Lab #6 – Calorimetry
Name
Background
Most of the energy absorbed or released during chemical and physical processes is thermal energy, or heat.
Calorimetry is the lab technique for measuring the heat exchanged during a process. In Calorimetry, a process
is done in water, we measure the temperature change of the water, and calculate the heat absorbed or released
by the water using q = mcΔT. The q for the process is then –qwater; if the water gained heat, the process lost
heat, and vice-versa.
In Part A of the procedure we will determine the specific heat of a metal. Specific heat is a quantity that
indicates how readily a material undergoes a temperature change (“heats up or cools down”) as it gains or loses
heat. Water has a specific heat of 4.184 J/gºC, oils have specific heats that are typically 2 or 3 times higher than
water’s, and air, like most gases, has a specific heat that is a `fraction of water’s. Specific heat is used in the
aforementioned formula q = mcΔT. We will use calorimetry to measure q, an electronic scale to measure m,
and a thermometer to measure T. We will measure all three for both the water and the metal, but we already
know water’s “c”. We can therefore calculate qwater which is equal to –qmetal.
In Part B we will determine the Heat of solution of an ionic compound, the heat absorbed or released as a solute
dissolves in a solvent. In a particularly efficient application of calorimetry, the water in our calorimeter will act
as our solvent. Once again, qwater = mcΔT = -qdissolving.
In Part C we will verify the validity of Hess’ Law. We will measure the heat involved in dissolving solid
NaOH, we will measure the heat involved in reacting predissolved NaOH solution with HCl solution, and we
will measure the heat involved in dissolving and reacting solid NaOH with HCl solution.
Far and away the greatest source of experimental error in calorimetry is heat transfer with the surroundings. We
will do calculations assuming that all of the heat released by a process is transferred to the water, when in fact
some heat is transferred to the thermometer, the calorimeter, and the surroundings. We will ignore heat transfer
to the calorimeter and thermometer (professionals don’t), and we will do two things to mitigate heat exchange
with the surroundings: We will use an insulated container, which we will assume neither absorbs nor transmits
heat, and we will do everything quickly. Speed is essential to good data; heat takes time to leak in or out, so if
we do the processes as quickly as possible, we can limit heat leakage.
Procedure
Part A – Specific Heat
Begin your hot water bath by filling a 400 mL beaker 2/3 full of water and heating on a ring stand. Obtain a
calorimeter with lid and a metal sample in a large stoppered test tube. Be sure to record the unknown number
of the metal sample! Weigh your sample in a zeroed beaker and record the mass of your sample. Return the
sample to the large test tube, loosely stopper the tube, and place it in the boiling water bath for at least 10
minutes. Make sure the water level is always sufficient to completely cover the metal in the test tube – add
water as needed to keep the metal covered. Using your regular thermometer (NOT the $50 ASTM
thermometer you borrowed), measure and record the temperature of the boiling water bath.
While the water bath is percolating, weigh and record the mass of your empty calorimeter. Add approximately
40 mL of water to the calorimeter, then weigh and record the mass of the calorimeter and water. Use the ASTM
thermometer to measure and record the temperature of the water in the calorimeter. When the metal is ready,
remove the test tube from the water bath and pour the metal into the calorimeter (be sure no hot water drips into
the calorimeter). Place the lid on the calorimeter and swirl as vigorously as possible without spilling or
breaking the ASTM thermometer. Watch the temp as it rises; record the highest temp attained.
Before returning the metal sample to the test tube, spread it out on a multilayered pile of paper towel to air dry.
Change the paper towel as needed. Do NOT return the metal wet, or it will oxidize horrendously.
Part B – Heat of Solution
Obtain an unknown salt. Weigh and record about 5 g of the sample on a piece of zeroed weighing paper. Place
about 50 mL of water in the calorimeter, weigh and record the mass of the calorimeter and water, then measure
and record the temperature of the water with the ASTM thermometer. Pour the 5 g of unknown sample into the
calorimeter, cover with the lid, and swirl as aggressively as possible. Record the highest temperature attained.
Part C – Hess’ Law with NaOH
1. Heat of dissolving - Place about 100 mL of water in the calorimeter, weigh and record the mass of the
calorimeter and water, then measure and record the temperature of the water with the ASTM thermometer.
Weigh approximately 2 grams of solid NaOH on a piece of zeroed weighing paper, and quickly return to your
bench and add the NaOH to the water in the calorimeter. [NaOH is very caustic, and also absorbs moisture in
the air quickly, a major potential source of error.] Immediately cover with the lid, swirl as aggressively as
possible, and record the highest temperature attained.
2. Heat of reaction with HCl – Place 50.0 mL of 1.0 M NaOH in the calorimeter and record the temperature.
Add 50.0 mL of 1.0 M HCl to the calorimeter, cover with the lid, swirl as aggressively as possible, and record
the highest temperature attained. Weigh and record the mass of the calorimeter and the 100.0 mL of solution.
3. Heat of dissolving and reacting NaOH – Place 100.0 mL of 0.5 M HCl in the calorimeter, weigh and record
the mass of the calorimeter and HCl, then measure and record the temperature of the HCl with the ASTM
thermometer. As per previous directions, add 2.0 grams of solid NaOH to the HCl in the calorimeter, cover
with the lid, swirl as aggressively as possible, and record the highest temperature attained.
Prelab summary
Procedural Tips: You can read the ASTM thermometers to the nearest ½ of a graduation.
Speed is of the essence. Swirling aggressively but safely is crucial.
Always record the highest temperature attained.
Safety: The ASTM thermometers used in this lab are very expensive (MSRP ~ $50). Be very careful in
handling your thermometer. Don’t let it roll off the table, don’t swing it into the plumbing.
The ASTM thermometer is not calibrated to 100ºC, so do NOT use it in the boiling water bath.
NaOH is extremely caustic, Always wash your hands after working with its solid or dissolved forms.
Never heat a tightly stoppered test tube.
Disposal: All solutions can be disposed of in the sink. Return the dry metal.
Data
Mass of empty calorimeter
g
Part A:
Unknown metal
Part B:
#
Unknown salt
Mass of metal
g
Temp of boiling water bath
ºC
Mass of calorimeter and H2O
#
Mass of salt
g
g
Mass of calorimeter and H2O
g
Initial temp of calorimeter H2O
ºC
Initial temp of calorimeter H2O
ºC
Final temp of calorimeter H2O
ºC
Final temp of calorimeter H2O
ºC
Part C1:
Part C2:
Mass of calorimeter and sol’n
g
ºC
Initial temp of NaOH
ºC
ºC
Final temp of calorimeter sol’n
ºC
Mass of calorimeter and H2O
g
Mass of NaOH(s)
g
Initial temp of calorimeter H2O
Final temp of calorimeter H2O
Part C3:
Mass of calorimeter and HCl
g
Mass of NaOH(s)
g
Initial temp of HCl
ºC
Final temp of solution
ºC
Calculations – Show all work in the space provided
Part A:
1. Calculate the heat gained by the water in the calorimeter, in Joules.
2. Calculate the specific heat of the metal, in J/(gºC)
Part B:
3. Calculate qwater, in Joules. Be sure to include the correct sign.
4. Calculate ΔHsolution of your unknown salt, in J/g.
Part C:
5. Calculate the ΔHsolution of NaOH in J/(mole NaOH).
6. Calculate the ΔHneutralization for the reaction between the NaOH(aq) and HCl(aq) solutions, in J/(mole NaOH).
7. Calculate the ΔHneutralization for the reaction between the NaOH(s) and HCl(aq) solution, in J/(mole NaOH).
8. Write the equation for each of the processes, followed by its ΔH. Use the data to verify Hess’ Law.
Dissolving NaOH:
Reacting NaOH(aq) with HCl:
Reacting and dissolving NaOH:
9. Why did we use 0.5 M HCl in Part C3 but 1.0 M HCl in Part C2?
Prelab Questions
1. A metal sample weighing 37.58 g was heated for 10 minutes in a 98ºC water bath. The metal was then
poured into calorimeter containing 40.00 grams of water at 22.04ºC . The temperature of the water rose to
27.24ºC.
(a) How much heat, in Joules, was absorbed by the water in the calorimeter?
(b) How much heat, in Joules, was lost by the metal?
(c) What was the ΔT of the metal?
(d) Calculate the specific heat of the metal, in J/(gºC).
(e) An obscure law called the Law of Dulong and Petit says the the molar mass of a metal can be approximated
by dividing 25 by the metal’s specific heat, in J/(gºC). Use this to estimate the molar mass of this metal.
2. When 1.25 g of NaOH is dissolved in 98.0 g of water in a calorimeter, the temperature of the water increases
from 22.0ºC to 25.3ºC.
(a) Calculate qwater, in Joules. Be sure to include the correct sign of q.
(b) Calculate ΔHsolution for the NaOH, in Joules/mole.
(c) Is the process endothermic or exothermic?