# Enthalpy of Reaction and Hess's Law

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• Published : February 25, 2013

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Joseph
Corey, Chris, Grant

Thermodynamics – Enthalpy of Reaction and Hess’s Law

I. Purpose

The purpose of this lab is to verify Hess’s Law through the three reactions of NaOH and HCl, NH4Cl and NaOH, and NH3 and HCl. The sum of the enthalpies of the first two reactions should equal the enthalpy of the third reaction.

II. Background

Hess’s Law is used to determine the enthalpy of a reaction from adding two or more preceding reactions. To determine the enthalpies of certain chemical reactions, the change in must be measured. This is best done using a calorimeter to prevent heat loss to the surroundings. To calculate the heat change in solution, the q=mct equation must be used. A positive value for “q” means the solution gains heat, a negative value means the solution loses heat and is exothermic. The reactions used in the lab are exothermic acid-base neutralizations. By calculating “q”, heat, the enthalpy of reaction can be found by knowing the Molarity of the reactants. The specific heat of the calorimeter can be found by the equation qcal= t x heat capacity. Since a calorimeter is used the heat released from the reaction will be absorbed in the solution, while some heat is transferred to the calorimeter. So the “q” of the reaction is given by this equation: qrxn= - (qsol+qcal).

III. Summary of Procedure

Part 1:
A calorimeter is to be arranged using Styrofoam cups and a hole on top of a cover to take the temperature while also preventing heat loss. 50mls of distilled is to be added to the calorimeter. 75mls must then be heated to 70 degrees Celsius and 50mls of that water is to be added to the calorimeter. The calorimeter has to be covered and then the temperature taken every 20 seconds for 3 minutes.

Part 2:
50mls of 2.0M HCl must be put into the calorimeter. A 50mls solution of 2.0M NaOH should then be added to the HCl solution and stirred. The temperature needs to be recorded every 20 seconds for 3 minutes. For the second reaction the same process has to be repeated using 2.0M NH4Cl and 2.0M NaOH. The third reaction requires the same process using 2.0M solutions of NH3 and HCl.

IV. Observations

* Styrofoam cup becoming hot after mixture
* thermometer scraping the Styrofoam cup sides
* cover doesn’t completely cover the cup
* small amount of time between pouring the chemicals and covering and mixing

V. Data
Part 1 Data

50.0mL H20 -- room temp: 22.4 C
50.0mL H20 – heated: 60.3 C

Time (Sec)| Temperature ( C )|
20| 40.3|
40| 40.2|
60| 40.1|
80| 40.0|
100| 39.9|
120| 39.8|
140| 39.7|
160| 39.6|
180| 39.5|
| |

Tmix: 40.4 Cqcal: 397.48 J
Tave: 41.35 CCcal: 22.08 J/C

Reaction 1:

50.0 mL 2.0M HCl: 21.3 C
50.0 mL 2.0M NaOH: 21.4
Time (Sec)| Temperature (C )|
20| 35.1|
40| 35.1|
60| 34.9|
80| 35.0|
100| 34.9|
120| 34.9|
140| 34.8|
160| 34.7|
180| 34.7|
| |

Tmix: 35.15 CH: -59.05 kJ/mol
qrxn: -5905 J
Reaction 2:

50.0 mL 2.0M NH4Cl: 21.4 C
50.0 mL 2.0M NaOH: 21.7 C

Time (Sec)| Temperature (C )|
20| 22.7|
40| 22.7|
60| 22.7|
80| 22.7|
100| 22.7|
120| 22.7|
140| 22.7|
160| 22.7|
180| 22.7|
| |

Tmix: 22.7 CH: -4.92 kJ/mol
qrxn: -492.05 J

Reaction 3:

50.0 mL 2.0M NH3: 21.6 C
50.0 mL 2.0M HCl: 21.8 C

Time (Sec)| Temperature (C )|
20| 34.8|
40| 34.7|
60| 34.7|
80| 34.6|
100| 34.5|
120| 34.5|
140| 34.4|
160| 34.4|
180| 34.3|
| |

Tmix: 34.84 CH: -56.22 kJ/mol
qrxn: -5622.2 J

VI. Results and Questions

Calculations please see attached graphs and work.
Post Lab Questions

1. What is meant by calorimetry?
Calorimetry is the scientific measuring of heat released during chemical and physical changes. It ensures that minimal heat is lost so the heat of reaction can be found accurately....