Q: Which of the following is the correct expression for calculating the standard enthalpy change ($$\Delta H^\circ) of a $$reaction using standard heats of formation?

$$\Delta H^\circ = \sum \Delta H^\circ_f [\text{Products}] - \sum \Delta H^\circ_f [\text{Reactants}]$$

Question 1

Which of the following statements best describes the relationship between enthalpy ($$\Delta H$$), entropy ($$\Delta S$$), and spontaneity of a chemical reaction?

Accepted Answer

A reaction is spontaneous if $$\Delta G = \Delta H - T\Delta S \u003c 0$$; both $$\Delta H$$ and $$\Delta S$$ contribute to spontaneity.

Question 2

A sample of nitrogen gas ($$N_2) $$occupies a volume of 5.81 L at a pressure of 0.900 atm. If the volume is decreased to 1.22 L at constant temperature, what is the final pressure?

Accepted Answer

$$4.29$$ atm

Question 3

Which of the following processes results in a decrease in entropy ($$\Delta S \u003c 0$$)?

Accepted Answer

Nitrogen gas being liquefied

Question 4

Given the reaction: $$Ba(OH)_2 \cdot 8H_2O(s) + 2NH_4Cl(s) \rightarrow $$BaCl_2$$(aq) + 2NH_3(aq) + 10H_2O(l)$$ with $$\Delta H^\circ = 80.3 kJ $$and $$\Delta S^\circ = 428 J/K$$, at what temperature (in K) is the reaction at equilibrium?

Accepted Answer

$$188 K$$

Question 5

Which of the following best explains why most exothermic reactions are spontaneous, but not all?

Accepted Answer

Because entropy changes ($$\Delta S) $$and temperature also affect spontaneity.

Question 6

A balloon filled with helium has a volume of 22.0 L at 298 K. What volume will the balloon occupy at 273 K if the pressure remains unchanged?

Accepted Answer

$$20.2 L$$

Question 7

Which of the following is the correct value for the gas constant $$R in $$the ideal gas law $$PV = nRT$$ when pressure is in atm and volume in liters?

Accepted Answer

$$0.08206 L-$$atm/mol-K

Question 8

How many grams of phosphorus ($$P_4) $$react with 35.5 L of $$O_2 at $$STP to form tetraphosphorus decoxide ($$P_4O_{10}$$)? The molar mass of $$P_4 is 123.895 g/$$mol. $$P_4(s) + 5O_2(g$$) \rightarrow P_4O_{10}(s)$$

Accepted Answer

$$78.2 g$$

Question 9

Which of the following best describes the physical behavior of gases?

Accepted Answer

It is described by four variables: pressure ($$P$$), temperature ($$T$$), volume ($$V$$), and amount in moles ($$n).$$

Question 10

Which of the following is NOT a correct unit conversion for pressure?

Accepted Answer

$$1$$ atm $$= 100$$ kPa

Question 11

A steel tank contains 6.9 mol of $$N_2 in a 1.5 L $$container at a pressure of 42 atm. What is the temperature of the gas in Kelvin?

Accepted Answer

$$110 K$$

Question 12

Which of the following best describes a spontaneous process in chemistry?

Accepted Answer

A process that, once started, proceeds on its own without continuous external influence.

Question 13

Which law states that the volume of a fixed amount of gas at constant pressure varies directly with its absolute temperature?

Accepted Answer

Charles's Law

Question 14

A 750 mL canister contains krypton gas at 298 K and 1.10 atm. How many moles of krypton are present?

Accepted Answer

$$0.034$$ mol

Question 15

Which of the following is true about the effect of temperature on spontaneity for a reaction with $$\Delta H \u003e 0$$ and $$\Delta S \u003e 0$$?

Accepted Answer

The reaction is spontaneous only at high temperatures.

Question 16

Which of the following best explains why the oxidation of sugar (as in the 'gummi bear sacrifice') is considered spontaneous?

Accepted Answer

Because it proceeds on its own once started, regardless of the rate.

Question 17

Which of the following is the correct expression for calculating the standard enthalpy change ($$\Delta H^\circ) of a $$reaction using standard heats of formation?

Accepted Answer

$$\Delta H^\circ = \sum \Delta H^\circ_f [	ext{Products}] - \sum \Delta H^\circ_f [	ext{Reactants}]$$

Question 18

Which of the following is a correct statement about the entropy ($$S) of a $$system?

Accepted Answer

Entropy increases as the system becomes more disordered.

Question 19

A laboratory method for reducing copper(II) oxide uses hydrogen gas. What is the first step in calculating the volume of $$H_2$$ gas needed to produce a given mass of copper?

Accepted Answer

Convert grams of $$Cu to $$moles of $$H_2$$ using the molar mass of $$Cu$$ and the coefficients in the balanced equation.

Question 20

Which of the following is the correct form of the ideal gas law?

Accepted Answer

$$PV = nRT$$

Thermochemistry, Spontaneity, Free Energy, and Gas Laws: Structured Study Notes

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