Illustrated Guide to Home Chemistry Experiments

Chapter 15 Laboratory: Thermochemistry and Calorimetry 281

CUTIOA nS

Sodium hydroxide and hydrochloric acid solutions are

corrosive and toxic, including the dilute solutions used

in this laboratory. Wear splash goggles, gloves, and

protective clothing.

z

TABLE 15-4: Determine the enthalpy change of a reaction—

observed and calculated data

Item Data

A. Volume of HCl solution _________.___ mL

B. Temperature of HCl solution_________.___ °C

C. Volume of NaOH solution _________.___ mL

D. Temperature of NaOH solution_________.___ °C

E. Temperature of solution (final)_________.___ °C

F. Temperature change [E – (B+D)/2]_________.___ °C

G. Heat of reaction _________.___ cal

H. Moles of sodium hydroxide ____.________ mol

I. Enthalpy change of reaction_________.___ cal/mol

oALIpToN ACTIvITIES

If you have time and the required materials, consider

performing these optional activities:

• Repeat the experiment using 1.2 M sulfuric acid in
  place of the 1.2 M hydrochloric acid. Look up the
  published value for the enthalpy change of reaction
  in the CRC handbook or an online reference and
  compare the known value to your experimentally
  determined value.


• Repeat the experiment using 1.2 M acetic acid in place
  of the 1.2 M hydrochloric acid. Look up the published
  value for the enthalpy change of reaction in the CRC
  handbook or an online reference and compare the
  known value to your experimentally determined value.

calories per mole or Joules per mole. To determine the enthalpy
change of reaction, ΔH°reaction, we need to divide Q, the heat of
reaction we observe experimentally, by the number of moles that
yielded that value of Q. (If we had a large calorimeter and reacted
1.0 mole of sodium hydroxide, the values of Q and ΔH°reaction
would be equal. Because we’re reacting only a small fraction of a
mole of sodium hydroxide, the absolute value of ΔH°reaction will be
much larger than Q.)

POCEDURER
1.ou have not already done so, put on your splash If y
goggles, gloves, and protective clothing.

2. If you have just made up the sodium hydroxide solution
   and/or the hydrochloric acid solution, make sure that
   both are at room temperature before proceeding.


3. Use a clean, dry graduated cylinder to accurately
   measure 50.0 mL of 1.2 M hydrochloric acid, transfer it
   to the calorimeter, and replace the lid of the calorimeter.
   Record the volume of HCl solution on line A of Table 15-4.
   (Although the exact molarity of the hydrochloric acid is
   unimportant, it is important to record the volume of the
   HCl solution that you use as accurately as possible.)


4. Measure the temperature of the HCl solution to 0.1°C and
   record the temperature on line B of Table 15-4.


5. Rinse and dry the graduated cylinder, and then use it to
   accurately measure 50.0 mL of 1.0 M sodium hydroxide
   solution. Record the volume on line C of Table 15-4.


6. Rinse and dry the thermometer and then use it to measure
   the temperature of the sodium hydroxide solution to
   0.1°C. Record the temperature on line D of Table 15-4. (The
   temperature of both solutions should be the same, but
   if they are not, you can average the two temperatures to
   arrive at a valid initial temperature. For example, if you start
   with 50.0 mL of hydrochloric acid at 22.3°C and 50.0 mL
   of sodium hydroxide solution at 21.1°C, your actual starting
   point is 100.0 mL of solution at 21.7°C.)


7. Remove the lid of the calorimeter, quickly add the sodium
   hydroxide solution, and replace the lid.


8. Stir the contents (or swirl the calorimeter gently) to mix
   the solutions thoroughly. Observe the temperature of the
   mixed solutions for several minutes until the temperature
   reaches its highest point. Record that temperature to
   0.1°C on line E of Table 15-4.

RQEWEvI UESTIOnS

q 1 : Calculate the heat of reaction and enter the value on line G

of Table 15-4. (Hint: remember to use the proper sign.)

q: 2 Calculate the number of moles of sodium hydroxide that

reacted and enter that value on line H of Table 15-4.

q: 3 Calculate the enthalpy change of reaction and enter that

value on line I of table 15-4.