Chapter 11
Heat and temperature
 Heat is a thermal energy.
 The Universe is made up of matter and energy. Matter
is made up of atoms and molecules and energy causes
the atoms and molecules to always be in motion either bumping into each other or vibrating back and
forth.
 The motion of atoms and molecules creates a form of
energy called heat or thermal energy which is present
in all matter.
 Temperature is a measure of the average heat or thermal
energy of the particles in a substance. Since it is an average
measurement, it does not depend on the number of
particles in an object. It does not depend on the size of it.
For example, the temperature of a small cup of boiling
water is the same as the temperature of a large pot of
boiling water even if the large pot is much bigger than the
cup and has millions and millions more water molecules.
 We experience temperature every day. When it is very hot
outside or when we have a fever we feel hot and when it is
snowing outside we feel cold. When we are boiling water,
we wait for the water temperature to increase and when we
make popsicles we wait for the liquid to become very cold
and freeze.
Heat
Temperature
 The unit for heat is Joules.

 It is the total random kinetic
energy of its atoms.

The unit for temperature is
Kelvin.
It is the average random
kinetic energy of the atoms.
 When water is heated in a beaker its temperature
gradually changes. If the beaker contains 360.3 g of
H₂O (20.0 mol) initially at 15⁰C and the heater is switched
on , the water temperature begins to rise . If the
temperature is recorded every 15 seconds till it reaches a
particular temperature( degree celcius) you can record
the data by converting the temperature to kelvin. After an
initial lag the temperature slowly rises with time. We can
calculate the slope=(y2-y1)/(x2-x1)=rise/run=∆T/∆t
 This is the change in temperature, ∆T, that occurs while
time is changing by an interval,∆t.
Molar heat capacity
 The heat required to increase the temperature of 1 mol
of a substance through 1K. It is the same as specific
heat capacity. Given by the formula q=nC∆T
 C is the specific heat capacity of a substance
 q=heat needed to increase the temperature of n
moles of a substance by ∆T.
Class Practice
 A 100 W heater is used to heat 20.0mol of water. The
temperature is recorded every 15 sec and data is
recorded and graph is plotted. The slope of the graph
is calculated to be 0.066K/s. Calculate the molar heat
capacity for water.
 Molar heat capacity depends on the number of atoms.
A mole of tungsten has a mass of 184 g and a mole of
aluminium has a mass of about 27 g. As predicted the
more heat would be needed to increase the
temperature of 1 mol of tungsten than 1 mol of
aluminium by 1K; but they have the same molar heat
capacity.Heat energy is taken up increasing the kinetic
energies of the atoms there are same number of atoms
in 1 mol of each metal.
Thermodynamics
 Thermodynamics is the study of the connection
between heat and work and the conversion of one into
the other. This study is important because many
machines and modern devices change heat into work
(such as an automobile engine) or turn work into heat
(or cooling, as in a refrigerator).
 The science of thermodynamics is founded on two
principles, both of which involve the concept of
energy. The first principle asserts that energy is
conserved, i.e., energy can neither be created nor
destroyed. The second principle asserts that the
overall distribution of energy tends to become more
uniform, never less uniform. These two principles are
called the first and second laws of thermodynamics
Entropy and enthalpy
 Entropy is a measure of the disorder of a system.
Example ice has a less entropy than liquid water
because it has more order.The entropy of a substance
increase with temperature.The gaseous form of water
has a greater entropy than liquid form which in turn
has a greater entropy that solid form.
 Molar entropies are always positive except at 0.0K. The
entropy for pure substances are o at absolute o Kelvin.
No disorder no entropy.
 Entropy ∆S=Q/T
Enthalpy
 The first law of thermodynamics is the application of
the conservation of energy principle to heat and
thermodynamic processes:
 The change in internal energy of a system is equal to
(the heat added to the system)- (the work done by the
system)= ∆U=Q-W
 When something is heated at constant pressure, its
enthalpy increase equals the thermal energy it
receives. The symbol for molar enthalpy is H because
enthalpy was also called as “heat content”.
 Temperature affects molar enthalpies.The total
enthalpy increase is equal to q ( the heat input), so the
molar enthalpy increase ∆H, can be found by dividing
by n the moles of any substance: ∆H=q/n
 From slide # 6 we saw that q=nC∆T; substitute in the
above equation we get ∆H=nC∆T/n=C∆T
Class Practice
 Page 393 sample problem 11B
Homework
 Page #395 do # 8,9,10,11,12
Changing states for water
 Water has 3 states of matter:
 Thermal energy is needed to melt ice and to boil water.
Thermal energy increases the kinetic energy of
molecules, allowing them to break free of
intermolecular forces.
The molar enthalpy changes that occurs during melting is called the molar
enthalpy of fusion or heat of fusion ∆Hfus.
The molar enthalpy of vaporization , (∆H vap ) or heat of vaporization is
the difference between the molar enthalpies of 1 mol of a substance in its
gaseous and its liquid states.
gas
Molar
enthalpy of
vaporization
liquid
solid
Molar enthalpy of
fusion
Gibbs energy
 A thermodynamic property incorporating both






enthalpy and entropy. It is expressed as
G=H-TS
T is the temperature (SI unit: kelvins)
S is the entropy (SI unit: joules per kelvin)
H is the enthalpy (SI Units: joules)
Change in Gibbs energy can be used to compare the
change in the enthalpy and entropy using the
following equation
∆G=∆H-T∆S
 A process is spontaneous if ∆G is negative. All
spontaneous process occurs with a decrease in Gibbs
energy.
 Spontaneous process takes place ‘naturally’ with no
apparent cause or stimulus.
 Non spontaneous process requires that something be
done in order for it to occur.
Homework
 Page 401
 Q.4,q.8,q.12,q.17
Hess’s Law
 This law states that that the overall enthalpy change in
a reaction is equal to the sum of the enthalpy changes
of the individual steps in the process.
Class Practice
 1. Calculate the change in enthalpy ,∆H , for the
following reaction
H2(g) + CO2(g) -H2O(g) + CO(g)
 Is the reaction exothermic or endothermic?
 2. Calculate the entropy change for the following reaction
H2(g) +CO2(g) H2O(g) + CO(g)
Does the reaction proceed towards a more ordered or
disordered state?
Homework
 Page 419
 Q.34,35
 Test prep all