AP Chemistry Entropy Question

Question

3Ag(s) + 4HNO3(aq) = 3AgNO₃(aq) + NO(g) + 2H₂O(l)

a) Predict the sign of the entropy change, Delta S, for the reaction. Justify your answer.

b) Use the information the table below to calculate the value of the standard enthalpy change for the reaction in kJ/mol.

c) Based on your answers to parts a and b, is the reaction more likely to be thermodynamically favorable at 25 degrees C or at 95 degrees C? Justify your answer?

d) The student runs the reaction using a 3 to 4 mole ratio of Ag(s) to HNO3(aq). Suggest a method the student can use to isolate solid AgNO3 from the other products of the reaction.

I got a positive entropy change for A since the moles of gas in the reaction go from zero to one. For B I got 43kJ/mol using the values provided and the moles of each substance in reaction. For the other 2 questions I have no idea, so I'd appreciate any help and checking of my previous work. Thanks!

Answer 1

Your answer to (a) looks good. I can’t see the values for (b) but a positive change in enthalpy seems reasonable.

Part (c) is asking you to think about the change in free energy equation. A reaction is more thermodynamically favorable as the ΔG becomes more negative. The ΔH is positive, which is not a favorable condition, however ΔS is positive, which is favorable. ΔG = ΔH - TΔS, so as temperature increases, the reaction will become more favorable as the ΔG will become more negative.

For part (d), the NO is a gas, so it will leave as it is produced as long as it’s an open container. Given the reactants are used in the given ratio from the equation (and assuming the reaction goes to completion) there should only be silver nitrate and water left at the end, so you could just evaporate the water and be left with just silver nitrate solid.

I hope this helps!