Free Energy & Equilibrium (College Board AP Chemistry)

2 NO(g) + O₂(g) →  2 NO₂(g)

A student investigates the reactions of nitrogen oxides. One of the reactions in the investigation requires an equimolar mixture of NO(g) and NO₂(g) , which the student produces by using the reaction represented above.

The particle-level representation of the equimolar mixture of NO(g) and NO₂(g) in the flask at the completion of the reaction between NO(g) and O₂(g) is shown below in the box on the right. In the box below on the left, draw the particle-level representation of the reactant mixture of NO(g) and O₂(g) that would yield the product mixture shown in the box on the right. In your drawing, represent oxygen atoms and nitrogen atoms as indicated below.

The student reads in a reference text that NO(g) and NO₂(g) will react as represented by the equation below. Thermodynamic data for the reaction are given in the table below the equation.

NO(g) + NO₂(g)   N₂O₃(g)

The student begins with an equimolar mixture of NO(g) and NO₂(g) in a rigid reaction vessel and the mixture reaches equilibrium at 298 K.

The student hypothesizes that increasing the temperature will increase the amount of N₂O₃(g) in the equilibrium mixture. Indicate whether you agree or disagree with the hypothesis. Justify your answer.

N₂O₃(g) reacts with water to form nitrous acid, HNO₂(aq), a compound involved in the production of acid rain. The reaction is represented below.

N₂O₃(g) + H₂O(l) →  2 HNO₂(aq)

The skeletal structure of the HNO₂ molecule is shown in the box below.

• To produce an aqueous solution of HNO₂ , the student bubbles N₂O₃(g) into distilled water. Assume that the reaction goes to completion and that HNO₂ is the only species produced. To determine the concentration of HNO₂(aq) in the resulting solution, the student titrates a 100. mL sample of the solution with 0.100 M KOH(aq). The neutralization reaction is represented below.

HNO₂(aq) + OH⁻ (aq) →  NO₂⁻(aq) + H₂O(l)

The following titration curve shows the change in pH of the solution during the titration.

Use the titration curve and the information above to

During the titration, after a volume of 15 mL of 0.100 M KOH(aq) has been added , which species, HNO₂(aq) or NO₂^–(aq) , is present at a higher concentration in the solution? Justify your answer.