Br2 molecules are greater than those among Cl2 molecules