6.1.4 Work

Question 1

work

Answer 1

• By convention, energy, heat, or work entering a system has a positive sign, while energy, heat, or work coming out of a system has a negative sign.
• PV work is work performed by a gas expanding or contracting.

Question 2

note

Answer 2

• If energy flows from the surroundings to the system, the change in internal energy ( E) is positive. If energy flows from the system to the surroundings, E is negative.
• If heat flows from the surroundings to the system, heat (q) is positive. If heat flows from the system to the surroundings, q is negative.
• If work (w) is done on the system by the surroundings, w is positive. If work is done on the surroundings by the system, w is negative.
• PV work is work performed by a gas expanding or
  contracting.
• In general, work (w) equals force (F) multiplied by distance (d). For PV work, the force is equal to the external pressure (PexΔ) times the area (A), and the distance is equal to the change in height (Δh). Area multiplied by change in height equals change in volume (ΔV). Therefore, the work done is equal to PexΔ multiplied by V. Since PV work is done on the system by the surroundings, w = –P exΔV.
• Problem: Calculate the work done in joules when a gas
  expands from 1.00 L to 2.00 L against a constant external pressure of 1.00 atm.
• Recall: 1 L · atm = 101.325 J.
• Plugging the given quantities into the equation
  w = –P ex V yields the work (–101 J).

Question 3

When a gas expands against a constant external pressure, what is true of work, w?

Answer 3

It is negative.

Question 4

Which of the following is true when the value of q is positive?

Answer 4

Heat flows from the surrounding into the system.

Question 5

When w is positive, which of the following is true?

Answer 5

Work was done by the surroundings on the system.

Question 6

When q is negative, which of the following is true?

Answer 6

Heat flowed from the system to the surroundings.

Question 7

What is the work in joules when a gas is changed from 7.50 L to 3.00 L against a constant external pressure of 3.00 atm?
Use 1 L-atm = 101.325 J.

Answer 7

1370 J

Question 8

What is the sign on w when a gas is compressed at constant pressure?

Answer 8

It is positive.

Question 9

What is the work, in joules, when a gas expands from 2.5 L to 4.0 L against a constant external pressure of 1.0 atm? 1 L-atm = 101.325 J

Answer 9

−150 J

Question 10

Which of the following best explain what PΔV is?

Answer 10

It is equal to w due to expansion of a gas against a constant pressure.

Question 11

The figure below shows a cylinder with a movable piston (green). A gas is trapped inside the cylinder.Which of the following is true?

Answer 11

From A to B the gas is compressed, work is done on the system, w is positive.

Question 12

What is the work in joules when a gas expands from 3.10 L to 6.40 L against a constant external pressure of 2.00 atm?
Use 1 L-atm = 101.325 J.

Answer 12

−669 J