How do you write chemical equations?

1 Answer
Aug 11, 2016

Stoichiometrically: mass is always conserved.


Chemical equations reflect a real chemical or physical change. It is a fundamental underlying principle of chemistry that mass is conserved. What does this mean? It means that if I start with 10 g of reactant, from all sources, at most I am going to get 10 g of product. In practice I am not even going to get that.

Let's take a simple hydrocarbon combustion reaction, the which drives your motor:

#C_6H_14(g) + 19/2O_2(g) rarr 6CO_2(g) + 7H_2O(g)#

Is mass conserved here? Well, it seems to be. Because for every reactant particle there is a corresponding product particle. If there were not, I would know that the equation was unbalanced and therefore unsound. In practice of course, hexane would not burn so cleanly. Some of the product would be #CO# or particulate #C#, the products of incomplete combustion. The masses, however, would be inevitably still be conserved.

This principle of conservation may be pushed farther. Not only is mass conserved, but charge and energy is conserved as well. I started with neutral reactants; I finished with neutral products. The energy content of the starting hydrocarbon was also conserved; I could use some of the energy evolved in the reaction above to do quantitfiable work.