The textbook (p. 161) describes an experiment in which Silver Nitrate (AgNO₃) solution is mixed with hydrochloric acid (HCl) solution in a constant-pressure calorimeter. (We assume that the calorimeter loses only a negligible quantity of heat.) The temperature of the resulting solution is observed to increase, due to the following reaction:

AgNO₃(aq) + HCl(aq) ® AgCl(s) + HNO₃(aq)

1. During this reaction, does energy flow into the resulting solution (if so, where did the energy come from?), out of the solution (if so, where did it go?), or is there no net flow of energy into or out of the solution (if so, how do you know?).

 

 

 

 

2. Three students are discussing this experiment. Here is part of their discussion:

 

Mary: The silver nitrate was originally a solid. When it’s put into solution along with the HCl, I think that heat flows out from the AgNO₃ and into the HCl solution, and that’s why the temperature increases.

Bob: Well, the hydrochloric acid is the more powerful reactant; it’s a strong acid, so it must be the one that reacts most strongly. I think that the heat must come out of the HCl.

Lisa: I don’t really think that the heat flows into either of those two. I think heat flows out of both the silver nitrate and the hydrochloric acid solution, and that’s why the temperature rises.

Mary: But how could heat flow out of both of the reactants? Where is it coming from then? Doesn’t that violate conservation of energy?

 

 

Comment on the students’ statements. Do you agree with one of them more than the others? If so, explain why. If you don’t think that any of them are completely correct, give your own opinion.

 

 

 

 

 

3. In the reactants, hydrogen atoms were bound to chlorine atoms, while silver, nitrogen, and oxygen atoms were bound together. During the reaction, these atoms become separated from each other (on their way to binding together in a new combination). When atoms that were bound together in a molecule become separated, does energy have to flow into the molecule, does energy flow out of the molecule, or is there no net flow of energy into or out of the molecule? Hint: Think of two magnets being stuck together.

 

 

 

4. In the products, the silver atoms are now combined with the chlorine atoms. In order for the silver atom to bind together with the chlorine atom into a silver chloride molecule, does energy have to be supplied to them, or will energy flow away from the atoms as they bind together? Or, will there be no net flow of energy one way or the other?

 

 

 

5. Is the answer you gave to #4 consistent with your answer for #3? Explain. Check your answer with an instructor before proceeding.

 

 

 

 

 

 

6. The textbook says (p. 147), "When we observe a chemical reaction in the laboratory, the chemicals usually constitute the system. The container and everything beyond it are considered the surroundings." In the present case, the word "chemicals" refers to the reactant molecules (silver nitrate and hydrochloric acid) and the product molecules (silver chloride and hydrogen nitrate). With that in mind, would you say that energy flows into the system, out of the system, or that there is no net flow of energy into or out of the system? Explain your answer. Check with one of the instructors before proceeding.

 

 

 

 

 

 

7. Let’s suppose that 2 liters of silver nitrate solution reacts with 2 liters of hydrochloric acid solution, and the temperature of the resulting 4 liter solution rises by 10° C. Suppose that during this process x joules of energy flow either into or out of the system. (Your answer to #6 should have decided whether it’s into or out of, or if instead x is really just 0 joules.)

Suppose now that we react 4 liters of silver nitrate solution with 4 liters of hydrochloric acid solution, and assume that the concentration of the solutions in this case is the same as it was when we only used 2 liters of each. The amount of energy that flows is now y joules. Is y going to be equal to: (i) 2x, (ii) 0.5x, (iii) x (but not zero), or (iv) equal to x because x = y = 0? Explain your answer.

 

 

 

8. Keep in mind your answer to #7, and answer these questions:
1. Will the temperature of the resulting solution (8 liters total) rise, fall, or not change at all? Explain.

 

 

 

2. If you said that the temperature will change, will the temperature change in this case be equal to, greater than, or less than 10° C? Explain your answer.

 

 

 

9. Let’s go back to the case when we had 2 liters of each reactant solution. Now, though, let’s suppose that the concentration of each solution is five times greater than before (so, e.g., instead of a 0.2 M solution, we have a 1.0 M solution). Remember that the total volume of solution is still just 4 liters.
1. Will the temperature of the resulting 4 liter solution rise, fall, or not change at all? Explain.

 

 

 

 

 

2. If you said that the temperature will change, will the temperature change in this case be equal to, greater than, or less than 10° C? Explain your answer.