Heat and Enthalpy - Thermochemistry Video Lecture | Physical Chemistry

hey guys its professor Dave, let's talk
about thermochemistry
thermochemistry is the branch of thermodynamics that
deals with the heat absorbed or produced by a
chemical reaction. a reaction can either be
exothermic, one that releases energy, or
endothermic, one that absorbs energy. this
change in energy is represented by Delta
H, or the change in enthalpy for the
reaction. for our purposes we can think
of the change in enthalpy as being the
change in energy for the reaction, or the
energy stored in all the bonds of the
products minus the energy stored in all
the bonds of the reactants. if there is
more energy in the products Delta H will
be positive, meaning the additional
energy had to be provided
making it an endothermic reaction. if
there is less energy in the products
then that additional energy was released
making it an exothermic reaction. energy
which is the capacity to do work can
have many forms like mechanical,
electrical, chemical, heat, and so forth.
and besides quantum-mechanical
exceptions, energy is conserved. that
means it can change forms like heat
energy that evolves when the chemical
energy in a broken bond is released, but
it won't be created or destroyed. heat,
represented by the letter q, is a
trickier thing to define, but a good way
to put it is that heat is the energy that
flows between a system and its
surroundings due to a difference in
temperature. heat will always flow
spontaneously from an area of high
temperature to an area of low
temperature. for a system, q will be
positive if heat is being absorbed by the
system, and it will be negative if heat is
being released by the system. when
temperature and pressure are constant
q = Delta H,
the change in enthalpy. when we discuss
thermochemistry we will discuss energies
in terms of
joules or kilojoules. another common unit
used by chemists is the calorie, which is
equal to 4.184 joules. one
calorie is the amount of energy needed
to raise the temperature of one gram of
water by one degree Celsius. this is not
to be confused with the food calorie
which is Calorie with a capital C. this
is actually equal to a kilocalorie or
1000 calories, also abbreviated as kcal.
a thermochemical equation reports
the change in enthalpy for a given
reaction in stoichiometric terms. here
the Delta H listed is specific to the
molar quantities given by the balanced
equation so if the number of moles of each
substance as given were to react, the
change in enthalpy would be as listed. to
find the enthalpy change for a specific
amount of material just do some
stoichiometry. if twice as many moles of
material as are listed were to react, the
enthalpy change would also be twice as
much as listed. let's check comprehension
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