Heat of Combustion

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Heat of Combustion

Introduction

Every chemical and physical transformation involves some change in energy. These changes may involve heat, light, electricity, or some other form of energy. The branch of chemistry, which deals with the measurement of these changes, is called thermochemistry. Enthalpy is a word which means 'heat content'. Every substance has some characteristic enthalpy because of its chemical makeup. It depends upon several factors: the total number of subatomic particles in each atom and how they are arranged; the total number of atoms in the substance and how these atoms are arranged; and the total number of molecules and how they are arranged (including any interactions between them). Because it is impossible to know all of these factors, we cannot calculate the absolute value for the heat content of a substance. However, we can measure the change in heat content, which accompany chemical or physical transformations.

The energy change of a reaction that occurs at constant pressure is termed the heat of reaction or the enthalpy change. The symbol Hrxn is used to denote the enthalpy change. If heat is evolved, the reaction is exothermic (Hrxn < 0) and if heat is absorbed the reaction is endothermic (Hrxn > 0).

If the transformation only involves a change in heat energy, it is a simple matter to monitor the energy change with a thermometer. The quantity of heat either evolved or absorbed may be calculated using the equation:

q  =  m · s · T

Where q is the quantity of heat, m is the mass of the substance undergoing the change, s is the specific heat of the substance, and T is the change in temperature in °C.

Purpose

In this experiment, we will determine the enthalpy of combustion, Hcomb for magnesium reacting with oxygen to form magnesium oxide:

Mg(s)  +  ½ O2(g)      MgO(s)

The most straight forward method of determining this value would be to weigh a piece of magnesium, place it in a 'bomb' calorimeter with excess ...