J . S o p o u š e k ú l o h a
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3.a-c
3. Thermochemical measurements
3.a. Determination of neutralization and dilution heats
When neutralizing aqueous solutions of acids with alkaline solutions, an
elementary reaction of dissociated ions occurs:
OHOHH 2→+ −+
∆ 𝐻𝐻𝑟𝑟
𝑚𝑚(25°𝐶𝐶, 1𝐴𝐴𝐴𝐴𝐴𝐴) = −57.321𝐾𝐾𝐾𝐾/𝑚𝑚𝑚𝑚𝑚𝑚 (1.1.)
However, the actual heat released during the neutralization (ie, total enthalpy change
∆ 𝐻𝐻𝑛𝑛𝑛𝑛) of strong acids with strong bases is also affected by other effects, such as heat of
dilution ∆ 𝐻𝐻ℎ𝑑𝑑. In addition, when neutralizing weak acids or weak bases, a certain amount
of energy is consumed to dissociate ∆ 𝐻𝐻𝑑𝑑𝑑𝑑𝑑𝑑 to release ions [H+
] or [OH].
Therefore, the
resulting neutralizing heat effect is moreover affected by the dilution and dissociation
heats of weak acid or base:
∆ 𝐻𝐻 = ∆ 𝐻𝐻𝑛𝑛𝑛𝑛 = ∆ 𝐻𝐻𝑟𝑟 + ∆ 𝐻𝐻ℎ𝑑𝑑 + ∆ 𝐻𝐻𝑑𝑑𝑑𝑑𝑑𝑑 (3.2.)
The total amount of the released heat Q during neutralization in the calorimeter is
determined by the temperature rise in the calorimeter according to the formula:
HTCQ ∆−=∆⋅= (3.3.)
where ∆T is the temperature rise during neutralization, C is the heat capacity of the
calorimeter, which indicates the amount of heat required to heat the calorimetric vessel
with its content of 1
o
C. In the case of an aqueous solution experiment, the C constant
depends mainly on the amount of water (𝑐𝑐𝐻𝐻2 𝑂𝑂
𝑝𝑝
)1,25( AtmC° =4.1796 J K-1
kg-1
).
We calibrate the heat capacity of the calorimeter C by measuring the temperature rise
∆T' in the calorimeter after releasing heat QE , which is produced by the passage of the
current I through the electrical resistor immersed in the calorimeter filling. The heat
capacity is given by eqn
C
Q
T
E I t
T
E
=
′
=
⋅ ⋅
′∆ ∆
(3.4.)
where E is the voltage at the resistance heater terminals and t is the flow time of the
electric current I.
TASK: Determine the neutralization ∆ 𝐻𝐻𝑛𝑛𝑛𝑛 and dilution ∆ 𝐻𝐻ℎ𝑑𝑑 heats of both strong
(HCl) and weak acid (acetic acid) when reacting with a strong base (NaOH).
Determine the heat of reaction ∆ 𝐻𝐻𝑟𝑟 when neutralizing strong acid with a strong base.
Estimate the dissociation heat ∆ 𝐻𝐻𝑑𝑑𝑑𝑑𝑑𝑑 of the weak acid.
LABORATORY AIDS AND CHEMICALS: Dewar vessel, digital thermometer equipped by
external sensor, operator PC, rotary el. stirrer, heating element in a glass case,
automatic dispenser(10 cm3
), ammeter, voltmeter, rheostat, manual switch, pipettes (10
and 50 cm3
), 2M HCl, 2M CH3COOH (HAc), 0,1M NaOH, 2M CH3COOH (HAc), 0.1M
NaOH, phenolphthalein indicator.
ASSEMBLING OF CALORIMETER. The main part of simple calorimeter according
FIGURE 1 is the Dewar vessel of about 500 cm3
. Fill the vessel with 0,1M NaOH or
water. The choice depends on the thermal effect that will be measured. The tube goes
from the dispenser with 2M HCl or HAc acid to the Dewar vessel. Check electric

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ú l o h a J . S o p o u š e k
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3.a-c
connection between temperature sensor,
digital thermometer, and operator PC. Switch
on thermometer and PC. Test the software for
collection of data before experiment.
INSTRUCTIONS:
1. Preparation of acid dispenser. Because
the dispenser tube has a considerable
volume, there is always a certain amount of
air or liquid in it before calorimetric
measurement. Thus a multiple dose
(10 cm3
) of the selected acid has to be
eliminated to the waste. The exact amount
of the acid solution should be determined by
titrating the dose volume with the NaOH
solution of known concentration.
2. Determination of both neutralization heat
∆𝑯𝑯𝑛𝑛𝑛𝑛 of strong acid and calorimeter
constant C. Fill the Dewar vessel with 0,1M
NaOH solution of the volume that will be
needed to neutralize the later dosed acid.
(ie. exact amount 10 ml 2M HCl needs 200ml 0.1M NaOH). Put the stirrer, the acid
dispenser capillary, the heating element, and the temperature sensor into the
calorimeter stopper. Turn on the rotary
stirrer. During the shortest period of time
between switching on and off, set the
current intensity through the heating
element to the value (0.5 A) using the
rheostat. Start data collection using PC.
Record the temperature in the calorimeter at
its initial relaxation (approx. 5-7 min) as well
as after adding the 10 ml dose of 2M HCl
(approx. 4 min). Then switch on the heating
and write down immediately the start time
(±1 sec) from the PC monitor and the
electric current. After the temperature rises
by approx. 0,7ºC, turn off the heating and
write down the exact time again from the PC
monitor. Leave the temperature in the
calorimeter relaxing for another 10 minutes
and then stop the measurement. This all will
give you a temperature record similar to the
FIGURE 2. After the measurement, empty
and rinse the calorimeter by water.
3. Determination of both diluting heat ∆𝑯𝑯hd
of strong acid and calorimeter constant

FIGURE 1: Assembly of a simple
calorimeter. D - Capillary with tube to
dispenser, T - Temperature sensor, M Mixer,
H - Heating, V – Voltmeter, A Ammeter,
R - Reostat, B - DC voltage
source, DT – digital thermometer.
FIGURE 2: Dependence of temperature
on time when the strong acid is
neutralized with a strong hydroxide,
including a further course of
determination of the heat capacity C of
the calorimeter.
J . S o p o u š e k ú l o h a
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3.a-c
C. Measure and put into the Dewar vessel as much water as used of 0,1M NaOH
solution in the previous neutralization experiment. Check the dispenser of 2M HCl
solution and follow the same procedure as done during the neutralization experiment
using water instead of 0,1M NaOH solution.
4. Determination of neutralisation and diluting heats of weak acid (HAc). Work
exactly in the same way as it was done before when working with strong acid,
including determining the calorimeter heat capacity and dilution heat.
DATA ANALYSIS: For all experiments: fit the lines on the linear regions of the
temperature – time dependence according to FIGURE 2. Evaluate the values ∆T
and T′∆ as accurately as possible. You should perform linear interleaving and
evaluation using a computer if a suitable SW is available.
REPORT: For both strong and weak acids: Graph 1-4: Dependence of temperature
on time obtained during measuring of neutralization and dilution heats. Table 1:
for all experiments: NaOH or water wolume, molar masses of NaOH and acid used at
neutralization, temperature increase of neutralization ∆T and of dilution T′∆ , the
voltage, current, and time values used in the eqn (3.4.), heat capacity of the calorimeter
C, molar neutralization enthalpy ∆ 𝐻𝐻𝑛𝑛𝑛𝑛
𝑚𝑚
or molar diluting enthalpy ∆ 𝐻𝐻ℎ𝑑𝑑
𝑚𝑚
. NEXT:
comparison of measured molar reaction enthalpy for strong acid (HCl) with its tabulated
value ∆ 𝐻𝐻𝑟𝑟
𝑚𝑚
. Estimation of molar dissociation enthalpy of weak acid (HAc).

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