I. 3 Heat Capacity, Latent Heat, Enthalpy

Question 1

Heat capacity

Answer 1

The amount of heat needed to raise its temperature per degree temperature increase.

C = Q/ΔT = (ΔU- W)/ ΔT

Question 2

Specific Heat Capacity

Answer 2

Heat capacity per unit mass.
c = C/m

Question 3

Heat Capacity at Constant Volume

Answer 3

Since W = -PdV, work is 0.

Cv = (ΔU/ΔT)v = (dU/dT)v = d/dT (NfkT/2) = Nfk/2

Energy needed to raise the object’s temperature, per degree regardless of whether heat energy enters or not.

Question 4

Heat Capacity at Constant Pressure

Answer 4

Objects often expand as they are heated, doing negative work on their surroundings. Thus heat must be added
to compensate for energy lost by work. (last term)

If pressure is constant during this:

Cp = (ΔU-(-PΔV)/ΔT)p = (dU/dT)p + P(dV/dT)p = (dU/dT)p + Nk/P = (dH/dT)p

This is for ideal gas.

Question 5

What is the relationship between Cp and Cv?

Answer 5

Cp = Cv + Nk = Cv + nR

Question 6

What is a phase transformation? What is its heat capacity?

Answer 6

Caused by putting heat into a system without increasing its temperature at all. This means the heat capacity is infinite.

C = Q/ΔT = Q/0 = inf.

Question 7

Latent Heat & Specific Latent Heat

Answer 7

The amount of energy absorbed or released by a substance to experience a phase transition. (L)

Specific Latent Heat (l) is amount of energy required to change state of one unit of mass w/o changing its temperature.

l = L/m = Q/m

Question 8

Enthalpy

Answer 8

The total energy to create system out of nothing. (add work to account for/make up room for compression-expansion that is done)

H = U + PV

Internal Energy + Energy required to push against the surroundings

Question 9

How can enthalpy increase?

Answer 9

1. Energy increases.
2. System expands and work is done on the atmosphere to make room for it.

ΔH = ΔU + PΔV (constant P)

Question 10

How can you implement Enthalpy into First Law of Thermodynamics? What does it tell us?

Answer 10

ΔU = Q + (-PΔV) + W other

ΔH = Q + W other

This tells us that the change in Enthalpy is only caused by heat and other forms of work. (constant-pressure process).

If no other types of work are present, it can directly tell us how much heat has been added to a system.