Energy, Enthalpy, and Entropy
So Easy a 10th Grader Can Do It
Brian Anderson
University of Texas - Austin
Energy Changes in Chemical Reactions
All chemical reactions either absorb energy
from their surroundings or give off energy to
their surroundings.
That energy change shows up in two ways, as
HEAT (q) and WORK (w):
ΔE = q + w
Calculating Change in Energy
ΔE = q + w
Heat = mass × specific heat × ΔT = mcΔT
Work = –(pressure × change in volume) = –PΔV
ΔE = mcΔT – PΔV
So if we measure the change in temperature
and the change in volume, we can calculate
the change in total energy for the reaction.
Two Quick Demos
• Demo 1: Put 1 Tbs baking soda into a balloon,
seal the balloon over the top of a flask
containing 200 mL vinegar, then shake the
baking soda into the flask.
CH3COOH + NaHCO3
CH3COONa + H2O + CO2
Measure the temperature change
and the volume change of the system.
Two Quick Demos
• Demo 2: Fill a 2-Liter bottle with gaseous CO2.
Add 30 mL of 3M NaOH, cap the bottle, and
shake.
CO2 (g) + NaOH (aq)
NaHCO3 (aq)
Measure the temperature change
and the volume change of the system.
What About an Open Beaker?
ΔE = heat + work
ΔE = mcΔT – PΔV
We can measure the temperature change and calculate the
heat term.
But how do we measure the volume change if the reaction
is not contained in any way?
If we can’t measure the volume change, we can’t calculate
the total energy change. We’re stuck.
Let’s Just Ignore It!
ΔE = q – PΔV
If we ignore the PΔV term, we can’t call it Energy any
more. So let’s make up a new word:
ENTHALPY (ΔH).
ΔE = q – PΔV
Does this mean Enthalpy is just a
fancy word for heat? Yes and no.
Is Enthalpy the Same Thing as Heat?
Technically, no, and here’s why:
We didn’t actually ignore the work term, what we really
did was add in the work done by the surroundings,
which is identical to the work done by the system, but
is opposite in sign:
ΔE = q – PΔV
ΔH = q – PΔV + PΔV
So technically, enthalpy is defined as the energy of the
system PLUS the work done on the system by the
surroundings.
No, Really -- Is Enthalpy the Same
Thing as Heat?
Effectively, yes, enthalpy and heat are the same
thing.
ΔH = q – PΔV + PΔV
ΔH = q
Same number, same units.
So is it Heat of Reaction or
Enthalpy of Reaction?
Technically, it should be enthalpy of reaction,
but it has become common to say heat of
reaction. It’s even in many textbooks that way,
so that battle is lost.
If it helps the students’ understanding, just say
enthalpy and heat are the same thing.
Clearing Up the Confusing Part
At constant volume (for example, a stoppered
flask or a calorimeter), ΔE = heat:
ΔE = heat – PΔV
ΔV = 0
ΔE = heat
At constant pressure (like an open beaker),
ΔH = heat:
ΔE = heat – PΔV
ΔH = heat
Quick Review
Ideally we would like to know the total energy of
a reaction (Energy), but most of the time we
can’t, so instead we do all our calculations using
only the heat term (Enthalpy).
Usually the work is small anyway and heat is
95% or more of the total energy.
Gibbs Free Energy
ΔG = ΔH – TΔS
ΔH is typically in units of kJ
ΔS is typically in units of J/K
TΔS term is usually lesser magnitude than ΔH.
Most exothermic reactions are spontaneous.
Endothermic reactions are spontaneous when the
entropy increase is high or when you raise the
temperature.
Further Reference
University of Wisconsin – Madison
Student Resources for General Chemistry
http://chemed.chem.wisc.edu/chempaths/GenC
hem-Textbook/Measuring-the-Enthalpy-Change826.html
Scroll down to where it says ΔH vs ΔU
Who is This Guy?
Brian Anderson has a B.A. in Chemistry from Knox College, an M.A.
in Marine Science from UT-Austin, and a Ph.D. in Chemistry from
UT-Austin.
Brian has held a number of positions in science education ranging
from pre-school through university level. He currently serves as
a BLOCKS pre-school science mentor on an NSF-funded grant
and runs the UTeach 4-8th Grade Master Science Teacher
Preparation Academy.
He is also the author of an outer space comedy chapter book series
called The Adventures of Commander Zack Proton, and
occasionally contributes to Highlights magazine.