# Hess’s Law and Heats of Formation

professor Dave here ,I want to tell you
about Hess’s law thermochemical equations can be manipulated to give
important data about chemical reactions. we can use them to predict exactly how
much energy will be absorbed or released by a reaction, which is very important
because we don’t want any unexpected explosions. there are two ways we can use
tabulated thermochemical data to calculate the Delta H of any reaction.
the first way requires that we manipulate thermochemical equations in
specific ways, so let’s be aware of some rules. first if the reaction has a
particular Delta H, the reverse of the reaction will have
the opposite Delta H, or the same number with reversed sign. second, if molar
quantities in the equation are multiplied by a coefficient, so is the
Delta H. so if you double all the substances, double the Delta H. these are things we can do to
thermochemical data to be able to add equations together to result in a
reaction we are curious about. here’s what I mean. let’s say we want to
know about the change in enthalpy associated with a reaction like this one,
but it is difficult to measure experimentally. we can take other
reactions with known enthalpy changes and rearrange them to align with our
equation and get the data we want. the first reaction provided has carbon
graphite on the left, which is where we want it, but the equation we want has two
moles of graphite, so let’s double this one, we get two moles of everything
instead of one, and we double the Delta H. next, this other one has CO on the right
where we want it, and in the right amount so all we need to do is add these
equations together. the O2 and CO2 will cancel because they are present in the
same amount on both sides, and we are left with the substances in our original
equation. since we added the equations we also add
the Delta H’s to get the Delta H for our equation. this kind of manipulation
is allowed by Hess’s law. we can manipulate the coefficients of a
reaction or reverse its direction in any way necessary as long as we change
the Delta H associated with it in the appropriate way. then we add or subtract
the equations as necessary to give us precisely the equation in question. the
Delta H you get by doing the arithmetic will be the Delta H for the reaction.
another way to calculate an unknown Delta H is to use standard enthalpies of
formation. this is denoted by the following symbol and it represents the
enthalpy associated with forming one mole of a substance from its respective
elements in their standard and most stable state. most stable state means the
most common allotrope, or physical form of an element. so carbon graphite instead
of diamond, diatomic oxygen instead of ozone and so forth. standard state just refers to standard
temperature and pressure which is room temperature and atmospheric pressure at
sea level, that’s what is meant by the degree symbol. we can calculate the
change in enthalpy for a reaction by adding up the standard heats of
formation of the products and then subtracting the sum of the standard
heats of formation of the reactants. the heats of formation can be found in your
textbook or online, and you just plug them in, multiplying each value by the
coefficients in the balanced equation. let’s check comprehension thanks for watching guys subscribe to my
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