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# hess's law

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Introduction

LAB REPORT 12 - HESS'S LAW OF CONSTANT HEAT SUMMATION INTRODUCTION DESIGN Aim: To determine the molar enthalpy change of formation of hydrated copper sulphate from anhydrous copper sulphate by referring to Hess's Law of constant heat summation. General Background: The standard enthalpy of formation or "standard heat of formation" of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 bar of pressure and the specified temperature, usually 298.15 K or 25 degrees Celsius). Its symbol is ?HfO. The standard enthalpy change of formation is measured in units of energy per amount of substance. Most are defined in kilojoules per mole, or kJ mol-1, but can also be measured in calories per mole, joules per mole or kilocalories per gram (any combination of these units conforming to the energy per mass or amount guideline). All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation. The standard enthalpy change of formation is used in thermochemistry to find the standard enthalpy change of reaction. This is done by subtracting the sum of the standard enthalpies of formation of the reactants from the sum of the standard enthalpies of formation of the products, as shown in the equation below: ?HreactionO = ??HfO (Products) ...read more.

Middle

5 H2O = 249.5 g No. of moles of CuSO4 = [Given mass of CuSO4] � [Molar mass of CuSO4] = [3.19 g] � [159.5 g] = 0.02 mole No. of moles of CuSO4 . 5 H2O = [Given mass of CuSO4 . 5 H2O] � [Molar mass of CuSO4 . 5 H2O] = [4.99 g] � [249.5 g] = 0.02 mole Enthalpy Cycle for the Reaction: From the enthalpy cycle included in the previous page, the enthalpy change that we are going to determine is ?H. From the knowledge of Hess's Law of constant heat summation, we can say: ?H + ?Hp = ?Ha Rearranging the above expression, we can get: ?H = ?Ha - ?Hp Therefore, in order to obtain ?H, we will have to calculate the difference between ?Ha and ?Hp. Calculating Change in Temperatures: Change in Temperature = Highest Temperature - Lowest Temperature From the data collected while conducting the experiment, the lowest temperature reached = 27 �C. However the estimated lowest temperature obtained from graph 1 attached earlier = 25 �C. Therefore this estimated temperature from the graph compensates for the amount of heat energy gained from the surroundings while conducting the experiment and hence, the lowest temperature reached is 25 �C and this value will be used hereafter in order to carry out the necessary and appropriate calculations. Change in Temperature for Hydrated Copper (II) Sulphate = (29 � 0.5) - (25 � 0.5) ...read more.

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