# Essay on Hess Law

1557 Words Jul 2nd, 2013 7 Pages
EXPERIMENT 3
Title : EXPERIMENTAL APPLICATIONS OF HESS’S LAW

Objective : To determine the heat of reaction by applying Hess’s Law for reactions whose energy cannot be easily measured directly.
Introduction
An exothermic reaction releases heat into the surroundings and is usually observed as temperature increases in the solvent, container, and other immediate surroundings. The amount of heat released from the reaction can be expressed as q reaction = q solution + q calorimeter ……..(1)
In this experiment, the calorimeter is assumed to absorb so little heat that q calorimeter is nearly zero and thus the equation simplifies to q reaction = q solution ……...(2)
The temperature change caused by the addition
Method
Part 1 : The reaction of HCL (aq) and NH3 (aq).
About 25 ml of 2.0 M NH3 was added into a 50 ml beaker. And about 25 ml of 2.0 M of HCL was added directly into Dewar flask. Both solution was kept in room temperature. The temperature of the both solution was recorded. Next, NH3 in the beaker is added into the Dewar flask. The mixture is then stirred slowly and continuously until the maximum temperature is reached and recorded. The experiment is repeated twice and average reading was obtained.

Part 2 : The dissolving of solid ammonium chloride in water
About 25 ml of distilled water was added into the Dewar flask. The solution is then stirred until constant temperature is obtained and the temperature was recorded. About 5 g of NH4CL (s) was weight and by using spatula NH4CL (s) placed into the calorimeter. The solution is then stirred gently with thermometer until the solid is dissolved completely and the maximum temperature was recorded. The experiment was repeated once again and average reading is obtained.
Results
Part 1 : The reaction of HCL (aq) and NH3 (aq) Number of experiment | 1 | 2 | 3 | HCL (initial temp) ° C | 25.5 | 26.0 | 25.5 | NH3 (initial temp) ° C | 24.0 | 24.0 | 24.0 | HCL + NH3 (max temp) ° C | 27.5 | 36.0 | 36.0 | Part 2 : The dissolving of solid ammonium chloride in water Number of experiment

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