Q: Calculate the entropy change ($\Delta S$) for the vaporization of 1 mol of water at 100°C, given $\Delta H_{vap} = 40.7$ kJ/mol and $T = 373$ K.

$\Delta S = \dfrac{40.7 \times 10^3}{373} = 109$ J/K

Question 1

Which of the following changes would result in an increase in entropy ($\Delta S > 0$) for a chemical system?

Accepted Answer

Dissolving NaCl in water

Question 2

Calculate the entropy change ($\Delta S$) for the vaporization of 1 mol of water at 100°C, given $\Delta H_{vap} = 40.7$ kJ/mol and $T = 373$ K.

Accepted Answer

$\Delta S = \dfrac{40.7 	imes 10^3}{373} = 109$ J/K

Question 3

A reaction has $\Delta H = -125$ kJ and $\Delta S = -200$ J/K. At what temperature will the reaction become nonspontaneous?

Accepted Answer

$T > \dfrac{\Delta H}{\Delta S} = \dfrac{-125,000}{-200} = 625$ K

Question 4

Which equation correctly relates Gibbs free energy to enthalpy and entropy?

Accepted Answer

$\Delta G = \Delta H - T\Delta S$

Question 5

Given the following reactions and their $\Delta G^\circ$ values:

1. $A \rightarrow B$, $\Delta G^\circ = -50$ kJ
2. $B \rightarrow C$, $\Delta G^\circ = +30$ kJ

What is $\Delta G^\circ$ for $A \rightarrow C$?

Accepted Answer

$\Delta G^\circ = -20$ kJ

Question 6

Which of the following statements about standard cell potential ($E^\circ_{cell}$) is correct?

Accepted Answer

$E^\circ_{cell} = E^\circ_{cathode} - E^\circ_{anode}$

Question 7

A voltaic cell is constructed using a Zn/Zn$^{2+}$ half-cell and a Cu/Cu$^{2+}$ half-cell. Given $E^\circ_{Zn^{2+}/Zn} = -0.76$ V and $E^\circ_{Cu^{2+}/Cu} = +0.34$ V, what is the standard cell potential?

Accepted Answer

$E^\circ_{cell} = 0.34 - (-0.76) = 1.10$ V

Question 8

Which of the following correctly describes the process of balancing a redox reaction in acidic solution?

Accepted Answer

Balance all elements except H and O, then balance O with H$_2$O, H with H$^+$, and electrons last.

Question 9

For the reaction $2Fe^{3+} + 2I^- \rightarrow 2Fe^{2+} + I_2$, which species is oxidized?

Accepted Answer

I$^-$

Question 10

Using the Nernst equation, calculate the cell potential at 25°C for a cell with $E^\circ_{cell} = 1.10$ V and $Q = 10$.

$E_{cell} = E^\circ_{cell} - \dfrac{0.0592}{n} \log Q$, where $n = 2$.

Accepted Answer

$E_{cell} = 1.10 - \dfrac{0.0592}{2} \log 10 = 1.10 - 0.0296 = 1.070$ V

Question 11

Which of the following is true about alpha decay?

Accepted Answer

The nucleus emits a $^4_2He$ particle, decreasing its mass number by 4 and atomic number by 2.

Question 12

Which equation is used to calculate the binding energy from mass defect?

Accepted Answer

$E = \Delta m c^2$

Question 13

A radioactive isotope has a half-life of 10 years. How much of a 100 g sample remains after 30 years?

Accepted Answer

12.5 g

Question 14

Which of the following is a correct statement about beta decay?

Accepted Answer

A neutron is converted to a proton, and a $^0_{-1}e$ particle is emitted.

Question 15

Which process is used in radiocarbon dating to determine the age of ancient artifacts?

Accepted Answer

Comparing the ratio of $^{14}C$ to $^{12}C$ in the sample

Question 16

Which of the following is true about the spontaneity of a reaction at standard conditions?

Accepted Answer

If $\Delta G^\circ < 0$, the reaction is spontaneous.

Question 17

Which of the following is the correct formula for calculating the standard entropy change ($\Delta S^\circ$) of a reaction?

Accepted Answer

$\Delta S^\circ = \sum S^\circ_{products} - \sum S^\circ_{reactants}$

Question 18

Which of the following is a correct application of Faraday's law in the stoichiometry of electrolytic cells?

Accepted Answer

The amount of substance produced is proportional to the total charge passed: $n = \dfrac{Q}{Fz}$

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