[solomon_pup]

and energy is either lost or gained in a chemical reaction right?

Energy kind of confuses me.

[Donaldson Tan]

The sum of all energy (chemical, electromagnetic, kinetic, etc) in the universe is constant.

The energy which you observe is either lost or gained in a chemical reaction is chemical energy.

Let's consider an exothermic reaction. When it is lost, it is released to the surroundings as heat/light/sound usually. Heat, light and sound are alternate forms of energy. This is just conversion of energy from one form to another. Although chemical energy is lost, it is converted to another form(s), which has same the measure of energy. Hence, the sum of energy remain constant.

We only consider conservation of mass and energy in nuclear reactions, where mass and energy are inter-changeable. However, this interchangability does not apply to chemical or physical processes.

[xiankai]

Energy kind of confuses me.

i would be surprised if it doesnt. energy is kind of an vague concept, because it has many forms, all of which are not directly perceptible. it is a kind of book-keeping measure that we made up in order to keep track of changes. do remember that energy does not really exist in the 'true' sense.

However, this interchangebility does not apply to chemical or physical processes.

indeed, for it takes place in amounts that are not appreciable.

[solomon_pup]

what I don't understand is the conept of energy being destroyed.  I have read before that people think energy can be destryed, I don't see why it can't.  That's why the question threw me off.  What's the proof that energy remains constant, whose to say it can't destroyed or even created at that matter?

Energy confuses me a bit but it's intrigued me for a long time

[P-man]

I don't know about destroyed. Maybe absorbed in such a way as it is not active any more, but someone told me it can't be destroyed, just altered.

[tamim83]

Energy can be transformed, but not created or destroyed.  Consider a car accident.  The kinetic energy of the car gets transfered into heat, sound, light, and many other types of energy. 

[Mitch]

what I don't understand is the conept of energy being destroyed. I have read before that people think energy can be destryed, I don't see why it can't. That's why the question threw me off. What's the proof that energy remains constant, whose to say it can't destroyed or even created at that matter?

Please provide one conceptual thought problem you can give where energy is not conserved in the universe.

[P-man]

Well, the universe itself stores energy, does it not?

[Mitch]

Store energy is different than conserve energy.

[Borek]

Store energy is different than conserve energy.

But it is possible thanks to the energy conservation

[P-man]

Yes, I see. So through conserving energy, the universe stores energy. Therefore there must be a way to extract that energy and use it, right?

[constant thinker]

Hello nuclear reactor. 

[FeLiXe]

we don't know that energy always stays the same. but nothing different has ever been observed. so we assume it always stays the same make up the first law of thermodynamics.

My physics teacher always said you have to save entropy not energy because entropy is what changes. even though the energy is not lost it's unusable because of high entropy. that's why you can't just use energy out of space.

[english]

For our purposes as students, just think of it in relation to mass conservation.  Mass cannot be created or destroyed, it just changes from one form to another through reactions right?

Well the same concept applies to energy.  Energy just cannot be seen.  It's a state of matter.

[Borek]

Mass cannot be created or destroyed, it just changes from one form to another through reactions right?

Right, although it requires small comment...

It's a state of matter.

E=mc²

so in the exothermical reaction mass of products is slightly smaller then the mass of reactants - mass difference was carried out by energy.

For all practical purposes that means that mass is conserved. Period.

But to be absolutely correct instead of talking about mass conservation and energy conservation we should talk about conservation of them both together.