Calorimetry: Enthalpy of Neutralization

Topics: Thermodynamics, Electrolyte, Heat Pages: 8 (2370 words) Published: March 10, 2013
Calorimetry: Enthalpy of Neutralization

The purpose of this lab experiment was to determine the molar concentration of the unknown HCl by using a coffee cup calorimeter. A coffee cup calorimeter is made of Styrofoam cups with a thermometer that is placed from the top and into the calorimeter to measure the temperature as the reaction happens. The Styrofoam cup used for the calorimeter creates an isolated system as it acts as a heat insulator between the cup and the surrounding. When a reaction happens bonds are created and destroyed causing energy to be released or absorbed. In a constant pressure, like in the coffee cup calorimeter, the change in energy equals to the enthalpy change of the reaction. Because enthalpy change of the reaction equals to the specific heat times the mass times the change in temperature, the amount of heat released or absorbed can be determined by measuring the change in temperature. If the enthalpy change is negative then the reaction is exothermic and if the enthalpy change is positive it is an endothermic reaction (Petrucci, Herring et al. 2011). The heat of the reaction was measured for a the neutralization reaction of four different acids which were 1.994 M HCl, 2.035 M HNO3 , 0.5115 M phenol and HCl with unknown molar concentration. There are two different situations for the neutralization reaction because the acid used can either be a strong electrolyte or a weak electrolyte. If the acid is a strong electrolyte it will dissociate into ions completely and so if neutralization happens the reaction taking place will be H+ + OH- -> H2O. The heat released for the reaction is the formation of water so the neutralization reaction of the acid will have a heat of the reaction value of -55.90kJ/mol. The neutralizations involving weak electrolytes will partially dissociate into ions. This will mean multiple reactions will govern the overall heat of the neutralization making the heat of the reaction be either smaller or larger than -55.90 kJ/mol. The temperature of the neutralization reaction was recorded for a set of intervals of time and from the data collected the change in temperature was found from the initial and the maximum temperature of the reaction. The heat of the reaction equals to the specific heat times the mass times the change in temperature. The mass is the volume of acid and base added together and using the conversion ratio of 1 ml equals to 1 g, the mass can be calculated and be used in the formula. And the specific heat capacity is the same as water 4.18 J/∘Cg. The concentration of unknown HCl can be calculated by using value of the heat released per mole of water. Procedure:

The experimental procedure used for this experiment is outlined in the CHEM 120L lab manual, experiment 4 page 52-58. The one changes done were the values of the concentrations of the acid and base.

Table 1: Part A NaOH (2.008 M) and HCl (1.994 M)
| 50 mL NaOH (2.008 M)| 40 mL HCl (1.994 M)|
| Trial 1| Trial 2| Trial 1| Trial 2|
Temperature (°C) before mixing| 23.1| 23.0| 23.1| 22.8| Temperature (°C) after mixing| 23.1| 23.1| 23.1| 23.1| Time (s)| Trial 1:Temperature (°C)| Trial 2:Temperature (°C)| 1.0| 35.0| 35.0|

2.0| 34.8| 35.0|
3.0| 34.9| 35.0|
4.0| 34.2| 35.0|
5.0| 34.2| 35.0|

Table 2: Part A NaOH (2.008 M) and HCl (1.994 M) Temperature recorded every 10 seconds. Time (s)| Trial 1:Temperature (°C)| Trial 2:Temperature (°C)| 0.0| 34.2| 35.0|
10| 34.1| 35.0|
20| 34.0| 35.0|
30| 34.0| 35.0|
40| 34.0| 35.0|
50| 34.0| 35.0|
60| 34.0| 34.8|

Table 3: Part A NaOH (2.008 M) and HCl (1.994 M) Temperature recorded every 30 seconds. Time (s)| Trial 1:Temperature (°C)| Trial 2:Temperature (°C)| 0.0| 34.0| 34.8|
30| 33.8| 34.8|
60| 33.7| 34.7|
90| 33.7| 34.5|
120| 33.3| 34.4|
150| 33.0| 34.2|
180| 33.0| 34.0|
210| 33.0| 33.0|
240| 33.0|...
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