Download Thermochemistry www.AssignmentPoint.com Thermochemistry is

Thermochemistry
www.AssignmentPoint.com
www.AssignmentPoint.com
Thermochemistry is the study of the energy and heat associated with chemical
reactions and/or physical transformations. A reaction may release or absorb
energy, and a phase change may do the same, such as in melting and boiling.
Thermochemistry focuses on these energy changes, particularly on the system's
energy exchange with its surroundings. Thermochemistry is useful in predicting
reactant and product quantities throughout the course of a given reaction. In
combination with entropy determinations, it is also used to predict whether a
reaction is spontaneous or non-spontaneous, favorable or unfavorable.
Endothermic reactions absorb heat. Exothermic reactions release heat.
Thermochemistry coalesces the concepts of thermodynamics with the concept
of energy in the form of chemical bonds. The subject commonly includes
calculations of such quantities as heat capacity, heat of combustion, heat of
formation, enthalpy, entropy, free energy, and calories.
The world’s first ice-calorimeter, used in the winter of 1782-83, by Antoine
Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various
chemical changes; calculations which were based on Joseph Black’s prior
discovery of latent heat. These experiments mark the foundation of
thermochemistry.
History
Thermochemistry rests on two generalizations. Stated in modern terms, they are
as follows:
www.AssignmentPoint.com
 Lavoisier and Laplace’s law (1780): The energy change accompanying
any transformation is equal and opposite to energy change accompanying
the reverse process.
 Hess' law (1840): The energy change accompanying any transformation
is the same whether the process occurs in one step or many.
 These statements preceded the first law of thermodynamics (1845) and
helped in its formulation.
Lavoisier, Laplace and Hess also investigated specific heat and latent heat,
although it was Joseph Black who made the most important contributions to the
development of latent energy changes.
Gustav Kirchhoff showed in 1858 that the variation of the heat of reaction is
given by the difference in heat capacity between products and reactants: dΔH /
dT = ΔCp. Integration of this equation permits the evaluation of the heat of
reaction at one temperature from measurements at another temperature.
Calorimetry
The measurement of heat changes is performed using calorimetry, usually an
enclosed chamber within which the change to be examined occurs. The
temperature of the chamber is monitored either using a thermometer or
thermocouple, and the temperature plotted against time to give a graph from
which fundamental quantities can be calculated. Modern calorimeters are
www.AssignmentPoint.com
frequently supplied with automatic devices to provide a quick read-out of
information, one example being the DSC or differential scanning calorimeter.
Systems
Several thermodynamic definitions are very useful in thermochemistry. A
system is the specific portion of the universe that is being studied. Everything
outside the system is considered the surrounding or environment. A system may
be: an isolated system — when it cannot exchange energy or matter with the
surroundings, as with an insulated bomb calorimeter; a closed system — when
it can exchange energy but not matter with the surroundings, as with a steam
radiator; an open system — when it can exchange both matter and energy with
the surroundings, as with a pot of boiling water.
Processes
A system undergoes a process when one or more of its properties changes. A
process relates to the change of state. An isothermal (same temperature) process
occurs when temperature of the system remains constant. An isobaric (same
pressure) process occurs when the pressure of the system remains constant. An
adiabatic (no heat exchange) process occurs when no heat exchange occurs.
www.AssignmentPoint.com