Section 10.2: Using Enthalpy
Objectives:
1. Define thermodynamics.
2. Calculate the enthalpy change for a given amount of substance for a given change in
temperature.
Vocabulary: thermodynamics
Molar Enthalpy Change
Because enthalpy is the total energy of a system, it is important. However, the only way to
measure energy is through a change. In fact, there’s no way to determine the true value of H. But
ΔH can be measure as an energy change occurs.
The enthalpy change for one mole of a substance is the molar enthalpy change.
A blacksmith heats metal in order to soften it so it
can be shaped. The heating process can be described
as:
Fe(s, 300K) →Fe(s, 1100K) ΔH = 20.1 kJ/mol
Therefore, when 1 mol of iron is heated from 27oC
to 827oC, its molar enthalpy increases by 20.100
joules.
Recall that q is the heat needed to change the temperature of a substance. When a pure substance
is only heated or cooled, the amount of heat involved is the same as the enthalpy change. In other
words, ΔH = q/mol for the heating or cooling of a substance.
Note that energy is required to heat a substance, so that heating is an endothermic process.
Cooling removes energy from a substance, so that cooling is an endothermic process.
Try these sample problems:
1. How much does the molar enthalpy chant when ice warms from -5.4oC to -0.2oC? The
molar heat capacity for ice is 37.4 J/K∙mol.
2. Calculate the molar enthalpy change when an aluminum can that has a temperature of
19.2oC is cooled to a temperature of 4.0oC. For Al, C = 24.2 J/K∙mol.
Thermodynamics is a science that examines various processes and the energy changes that
accompany the processes. By studying and measuring thermodynamic properties, chemists have
learned to predict whether a chemical reaction can occur and what kind of energy change it will
have.
Chemical thermodynamics is the study of the interrelation of heat and work with chemical
reactions or with physical changes of state within the confines of the laws of thermodynamics.
Chemical thermodynamics involves not only laboratory measurements of various
thermodynamic properties, but also the application of mathematical methods to the study of
chemical questions and the spontaneity of processes.
Enthalpy Level Diagrams...
Enthalpy level diagrams can be used to illustrate overall exothermic and endothermic changes.
They show the difference in total enthalpy of the reactants and products for a reaction:
For an exothermic reaction the total enthalpy of the products is less than that of reactants. For an
endothermic reaction the total enthalpy of the reactants is less than that of the products. For each,
the difference in these total enthalpies is equal to the overall enthalpy of the reaction, DH.