AP Chemistry Unit 9 practice questions

Question 1

A researcher plots the standard Gibbs free energy change (ΔG°) on the y-axis versus temperature (T) on the x-axis for a chemical reaction. The resulting line has a negative slope and a positive y-intercept. Based on the linear relationship ΔG° = ΔH° − TΔS°, which statement correctly identifies the signs of the enthalpy and entropy changes indicated by the graph?

Accepted Answer

The enthalpy change is positive and the entropy change is positive.

Answer

The enthalpy change is positive and the entropy change is negative.

Answer

The enthalpy change is negative and the entropy change is positive.

Answer

The enthalpy change is negative and the entropy change is negative.

Question 2

A heating curve plots temperature versus heat added for a pure substance. The horizontal segment representing vaporization is significantly longer than the horizontal segment representing melting. Which conclusion about the standard entropy of vaporization (ΔS°vap) compared to the standard entropy of fusion (ΔS°fus) is best supported by this graphical difference?

Accepted Answer

ΔS°vap > ΔS°fus because the gas phase has greater matter dispersal than the liquid.

Answer

ΔS°vap < ΔS°fus because the higher temperature reduces the calculated entropy change.

Answer

ΔS°vap = ΔS°fus because the mass of the substance remains constant during heating.

Answer

ΔS°vap ≈ 0 because the temperature remains constant throughout the phase transition.

Question 3

A graph of ln K versus 1/T for the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) yields a straight line with a negative y-intercept. Based on the relationship ln K = (−ΔH°/R)(1/T) + (ΔS°/R), what does the y-intercept indicate about the entropy change of the system?

Accepted Answer

The entropy change is negative, consistent with the decrease in moles of gas.

Answer

The entropy change is negative, consistent with the increase in moles of gas.

Answer

The entropy change is positive, consistent with the decrease in moles of gas.

Answer

The entropy change is positive, consistent with the increase in moles of gas.

Question 4

A student constructs a Nernst plot ($$E_{cell}$$ versus $$\log Q$$) for a reaction between metal M and metal ion $$Z^{n+}$$. The slope of the line is determined to be $$-0.010$$ V at 298 K. Given that the theoretical slope is $$\frac{-0.0592}{n}$$ V, what is the balanced net ionic equation represented by this data?

Accepted Answer

$$2M(s) + 3Z^{2+}(aq) \rightarrow 2M^{3+}(aq) + 3Z(s)$$

Answer

$$M(s) + Z^{3+}(aq) \rightarrow M^{3+}(aq) + Z(s)$$

Answer

$$M(s) + Z^{2+}(aq) \rightarrow M^{2+}(aq) + Z(s)$$

Answer

$$M(s) + Z^{+}(aq) \rightarrow M^{+}(aq) + Z(s)$$

Question 5

A graph of current (A) versus time (s) for the electrolysis of a molten chloride salt $XCl_n$ is a rectangle with a height of 10.0 A and width of 965 s. If 0.90 g of metal X is deposited, what is the balanced reduction half-reaction? (F = 96,500 C/mol)

$Al^{3+} + 3e^- \rightarrow Al(s)$

$Mg^{2+} + 2e^- \rightarrow Mg(s)$

$Na^{+} + e^- \rightarrow Na(s)$

$Ca^{2+} + 2e^- \rightarrow Ca(s)$

Accepted Answer

$$Al^{3+} + 3e^- \rightarrow Al(s)$$

Answer

$$Mg^{2+} + 2e^- \rightarrow Mg(s)$$

Answer

$$Na^{+} + e^- \rightarrow Na(s)$$

Answer

$$Ca^{2+} + 2e^- \rightarrow Ca(s)$$

Question 6

In a galvanic cell, a graph of the mass of the anode (Metal A) lost versus the mass of the cathode (Metal B) gained is linear with a slope of 0.30. The atomic mass of A is 65.4 g/mol and B is 107.9 g/mol. What is the balanced cell reaction?

Accepted Answer

$$A(s) + 2B^+(aq) \rightarrow A^{2+}(aq) + 2B(s)$$

Answer

$$2A(s) + B^{2+}(aq) \rightarrow 2A^+(aq) + B(s)$$

Answer

$$A(s) + B^{2+}(aq) \rightarrow A^{2+}(aq) + B(s)$$

Answer

$$3A(s) + 2B^{3+}(aq) \rightarrow 3A^{2+}(aq) + 2B(s)$$

Question 7

A model of a galvanic cell anode shows cadmium atoms ($$Cd$$) becoming cadmium ions ($$Cd^{2+}$$). Which combination correctly describes the half-reaction components and the quantitative flow of electrons for every 10 atoms oxidized?

Accepted Answer

20 electrons flow from the anode to the cathode

Answer

10 electrons flow from the anode to the cathode

Answer

20 electrons flow from the cathode to the anode

Answer

5 electrons flow from the cathode to the anode

Question 8

A reaction vessel initially contains a mixture of $$I_2$$ molecules and $$F^-$$ ions. Based on standard reduction potentials ($$E^\circ$$ for $$F_2$$ is +2.87 V, $$E^\circ$$ for $$I_2$$ is +0.54 V), which statement describes the correct final particulate representation?

Accepted Answer

The initial $$I_2$$ and $$F^-$$ particles remain unchanged

Answer

The initial F⁻ ions are oxidized to F₂ molecules while I₂ remains unchanged

Answer

The initial I₂ molecules are reduced to I⁻ ions while F⁻ remains unchanged

Answer

The initial I₂ and F⁻ particles react to form IF₅ molecules and electrons

Question 9

A galvanic cell operates using the reaction $$Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)$$. The atomic mass of Zn is 65.38 g/mol and Cu is 63.55 g/mol. Which statement best explains the change in electrode mass based on particulate electron transfer?

Accepted Answer

For every mole of reaction, 2 moles of electrons transfer, causing the anode to lose slightly more mass than the cathode gains.

Answer

For every mole of reaction, 1 mole of electrons transfers, causing the anode to gain mass while the cathode loses mass.

Answer

The transfer of electrons from Cu to Zn causes the zinc electrode to increase in mass due to electron accumulation.

Answer

The stoichiometry is 1:1, so the mass lost by the zinc electrode must be exactly equal to the mass gained by the copper electrode.

Question 10

Electrolysis of a molten metal chloride $MCl_n$ using a current of 5.00 A for 386 seconds deposits 0.010 mol of metal M at the cathode. (Faraday's constant = 96,500 C/mol $e^-$ ). Which half-reaction represents the reduction of M consistent with this data?

$M^{2+} + 2e^- \rightarrow M$

$M^+ + e^- \rightarrow M$

$M^{3+} + 3e^- \rightarrow M$

$M^{4+} + 4e^- \rightarrow M$

Accepted Answer

$$M^{2+} + 2e^- \rightarrow M$$

Answer

$$M^+ + e^- \rightarrow M$$

Answer

$$M^{3+} + 3e^- \rightarrow M$$

Answer

$$M^{4+} + 4e^- \rightarrow M$$

AP Chemistry Unit 9 Practice Test

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