okay let's take a look at a couple questions regarding enthalpy of
formation so the first question is just about the definition whereas asking
which reaction defines the standard enthalpy of formation so we have some
options here and only one of them is going to appropriately represent the
definition of the standard heat of formation so there's a very specific set
of criteria that go along with standard heat of formation and only one of these
complies with that so that's the first question and so the second question asks
if the standard heats of formation for carbon dioxide carbon monoxide and water
vapor are negative 393 point 5 negative 110 point 5 and negative 240 1.8
kilojoules per mole respectively what is the standard enthalpy change for
the following reaction so we're going to use standard heats of formation to
calculate Delta H or the enthalpy change for this reaction so if you don't
remember how to do this I have a few tutorials that can help refresh your
memory and when you're ready give it a try
so looking at this first one let's
refresh our memories as to the definition of the standard heat of
formation the first thing we have to remember is that the reactants must be
elements in their most common form the most common allotrope of that element so
elements can come in different allotropes it has to be the most common
one and it certainly must be an element we cannot have any compounds as a
reactant that would defy the definition of the heat of formation we're forming
some compound from its constituent elements in their most common form and
then the second thing is that we are specifically forming one mole of the
product in question it is a it is a situation where we are trying to get a
per mole value for enthalpy data we're regarding the
formation of one mole of a compound so these are the two things that must be
kept in mind as we're looking at these so looking at option a what do we notice
well we see that carbon is in the diamond allotrope and the problem is
that diamond is not a standard state of carbon it is not the most common
allotrope of carbon we would expect to see charcoal there so this one is
actually not going to work because of the word diamond there that doesn't
comply with the definition now skipping over to option D we can very clearly see
that we have a compound as a reactant we have carbon monoxide and that is not an
element so this does not qualify as a standard heat of formation for carbon
dioxide this would have to be something closer to option a just with charcoal
instead of diamond because we need the elements on the left side now looking at
option C this looks pretty good but the only problem is that on the right we are
forming two moles of ammonia so if we cut all the coefficients in half and we
had one half and two plus three halves h2 yields nh3 then that would be good
we'd be solid but we don't we have two moles of ammonia on the right and so
that doesn't work and that leaves option B and option B is
solid because we have hydrogen as an element we have fluorine as an element
and we are forming one mole of HF gas and so that one works and so B's the
answer there now for number two we're looking at this reaction and we want to
find the Delta H of this reaction and we have heat of formation data so we should
recall that we can get the Delta H of the reaction by taking the sum of the
heats of formations of the products and subtracting from that the sum of the
heats of formation of the reactants and so all we have to do is plug in the
values that we have so plugging in the products first we have negative 110.5
for carbon monoxide and we have negative 241.8 for the water vapor and so
adding those up will represent the sum of the heats of formation of the
products now subtracting from that the heats of formation of the reactants
we have negative 393.5 for co2 and then hydrogen as with any other element is
going to be zero remember we don't have heats of formation for elements elements
of what we use as reactants in heats of formation reactions to generate
compounds so any element is going to always be zero and so we just plug that
into the calculator do the arithmetic and we get a delta H value of 41.2
kilojoules per mole so that's a little bit of information about heats of formation.