Heat Capacity

Basic Physical Chemistry - Thermodynamics 1

2 Basic Physical Chemistry - Thermodynamics

Heating Objects

Heat Capacity, C, is the heat energy required to

raise the temperature of an object by 1 °C or 1 K.

Low

Heat

Capacity

High

Heat

Capacity

C = q / DT [6]

3 Basic Physical Chemistry - Thermodynamics

Definitions of Heat Capacity

Heat capacity is an extensive property.

Specific Heat Capacity, Cs = C / m units J K–1 kg–1

Molar Heat Capacity, Cm = C / n units J K–1 mol–1

Specific Heat capacity and Molar Heat Capacity

are intensive properties.

130

140 390

440

710

710

1340 2000

2010

4 Basic Physical Chemistry - Thermodynamics

520

Specific heat capacities of substances / J K–1 kg–1

880

1670

5 Basic Physical Chemistry - Thermodynamics

Isochoric and Isobaric Heat Capacities

Isochoric heat capacity is defined as:

CV = qV / DT = DU / DT [7]

Isobaric heat capacity is defined as:

Cp = qp / DT = DH / DT [8]

Hm = Um + pVm

H = U + pV

For an ideal gas: pV = nRT

pVm = RT

6 Basic Physical Chemistry - Thermodynamics

RT = 8.314 J K–1 mol–1 × 298 K ~ 2.5 kJ mol–1

Hm = Um + RT

This is not negligible for a gas.

Cp,m & CV,m Relationship for an Ideal Gas

For a change of temperature DT

Hm = Um + RT

DHm = DUm + RDT

DHm

DT=DUm

DT+ R

7 Basic Physical Chemistry - Thermodynamics

Cp,m = CV,m + R [9]

Cp,m & CV,m Relationship for an Ideal Gas

where R = 8.314 J K–1 mol–1

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