AP Chem practice MCQ

Based on the information in the table above, which of the compounds has the highest boiling point, and why?

Propanoic acid, because it can form intermolecular hydrogen bonds

2 NO2(g) → N2O4 ← (g) dark brown ,colorless The dimerization of NO2(g), an exothermic process, is represented by the equation above. Which of the following experimental techniques will allow the most accurate determination of the concentration of NO2(g) at equilibrium?

Spectrophotometry

Which of the following best explains why the combustion reactions represented in the table are exothermic?

The energy required to break the bonds in the reactants is less than the energy released in forming the bonds in the products.

. A 50.0 g sample of Feat 100°C is added to 500.0 mL of water at 35°C in a perfectly insulated container. Which of the following statements is true?

The heat energy lost by the Fe will be equal to the heat energy gained by the water.

C9H8O4(aq) + NaOH(aq) C9H7O4 (aq) + Na+(aq) + H2O(l) The reaction represented above occurs when 2.00 × 10-4 mol of pure acetylsalicylic acid, C9H8O4 , is completely dissolved in 15.00 mL of water in a flask and titrated to the equivalence point with 0.100 M NaOH(aq). Which of the following statements about the titration is true at the equivalence point?

[C9H8O4] is less than [C9H7O4 ] . *At the equivalence point, the number of moles of OH aq − added is equal to the initial number of moles of C9 H8O4 aq and the reaction shown has gone essentially to completion. Therefore, the concentration of C9H7O4 − is much greater than that of C9 H8 O4

. Reactions with high activation energies

are very slow

Which of the following particle diagrams best represents the products when four molecules of H2O2(l) decompose into water and oxygen gas at room temperature?

diagram will obtain 4 H2O molecules and 2 pairs of O2 molecules

metalllic bonding has

high Electrical Conductivity in Solid Phase AND high Electrical Conductivity in Liquid Phase

A student titrates 10.0 mL samples of 1.0 M solutions of each of the haloacetic acids with a standard solution of NaOH. Which of the following statements correctly predicts the volume of NaOH(aq) needed to reach the equivalence point?

All of the acids will need the same volume of NaOH(aq) to reach the equivalence point. *At the equivalence point, the number of moles of each monoprotic acid used is equal to the number of moles of NaOH( ) aq added. Since equimolar amounts of the acids are used and the same NaOH( ) aq solution is used for all four titrations, the volume of NaOH( ) aq needed to reach the equivalence point is the same for all four titrations.

The pH of a solution made by combining 150.0 mL of 0.10 M KOH(aq) with 50.0 mL of 0.20 M HBr(aq) is closest to which of the following?

12

Which of the following equations best represents the species that react and the species that are produced when CaCO3(s) and HCl(aq) are combined?

2 H+(aq) + CaCO3(s) Æ Ca2+(aq) + H2O(l) + CO2(g)

CaF2(s) ←→ Ca ΔH> 0 2+(aq) + 2 F −(aq) Dissolution of the slightly soluble salt CaF2 is shown by the equation above. Which of the following changes will decrease [Ca2+] in a saturated solution of CaF2 , and why? (Assume that after each change some CaF2(s) remains in contact with the solution.)

Adding NaF(s), because the reaction will proceed toward reactants

Equal volumes of solutions in two different vessels are represented above. If the solution represented in vessel 1 is KCl(aq), then the solution represented in vessel 2 could be an aqueous solution of

CaCl2 with twice the molarity of the solution in vessel 1

As a sample of KNO3(s) is stirred into water at 25°C, the compound dissolves endothermically. Which of the following best helps to explain why the process is thermodynamically favorable at 25°C?

Dissolving the salt increases the entropy of the system.

When a small amount of 12 M HNO3(aq) is added to a buffer solution made by mixing CH 3NH2(aq) and CH3NH3Cl(aq), the pH of the buffer solution changes from 10.64 to 10.62. Which of the following equations represents the reaction that accounts for the fact that the pH does not change significantly when the HNO3(aq) is added?

CH3NH2(aq) + H+(aq) → CH3NH3 +(aq)

*important formula

Ecell = Ecathode - Eanaode Ecell = Ecopper - Ezinic

A 20. mL sample of 0.50 M HC2H3O2(aq) is titrated with 0.50 M NaOH(aq). Which of the following best represents the species that react and the species produced in the reaction?

HC2H3O2(aq) + OH−(aq) → C2H3O2 −(aq) + H2O(l)

The photoelectron spectra for H and He are represented above. Which of the following statements best accounts for the fact that the peak on the He spectrum is farther to the left and higher than the peak on the H spectrum?

He has a greater nuclear charge than H and an additional electron in the same energy level.

A student observes that the equilibrium constant for a reaction is greater than 1.0 at temperature below 500 K but less than 1.0 at temperatures above 500 K. What can the student conclude about the values of H and S for the reaction? (Assume that ∞ and S are independent of temperature.)

H∞ < 0 and S∞ < 0 *When the equilibrium constant is greater than 1.0, ΔG is less than 0 (the reaction is thermodynamically favorable), and when the equilibrium constant is less than 1.0, ΔG is greater than 0 (the reaction is not thermodynamically favorable). Since ΔG=ΔH-TΔS, HΔ<0 and ΔS< 0 results in a negative ΔG at low temperatures and a positive ΔG at high temperatures.

A solution is prepared by mixing equal volumes of 0.20 M HC2H3O2 and 0.40 M NaC2H3O2 . Which of the following correctly describes what occurs if a small amount of HCl(aq) or NaOH(aq) is added?

If HCl(aq) is added, the pH will decrease only slightly because the H+ ions will react with C2H3O2 − ions.

Which of the following most likely describes the solid represented in the diagram above? (big negative molecule and small positive molecule's every other)

It is a brittle, water-soluble electrolyte that is a poor thermal and electrical conductor as a solid.

H2(g) + I2(g) 2 HI( g) Kc = 50. at 600°C Equimolar samples of each of three gases, H2(g), I2(g), and HI(g), are introduced into a 3.0 L container that is heated to 600°C. According to the information above, which of the following will occur as the system approaches equilibrium?

More HI(g) will be produced because Q < Kc .

Samples of NaF(s) and NH4Cl(s) are dissolved in separate beakers that each contain 100 mL of water. One of the salts produces a slightly acidic solution. Which of the following equations best represents the formation of the slightly acidic solution?

NH4+(aq) + H2O(l) NH3(aq) + H3O+(aq) *NH 4 aq + partially hydrolyzes (reacts with water) to form NH3 aq and H3O aq + Since H3O aq + is a product, the resulting solution is slightly acidic

N2O(g) + CO(g) → N2(g) + CO2(g) The rate of the reaction represented above increases significantly in the presence of Pd(s). Which of the following best explains this observation?

One of the reactants binds on the surface of Pd, which introduces an alternative reaction pathway with a lower activation energy.

2 NOBr(g) → 2 NO(g) + Br2(g) The equation above represents an elementary step in a chemical reaction. Which of the following is the correct expression for the rate law of the elementary step?

Rate = k[NOBr]2

The step-wise dissociation of selenous acid, H2SeO3(aq), is represented by the equations above. Which of the following best helps explain why the value of Ka2 is so much smaller than the value of Ka1 ?

Removing the first H+ from H2SeO3(aq) requires less energy than removing the second H+, because the second H+ is removed from a negatively charged species.

Which of the following best helps to explain why CCl4 is a liquid whereas CI4 is a solid when both are at 25°C?

The London dispersion forces are stronger in CI4 than in CCl4 because CI4 has a more polarizable electron cloud than CCl4 .

Which of the following correctly compares periodic properties of two elements and provides an accurate explanation for that difference?

The atomic radius of Cl is smaller than that of S because Cl has a larger nuclear charge than S does.

A sealed 10.0 L flask at 400 K contains equimolar amounts of ethane and propanol in gaseous form. Which of the following statements concerning the average molecular speed of ethane and propanol is true? *ethane molecules are less massive than propanol molecules

The average molecular speed of ethane is greater than the average molecular speed of propanol.

The saturated CuSO4(aq) shown above is left uncovered on a lab bench at a constant temperature. As the solution evaporates, 1.0 mL samples of the solution are removed every three days and the [SO42] in the samples is measured. It is observed that the [SO42] in the solution did not change over time. Which of the following best helps to explain the observation?

The concentration of ions in a saturated solution of an ionic solid at a given temperature is constant. As water evaporates from the solution, enough of the solute precipitates to keep the concentration of the ions in solution constant.

A mixture of two gases, 0.01 mol of C4H10(g) and 0.065 mol of O2(g), is pumped into a cylinder with a movable piston, as shown above. The mixture, originally at 200C and 1.0 atm, is sparked and the reaction represented below occurs. 2 C 4H10(g) + 13 O2(g) Æ 8 CO2(g) + 10 H2O(g) Which of the following is true after the product gases return to the original temperature and pressure, and why will the change occur? (Assume all gases behave ideally.)

The piston will be higher than its original position, because the cylinder will contain a greater number of gas molecules.

C(diamond) C(graphite) G = 2.9 kJ/molrxn Which of the following best explains why the reaction represented above is not observed to occur at room temperature?

The reaction has an extremely large activation energy due to strong three-dimensional bonding among carbon atoms in diamond.

A balloon filled with 0.25 mol of He(g) at 273 K and 1 atm is allowed to rise through the atmosphere. Which of the following explains what happens to the volume of the balloon as it rises from ground level to an altitude where the air temperature is 220 K and the air pressure is 0.1 atm?

The volume will increase because the decrease in air pressure will have a greater effect than the decrease in temperature.

A 1.0 L sample of a pure gas is found to have a lower pressure than that predicted by the ideal gas law. The best explanation for the observation is that the molecules of the gas

are attracted to each other and do not exert as much force on the container walls as they would if they had no mutual attractions

The average kinetic energy of a sample of gas particles is greater

at a higher temperature.

A 1.0 mol sample of He(g) at 25°C is mixed with a 1.0 mol sample of Xe(g) at 50°C. Which of the following correctly predicts the changes in average kinetic energy and the average speed of the Xe(g) atoms that will occur as the mixture approaches thermal equilibrium?

av. kinetic energy and speed will decrease *When the two samples are combined, the particles of the two different gases collide, and the net effect is that kinetic energy is transferred from the particles at the higher temperature to the particles at the lower temperature until the particles of the two gases have the same average kinetic energy and are at the same temperature. The Xe atoms started at the higher temperature, so the average kinetic energy of the Xe atoms decreases when the two gases are combined

To catalyze a biochemical reaction, an enzyme typically

binds temporarily to reactant molecules to lower the activation energy of the reaction

Network covalent bonding has

no Electrical Conductivity in Solid Phase AND no Electrical Conductivity in Liquid Phase

ionic bonding has

none Electrical Conductivity in Solid Phase AND high Electrical Conductivity in Liquid Phase

HCHO2(aq) + H2O(l) → H3O+(aq) + CHO2 HCHO2(aq), a weak acid (K = 2 × 10−4) a , dissociates in water according to the equation above. Which of the following provides the best estimate of the pH of 0.5 M HCHO2(aq) and identifies the species at the highest concentration (excluding H2O) in the solution?

ph 2 species at highest concertation: HCHO2(aq)

if the average kinetic energy decreases

then the average speed must also decrease.