BioPHYSICAL chemistry

Specific heat

The application of heat to an object will cause
the object’s temperature to increase (Figure 2.5).
Since heat represents the transfer of energy
and temperature is a measure of the kinetic
energy of molecules, the change in temperature,
ΔT, is directly proportional to the heat applied
(provided that heat does not result in a
chemical change):

q=CΔT (2.11)

The proportionality constant Cis termed
the heat capacitybecause it is a measure of
how much heat is absorbed or emitted with a given temperature change.
The value of Cdepends upon the properties of the object; for example, the
heat capacity of water is very different to that of a metal rod. The heat
capacity also is dependent upon the conditions under which heating
and cooling are performed. The simplest case is when the volume of the
system is held constant, as denoted by the inclusion of a subscript Vwith
the heat capacity, CV. The other common condition is when pressure is
held constant, which is denoted by the subscript P, CP.
As an example of the use of heat capacity, consider the effect of heat-
ing water compared to a piece of copper. Under constant pressure, at
4.18 J K−^1 g−^1 , the heat capacity of liquid water is much larger than that
of copper (0.38 J K−^1 g−^1 ). The smaller value of copper’s heat capacity means
it will become much hotter than an equivalent amount of water when a
given amount of heat is applied. For example, applying 1 J of energy to
1 g of water will produce a temperature change of 0.24 K:

(2.12)

The same amount of heat will result in a temperature change of 2.63 K
for 1 g of copper. Due to its relatively small specific heat, copper is a good
heat conductor and so samples are often embedded in a copper block (or
one made of another metal) to set a sample at a specific temperature in
experiments.

Internal energy for an ideal gas

Energy can be transferred into a system by either a heat flow or by work.
Regardless of how the energy was supplied, it can be released in either

ΔT

q

CP.

== −−=.

1

418

11024

J

JK g

K

CHAPTER 2 FIRST LAW OF THERMODYNAMICS 31

Add heat

Heat source

T initial T initial  q

q

CP

Figure 2.5

Specific heat

at constant

pressure, CP, is a

thermodynamic

parameter that

describes how

much a substance

will change in

temperature in

response to the

addition of heat, q.