This content explains how to measure heat changes in chemical reactions, focusing on the units used and two primary methods for calculating heat (q): using specific heat capacity and heat capacity.
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an important concept is for us to think about
about
how we measure our heat changes
and what kind of units that we're going
to use to do that measurements
so the first one is a calorie and this
is the the energy
it takes to heat one gram of water one
degree celsius
so this is a definition that is classic
in terms of what we use
and you will see this come up quite a
bit in one particularly common place
you will see the term calorie is when
you look at your food packaging
but recognize that the calorie that is
on the packaging
is actually with a capital c and the
capital c
is actually a k calorie so make sure you do
do
understand that in terms of thinking
about something that is more real
um that you might accomp uh encounter
every day
but the most common unit that we're
going to use is actually in the si
unit and that's our joule so in our joule
joule
one joule is going to be equal to the
units for that is one kilogram
meter squared per second squared
so that is our typical si unit that goes
back to
uh killer to mass and and speeds
so we will go ahead and use this joule
term uh
probably the most but you'll also see
calories come up on occasion
but we're going to go ahead and
calculate q in a couple of different ways
ways
so our q is how we are going to measure heat
heat
in our reactions but we need to go over
a couple of different ways of calculating
calculating
heat so we can calculate heat using a
specific heat capacity
and this value is actually an intensive property
so bringing back from the earlier
concept of extensive versus
intensive this value doesn't matter how much
much
material or matters there so we can use
a specific heat capacity we're going to
use a lowercase c
from this just so you know um
if you talk to a previous student of gen
chem one
you will also see this as a lower case s for
for
specific heat capacity but our textbook
uses a lowercase c
so the heat using um a specific
heat capacity can be calculated by
measuring the mass
times the specific heat capacity times
the change in temperature
so this is our mass specific heat
capacity and this temp delta the delta t
is t final minus your t
initial so the final temperature of the material
material
and then the initial temperature of this material
material
and that makes sense for this uh the
specific heat capacity the units for this
this
is going to be joules per gram degrees celsius
celsius
and that makes sense because our mass
will be able to
cancel out the grams over here and our temperature
temperature
change in temperature can cancel out our
degrees celsius
so then we get joules at the end for our q
q
which are our s i units for measuring heat
heat
so make sure you check your units
so one thing that comes up quite a bit
your mass
may be in some other unit
so it may be in something like kilograms or
or
um or milligrams and then you'll have to
convert it based on whatever your value
your lowercase c is and same thing for temperature
temperature
um it's actually most common to use
things like in kelvin
but since we're taking a difference in
the temperature
degrees celsius also works quite well
for these kinds of calculations
we can also use what we call a heat capacity
capacity
a heat capacity is is an extensive property
and you will most commonly encounter the
heat capacity as a capital c
and this will be like the heat capacity
your textbook goes really uh detailed
into this like for example the heat capacity
capacity
of a small frying pan is different than
the heat capacity of a large frying pan
just because there's more mass in a
large frying pan
so but what we will typically use is
this heat capacity will come
into something that we are going to call
a bomb calorimeter
calorimeter
and you will see the heat capacity of
that object and
you know you have a capital c by looking
at the units
the units for this are going to be
joules per degree celsius
so notice in there there isn't any mass
because it deals with a specific
object uh that we are using so the q
to calculate for this is going to be our
capital c
times delta t so we get our joules per
degree celsius
here and we get our t final minus our
t initial so based on these couple of uh
definitions we have uh plenty to calculate
calculate
in terms of our next topic which is
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