7.E: Exercises

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Additional Exercises

Ammonia (NH₃) is made by the direct combination of H₂ and N₂ gases according to this reaction:

N₂(g) + 3H₂(g) → 2NH₃(g) + 22.0 kcal
(A)Is this reaction endothermic or exothermic? (B) What is the overall energy change if 1,500 g of N₂ are reacted to make ammonia?
A 5.69 g sample of iron metal was heated in boiling water, then dropped into a beaker containing 100.0 g of H₂O at 22.6°C, both reaching a final temperature of 23.1°C. Assuming that the water gained all the heat lost by the iron, what is the temperature change for the iron? (Hint: You will first need to find the heats.)
(A) When 1 g of steam condenses, how much energy (heat) is released. (B) How many grams of ice can be melted with this amount of heat? (Hint: You will need the heats of vaporization and fusion for water.)
The change in energy is +65.3 kJ for each mole of calcium hydroxide [Ca(OH)₂] according to the following reaction:

Ca(OH)₂(s) → CaO(s) + H₂O(g)

If 652 grams of Ca(OH)₂ react, calculate the amount of heat absorbed.
The volume of the world’s oceans is approximately 1.3 × 10²⁴ cm³.
(A) How much heat energy would be needed to increase the temperature of the world’s oceans by 1°C? Assume that, for the oceans, the heat capacity is 4.2 J/g⁰C and the density is 1.0 g/mL. (B) If Earth receives 6 × 10²² J of heat energy per day from the sun, how much could the ocean be warmed by the sun in one day (temperature change), assuming all the energy went into warming the water?
Section 7.5 indicates that the conversion of ATP to ADP gives off about 7.5 kcal/mol. (A) What is the sign of that change, and is that process exothermic or endothermic? (B) What would be the value of ΔH if the reaction is reversed? (C) What is the maximum number of ADP that could be converted to ATP using the energy of oxidation of glucose, Reaction 7.5.1?
The molecule borneol can rearrange to a different form called isoborneol in a process called isomerization. A container initially contains only borneol gas at a pressure of 553 torr. At equilibrium, the pressure of borneol is 500.0 torr and the pressure of isoborneol is 53.0 torr. (A) Calculate the value of the equilibrium constant for the reaction, borneol ⇔ isoborneol. (B) Is the isomerization reaction exergonic or endergonic? (C) Which is more stable at this temperature, borneol or isoborneol?

Answers

(A) Exothermic; (B) –1200 kcal
ΔT = 80°C (for iron), rounded to one significant figure based on the water temperature change
(A) 540 cal; (B) 6.76 g
575 kJ
(A) 5.5 × 10²⁴ J to increase 1 ⁰C; (B) ΔT = 0.01 °C; the heat from the sun per day is about a hundred times less than what is required to change the oceans by 1 ⁰C.
(A) Negative because "gives off" so ΔH = - 7.5 kcal/mol, exothermic. (B) ΔH = + 7.5 kcal/mol, endothermic (C) (670 kcal/mol) / (7.5 kcal/mol) = 89.3, so the maximum number would be eighty nine.
(A) K = (pressure of product) / (pressure of reactant) = (P isoborneol) / (P borneol) = (53.0) / (500.0) = 0.106; (B) Small equilibrium constant (K < 1) means endergonic (ΔG > 0); (C) Positive energy change means the product is higher-energy, so the reactant (borneol) is lower-energy and more stable.