Open Question
(c) For a given process at constant pressure, _x001F_H is negative. Is the process endothermic or exothermic?
Ch.5 - Thermochemistry
Chapter 5, Problem 36
Suppose that the gas-phase reaction 2 NO(g) + O2(g) → 2 NO2(g) were carried out in a constant-volume container at constant temperature. (a) Would the measured heat change represent _x001F_H or _x001F_E? (b) If there is a difference, which quantity is larger for this reaction? (c) Explain your answer to part (b).
Verified step by step guidance1
Step 1: Understand the difference between enthalpy change (ΔH) and internal energy change (ΔE). ΔH is the heat change at constant pressure, while ΔE is the heat change at constant volume.
Step 2: Recognize that the problem states the reaction occurs in a constant-volume container, which means the measured heat change represents ΔE, the change in internal energy.
Step 3: Recall the relationship between ΔH and ΔE: ΔH = ΔE + ΔnRT, where Δn is the change in moles of gas, R is the ideal gas constant, and T is the temperature in Kelvin.
Step 4: Calculate Δn for the reaction: 2 NO(g) + O2(g) → 2 NO2(g). The change in moles of gas, Δn, is (moles of products) - (moles of reactants) = 2 - (2 + 1) = -1.
Step 5: Since Δn is negative, ΔH = ΔE + ΔnRT will be less than ΔE, meaning ΔE is larger than ΔH for this reaction.
Related Practice
Open Question
(a) Under what condition will the enthalpy change of a process equal the amount of heat transferred into or out of the system? (b) During a constant-pressure process, the system releases heat to the surroundings. Does the enthalpy of the system increase or decrease during the process? (c) In a constant-pressure process, _x001F_H = 0. What can you conclude about _x001F_E, q, and w?
Open Question
Assume that 2 moles of water are formed according to the following reaction at constant pressure (101.3 kPa) and constant temperature (298 K): 2 H2(g) + O2(g) → 2 H2O(l). (b) Calculate _x001F_E for the reaction using your answer to (a).
Textbook Question
A gas is confined to a cylinder under constant atmospheric pressure, as illustrated in Figure 5.4. When 0.49 kJ of heat is added to the gas, it expands and does 214 J of work on the surroundings. What are the values of H and E for this process?
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Textbook Question
The complete combustion of methane, CH4(g), to form H2O(l) and CO2(g) at constant pressure releases 890 kJ of heat per mole of CH4. (a) Write a balanced thermochemical equation for this reaction.
Textbook Question
The complete combustion of methane, CH4(g), to form H2O(l) and CO2(g) at constant pressure releases 890 kJ of heat per mole of CH4. (b) Draw an enthalpy diagram for the reaction.
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