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.
-Based on the information given in the passage, which of the following is the standard molar enthalpy of formation for sodium hydroxide?

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. -Based on the information given in the passage, which of the following is the standard molar enthalpy of formation for sodium hydroxide? A) −590 kJ/mol B) −470 kJ/mol C) −940 kJ/mol D) 275 kJ/mol

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.
-Based on the information given in the passage, which of the following is the standard molar enthalpy of formation for sodium hydroxide?

Quizplus · Accepted Answer

11ecba2d_11af_52d7_a3bf_13d5699359e0_MD0008_00 According to Hess's law, the overall enthalpy change ΔH° of a reaction is equal to the sum of changes in enthalpy of each component. Table 1 gives the standard molar enthalpies of formation of each component except sodium hydroxide (NaOH). Therefore, the enthalpy of formation for NaOH can be calculated by subtracting the enthalpies of bleach, ammonia, and dichloramine from the overall enthalpy of the reaction (−55 kJ/mol), applying the following: The signs of the reactants must be changed. Table 1 gives enthalpies of formation, but ammonia and bleach are consumed, not formed. Therefore, they contribute +350 kJ/mol and +45 kJ/mol of enthalpy, respectively. The standard enthalpies of formation for each component must be multiplied by their stoichiometric coefficients. In the case of bleach, two molecules are consumed per reaction, so the standard enthalpy must be multiplied by 2. Similarly, two molecules of sodium hydroxide are formed per reaction, and these must be accounted for. To determine the standard enthalpy of formation of NaOH (x), set up the equation: 11ecba2d_11af_79e8_a3bf_d755e7c65980_MD0008_00 Solve for the enthalpy of formation of NaOH (x). 11ecba2d_11af_79e9_a3bf_65dc22278d42_MD0008_00 11ecba2d_11af_79ea_a3bf_13fa87987247_MD0008_00 Alternatively, the answer can be found without directly changing the signs of the given enthalpies by using Hess's law and stating the overall enthalpy as the difference between the sums of the product and reactant enthalpies. 11ecba2d_11af_79eb_a3bf_af2e86810647_MD0008_00 11ecba2d_11af_79ec_a3bf_e782c3711e12_MD0008_00 11ecba2d_11af_79ed_a3bf_79db7b463e2c_MD0008_00 11ecba2d_11af_a0fe_a3bf_09b1cc77ce55_MD0008_00 (Choice A) Using the enthalpies of formation for 1 mol of bleach and NaOH instead of multiplying by 2 yields this answer. (Choice C) Solving for 2x rather than x yields this answer, which is the enthalpy of formation for 2 mol of NaOH. (Choice D) Keeping the signs of the enthalpies for bleach and ammonia negative instead of switching them to positive yields this answer. Educational objective: Hess's law states that the overall enthalpy change of a reaction is equal to the sum of the enthalpies of formation for each component. Because reactants are consumed rather than formed, the signs of their enthalpies must be changed. Each component must be multiplied by its stoichiometric coefficient to account for the total number of molecules consumed and formed in each reaction.

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

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