INSTITUTE OF CHEMISTRY, COLLEGE OF SCIENCE
UNIVERSITY OF THE PHILIPPINES, DILIMAN QUEZON CITY, PHILIPPINES RECEIVED NOVEMBER 20, 2012
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
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.
d.Calculate the enthalpy change (per mole) for the displacement reaction.
5.Given that the standard enthalpy of formation of liquid...