Video Transcript
In an experiment, it was found that
a reaction resulted in 80 grams of water changing temperature by 15 degrees
Celsius. What is the value in joules for the
heat energy transferred in this reaction? Use a value of 4.2 joules per gram
per degree Celsius for the specific heat capacity of water.
The experiment that this question
is talking about is most likely a calorimetry experiment. Calorimetry experiments are
performed in devices called calorimeters, and their goal is often to find the change
in energy that’s associated with the process. To perform a calorimetry
experiment, we place our sample, in this case our reactants, inside the calorimeter,
which will give off heat that the water will absorb. We can then measure the change in
temperature of the water to calculate the amount of heat that the reaction gave off,
which is what we’re being asked to calculate in this question.
We can calculate the amount of heat
that’s transferred by using the results of a calorimetry experiment through this
formula that tells us that the heat is equal to mass times the specific heat
capacity times the change in temperature. We can calculate the amount of heat
that’s transferred by a reaction using a calorimeter by using the temperature change
of the water because the amount of heat that the reaction gives off will be equal to
the amount of heat that the water absorbs.
So, the problem tells us that we
have 80 grams of water, and the specific heat capacity is given to a value of 4.2
joules per gram per degree Celsius. And the problem tells us the change
in temperature is 15 degrees Celsius. We’ll notice that our units cancel,
leaving us in units of joules, which is what the problem asked for. And multiplying everything through,
we’ll find that 5040 joules of heat were transferred as a result of this
reaction.
Usually, the goal of calorimetry
experiments is to calculate the change in energy as a result of a reaction. Under constant-pressure conditions,
the change in energy will be equal to the heat that we calculated. But this problem didn’t quite give
us enough information to determine that, since we’re not told if the change in
temperature was an increase or a decrease. So, the change in energy could be
5040 joules or it could be negative 5040 joules. We just wanted to be sure, given
this amount of information. But this question didn’t ask us for
the change in enthalpy; it just asked us to determine the amount of heat energy that
was transferred in this reaction, which is 5040 joules.
