Thermochemistry: Heat and Enthalpy


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 thanks for watching guys subscribe to my
channel for more tutorials and as always feel free to email me

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