Heat Capacity, Enthalpy and Thermochemistry

Question 1

Define molar heat capacity (Cm)

Answer 1

Molar heat capacity is the amount of energy required to raise 1 mol of s substance by 1K

q = nCmdeltaT

Question 2

How can C be expressed in terms of differentials?

Answer 2

The heat capacity is the increment in heat dq required to increase the temperature of s system by an amount dT

C = dq/dT

This is an extensive property so it is often divided by n to give molar heat capacity.

Question 3

What is isochoric heat capacity?

Answer 3

Heat capacity when the volume of the system is constant

No pV work occurs as dV = 0

hence dU = (dq)
Cv = (dU/dT)

Question 4

What is isobaric heat capacity?

Answer 4

The pressure of the system is constant

If heat is supplied at a constant sample pressure, the sample will expand and work will be done against the external pressure (pext)

Not all the heat will be used to raise the temperature

Cp>Cv

dU = dq -pextdV
dq = dU + pextdV
Cp = ((dU + pextdV)/dT)

Question 5

How does Cp vary with temperature?

Answer 5

For some systems (monoatomic gases) C is temperature independent

However for most systems, the heat capacity varies with temperature

At phase transitions Cp tends to infinity

As heat flow dq does not change with temperature until latent heat of the the transition has been supplied

Within the gas phase it is expressed as a power series:
Cp = a + bT + cT^2 +dT^3

Question 6

What is enthalpy?

Answer 6

Enthalpy is a state function
dH = (dq) at constant p
H = U + pV

The change in enthalpy is equal to the heat flow q at constant pressure

For nearly incompressible solids and liquids, V is approximately 0 so H is approximately U

For an ideal gas,
H = U + nRT

Question 7

What does it mean if H is negative

Answer 7

Heat is released

Reaction is exothermic

Question 8

What does it mean if H is positive

Answer 8

Heat is absorbed

Reaction is endothermic