Law of conservation of mass and chemical reactions

Law of conservation of mass states that mass is conserved in a closed system such as our universe. Meaning that no matter what happens, mass can only transform from mass to mass or heat, energy etc.

Consider this real world example, you completely burn a paper, it turns into ash which although might not be equal in mass to that of paper’s because some of the paper’s mass is reduced because most carbon present in the paper reacts with the surrounding oxygen, becomes Carbon Dioxide and drifts away.

Same is the case with hydrogen present in the paper, it reacts with oxygen to produce water that evaporates and is not noticeable.

However, the overall mass is never reduced or increases, it just stays constant. In classical mechanics, there’s no correlation between mass and energy.

Special Relativity, on the other hand, is a different ball game, where law of conservation of mass and energy become corresponding due to mass-energy equivalence.

Relativistic Physics proves that mass is actually a form of energy that is in rest frame. So, what were two laws in classical mechanics are essentially described by only one law in relativistic physics.

How chemical reactions are affected

Law of conservation of mass has many implications, one of those implications relates to chemical reactions that make sure to conserve mass of reactants in products.

This happens to satisfy thermodynamics requirements as well as those of law of conservation of mass.

Otherwise, these reactions would be impossible in nature. Therefore, it is of prime importance to balance chemical equations. Let’s look at an example to get a clear picture.

Example

C2H6 + O2 ---> CO2 + H2O

There are two carbons on the left side but only one on the right, and it is in a single chemical species on both sides. Therefore, we put a 2 in front of the CO2 on the right hand side.

C2H6 + O2 ---> 2CO2 + H2O

Also, as the oxygen is in two compounds on the right side, we would look at the hydrogen next as it is in one compound on both sides of the equation. There are 6 hydrogens on the left side and two on the right - we put a 3 in front of the H2O on the right side.

C2H6 + O2 ---> 2CO2 + 3H2O

Now we’ve got two oxygens on the left side and seven on the right. We place a 3 1/2 in front of the O2 on the left hand side.

C2H6 + 3 1/2O2 ---> 2CO2 + 3H2O

However, we don't want a half in a chemical equation, it doesn’t work that way so we multiply every coefficient on both sides with two.

2C2H6 + 7O2 ---> 4CO2 + 6H2O

And this is now a perfectly balanced chemical equation.

Wrapping up

We’ve learned that it's impossible to seperate thermodynamics and law of conservation of mass and how a unified understanding of both in relativistic physics makes sense in chemical reactions.

This is the origins of equation balancing in chemistry.