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  that accompanies the formation of 1  of a substance in its  from its constituent elements in their  (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 ΔHfO.

The standard enthalpy change of formation is measured in units of energy per amount of substance. Most are defined in kilojoules per , or kJ mol-1, but can also be measured in  per mole,  per  or kilocalories per  (any combination of these units conforming to the energy per mass or amount guideline).

All elements in their standard states ( gas, solid carbon in the form of , 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:

ΔHreactionO = ΣΔHfO (Products) - ΣΔHfO (Reactants)

The standard enthalpy of formation is equivalent to the sum of many separate processes included in the  of synthesis reactions.

 (1802 - 1850) is important primarily for his thermochemical studies. Hess' Law states that the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps. This is also known as the law of constant heat summation.

Hypothesis:

The reaction being investigated is summarized in the chemical equation below:

CuSO4(s) + 5 H2O(l) →  CuSO4 . 5 H2O(s)

It can be seen from the above equation that the product formed from the reaction i.e. hydrated copper sulphate comprises of new bonds. We know that the process of making bonds is an exothermic process, which means that energy is released or given out when these new bonds are formed. Hence, on the basis of this, I predict that the change in enthalpy for formation of hydrated copper sulphate according to the above reaction will be negative, signifying that the overall reaction is exothermic.

Therefore, I predict that the molar enthalpy change of formation of hydrated copper sulphate from anhydrous copper sulphate will be negative, indicating that the reaction is exothermic.

Independent Variables:

There were no independent variables in this experiment as none of the parameters or factors in the experiment were varied.

Dependent Variables:

In this experiment, the dependent variable was the temperature of the system measured in °C using a thermometer, as the reaction outlined above occurs. The temperature of the system was constantly monitored, measured and recorded at set time intervals.

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Controlled Variables:

In this experiment, the controlled variables were: the volume of distilled water used i.e. 50 cm3 in both the reactions, the temperature and pressure at which the experiment was conducted i.e. room temperature and pressure in the school laboratory and the time interval after which the temperature of the system was measured and noted down i.e. 0.5 minute or 30 seconds.

MATERIALS AND METHOD

Apparatus Used:

  • 1 polystyrene cup
  • Glass rod
  • Tripod stand
  • Electronic mass balance
  • 2 spatulas
  • A 50 cm3 measuring cylinder
  • Ceramic gauze
  • A 100 cm3 beaker
  • ...

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