What is the physical meaning of heat of formation?

 

#engineering #heat

It takes energy to do anything. It takes energy to move, to write,to watch, to read. Similarly it takes energy to make something. Without energy nothing  can be done and nothing can be made. Heat of formation is that energy which is required to make a product out of its constituents. 

In  chemistry we usually deal with what are known as chemical reactions. In a reaction, reactants combine with each other in different ways to form products. 

The difference in energy of formation of products and reactants is called heat of reaction. A reaction may release energy such as the one which involves combustion of fuels . In such a reaction the fuel is burned in oxygen to create new products. Since energy is given off as heat and light in a  combustion reaction the heat of formation of products is lower than the heat of formation of reactants.This means that the products are much more energetically favorable than reactants. 

The elements which exist naturally  in nature are said to have a 0 heat of formation. They already exist, no energy is needed to create them. 

The heat of formation dictates the possibility of a reaction happening at all. The heat of formation defines the existence or nonexistence of certain materials. It is incredibly difficult to burn water or sand. Even carbon in its graphite form needs hundreds of degrees to combust to co2. 

The more the heat of formation of a certain substance the less likely is its existence. Obtaining silicon by decomposition of silicon dioxide is incredibly difficult because in nature silicon dioxide is more stable than silicon. Consequently a lot of energy is required to reduce silicon dioxide to silicon. As a result elemental silicon is hardly found.

Similarly water is a very stable  compound and thus extracting hydrogen from water is an extremely energy intensive process. Elemental hydrogen is hardly found in nature.Although there are a few deposits of green hydrogen they are not proven to exist in sufficiently large quantities.Try as much as you want helium won't react. On the other hand, sodium or potassium are hard to preserve in their natural states. 

But we need to provide some sort of definition for "nature". Let us take hydrogen and define nature as it sees. On our planet earth hydrogen sees oxygen and burns to form water releasing tremendous amounts of energy. This indicates that in this particular nature water is more energetically favorable than hydrogen or oxygen  molecules. But in the universe hydrogen molecules are the most abundant element,no water is formed until elemental oxygen also exists. 

Heat of formation only determines the existence of products when reactants are available and not otherwise. No matter how much energy you supply to pure hydrogen, water won't be formed until oxygen is also present. When both hydrogen and oxygen and present heat of formation predicts the existence of water. 

Why do we care? What does it matter? Doesn't matter much but it's something you need to be aware about when working with materials. Compounds are not the only way to make materials. Even two unreactive materials can be combined to form new materials as composites that have different properties than the original materials. Stainless steel is one very common example of a composite alloy composed of materials that won't otherwise react. Brass and bronze are other alloys composed of materials that are unreactive otherwise. 

Heat is a very useful tool that can help you engineer materials the way you want. When the energy required to form a material is prohibitively high there's  always an opportunity to go for a combination of materials that add up to more or less the same result.