Rdr 1

DATE PERFORMED: NOVEMBER 14, 2012

CALORIMETRY

INSTITUTE OF CHEMISTRY, COLLEGE OF SCIENCE
UNIVERSITY OF THE PHILIPPINES, DILIMAN QUEZON CITY, PHILIPPINES
RECEIVED NOVEMBER 20, 2012

INTRODUCTION Heat is a form of energy that can be passed from an object with high temperature to an object with low temperature.
The heat required to change the temperature of a substance by 1 K is called heat capacity.
A calorimeter is a device commonly used to measure the amount of heat that may be produced or assimilated by the system. This device encloses a reaction inside an insulated wall to prevent the exchange of heat between the calorimeter, and its surroundings.
The heat capacity of a calorimeter is determined by reacting a known amount of heat inside, and measuring the temperature change by placing a thermometer held by a rubber stopper on top of the device.
Preferably, it is better to know the heat capacity of the calorimeter itself rather than the entire calorimeter system.
The computed value of calorimeter’s heat capacity, Ccal, can be used to calculate the ∆Hrxn, the heat released at constant volume, of other compounds reacting inside the same calorimeter.

ANSWERS TO QUESTIONS

A.
1. After obtaining experimental values of ∆Hrxn, explain any discrepancy of the values to the theoretical. Give some possible sources of errors.
- There are many potential errors in doing the calorimeter experiment. The set up may not be a perfect insulator. There may be a leak of heat inside the reaction since the students opened the test tube while adding the HCl, or there is a hole in the styrofoam where heat can escape. This will result to lower ∆H reading. Also, there might be an error in preparing the solutions. The chemicals might have been measured more or less than the required value. In addition, the students may have not waited long enough, or stirred the solutions enough before taking the thermometer reading. 2. In the procedure for the determination of ∆H, explain why it is important:
a. that the total volume of the resulting solution be 15 mL?
- The resulting solution should be 15 mL because if it is not so, the measured ∆T will change. There will be different amount of chemicals reacting inside the test tube so the experiment’s result would change.

b. to know the exact concentrations of the reactants?
- The exact concentration of the chemicals is important since it is involved in the computation of ∆H. The increase or decrease of the concentration affects the number of reactant particles that collides with each other, affecting the temperature reading. In addition, knowing the concentration of a reactant would determine which is the limiting reactant.

c. to know the exact weight of the metal solids used?
- The students should know the exact weight of the metal solids used to know how much liquid reactant is needed to completely dissolve the solid being used. This is important to avoid having an incomplete reaction.

3. The neutralization of 200 mL of 0.5 M HA by sufficient amount of NaOH evolves 6.0 kJ or heat.
a. Calculate the enthalpy change for the neutralization of 1 mole HA.

b. Is HA a weak or strong acid? Justify your answer using thermochemical equations?

c. Write the net ionic equation for the reaction between HA and NaOH.

4. A calorimeter similar to your Styrofoam-ball calorimeter was used to determine the enthalpy change associated with the reaction between Cu2+ and Zn(s). The reaction between 20 mL of 0.450 M CuSO4 and 0.264 g Zn(s) resulted to a temperature change of 8.83 °C.
a. Write the net ionic equation for the calibration reaction.

b. Write the net ionic equation for the displacement reaction.

c. Calculate Ccal.

d. Calculate the enthalpy change (per mole) for the displacement reaction.

5. Given that the standard enthalpy of formation of liquid water, ∆H°f,H2O is -285 kJ/mol, calculate the ∆H°f of OH-(aq).

B.
1. Give the net ionic thermochemical equation of the reaction used to calibrate the calorimeter.

a. Is the reaction endothermic or exothermic?
- The reaction is exothermic.

b. Which is limiting reactant?
- The limiting reactant is HCl.

c. How much (in moles) limiting reactant was used?

d. How much heat was generated (or absorbed) by the reaction?

2. Relate the sign of the ∆T to the ∆H of the reaction used for calibration.
- The experiment yielded a positive ∆T suggesting that the temperature after the reaction is higher than before. This result is due to the negative value of ∆H, which means that the system releases heat to the surroundings, thus, the increase in temperature read by the thermometer. In short, a positive ∆T will result to negative ∆H.
3. What is the heat capacity of the calorimeter? Relate the sign to the sign of the ∆T.
- The heat capacity of the calorimeter used is 74.79 J/°C.The Ccal have a positive sign since the measured ∆T is positive.

• NOTE: The complete solutions from numbers 4-7 is in the APPENDIX, B.

4. In the appendix, show the derivation to obtain the equation used to calculate the heat capacity of the calorimeter.

5. Give the net ionic equation for each reaction.
1. NH3 + H+ NH4
2. CH3COOH + OH- H2O + CH3COO-
3. NH3 + CH3COOH CH3COO- + NH4
4. OH- + H+ H2O
5. 2H+ + Mg Mg2+ + H2
6. CH3COOH + Mg 2CH3COO- + Mg2+ + H2
7. Cu2+ + Zn Zn2+ + Cu
8. CO32- + Ca2+ CaCO3

6. Determine the limiting reactant and the amount of the limiting reactant in each of the reaction performed.
1. HCl
2. CH3COOH
7. Calculate for the theoretical and experimental enthalpy of each reaction.
a. Determine whether the reaction is endothermic or exothermic.
b. Give the % error of your experimental values.

REFERENCES

[1] Silberberg. Principles of General Chemistry.
New York: McGraw-Hill Companies, Inc. 2010.
[2] Wilbraham, Antony, Dennis Staley, Michael
Matta, and Edward Waterman. Chemistry. Jurong: Pearson Education Asia Ptd Ltd, 2000.
[3] “Heat Effects and Calorimetry.” Calorimeter.
College of DuPage. November 18, 2012.
[4] Chieh,Chung. “Measuring Heats of Reaction.”
Calorimetry. University of Waterloo. November 18, 2012.
[5] Blauch, David. “Heat Capacity of the
Calorimeter.” Calorimetery. 2009. Davidson College. November 18, 2012.

APPENDIX

A. WORKING EQUATIONS

B. SAMPLE CALCULATIONS

References: New York: McGraw-Hill Companies, Inc. 2010.