10. Hess’s Law
In the previous experiment you explored the heat of solution of potassium hydroxide.
Using Hess’s law, you will now use those results in conjunction with the data collected
in this experiment to determine the enthalpy of neutralization for the reaction between
potassium hydroxide (KOH) and hydrochloric acid (HCl).
10.1 Prelaboratory Assignment
Read this lab guide and Section 6.7 in your textbook. Then, complete the prelab assign-
ment in Chem21. The prelab assignment is due at the beginning of lab on the day your
lab meets.
10.2 Introduction
Enthalpy is a state function, meaning the enthalpy for a process only depends on the
initial and final states of the system. Hess’s law states that when several reactions are
added together, their enthalpies can be added together to give the enthalpy for the sum
of the reactions. For example:
A + B −→
C ∆H
1
C −→ D + E ∆H
2
A + B −→ D + E ∆H = ∆H
1
+ ∆H
2
If you need to reverse a reaction, the enthalpy is negated:
C −→ A + B ∆H = −∆H
1
Finally, if you need to multiply a reaction by a coefficient, the enthalpy is also multi-
plied by the same coefficient:
3(A + B −→ C) ∆H = 3∆H
1
In the previous experiment, you determined ∆H
soln
, the enthalpy for the dissolution
of KOH (Reaction 1, below). In this experiment, you will determine ∆H
neut
, the enthalpy
for the neutralization of solid KOH (Reaction 2, below). You will then combine these
1
values using Hess’s law to calculate ∆H for the neutralization of aqueous KOH with HCl
(Reaction 3, below). Finally, you will perform Reaction 3 and compare your experimental
result with the theoretical result you calculated.
10.3 Precautionary Measures
• KOH is a strong base and HCl is a strong acid. Both are extremely corrosive chem-
icals that will burn your skin and eat holes in your clothing. Wear your safety
goggles at all times and wear gloves when handling solid KOH, KOH solutions,
and HCl solutions.
• If small quantities of solid KOH, KOH solution or HCl solution are spilled on
you, rinse the chemicals off for several minutes with a stream of cold water from
the sink. Also inform your instructor right away.
• In the case of a small spill on a countertop, wipe up the chemicals with a paper
towel, and rinse the area thoroughly with water. In the case of a larger spill, seek
the assistance of your instructor.
10.4 Experimental
In this experiment, you will be graded on your accuracy. Set up your lab notebook. You
will be recording temperatures over time as you did in the previous experiment. This
data should be recorded in a table.
Insert a thermometer in a Styrofoam cup that has been placed on a magnetic stir plate.
(Do not turn on the heater!) Also, put a magnetic stirbar in the cup.
For each reaction you will start with a measured volume of hydrochloric acid solution
in the Styrofoam cup. You will turn on the stirrer and establish a baseline temperature by
recording the temperature of the solution (to the nearest tenth of a degree) every 15 sec-
onds for approximately two minutes. You will then add a known mass of solid KOH or a
known volume of KOH solution to the HCl solution and continue to record temperatures
every 5 seconds in a table in your notebook until the temperature of the water remains the
same for 1
1
2
minutes or decreases for 5 measurements in a row. The initial temperature (T
i
)
is the last baseline temperature reading before the KOH is added. The final temperature
(T
f
) is the highest temperature, not the last temperature reading recorded.
Reaction 1: KOH(s) −→ KOH(aq)
Reaction 1 was completed during the last experiment, so we only need the data from that
experiment for this part. You should have already recorded that data in your lab notebook
as part of your prelaboratory assignment.
2
10. Hess’s Law
Reaction 2: KOH(s) + HCl(aq) −→ H
2
O(l) + KCl(aq)
Use a graduated cylinder to obtain approximately 100 mL of 1.5 M HCl solution. Record
the exact volume and concentration, and then pour it into the cup. Establish a baseline
temperature as described above. In the meantime, weigh between about 3 g of KOH into a
weigh boat, being certain to record the exact mass of KOH. (Remove the weigh boat from
the balance pan when adding KOH to avoid spills.) As quickly as possible, start timing,
and add the KOH to the HCl solution. Record the temperature as described above. After
recording the temperature, use litmus paper to determine whether your final solution is
acidic or basic. Enter the result in your notebook and use that observation to determine
which is the limiting reagent, KOH or HCl.
Use tweezers to remove the stir bar, and pour the solution into an 800 mL beaker
so you can dispose of it properly later. DO NOT POUR THE SOLUTION DOWN THE
DRAIN. Rinse and dry your cup, stirbar, thermometer and weigh boat. Then, repeat this
process so you will have two values for ∆H
neut
to average together.
Reaction 3: KOH(aq) + HCl(aq) −→ H
2
O(l) + KCl(aq)
Use a graduated cylinder to obtain approximately 50 mL of 1.5 M HCl solution. Record the
exact volume and concentration, and then pour it into the cup. Establish a baseline tem-
perature as described above. In the meantime, rinse and dry the graduated cylinder, and
obtain about 50 mL of the 1 M KOH solution, being certain to record the concentration and
the exact volume. As quickly as possible, start timing, and add this solution to the HCl
solution. Record the temperature as described above. After recording the temperature,
use litmus paper to determine whether your final solution is acidic or basic. Enter the
result in your lab notebook and use that observation to determine which is the limiting
reagent, KOH or HCl.
Use tweezers to remove the stir bar, and pour the solution into an 800 mL beaker
so you can dispose of it properly later. DO NOT POUR THE SOLUTION DOWN THE
DRAIN. Rinse and dry your cup, stirbar, thermometer and weigh boat. Then, repeat this
process so you will have two values for ∆H to average together.
10.5 Hazardous Waste and Clean-up
10.5.1 Hazardous Waste
At the end of the lab, transfer all of your waste material to the provided receptacle in the
hood.
3
10.5.2 Clean-up
Dry your glassware (including your cup - don’t throw it away!) and return it to your
station. Then, wipe down your benchtop with a wet paper towel and dry it. Have your
instructor check your station before your leave.
BEFORE YOU LEAVE LAB: Tear out the carbon-copy pages of your notebook. Make sure
your name, your partner’s name and your section number are on each page. Staple these
pages together and turn them in to your instructor.
10.5.3 Lab Assignment
Enter all of your data, calculations and answers to questions in the Lab 10 Assignment
in Chem21. You will need to refer to your notebook for this. The Assignment is due
approximately 15 minutes before your next lab meeting.
10.6 Calculations
The calculations are somewhat repetitive, so you may want to set up a table in your lab
notebook to keep track of the results. Your table should look something like the table
below.
Reaction 2 Reaction 3
Trial 1 Trial 2 Trial 1 Trial 2
Volume of HCl
Volume of KOH - -
Total Volume
Mass of water
∆T
q
H
2
O
q
rxn
Mass of KOH - -
Volume of KOH (L) - -
Moles of KOH
Volume of HCl (L)
Moles of HCl
Litmus Color
Moles of Limiting Reagent
∆H
rxn
Average ∆H
rxn
For each trial of each reaction, calculate
• the total volume of the solution.
4
10. Hess’s Law
• the mass of water. (Assume this is the mass of the solution and the density is
1.00 g/mL.)
• ∆T (= highest temperature - initial temperature)
• q
H
2
O
(= m
H
2
O
C∆T where C = 4.184 J/g
◦
C)
• q
rxn
(= −q
H
2
O
)
• the moles of KOH used. (When solid KOH is used, convert mass to moles using
MW = 56.11 g/mol. When aqueous KOH is used, use the molarity of the solution to
convert volume to moles.)
• the moles of HCl. (Use volume and molarity.)
• which is the limiting reactant, HCl or KOH? (You should be able to tell by the num-
ber of moles of each that was used and your result should be confirmed by the
litmus test.)
• ∆H
rxn
(= q
rxn
/n where n is the number of moles of the limiting reagent.)
Average the ∆H
rxn
from each trial for each reaction.
Use Hess’s Law to calculate ∆H for Reaction 3 using your values for ∆H
soln
and
∆H
neut
according to the formula you wrote in your prelab.
Calculate the percent difference between the experimental ∆H for Reaction 3 in your
table and your calculated value above.
Percent difference =
|calculated value − experimental value|
|experimental value|
× 100%
5
