|MadSci Network: Chemistry|
The heat given off by combustibles is not only referred to as the Heat of Combustion but is also known as the Enthalpy of Combustion. There is a book which is called The CRC Handbook of Chemistry and Physics which is revised and published annually. This book contains a multitude of physical and chemical data on various chemicals. Since I have access to a copy, I know that it has a table titled "Enthalpy of Combustion of Hydrocarbons" that contains combustion data for about 150 common chemicals. This book should be available in your local library or a university library if you have access. The heat of combustion of materials is determined by a method known as calorimetry. Although the concept is pretty simple, the field has progressed significantly and has become very complex. A simple calorimeter consists of a container that contains a substance of known properties (for an easy example we'll use water). To find out the Heat of Combustion for a certain material the material is burned in such a way that all of the heat is transferred to the substance in the container (water in our case). Since we are burning a substance and transferring the heat to the water, the water should warm up. From the increase in the water temperature, the heat produced by the combustion can be calculated. There are a number of important things that we have to know before we can make the calculation. These include; - the exact amount of material combusted - the exact amount of water in your calorimeter - the temperature of the water before and after the combustion takes place - the heat capacity of the substance in the calorimeter (water in our case) Due to the properties of different substances, not all materials warm up at the same rate. The amount of energy needed to warm substances is known as the heat capacity. For water, it is known that it takes 0.9988 calories (~1 cal) to increase the temperature of 1 mL of water by 1 degree Celsius. The formula for figuring out the Heat of Combustion of the original substance would be; Heat of combustion = (T2-T1)*Cf*V Where; T1 = the initial temperature (deg. C) T2 = the final temperature Cf = the heat capacity (in units of cal/deg mL) V = the volume of the substance being heated As you might imagine, the science of calorimetry has advanced far beyond the level of heating a container full of water. Nevertheless this is how the original experiments were done and offers a good place to start when trying to understand the basic principles. As for the difference between the terms combustible and flammable, I believe it is the following (although I haven't been able to find out for sure); - Flammable is a substance which under normal conditions has the ability to catch fire with a minimal ignition source (such as a spark). An example of this might be a substance such as propane. - Combustible materials would be any material that will burn. In this category we could also place propane and the like but it would also include materials that need more vigorous conditions to burn and are not likely to catch fire with a simple spark. An example of a combustible material of this sort would be wood or paper. In my opinion therefore, all flammable materials are combustible, but all combustible materials are not necessarily flammable.
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