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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