AP Chemistry Unit 4 AP Exam Prep Quiz

A student was studying physical and chemical changes. The student carried out some procedures in the laboratory and recorded observations. For one of the procedures, the student concluded that a physical change took place, but not a chemical change. Which of the following could have been the results of the procedure?

A cube of metal was changed into a flat sheet of metal.

When the equation below is balanced and all coefficients are reduced to their lowest whole-number terms, what is the coefficient for H₃PO₄(l)?

 

. . . Ca₃(PO₄)₂(s) + . . . H₃PO₄(l) → . . . Ca(H₂PO₄)₂(s)

4

 

A mixture of CO(g) and O2(g) is placed in a container, as shown above. A reaction occurs, forming CO2(g). Which of the following best represents the contents of the box after the reaction has proceeded as completely as possible?

Which of the following describes the changes in forces of attraction that occur as H2O changes phase from a liquid to a vapor?

Hydrogen bonds between H₂O molecules are broken.

Table: Substances, their Lewis Diagrams, and Boiling Points

Subtance	Lewis Structure	Boiling Point
CH3OH		338 K
C2H5OH		351 K

 

Equimolar samples of CH₃OH(l) and C₂H₅OH(l) are placed in separate, previously evacuated, rigid 2.0 L vessels. Each vessel is attached to a pressure gauge, and the temperatures are kept at 300 K. In both vessels, liquid is observed to remain present at the bottom of the container at all times. The change in pressure inside the vessel containing CH₃OH(l) is shown below.

 

 

Which of the following best describes the change that takes place immediately after the CH₃OH(l) is introduced into the previously evacuated vessel?

A physical change takes place because intermolecular attractions are overcome.

Addition of sulfurous acid (a weak acid) to barium hydroxide (a strong base) results in the formation of a precipitate. The net ionic equation for this reaction is

H₂SO₃(aq) + Ba²⁺(aq) + 2 OH^-(aq) ⇄ BaSO₃(s) + 2 H₂O(l)

An experiment is performed to measure the mass percent of CaCO₃(s) in eggshells. Five different samples of CaCO₃(s) of known mass react with an excess of 2.0 M HCl(aq) in identical sealed, rigid reaction vessels. The pressure of the gas produced is measured with a pressure gauge attached to the reaction vessel. Since the reaction is exothermic, the reaction system is cooled to its original temperature before the pressure is recorded. The experimental data are used to create the calibrated line in the chart.

The experiment is repeated with an eggshell sample, and the experimental data are recorded in the table.

Mass of eggshell sample	0.200 g
Pressure prior to reaction	0.800 atm
Pressure at completion of reaction	0.870 atm

 

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) → Ca²⁺(aq) + H₂O(l) + CO₂(g)

A student mixes 20.0 g of white KCl crystals with distilled water in a beaker. After the mixture is stirred, no crystals are visible, and the solution is clear. After several days, all of the water evaporates and white crystals are found in the beaker. Which of the following pieces of experimental evidence would best help the student to confirm that a new compound had not been made and that only a physical change occurred?

After the water has evaporated, the white crystals in the beaker have a mass of 20.0 g

H₂ + F₂ → 2 HF

 

In the reaction represented above, what mass of HF is produced by the reaction of 3.0 x 10²³ molecules of H₂ with excess F₂? (Assume the reaction goes to completion.)

20. g

A 0.35 g sample of Li(s) is placed in an Erlenmeyer flask containing 100 mL of water at 25°C. A balloon is placed over the mouth of the flask to collect the hydrogen gas that is generated.

 

After all of the Li(s) has reacted with H₂O(l), the solution in the flask is added to a clean, dry buret and used to titrate an aqueous solution of a monoprotic acid. The pH curve for this titration is shown in the diagram below

 

What will be the effect on the amount of gas produced if the experiment is repeated using 0.35 g of K(s) instead of 0.35 g of Li(s) ?

Some gas will be produced but less than the amount of gas produced with Li(s).

K(s) + Cl₂(g) → KCl(s)           ΔH° = −437 kJ/mol_rxn

 

The elements K and Cl react directly to form the compound KCl according to the equation above. Refer to the information above and the table below to answer the questions that follow.

 

Process	ΔH° kJ/molrxn
K(s) → K(g)	v
K(g) → K+(g) + e-	w
Cl2(g) → 2 Cl(g)	x
Cl(g) + e- → Cl-(g)	y
K+(g) + Cl-(g) → KCl(s)	z

 

What remains in the reaction vessel after equal masses of K(s) and Cl₂(g) have reacted until either one or both of the reactants have been completely consumed?

KCl and Cl2 only

2 KClO₃(s) → 2 KCl(s) + 3 O₂(g)

 

What is the percentage yield of O₂ if 12.3 g of KClO₃(molar mass 123 g) is decomposed to produce 3.2 g of O₂ (molar mass 32 g) according to the equation above?

67%

Refer to the following.

 

PCl₅(g) ⇄ PCl₃(g) + Cl₂(g)

 

PCl₅(g) decomposes into PCl₃(g) and Cl₂(g) according to the equation above. A pure sample of PCl₅(g) is placed in a

rigid, evacuated 1.00 L container. The initial pressure of the PCl₅(g) is 1.00 atm. The temperature is held constant until the PCl₅(g) reaches equilibrium with its decomposition products. The figures below show the initial and equilibrium conditions of the system.

If the decomposition reaction were to go to completion, the total pressure in the container would be

2.0 atm

A particle view of a sample of H₂O₂(aq) is shown above. The H₂O₂(aq) is titrated with KMnO₄(aq), as represented by the equation below.

 

2 MnO₄⁻(aq) + 5 H₂O (aq) + 6 H⁺(aq) → 2 Mn²⁺(aq) + 5 O₂(g) + 8 H₂O(l)

 

Which of the following particle views best represents the mixture when the titration is halfway to the equivalence point? (H₂O molecules and H⁺ ions are not shown.)

Which of the following represents a process in which a species is reduced?

NO3-(aq) → NO(g)

When water is added to a mixture of Na₂O₂(s) and S(s) , a redox reaction occurs, as represented by the equation below.

 

 

 

Atoms of which element are reduced in the reaction?

O in Na2O2; each atom gains one electron

The reaction represented in the equation is observed to proceed spontaneously to the right in aqueous solution.

 

HSO₄⁻(aq) + CO₃²⁻(aq) ⇄ SO₄²⁻(aq) + HCO₃⁻(aq)

 

In this system the strongest base is

CO3^2-(aq)

Cu(s) + 2 AgNO₃(aq) → Cu(NO₃)₂(aq) + 2 Ag(s)

 

The reaction between solid copper and aqueous silver nitrate produces solid silver and a blue solution, as represented by the balanced equation shown above. Based on the balanced equation, which of the following identifies the oxidation and reduction half-reactions?

Oxidation Half-reaction	Reduction Half-reaction
Cu(s) → Cu2+(aq) + 2 e-	Ag+(aq) + e- → Ag(s)

An experiment was performed to investigate the reaction between Zn metal and Ni²⁺(aq) at different concentrations. Because the Ni²⁺(aq) ion is green, the extent of the reaction was determined using spectrophotometric analysis. Four standard solutions of Ni²⁺(aq) were prepared by dissolving NiC1₂ ∙ 6H₂0 (molar mass 240 g/mol) in water. The absorbance of each solution was measured; the results are shown both in the table below and in the following plot of the absorbance data.

Table: Absorbance of four solutions

Solution	[Ni2+]	Absorbance
1	0.020	0.12
2	0.040	0.25
3	0.060	0.39
4	0.080	0.51

 

Which species is being oxidized in the reaction between Zn metal and NiC1₂(aq)?

Zn

I₂(aq) + C₆H₈O₆(aq) → C₆H₆O₆(aq) + 2 I^-(aq) + 2 H⁺(aq)

 

The compound C₆H₈O₆ reacts with I₂ according to the reaction represented by the equation above. The reaction is correctly classified as which of the following types?

Oxidation-reduction, because I2 is reduced

Precipitation, because there are only two reactants but there are three products.

HC2O4-(aq) and C2O4^2- (aq)

Ni(s) → Ni²⁺(aq) + 2e^-

Ag⁺(aq) + e^- → Ag(s)

Which of the following is the balanced net ionic equation for an oxidation-reduction reaction between Ag⁺(aq) and Ni(s) based on the half-reactions represented above?

2Ag+(aq) + Ni(s) → 2 Ag(s) + Ni^2+(aq)