Objects or Substances with the Higher Energy

44. For each of the following situations, you are asked which of two objects or substances has the higher energy. Explain your answer with reference to the capacity of each to do work and say whether the energy that distinguishes them is kinetic energy or potential energy.

a. (1) A methane molecule, CH4, in the stratosphere or (2) a CH3 molecule and a hydrogen atom formed from breaking one of the carbon-hydrogen bonds in a CH4 molecule.

b. (1) A water molecule moving at 1.63 × 103 mi/h or (2) the same water molecule moving at 1.81 × 103 mi/h. (These are the average velocities of water molecules at 110 °C and 200 °C.)

c. (1) Iodine solid or (2) iodine gas. (Assume that the two systems are at the same temperature.)

d. (1) A nitrogen monoxide, NO, molecule and an oxygen atom in the stratosphere or (2) the NO2 molecule that can form when they collide.

e. (1) Two bar magnets pushed together with the north pole of one magnet almost touching the south pole of the other magnet or (2) the same magnets farther apart.

f. (1) A water molecule moving at 1.63 × 103 mi/h or (2) a uranium hexafluoride, UF6, molecule moving at the same velocity

49. For each of the following changes, describe whether (1) kinetic energy is being converted into potential energy, (2) potential energy is being converted into kinetic energy, or (3) kinetic energy is transferred from one object to another. (More than one of these changes may be occurring.)

a. A car in an old wooden roller coaster is slowly dragged up a steep incline to the top of the first big drop.

b. After the car passes the peak of the first hill, it falls down the backside at high speed.

c. As it goes down the hill, the car makes the whole wooden structure shake.

d. By the time the car reaches the bottom of the first drop, it is moving fast enough to go up to the top of the next smaller hill on its own.

63. Cinnabar is a natural mercury(II) sulfide, HgS, found near volcanic rocks and hot springs. It is the only important source of mercury, which has many uses, including dental amalgams, thermometers, and mercury vapor lamps. Mercury is formed in the following endothermic reaction when mercury(II) sulfide is heated. Consider a system in which a sample of HgS in a closed container is heated with a Bunsen burner flame. 8HgS(s) → 8Hg(l ) + S8(s)

a. Describe the changes in particle motion as heat is transferred from the hot gases of the Bunsen burner flame to the container to the HgS(s).

b. In general terms, explain why energy is absorbed in the reaction.

c. Into what form of energy is the heat energy converted for this reaction?

CHM 101 Module Four Additional Homework Problems

1. In order to verify the heat of reaction, a chemist mixes 1 liter of 1 M NaOH and 1 liter of 1 M HCl. The initial temperature of the two solutions is 25C. The specific heat of water is 4.184 J/gC.

The enthalpy of reaction of the following is –58 kJ/mol.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

a) Is this reaction exothermic or endothermic? (Will the temperature increase or decrease?)

b) Considering only the specific heat of the 2 liters of water and no heat transfer to/from surroundings, what is the final temperature of the solution (notice that the specific heat is in terms of mass, not volume)?

2. The same reaction takes place in two different vessels, which are initially at atmospheric pressure (1 atm). The first vessel is rigid and does not expand with change in pressure; the second vessel will expand to maintain a pressure of 1 atm inside. Over the course of a reaction, the second vessel expands by 1 liter.

a) What is the amount of PV work done by the reaction for each of the vessels?

b) Which vessel’s content has a higher enthalpy?

c) Based on the answer to part b, which vessel would you expect to have a higher temperature?

3. Use the enthalpies of formation in the table below to answer the following questions.

a) Calculate the change in enthalpy for the combustion of methane using the values in the table above (assuming that the system remains at 298 K) for the combustion of methane to form carbon dioxide and gaseous water.

b) Repeat this calculation assuming that the product is liquid water.

c) Explain what this difference represents.