Lab Activity
08 Entropy and Enthalpy
Background
All reactions require an input of energy, called the activation energy, and all reactions
release some energy. The energy that is absorbed and released as heat is called
enthalpy, symbolized with a capital H. When the reaction gives out more heat than it
takes in, the reaction is called exothermic. In that case the change in enthalpy, H, is
negative (the products have less energy than the reactants). When the heat that goes
into the reaction is greater than the heat that comes out the reaction is endothermic
and the H is positive. Because exothermic reactions provide activation to other
molecules to react, exothermic reactions are favored, that is they generally occur
readily.
Reactions also involve in something called entropy, symbolized with a capital S.
Entropy is the amount of chaos or disorder present. Gases are highly disordered,
while solids are highly ordered, liquids being somewhere in between. Thus, when a
solid melts the amount of entropy increases, meaning that S is positive. Although the
reason why is somewhat obscure, it is clear that nature favors reactions that increase
entropy. This can be clearly seen in the world around you; a dropped plate breaks, but
dropped pieces of porcelain never form a plate; your room gets messier, but never
cleaner without effort.
These two factors (entropy and enthalpy) both influence whether a reaction will occur
spontaneously. Spontaneous, in this context, does not mean suddenly and without
warning, it means that the reaction continues to occur once it is started. A simple
example would be forest fires. Forests do not randomly burst into flames, however
when a fire is started it is very difficult to stop it. Non-spontaneous reactions occur, but
they need to be constantly "pushed" in order to keep going. Photosynthesis is one
such reaction, without the constant influx of solar energy, photosynthesis stops.
Exothermic reactions that increase entropy are always spontaneous (natures favors
both factors), while endothermic reactions that decrease entropy are never
spontaneous. Reactions that are mixed, those where nature favors one factor and
does not favor the other are sometimes spontaneous and sometimes not.
In this lab, you will investigate the factor that determines when a reaction, in this case
the freezing of a compound, is and is not spontaneous.
Purpose
Explore the concepts of entropy and enthalpy by measuring the change in
temperature of a substance as it slowly cools and undergoes a phase change from
liquid to solid. Investigate what factor determines whether a reaction is spontaneous
or not when the enthalpy and entropy disagree.
Materials
PASCO & Other Equipment
PASPORT Xplorer GLX
hot plate
stainless steel temperature probe
beaker, 250-mL
Small Tripod Base & Rod
test tube, large
Buret Clamp (2)
protective gear
Consumables
unknown substance, about 10.0 mL
water, 200.0 mL
Safety Precautions
Remember, follow the directions for using the equipment.
•
Wear safety glasses and follow standard laboratory safety procedures.
•
Be careful when handling chemicals.
Pre-Lab Questions
Does entropy favor a solid melting or a liquid freezing?
1)
What factor do you believe will determine when each process (melting and
freezing) will be spontaneous?
Procedure
Equipment Setup
Set up a hot-water bath. Fill a 250-mL beaker about 3/4 full of tap water.
Place the beaker on the hot plate and turn on the hot plate so it will begin warming the water to boiling.
1) Fill a test tube about 1/3 full of the substance that you are testing.
Xplorer GLX Setup
1)
Plug the temperature probe into
or
on the left side of the GLX.
Turn on the GLX.
1)
Set the GLX to take data samples every 10 Seconds.
•
From the Home screen. Press
•
Press
•
Press
sample.
•
Press
,
to open the Sensors screen.
to select seconds as the sample rate.
to high light Sample Rate. Press
,
,
to select 10 sec. per
to return to the Graph Display.
Record Data
1)
Press
to begin recording data.
Use a support rod and clamps to mount the test tube and the Temperature Sensor so that the test tube is in the water
bath and the Temperature Sensor is in the material in the test tube.
2) Continue heating the water until it reaches the boiling point and the material in the
test tube is completely melted.
3) Carefully remove the test tube and sensor from the hot-water bath. Turn off the hot
plate.
4) Continue to record the temperature until all of the material has completely
solidified (about 10 minutes).
5) Press
again to end data recording.
Do not try to remove the temperature probe from the solidified material. Instead, put the test tube back into the hotwater bath and turn on the hot plate. Warm the test tube until all of the material has melted and then remove the probe.
6) Follow your instructor’s directions for what to do with the test tube and the
temperature probe.
Analyze
Sketch or print out a copy of your graph of Temperature versus Time and label all the relevant parts.
Analysis and Synthesis Questions
Examine your graph of temperature versus time to determine what parts of the graph show changes in enthalpy and
entropy. Determine the melting point of the substance. Determine the freezing point of the substance. What is true
about these two points?
1)
Describe the changes taking place in entropy and enthalpy as the liquid cools and
freezes.
2)
During what parts of the graph is enthalpy the dominant factor? During what parts
is entropy dominant?
3)
What is the factor that determines whether or not the reaction is spontaneous?
4)
Do your results support your prediction?