Exercises

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Section 5-1

Q5-1

What is kinetic energy? Can a single molecule have a kinetic energy? Please explain

Q5-2

What do you think temperature is? Can a single molecule have a temperature? Please explain

Q5-3

What is thermal energy?

Q5-4

What is the difference between temperature and thermal energy?

Section 5-2

Q5-1

What happens to the average speed of gas molecules as the temperature increases? Why?

Q5-2

When gas molecules collide (for example molecules of O₂ or N₂ in the air), why don’t they stick together?

Q5-3

Imagine a system composed of two different types of molecules, one relatively light and one relatively heavy. At a particular temperature, how do their average velocities compare? Why?

Q5-4

Imagine a system composed of two different types of molecules, one relatively light and one relatively heavy. At a particular temperature, how do their average kinetic energies compare? Why?

Q5-5

You place a thermometer into a solution - why does it take time for the reading on the thermometer to correspond to the temperature of the solution?

Q5-6

Why don’t all the gas particles move with the same speed at a given temperature?

Q5-7

If gas molecules are moving so fast (around 500 m/s), why do most smells travel at significantly less than that? (if I open a container of smelly stuff at the front of the CH 101 auditorium it takes a few seconds to smell it – why?)

Section 5-4

Q5-1

Water has some weird properties. Use to your knowledge of the structure of water to explain why water has each of these properties – and why each of them is important for life as we know it?

A) High specific heat

B) High melting/boiling point

C) Low density of ice (solid water)

D) Low vapor pressure (the pressure exerted by a gas above the liquid at a given temperature)

Section 5-5

Q5-1

A) Draw a graph of the change in temperature when equal amounts of thermal energy are added at the same rate to equal masses of water (sp ht 4.18 J/g °C), ethanol (sp ht 2.44 J/g °C). Assume they both start at the same temperature (10 °C ) and the ethanol ends up at 50 °C.

B) Does each sample reach the same temperature? Why or why not?

Q5-2

A) Plot the temperature change vs time as a sample of water moves from a temperature of 90°C to 110°C as you add heat at a constant rate. Explain what is happening at each different part of your graph.

B) Draw a molecular level picture of what the sample looks like at 90°C and 110°C

Section 5-6

Q5-1

If you put a block of hot metal next to a block of cold metal (touching), what will happen? Why?

Q5-2

If you add a drop of dye to a beaker of water it will and stir – the dye mixes up with the water. Why? Will this process ever run in reverse (ie the dye and water will unmix?)

Q5-3

Tell us what you know about “entropy”.

Section 5-7

Draw a picture of a beaker with ice and water at 10 °C. Label the system and the surroundings.

Since the temperature is at 10 °C what will happen to the ice-water?

What will be the sign of ΔH for the system? (Which way does the energy transfer?) Show this on your diagram (use an arrow)

What will be the sign of ΔH for the surroundings?

What is the sign of ΔS for the system - explain your answer?

What is the sign of ΔS for the surroundings - explain your answer?

Section 5-8

Q5-1

You have two different experiments set up (at atmospheric pressure):

A. A mixture of liquid water and water vapor with a surrounding temperature of 90 °C		B. A mixture of liquid water and water vapor with a surrounding temperature of 110 °C
What is the sign of ΔH for the system?
What is the sign of ΔH for the surroundings?
What is the sign of ΔS for the system?
What is the sign of ΔS for the surroundings?
What is the sign of ΔS for the Universe?

Q5-2

What would ΔS for the Universe be if the temperature was 100 °C? (at atmospheric pressure)

Q5-3

Why does water condense below 100 °C and boil above 100 °C?

Q5-4

Rank the following compounds in order of increasing ΔH of vaporization (the energy required to change one mole of each compound from solid to liquid). Explain your answer in terms of the intermolecular forces in each compound (draw it out)

CH₃CH₂OH CH₃OCH₃ CH₃CH₂CH₃

Section 5-9

Q5-1

For the reaction: 2NO₂(g) à N₂O₄(g)

ΔS° = –175.8 J/K

ΔH° = –57.2 kJ

A) Calculate ΔG° (remember T = 298 K at the standard state) – what are the units?

B) Is this reaction thermodynamically favorable? (What is the sign of ΔG°)

C) At what temperature is the reaction at equilibrium (i.e. phase change)?