Experimental applications of Hesss Law

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Title: Experimental applications of Hess’s Law

Objectives: 1) To determine the enthalpy change of reaction,  by Hess’s law.

2) To study the relationship between change in temperature  and enthalpy change.

Introduction:

        A Chemical reaction not only involved in a single steps reaction change also involved in a series of intermediate steps of changing. In this experiment are used the hydrochloric acid react with ammonia aqueous solution and the dissolving of solid ammonium chloride in water to determine the enthalpy change of the reaction. Reaction can be divided to two categories which is exothermic and endothermic reaction. Every reaction involved either exothermic or endothermic reaction. Just a simple example, when water in liquid state change to solid state is involved releases heat (exothermic reaction) in to surrounding. While when water in solid state change to liquid state involved in absorb heat (endothermic reaction) from surrounding.

        Heats are produce when chemical reactions occur. In this experiment, the system is an isolated system which no exchanges heat and mass with surrounding. Hence, the amount of heat released or absorb from the reaction can be expressed as

In this experiment, the container that was used is calorimeter which can assume the heat of container () absorb is very litter as nearly zero. Thus, the equation simplifies to

        Heat of the solution will base on the change of the temperature,  of solution and depend on the specific heat,  of the substance. Thus, the heat of reaction can be express as

                         (m = mass of substance)

        Since the reaction are in isolated system and run at constant pressure, thus the heat of reaction will equal to the enthalpy change of reaction. Thus, Hess’s law can be used to calculate the change of the heat. According to the Hess’s law, if a reaction takes place in several steps then its standard reaction enthalpy is the sum of the standard enthalpies of the intermediate reactions into which the overall reaction may be divided at the same temperature. 

From the graph above, as Hess’s law state that the enthalpy heat of this reaction can be calculate as

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Chemicals: Spatula 2.00 M , 2.00 M HCl, solid

Glasswares: Dewar flask, thermometer (0 - 200)

Procedure:

Part 1 : The reaction of hydrochloric acid solution and ammonia aqueous solution

  1. Measured 25.0ml of 2.00 M  using a graduated cylinder into a 50 ml beaker and recorded the initial temperature.
  2. Measured 25.0ml of 2.00 M of  directly in to Dewar flask and recorded the initial temperature.
  3. Added theto the  in the calorimeter. Stirred the mixture slowly and continuously with thermometer and recorded the maximum temperature reached by the mixture. ...

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