8. Thermochemistry

Which of the following exchanges of energy has a positive sign for heat (relative to the system)?

If a cylinder equipped with a piston expands against an external pressure of 1.25 atm, increasing the volume of a cylinder by 0.500 L and releasing 25.0 J of heat, what is ΔE for the process?

A 0.1250-g sample of octane (C₈H₁₈) is burned in a bomb calorimeter with a heat capacity of 5.1 × 10² J/℃. The temperature inside the calorimeter increases by 11.8℃. What is the ΔE (in kJ/mol octane) for the combustion of octane?

Which of the following is the result of an exothermic process?

The contents of the closed box in each of the following illustrations represent a system, and the arrows show the changes to the system during some process. The lengths of the arrows represent the relative magnitudes of q and w. (a) Which of these processes is endothermic?

Several processes are given in the table and labeled as endo- thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process (d) Rubbing alcohol evaporates from your skin.

Several processes are given in the table and labeled as endo- thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process (c) Water freezes into ice in the freezer.

Several processes are given in the table and labeled as endo- thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process (b) Methane, the main component of natural gas, is burned to produce a flame on a stovetop.

Several processes are given in the table and labeled as endo- thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process (a) Ammonium nitrate dissolves in water, and the temperature of the solution decreases.

Imagine a container placed in a tub of water, as depicted in the accompanying diagram. (a) If the contents of the container are the system and heat is able to flow through the container walls, what qualitative changes will occur in the temperatures of the system and in its surroundings? From the system's perspective, is the process exothermic or endothermic?

Consider the vaporization of liquid water to steam at a pressure of 1 atm. (a) Is this process endothermic or exothermic?

Which of the following reactions in the stratosphere cause an increase in temperature there? (a) O(g) + O2(g) → O3+(g) (b) O3*(g) + M(g) → O3(g) + M*(g) (c) O2(g) + hv → 2 O(g) (d) O(g) + N2(g) → NO(g) + N(g) (e) All of the above

For the following processes, calculate the change in internal energy of the system and determine whether the process is endothermic or exothermic: (a) A balloon is cooled by removing 0.655 kJ of heat. It shrinks on cooling, and the atmosphere does 382 J of work on the balloon.

A piece of dry ice (solid CO2) is placed inside a balloon, and the balloon is tied shut. Over time, the carbon dioxide sub- limes, causing the balloon to increase in volume. Give the sign of the enthalpy change and the sign of work for the sublima- tion of CO2.

Imagine a reaction that results in a change in both volume and temperature:

(b) Has there been an enthalpy change? If so, what is the sign of ∆H? Is the reaction exothermic or endothermic?

Without referring to tables, predict which of the following has the higher enthalpy in each case: (c) 1 mol I21g2 and 1 mol H21g2 at 25 °C or 2 mol HI1g2 at 25 °C

Without referring to tables, predict which of the following has the higher enthalpy in each case: (b) 2 mol of iodine atoms or 1 mol of I2

Without referring to tables, predict which of the following has the higher enthalpy in each case: (a) 1 mol I21s2 or 1 mol I21g2 at the same temperature

At one time, a common means of forming small quantities of oxygen gas in the laboratory was to heat KClO3: 2 KClO31s2¡2 KCl1s2 + 3 O21g2 H = -89.4 kJ (c) The decomposition of KClO3 proceeds spontaneously when it is heated. Do you think that the reverse reaction, the formation of KClO3 from KCl and O2, is likely to be feasible under ordinary conditions? Explain your answer.

Consider the decomposition of liquid benzene, C6H61l2, to gaseous acetylene, C2H21g2: C6H61l2 ¡ 3 C2H21g2 H = +630 kJ (d) If C6H61g2 were consumed instead of C6H61l2, would you expect the magnitude of H to increase, decrease, or stay the same? Explain.

Consider the decomposition of liquid benzene, C6H61l2, to gaseous acetylene, C2H21g2: C6H61l2 ¡ 3 C2H21g2 H = +630 kJ (c) Which is more likely to be thermodynamically favored, the forward reaction or the reverse reaction?

(b) Is this process endothermic or exothermic?

Determine whether each process is exothermic or endothermic and indicate the sign of ΔH. a. natural gas burning on a stove b. isopropyl alcohol evaporating from skin c. water condensing from steam Indicate the sign of ΔH for the following processes.

Determine whether each process is exothermic or endothermic and indicate the sign of ΔH. a. natural gas burning on a stove b. isopropyl alcohol evaporating from skin c. water condensing from steam Determine whether each of the following is exothermic or endothermic.

(a) Is the dissociation of fluorine molecules into atomic fluorine, F2(g)Δ2 F(g) an exothermic or endothermic process?

Without doing any calculations, predict the sign of H for each of the following reactions: (a) NaCl1s2¡Na+ 1g2 + Cl-1g2

In the chemical process called electron transfer, an electron is transferred from one atom or molecule to another. (We will talk about electron transfer extensively in Chapter 20.) A simple electron transfer reaction is A1g2 + A1g2¡A+1g2 + A-1g2 For a representative nonmetal such as chlorine, is this process exothermic?

An important reaction for the conversion of natural gas to other useful hydrocarbons is the conversion of methane to ethane. 2 CH41g2¡C2H61g2 + H21g2 In practice, this reaction is carried out in the presence of oxygen, which converts the hydrogen produced into water. 2 CH41g2 + 12 O21g2¡C2H61g2 + H2O1g2 Use Table 8.3 to estimate H for these two reactions. Why is the conversion of methane to ethane more favorable when oxygen is used? Why is the conversion of methane to ethane more favorable when oxygen is used?

The thermite reaction, Fe2O3 + Al S Al2O3 + Fe produces so much heat that the Fe product melts. This reaction is used industrially to weld metal parts under water, where a torch cannot be employed. It is also a favorite chemical demonstration in the lecture hall (on a small scale). (d) If you performed the reverse reaction— aluminum oxide plus iron makes iron oxide plus aluminum—would that reaction have heat as a reactant or a product?

Which of the following processes is endothermic?

Which one of the following is an endothermic process?

Which of the reactions are exothermic?

In which one of the following processes is the system endothermic?