Textbook Question
A 2.85-g lead weight, initially at 10.3 °C, is submerged in 7.55 g of water at 52.3 °C in an insulated container. What is the final temperature of both substances at thermal equilibrium?
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Ch.6 - Thermochemistry
Chapter 6, Problem 72
Should you carry out a chemical reaction under conditions of constant volume or constant pressure to obtain the largest possible amount of heat, if there is a large increase in the number of moles of gas? Explain.
Verified step by step guidance1
<Understand the difference between constant volume and constant pressure conditions. Under constant volume, the heat exchanged is equal to the change in internal energy (\( \Delta U \)), while under constant pressure, the heat exchanged is equal to the change in enthalpy (\( \Delta H \)).>
<Recall that for reactions involving gases, the change in enthalpy (\( \Delta H \)) is related to the change in internal energy (\( \Delta U \)) by the equation: \( \Delta H = \Delta U + \Delta n_{gas}RT \), where \( \Delta n_{gas} \) is the change in moles of gas, \( R \) is the ideal gas constant, and \( T \) is the temperature.>
<Consider the scenario where there is a large increase in the number of moles of gas (\( \Delta n_{gas} > 0 \)). This means that \( \Delta n_{gas}RT \) is positive, making \( \Delta H > \Delta U \).>
<Since \( \Delta H > \Delta U \) when there is an increase in moles of gas, more heat is absorbed or released under constant pressure conditions compared to constant volume conditions.>
<Therefore, to obtain the largest possible amount of heat, the reaction should be carried out under constant pressure conditions.>
Related Practice
Textbook Question
Two substances, A and B, initially at different temperatures, come into contact and reach thermal equilibrium. The mass of substance A is 6.15 g and its initial temperature is 20.5 °C. The mass of substance B is 25.2 g and its initial temperature is 52.7 °C. The final temperature of both substances at thermal equilibrium is 46.7 °C. If the specific heat capacity of substance B is 1.17 J/g•°C, what is the specific heat capacity of substance A?
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Textbook Question
Exactly 1.5 g of a fuel burns under conditions of constant pressure and then again under conditions of constant volume. In measurement A the reaction produces 25.9 kJ of heat, and in measurement B the reaction produces 23.3 kJ of heat. Which measurement (A or B) corresponds to conditions of constant pressure? Explain.
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Textbook Question
When 0.514 g of biphenyl (C12H10) undergoes combustion in a bomb calorimeter, the temperature rises from 25.8 °C to 29.4 °C. Find ΔErxn for the combustion of biphenyl in kJ/mol biphenyl. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.86 kJ/°C.
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Textbook Question
Mothballs are composed primarily of the hydrocarbon naphthalene (C10H8). When 1.025 g of naphthalene burns in a bomb calorimeter, the temperature rises from 24.25 °C to 32.33 °C. Find ΔErxn for the combustion of naphthalene. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.11 kJ/°C.
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Textbook Question
Zinc metal reacts with hydrochloric acid according to the balanced equation: Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g) When 0.103 g of Zn(s) is combined with enough HCl to make 50.0 mL of solution in a coffee-cup calorimeter, all of the zinc reacts, raising the temperature of the solution from 22.5 °C to 23.7 °C. Find ΔHrxn for this reaction as written. (Use 1.0 g/mL for the density of the solution and 4.18 J/g•°C as the specific heat capacity.)
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