Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

As Thermometric Titraton

AS-Thermometric titration (With thermometers)

A2-Thermometric titration (With temperature probes)

 

You will use thermometric titration to determine the concentration of hydrochloric acid.

The reaction between the sodium hydroxide and hydrochloric acid is an exothermic neutralization reaction.

The maximum temperature is reached at the end-point of the titration.

Materials:

  •  Eye protection: goggles
  •  Cork, one-holed to fit thermometer
  •  Burette stand
  •  Beaker (to hold the cups)
  •  Thermometer (0 – 100 °C)
  •  Volumetric pipette 25 cm3
  •  Clamp and stand
  •  Burette (50 cm3)
  •  Two insulated (polystyrene) cups
  •  Pipette filler

 

Procedure: (read all steps before beginning)

1)     Place two polystyrene (Styrofoam) cups in a beaker for support.

2)     Using a pipette and pipette filler, transfer 25 cm3 of the sodium hydroxide solution into the cup.

3)     Fill the burette with the acid to the zero mark.

4)     Measure the temperature of the base in the cup and record ONLY when it is steady for
more than 1 minute. Record ZERO as the volume for this temperature.

5)     Using the burette, add a small portion (3 – 5 cm3) of dilute hydrochloric acid to the solution
in the cup.

6)     Record the actual volume reading. (be careful with the amount of significant  figures)

7)     Stir by swirling the cup and measure the highest temperature reached.

8)     Immediately add a second small portion of the dilute hydrochloric acid, stir, and again  measure the highest temperature and note down the volume reading.

9)     Continue adding acid until there are enough readings to decide the maximum temperature reached during this experiment. You will need to add at least 30 cm3 of the acid.

Analysis

(This section will not be provided in case you will have a thermometric titration in your lab exam).

  1. Plot a graph of temperature vs. total volume of acid added
  2. Draw straight lines of best fit and extend them until they cross (see diagram right).
  3. The point at which the two lines meet corresponds to the volume of acid needed for neutralization and to the maximum temperature. Your graph should be similar to the one on the right

Calculations

FOR THE FOLLOWING QUESTIONS:

 BE CAREFUL WITH THE SIG FIG AND THE UNITS. SHOW      YOUR WORK!

  1. How many moles of Base were present in the cup before the titration.
  2. How many moles of HCl were present in the volume that corresponds with the maximum temperature?
  3. Calculate the concentration of HCl based on questions 1 and 2.
  4. Suppose you had diluted the acid from a concentrated solution using 25 mL pippete and a 250 cm3 volumetric flask BEFORE adding it to the burette. Which would be the concentration of the original acid solution?
  5. You repeated the experiment by performing a regular acid base titration using an indicator. Which is the volume of acid used through this experiment?
  6. How do you compare the extrapolated value in the thermometric titration and the measured volume in the acid-base titration?
  7. Write down the following information to calculate the enthalpy change of neutralization for the reaction?
    1. Add up the volumes used in the acid base titration to calculate the total amount of solution used (= to the mass of the water since we consider the density of the water as 1g cm-3
    2. Which are the initial and final (maximum) temperatures
  8. Calculate the heat of neutralization for the amount of atoms that reacted.
    Assume that the obtained solution has a specific heat equivalent to the one in water. (4.18 J g-1 °C-1)
  9. How many moles of water were produced in the reaction in the beaker?
  10. Calculate the MOLAR ENTHALPY OF NEUTRALIZATION.

CONCLUSION:

  1. Which is the more accurate way to titrate an unknown solution?
  2. Which are the main sources of error in the experiment?
  3. Consider the limitations of the experiment, and possible improvements to it.

++++++++++++++++++++++++++++++++++++++++++++++++++++