AP Chemistry: 6.2 Energy Diagrams – Exam Style questions with Answer- FRQ

Question

4 Fe(s)+ 3 O2 (g)  → 2 Fe 2 O 3(s)                      ∆H° − 1650 kJ / molrxn 

A student investigates a reaction used in hand warmers, represented above. The student mixes Fe(s) with a catalyst and sand in a small open container. The student measures the temperature of the mixture as the reaction proceeds. The data are given in the following table.

(a) The mixture (Fe(s), catalyst, and sand) has a total mass of 15.0 g and a specific heat capacity of 0.72 J/(g·°C). Calculate the amount of heat absorbed by the mixture from 0 minutes to 4 minutes.

(b) Calculate the mass of Fe(s), in grams, that reacted to generate the amount of heat calculated in part (a).

(c) In a second experiment, the student uses twice the mass of iron as that calculated in part (b) but the same mass of sand as in the first experiment. Would the maximum temperature reached in the second experiment be greater than, less than, or equal to the maximum temperature in the first experiment? Justify your answer.

▶️Answer/Explanation

Ans:

(a) For the correct calculated value with units: 
q = mc ΔT = (15.0 g)(0.72 J/(g· 0C))(39.7 0C – 22.0 0C) = 190 J

(b) For the correct calculated value of the moles of reaction, consistent with part (a) (may be implicit):

qsys  = – qsurr

\(-190 J\times \frac{1 kJ}{1000 J}\times \frac{1 mol _{rxn}}{-650 kJ}=0.00012 mol_{rxn}\)

For the correct calculated value of the mass of iron:

\(0.00012mol_{rxn}\times \frac{4 mol Fe}{1 mol_{rxn}}\times \frac{55.85 g Fe}{1 mol Fe}=0.027 g Fe \) (0.026 g if decimals are carried)

(c) For the correct answer and a valid justification:
Greater than. A greater mass of iron provides a greater number of moles of reaction, which would transfer a greater quantity of thermal energy to the same mass of sand and therefore
lead to a greater maximum temperature.

 Question

2 C3H7OH(l) + 9 O2(g) → 6 CO2(g) + 8 H2O(g)

A student performs an experiment to determine the enthalpy of combustion of 2-propanol, C3H7OH(l), which combusts in oxygen according to the equation above. The student heats a sample of water by burning some of the C3H7OH(l) that is in an alcohol burner, as represented below. The alcohol burner uses a wick to draw liquid up into the flame. The mass of C3H7OH(l) combusted is determined by weighing the alcohol burner before and after combustion.

Data from the experiment are given in the table below.

(a) Calculate the magnitude of the heat energy, in kJ, absorbed by the water. (Assume that the energy released from the combustion is completely transferred to the water.)
(b) Based on the experimental data, if one mole of C3H7OH(l) is combusted, how much heat, in kJ, is released? Report your answer with the correct number of significant figures.
(c) A second student performs the experiment using the same mass of water at the same initial temperature. However, the student uses an alcohol burner containing C3H7OH(l) that is contaminated with water, which is miscible with C3H7OH(l). The difference in mass of the alcohol burner before and after the combustion in this experiment is also 0.55 g. Would the final temperature of the water in the beaker heated by the alcohol burner in this experiment be greater than, less than, or equal to the final temperature of the water in the beaker in the first student’s experiment? Justify your answer.

▶️Answer/Explanation

Ans:

(a)

q = mcΔT
=(125.00 g) (4.18 J/(g.0C))(51.1 0C – 22.0 0C)
= 15,200 J  = 15.2 kJ

(b)

1 mol \(C_{3}H_{7}OH\times \frac{60.09 gC_{3}H_{7}OH}{1 mol C_{3}H_{7}OH}\times \frac{15.2 kJ}{0.55 g C_{3}H_{7}OH}=1661 kJ\)
                                                                                           = 1.7 × 103 kJ

(c)

The final temperature measured by the second student would be less than that measured by the first student because:

the actual mass of C3H7OH(l) combusted will be less than 0.55 g
OR
combustion of the contaminated sample will also require vaporization of the water in the sample. 

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