Chemical Reactions

[psicicle]

In chemical reactions, the mass of the reactants and the product are the same right? But in fire, energy is released, so if the mass was still exactly the same after you burned something, then you would have gotten some energy free! I don't get this I learned this in science class, grade 8 so do you think that they just didn't bother explaining that?

[Skye]

The mass is so small that it can be ignored in chemistry.

 

You don't get any energy for free though, because it takes energy to form a molecule (called the heat of formation), and this what is released as when you have a fire (heat of combustion).

[raivo]

You lose a bit of mass in every chemical reaction where energy is released because energy has its mass. One joule has weight of aproximately 0.00000000001 mg. This is so small number that in chemistry you will never need to use it.

[The Thing]

Even in nuclear fission and fusion the mass loss isn't significant. But in antimatter vs matter reactions, almost all of the mass are destroyed and converted to energy, which is what makes them release so much energy.

[psicicle]

basicly then, my teacher and textbook didn't consider my class to be ummm advanced enough??? My teacher keeps saying stuff about how the mass is always the same... but anyway, how would the mass loss occur? what would be transformed into energy? Like maybe in this: Zn+CuSo4--->Cu+ZnSo4 would one of the atoms be changed or something?

[jdurg]

No, your teacher is just leaving out things that are of no use to you at this point in time. If your teacher told you every little detail about every subject in chemistry, it would be far too much information at one time and make it VERY difficult to understand the basics. Learning is about building foundations from which you then grow upon. You get taught the basics and then as you progress you start to learn the details. If some of those details won't do anything but cause confusion and make it difficult to learn those basics, then they should be left out.

 

When talking about chemical reactions, the mass does stay the same. Have you been introduced to significant figures yet? If you have, then I can just say that the mass loss is so infintesimally small that you would not be able to report that change in mass because of the limitations in significant figures. A good analogy would be if the entire population of the Earth moved to the moon, but one person died in the process. In reality, the population wouldn't be the same if comparing the Earth and Moon, but in terms of the overall change it's only about 0.00000004% (Assuming only 2.5 billion people on Earth). The change is so small that it cannot possibly be measured with any amount of certainty.

 

Because of the limitations in the accuracy with which you can report your results, the mass change does not affect your results at all. So when you're doing calculations and balancing things out, the mass change is best ignored rather than make your calculations far more complex even though the final answer does not change.

 

How does the mass change occur? Well, the particles in an atom are held together by various forces. As these forces rearrange when anything happens to the atom, it causes the particles to move and rearrange themselves. In order to get into the most energetically favorable state, small amounts of mass are converted into energy. This small amount of mass is so fantastically small that it cannot really be measured with any certainty unless you have ungodly amounts of the stuff. If a reaction is exothermic, it means that some mass is converted into energy and given off as heat. If a reaction is endothermic, then the reaction results in some energy being converted into mass. Again, the amount is so small that it cannot be measured with any certainty unless you are in a very well funded research lab. For any high school or university level chemistry class, you cannot observe this mass change.

[The Thing]

And think, E=mc^2. If the mass difference is detectable by, say, a centigram scale, then the energy release would be so powerful you wouldn't exist to write down the data. Nearly total annihlation of matter happens only in antimatter and matter reactions. Yay, hopefully thats the reaction for future rockets. But, how will they contain the antimatter? Hmm...