hello and welcome to thermal chemical equations and their application to energy mole mass
conversions the first thing we need to know about is the energy term and how it is used
and placed in the equation there are various pieces of information and the energy term can
give us and we will go through four reactions to show the different ways the energy term
expresses information and then go through several conversion problems the energy term tells us the
energy change occurring in the reaction and here it is placed on the product side for this
reaction the significance of which we will get to in a moment but first we need to understand
that the energy term can also be expressed apart from the reaction as its change in enthalpy Delta
H which is called the heat of reaction we see that this is a negative heat of reaction which
has the same meaning as placing the energy term in the equation on the product side both of which
tell us that this is an exothermic reaction an exothermic process occurs when energy is released
or lost during a chemical or physical change by placing the energy term on the product side
or by showing it as a negative heat of reaction that is expressing a release or loss of energy in
this reaction the energy term is on the reactant side which is a positive heat of reaction both of
these show an endothermic reaction which is when energy is absorbed or gained during a chemical or
physical process here the energy change is positive it is endothermic the energy term
on the reactant side shows that the reaction is gaining energy which is a positive energy change
here we see the decomposition of water but with two different heats of reaction here we can see
the importance of knowing the states of reactants and products for heat of reaction number one the
reactant water is in the liquid state while for the heat of reaction number two water is
in the gaseous state why does that give us two different heats of reaction let's look at it
from a graphical perspective to understand these different Delta H's will the reactant water be
placed differently on the energy axis depending on its state you're right the answer is yes for
the two lines representing the starting energy of water can you predict which represents the liquid
and which is the gas yes you're right again liquid water is at a lower temperature than gaseous water
liquid water has to absorb energy in order to become a gas what's interesting here is that the
products for both end up at this same energy so which reactant has to gain more energy to get to
the products gaseous water requires less energy than liquid water to get to the same products
so knowing the States is an important part of understanding the energy change in any chemical
or physical process let's look at something very interesting here is the decomposition of
hgo mercury to oxide and it's heat of reaction positive 180 kilojoules it is exothermic there
are many reactions which are reversible where the products react to produce the reactants
and this is a reversible reaction mercury and oxygen can react to form mercury to oxide but
notice the heat of reaction the magnitude is the same 180 kilojoules but now it is negative it is
exothermic so the decomposition of mercury to oxide requires the addition of 180 kilojoules
to D come to decompose while the synthesis of HG o releases 180 kilojoules the process is the same
just going in the opposite direction and so it's energy change is the same but also going in the
opposite direction the gain of 180 kilojoules versus the loss of 180 kilojoules importantly
this energy change is tied to the number of moles represented by the coefficients let's
explore that very important concept further so we need to account for the amount of substance
which results in any specific amount of energy change so the energy term the heat of reaction
gives the energy chain / amount of moles shown by each coefficient let's see what that means
for each species in this reaction so in this reaction the combustion of methanol ch4 Oh 1,450
kilojoules is released for every two moles of ch4 reacted and 1452 kilojoules is released for every
three moles of oxygen reacted and for every 4 moles of water produced and every two moles of
carbon dioxide produced I hope that you can see this relationship between energy and moles gives
us access to the conversions between energy and moles and mass in the equation let's use
those relationships to solve some conversion problems what amount of energy is produced in
the combustion of 100 moles of ch4 Oh the first statement gives us the relationship between the
energy term and the amount of ch4 oh it reacted okay so let's start with the amount given in the
problem 100 moles of ch4 oh and then we're going to multiply it by the relationship of the energy
term and the coefficient putting the unit we have on the bottom and the unit wanted on top
and here we have the relationship between the two units 1452 kilojoules for every two moles
so the reaction 100 moles of ch4 o gives of 70 2600 kilojoules of energy released the next
question gives us grams instead of moles but we still only have the relationship of kilojoules
to moles net grams so we have to first convert grams of ch4 O to moles so we multiply by 1 mol
over the molar mass and that gets us moles of ch4 o we now can multiply by the relationship
of moles to energy and that is still 14 hundred and 52 kilojoules for every two moles which gives
us 2200 seventy kilojoules released for every 100 grams of ch4 Oh reacted okay in the next
problem were given kilojoules and asked for how many moles of ch4 Oh must react to produce that
amount 100 kilojoules so here we multiply it by the relationship of kilojoules on the bottom in
moles on top so for every two moles we still have that same relationship 1452 kilojoules and so the
combustion of 0.14 moles of Ch 400 produces 100 kilojoules heats of reaction however are often
expressed per mole for a specific substance so let's see how that works you were back
to the synthesis of HG oh but the question is what is the heat of reaction per mole of the
decomposition of mercury to oxide HG oh we have the synthesis of HGO given to us so there's two
things to consider the fact that the equation gives us the heat of reaction for the synthesis
of mercury two oxide not its decomposition and to the energy term gives us the energy released
for the synthesis of two moles of HG oh so we need to reverse the given reaction to get the
decomposition reaction and that will give us the correct sign for the heat of reaction which
is a positive 180 kilojoules however that energy term is for 2 moles of HG OH and so we divide 180
kilojoules by 2 moles and that gives us the the heat of reaction per mole 90 kilojoules per mole
okay this has been a mad scientist production see ya