How do conservation of mass and conservation of charge apply to chemical reactions?

Feb 2, 2017

Conservation of mass, and conservation of energy applies ABSOLUTELY to all chemical reactions.

Explanation:

What does $\text{conservation}$ mean?

Well, in terms of mass, if I start with $10 \cdot g$ of reactant FROM ALL SOURCES, at most I can get $10 \cdot g$ of product. In practice, I am not even going to get that, because losses invariably occur on handling, and some mass is lost. And thus for a combustion reaction:

$C \left(s\right) + {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) \uparrow$

If I start with $12 \cdot g$ of carbon or coke, and burn it, I can get $44 \cdot g$ of carbon dioxide as a maximum. How is mass conserved here?

Moreover, this combustion reaction, has a certain latent heat, an enthalpy value: when strong $C - O$ or $C = O$ bonds are formed, energy is released to the environment. So I can modify the given equation to reflect this energy transfer:

$C \left(s\right) + {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) \uparrow + \Delta$

Where $\Delta$ represents the energy released; this is certainly quantifiable, and the energy released depends on the mass of carbon dioxide formed.

And likewise for a change of state:

${H}_{2} O \left(g\right) \rightarrow {H}_{2} O \left(l\right) + \Delta$

When steam, ${H}_{2} O \left(g\right)$ condenses, a certain amount (and certainly quantifiable!) of energy is released. This is the same amount of energy used to vaporize the water to form an equivalent quantity of steam:

${H}_{2} O \left(l\right) + \Delta \rightarrow {H}_{2} O \left(s\right)$

When we tabulate these values of $\text{latent heat}$, of $\text{enthalpy}$, typically we represent energy as a $\text{reactant}$ as a positive quantity. Accordingly, energy released by the system, is treated as a negative quantity. All KNOWN chemical reactions conform to the laws of $\text{(i) conservation of energy}$, and $\text{(ii) conservation of mass}$.

See here for more of the same with respect to conservation of mass.