THERMO5

Question 1

How can you raise the temperature of a system?

Answer 1

• adding heat

- doing work on it

Question 2

One Joule

Definition

Answer 2

-the mechanical (potential) energy required to raise the temperature of 1g of water by 1K

Question 3

First Law of Thermodynamics

Answer 3

ΔEint = Qin + Won

ΔEint = increase in internal energy of the system
Qin = heat transferred into the system
Won = work done on the system

Question 4

cv’

Answer 4

molar specific heat at a constant volume

Question 5

cp’

Answer 5

molar specific heat at constant pressure

Question 6

Ratio Between cv’ and cp’ as

Answer 6

cp’ > cv’ for substances that expand on heating since work is done against the surroundings during expansion
cp’ ≈ cv’ for liquids and solids as any expansion is very small

Question 7

Determining Won

Answer 7

-for a gas exerting a force F on a piston surface area A, the work done by the gas to move the piston a distance dx is
dWby = Fdx = PAdx = PdV
-the work done on the gas is the negative of the work done by the gas
-so dWon = -Pdv, giving

Won = -∫P dv

Question 8

Won

Constant Pressure

Answer 8

Won = -PΔV

Question 9

Won

Constant Volume

Answer 9

Won = 0

Question 10

First Law of Thermodynamics

Ideal Gas at Constant Volume

Answer 10

ΔEint = CvΔT

Question 11

First Law of Thermodynamics

Ideal Gas at Constant Pressure

Answer 11

ΔEint = CpΔT - PΔV

Question 12

Mayer’s Equation

Answer 12

Cp - Cv = nR
AND
cp’ - cv’ = R

Question 13

Monatomic Gas

cv’

Answer 13

cv’ ≈ 1.5R

Question 14

Diatomic Gas

cv’

Answer 14

cv’ ≈ 2.5R

Question 15

Polyatomic Gas

cv’

Answer 15

cv’ > 2.5R

Question 16

How to find cv’ for the general case

Answer 16

Cv = dEint/dT

and cv’ = Cv / n

Question 17

What does internal energy in an ideal gas depend on?

Answer 17

only temperature

Question 18

Joule Thompson Effect

Definition

Answer 18

a real gas cools when undergoing a free expansion

Question 19

Joule Thompson Effect

Expansion of Ideal Gas into Vacuum

Answer 19

-gas allowed to freely expand into the vacuum in apparatus insulated from the surroundings
-when the gas reaches equilibrium the temperature is found to have remained for constant (for gases at low density)
-shows that internal energy does not depend on the volume of a ideal gas
KEav = 3/2 nRT

Question 20

Joule Thompson Effect

Expansion of Real Gas into Vacuum

Answer 20

• real gases have attractions between molecules
• as the gas expands, the potential energy of the molecules is increased as they get further apart
• for a rigid insulated system this increase in PE can only come from reducing the KE
• so the temperature is reduced slightly

Question 21

Heat Capacity Ratio

Equation

Answer 21

γ = Cp / Cv = cv’ / cp’

Question 22

Monatomic Gas

Heat Capacity Ratio

Answer 22

γ = 5/3 ≈ 1.67

Question 23

Diatomic Gas

Heat Capacity Ratio

Answer 23

γ = 7/5 ≈ 1.40

Question 24

Polyatomic Gas

Heat Capacity Ratio

Answer 24

γ = 8/6 ≈ 1.33