Causes of Change
Using Enthalpy

Because enthalpy is the total energy of a
system, it is a very important quantity.

However, like we know, it can only be
measured through a changed.

There is no way to determine the true
value of H, but ΔH can be measured as a
change occurs.
Molar Enthalpy Change

The enthalpy change for one mole of a
pure substance is called the __________.

Consider the following (heating iron):
Fe(s, 300K)  Fe(s, 1100K) ΔH = 20.1 kJ/mol

This indicates that when 1 mol of solid
iron is heated from ___K to ___K, its
molar enthalpy increases by 20,100
Joules.
Molar Enthalpy Change

The iron does not change state OR goes
through a chemical reaction.

This means the change in enthalpy
represents only a change in the kinetic
energy of the iron atoms.

When a pure substance is heated or
cooled, the amount of heat involved is the
same as the change in enthalpy (ΔH)
ΔH = q
Molar Heat Capacity Governs the
Changes

With this said, the molar enthalpy change
is related to the molar heat capacity by
the following equation:
Molar enthalpy change = CΔT
=(molar heat cap)(change in T)
This equation DOES NOT apply to chemical
reactions or changes in state.
Molar Heat Capacity Governs the
Changes
Calculating Molar Enthalpy Change for
Heating

How much does the molar enthalpy change when
ice warms from -5.4ºC to -0.2ºC? The molar
heat capacity of water is 37.4 J/K*mol.
Practice!
What are the units??
0.19 kJ/mol.
Calculating Molar Enthalpy Change for
Cooling

Calculate the molar enthalpy change when an
aluminum can that has a temperature of 19.2ºC
is cooled to a temperature of 4.00ºC. The molar
heat capacity for Al is 24.2 J/K*mol.
Practice Some More!!
-368 J/mol
Lead has a molar heat capacity of 26.4
J/K*mol. What molar enthalpy change occurs
when lead is cooled from 302ºC to 275ºC?
-713 J/mol
Calculate the molar enthalpy change when
Hg is cooled 10 K. The molar heat capacity
of mercury is 27.8 J/K*mol.
-300 J/mol

Notice the molar enthalpy change for the
first problem was positive, meaning that
the heating of the sample requires energy
◦ The heating of a sample is an endothermic
process.

The second problem had a negative molar
enthalpy change, which means the
heating gives off energy (gets hotter)
◦ This implies that the process is ____________.
Signs of Molar Enthalpy Change

You can use the signs of the enthalpy
change to determine whether a process is
exo- or endothermic.

Processes that have positive enthalpy
changes are endothermic.

Processes that have negative enthalpy
changes are exothermic.
Exothermic and Endothermic

Enthalpy changes can be found for a system
of substances as well

Consider the following:
H2(g) + Br2(g)  2HBr(g) + heat

This indicates that the enthalpy of 2HBr is
LESS than the enthalpy of H2 and Br2, so the
enthalpy change is ________ (pos or neg)

This negative enthalpy change indicates a
_____________ process.
Enthalpy of a System

Enthalpy is the first of three
thermodynamic properties that we will
cover.

Thermodynamics – the science that
examines various processes and the
energy changes that accompany the
processes.

Allows chemists to predict whether a
chemical reaction can occur and what kind
of energy it will have.
Thermodynamics

Think about the heating of iron again:
Fe(s, 300K)  Fe(s, 1100K)
ΔH = 20.1 kJ/mol

Similarily, a phase change or a chemical
reaction can be written using the same
style.

Example:
H2(g, 298K) + Br2(g, 298K)  2HBr(g, 298K)
ΔH = -72.8 kJ
Writing Equations of H Changes

Notice that the enthalpy change for this
reaction and other chemical reactions are
written using the symbol ΔH.

Think about the last reaction:
Is it an exothermic or an endothermic
process??
___________________
Writing Equations of H Changes

Also look over your notes and slideshows
for a quiz involving all information and
calculations.