Wikipedia:Reference desk/Archives/Science/2014 June 15

Science desk
< June 14	<< May | June | Jul >>	June 16 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

June 15

Are Chemical Reactions originally derived from solar energy?

This question came about from my “Horses on Mars” query when one poster opined that horses get their energy from the "solar panels" of the grass they eat, and thus the energy they expend is solar energy converted to kinetic energy. I thought about this, and wondered about the nature of chemical reactions. Take this one: Put some potassium in water. Boom!! Now, is this solar energy? No, it is not. Well, where did all that energy come from?

Perhaps chemical energy of this kind is essentially nuclear energy. And in that case, perhaps it is wrong to say that all our energy, as physical organisms, is derived from solar energy. If we could take in potassium and water in a controlled way, we could be supermen, and the energy we would need would be only that miniscule amount we needed to open a new jar of potassium in the morning and swish it down with a glass of Adam's Ale. Myles325a (talk) 04:01, 15 June 2014 (UTC)

I'm not sure I follow where you are going? It sounds like you don't really have a good fundamental understanding of what energy is... Energy is simply a way to quantify change. If we have a system in two different states of being, energy is just a measure of how different those two states are from each other. Any of the ways we, as humans, classify energy (potential vs. kinetic... work vs. heat... enthalpy vs. entropy...) are just human created classifications which help us to understand how energy works. How you divide energy into different "forms" is only as important as it is useful. If you start with the "it's all solar energy", you sort of right to a point, except some small fraction is actually energy stored inside the earth from its own formation, some kinds of extremophile life is able to tap into this energy, as do humans when we use Geothermal energy. So if I take a potassium salt, and electrolyze potassium from it using energy from a geothermal power plant, I haven't really used any of the sun's energy. If you really want to harness solar energy for yourself, eat some spinach. Because the energy stored in the chemical bonds in the spinach came from the sun. Just remember, however, as you keep trying to work backwards, that since the sun and earth have not existed since forever, its energy had to come from somewhere first; the only antecedent for all energy is the Big Bang. --Jayron32 05:00, 15 June 2014 (UTC)

Put another way: where did you get your potassium? The OP incorrectly dismissed his reaction as one that doesn't derive any energy from the sun - because there is no apparent photochemistry going on - but he forgot to account for the energy in the atoms of the reactants that did derive from the sun many billions of years ago.

If we trace the history farther: where did the sun get its energy? Well, that's an easy, well-formed question with a straightforward answer, based on our scientific knowledge about stellar formation. Our sun got its energy for nuclear fusion from the gravitational collapse of its hydrogen gas.

Where did that hydrogen get its energy... - well, the cloud of gas had gravitational potential energy when it was separated; and it got that energy from the kinetic energy it had when it was formed. That kinetic energy came from an earlier time, when the gas was in a different configuration, and its constituent particles were sub-atomic... And if we keep tracing backwards toward the proverbial "dawn of time," we eventually reach a point where the question must be posed in metaphysical terms.

As scientists, we don't think in terms of "first cause." If anything, we're really only concerned with material cause and effect. As Jayron has described, energy is just one of the ways we speak quantitatively about cause and effect. When we are on Earth, talking about terrestrial processes, it is convenient to summarize almost every process as one whose common denominator is the stellar energy emitted by our sun. The original energy didn't "come from" the sun; but there is a clearly defined historical path that lets us trace the time-evolution of almost any terrestrial process back to the sun. Nimur (talk) 05:34, 15 June 2014 (UTC)

Well, if I must quibble with one thing in that excellent answer, I'm not sure energy really deals well with cause and effect. Energy is either a state function or a process function, and even process (or path) functions can be thought of as a bunch of microstates in succession (i.e. a calculus-sorta-thingy). Energy tells how how much something has changed, rather than what caused something to change... --Jayron32 05:49, 15 June 2014 (UTC)

It depends on what chemical reactions we are talking about whether they come from the sun. Potassium doesn't come from the sun. Potassium comes only from supernova nucleosynthesis. The energy that was used to electrolyze the potassium from a potassium salt (since potassium is never found as the native metal on Earth) may have come immediately from any of various sources. Many organic chemical compounds on Earth really are generated by plants in photosynthesis, and that is a use of energy that comes from solar fusion. Also, as Jayron32 says, energy is simply a way to quantify change. Robert McClenon (talk) 17:45, 15 June 2014 (UTC)

With so many competent contributors here I am not sure my ignorance will be helpful. I do believe though that the OP's question is somewhat misunderstood. He is describing this exothermal, extremely volatile reaction: 2K + 2H2O -> 2KOH + H2. What went into this reaction: (1) X-ray energy during supernova explosion, as one person said, that helped to combine protons and electrons to make potassium along with other elements. At this stage the OP is correct: it is all nuclear fusion. (2) during the eons of that particular potassium travel in space and inside the planets the energy could have been subtracted or added when potassium reacted with some anions to make salts (3) Some chemist extracted pure potassium investing some energy in it. (4) the final reaction of potassium with water came (hydrolysis). --AboutFace 22 (talk) 21:24, 15 June 2014 (UTC)

First, the nuclear fusion that produced the potassium was not in the sun, but in a star that exploded 4.7 billion years ago, the shell of which passed through the protostellar nebula, possibly causing planet and star formation. The sun has never produced any potassium and never will. Red giant nucleosynthesis doesn't produce elements as heavy as potassium. Second, the potassium probably reacted with anions to form a salt in the primitive solar system, not in space. FThird, the energy used to electrolyze the potassium was of solar origin; it probably came from fossil fuel, which is preserved solar energy. Fourth, while the reaction that AboutFace lists is the first reaction, but not the only one in that sequence, because the hydrogen then catches fire, and the remaining potassium often also catches fire. Robert McClenon (talk) 23:19, 15 June 2014 (UTC)

It might be helpful to break down energy sources to their origin:

SOLAR <- sunlight <- nuclear fusion
WIND <- uneven heating of the atmosphere <- sunlight <- nuclear fusion
WAVE <- wind <- uneven heating of the atmosphere <- sunlight <- nuclear fusion
HYDRO <- precipitation <- evaporation + wind <- sunlight <- nuclear fusion
BIOFUELS <- photosynthesis <- sunlight <- nuclear fusion
FOSSIL FUELS <- organic material <- photosynthesis <- sunlight <- nuclear fusion

Here are a couple that aren't from the nuclear fusion of the Sun:

TIDAL <- rotation of Moon around Earth <- uneven distribution of matter in the universe <- Big Bang
NUCLEAR REACTORS <- nuclear fission <- heavy elements created in supernovae

Geothermal energy actually has multiple sources:

GEOTHERMAL <- tidal heating <- rotation of Moon around Earth <- uneven distribution of matter in the universe <- Big Bang
           <- nuclear decay <- nuclear fission <- heavy elements created in supernovae
           <- residual heat from formation of the solar system <- uneven distribution of matter in the universe <- Big Bang

StuRat (talk) 23:56, 15 June 2014 (UTC)

Useful. I will note that the second category of geothermal energy has to do with the decay of uranium and thorium in the Earth. Robert McClenon (talk) 00:08, 16 June 2014 (UTC)

Thanks. Perhaps I should add that supernovae are powered by nuclear fusion, so nuclear fission, either in a nuclear reactor or in the Earth, can also be said to be powered by nuclear fusion, although not in the Sun. So, that leaves us with just 2 energy sources, fusion and the uneven distribution of matter in the universe due to the Big Bang. Of course, if matter had been evenly distributed, then there would be no stars and hence no fusion. StuRat (talk) 04:54, 16 June 2014 (UTC)