Passage
Household cleaners commonly contain either ammonia (NH₃) or bleach (NaOCl) as the principal active ingredient. Mixing ammonia-based and bleach-based cleaners can be hazardous, as the resulting reactions can form several toxic products. One of these reactions forms the irritant dichloramine (NHCl₂) as follows:
Reaction 1The overall reaction has a standard enthalpy change of −55 kJ/mol and a standard entropy change of −150 J/mol·K. Partial thermodynamic data for formation of Reaction 1 reagents from their elements are shown in Table 1.Table 1 Partial Data for Enthalpies, Entropies, and Free Energies of Formation of Several Compounds at 25 °C
After it is formed, dichloramine readily enters the gas phase, where it may be inhaled. In the lungs, dichloramine reacts with water to produce hydrochloric acid and ammonia, as well as reactive oxygen species. In small doses, these products cause mild irritation, but inhalation of large amounts of chloramines can lead to pneumonitis (inflammation of the lungs) and may require medical intervention.
Adapted from Trogolo D, Arey JS. Benchmark thermochemistry of chloramines, bromamines, and bromochloramines: halogen oxidants stabilized by electron correlation. Phys Chem Chem Phys. 2015;17(5) :3584-98.
-Assuming ΔH° and ΔS° are constant at all temperatures, at approximately which temperature (in degrees Celsius) will ΔG° of Reaction 1 equal zero (ie, switch from spontaneous to nonspontaneous) ?

Question

Passage
Household cleaners commonly contain either ammonia (NH₃) or bleach (NaOCl) as the principal active ingredient. Mixing ammonia-based and bleach-based cleaners can be hazardous, as the resulting reactions can form several toxic products. One of these reactions forms the irritant dichloramine (NHCl₂) as follows:

Passage Household cleaners commonly contain either ammonia (NH<sub>3</sub>) or bleach (NaOCl) as the principal active ingredient. Mixing ammonia-based and bleach-based cleaners can be hazardous, as the resulting reactions can form several toxic products. One of these reactions forms the irritant dichloramine (NHCl<sub>2</sub>) as follows: <strong>Reaction 1</strong>The overall reaction has a standard enthalpy change of −55 kJ/mol and a standard entropy change of −150 J/mol·K. Partial thermodynamic data for formation of Reaction 1 reagents from their elements are shown in Table 1.<strong>Table 1</strong> Partial Data for Enthalpies, Entropies, and Free Energies of Formation of Several Compounds at 25 °C After it is formed, dichloramine readily enters the gas phase, where it may be inhaled. In the lungs, dichloramine reacts with water to produce hydrochloric acid and ammonia, as well as reactive oxygen species. In small doses, these products cause mild irritation, but inhalation of large amounts of chloramines can lead to pneumonitis (inflammation of the lungs) and may require medical intervention. Adapted from Trogolo D, Arey JS. Benchmark thermochemistry of chloramines, bromamines, and bromochloramines: halogen oxidants stabilized by electron correlation. Phys Chem Chem Phys. 2015;17(5) :3584-98. -Assuming ΔH° and ΔS° are constant at all temperatures, at approximately which temperature (in degrees Celsius) will ΔG° of Reaction 1 equal zero (ie, switch from spontaneous to nonspontaneous) ? A) 95°C B) 370°C C) −270°C D) −640°C

Reaction 1The overall reaction has a standard enthalpy change of −55 kJ/mol and a standard entropy change of −150 J/mol·K. Partial thermodynamic data for formation of Reaction 1 reagents from their elements are shown in Table 1.Table 1 Partial Data for Enthalpies, Entropies, and Free Energies of Formation of Several Compounds at 25 °C

After it is formed, dichloramine readily enters the gas phase, where it may be inhaled. In the lungs, dichloramine reacts with water to produce hydrochloric acid and ammonia, as well as reactive oxygen species. In small doses, these products cause mild irritation, but inhalation of large amounts of chloramines can lead to pneumonitis (inflammation of the lungs) and may require medical intervention.
Adapted from Trogolo D, Arey JS. Benchmark thermochemistry of chloramines, bromamines, and bromochloramines: halogen oxidants stabilized by electron correlation. Phys Chem Chem Phys. 2015;17(5) :3584-98.
-Assuming ΔH° and ΔS° are constant at all temperatures, at approximately which temperature (in degrees Celsius) will ΔG° of Reaction 1 equal zero (ie, switch from spontaneous to nonspontaneous) ?

Quizplus · Accepted Answer

11ecba2d_11b0_b270_a3bf_25b24eda2b61_MD0008_00 The Gibbs free energy of a reaction determines whether it is spontaneous. When all reactants and products are initially at 1 M concentration, the reaction is said to be in the standard state and has a free energy denoted by ΔG°. At any given temperature T, ΔG° = ΔH° − TΔS° A reaction is spontaneous when ΔG° is negative whereas nonspontaneous reactions have a positive ΔG°. Therefore, the temperature at which ΔG° = 0 is the temperature at which the standard-state process switches from spontaneous to nonspontaneous. ΔS° is typically given in units of J/mol·K, and must be divided by 1,000 to convert to kJ/mol·K so that all components of the equation have the same units. In this case, −150 J/mol·K becomes −0.15 kJ/mol·K. Given ΔH° and ΔS° of the overall reaction (−55 kJ/mol and −0.15 kJ/mol·K, respectively), the temperature at which ΔG° = 0 can be calculated by setting up the equation: 0 kJ/mol = −55 kJ/mol − T(−0.15 kJ/mol·K) 55 kJ/mol = −T(−0.15 kJ/mol·K) Solving for the temperature T, 11ecba2d_11b0_b271_a3bf_5b8ced994f63_MD0008_00 The initial answer will be in kelvin. To convert the temperature to Celsius, subtract 273 from the answer in kelvin: 367 − 273 = 94°C ≈ 95°C (Choice B) Keeping the temperature in kelvin yields an answer of approximately 370 K, not 370°C. (Choice C) Keeping the entropy in J/mol·K instead of converting to kJ/mol·K yields an answer of approximately −270°C. (Choice D) Dropping a negative sign anywhere during the calculation will lead to an answer of approximately −640°C. Note that absolute zero is approximately −273°C, so −640°C is not physically possible. Educational objective: The Gibbs free energy of a reaction determines whether the reaction is spontaneous. It is related to both enthalpy and entropy by the equation ΔG° = ΔH° − TΔS°. A negative ΔG° corresponds to a spontaneous reaction whereas a positive ΔG° corresponds to a nonspontaneous reaction.

Passage Household Cleaners Commonly Contain Either Ammonia (NH3) or Bleach (NaOCl)

Passage Household Cleaners Commonly Contain Either Ammonia (NH₃) or Bleach (NaOCl)