Chemical bonds

[Anilkumar]

Hello,

 

Let me begin with this;

 

If I want to cool some object, say a slab of glass, I will have to extract heat/energy from it, right? Now suppose I cool it by say, some 10 degrees, then where does the heat, I just extracted from the glass slab, come from. Where was it hidden uptil now?

 

Thank you?

Edited May 22, 2011 by Anilkumar

[hypervalent_iodine]

I'm not sure I understand your question. Heat has a few definitions, but in science it's simply a form of energy which may spontaneously flow from one system to another. In your case, from the glass to the surroundings. Could you be more specific with what you're after here? I'm almost sure I haven't answered your question the way you intended it.

[Anilkumar]

I'm not sure I understand your question. Heat has a few definitions, but in science it's simply a form of energy which may spontaneously flow from one system to another. In your case, from the glass to the surroundings. Could you be more specific with what you're after here? I'm almost sure I haven't answered your question the way you intended it.

 

 

Hi,

 

Thank you hypervalent_iodine, for attending.

 

Let me make it clear with the help of an illustration.

 

Suppose I keep the glass slab in a refrigerator for cooling. After a while the temperature of the slab is lesser, because the refrigerator sucked some of the heat from its body. My question is; where [from which quarters of the slab's body] did that heat, which the refrigerator sucked out, come from. Where was that heat residing till now.

 

Thank you.

[hypervalent_iodine]

You need to look at it from a different angle. What you are describing is a process of heat transfer. The refrigerator isn't sucking anything out at all. The heat is being transferred from the glass, which is of a higher temperature, to the surroundings (the refrigerator), which is of a lower temperature. Simply put, the two systems are trying to reach thermal equilibrium.

 

So to answer your question is no different to how I answered it previously. The heat comes from the glass.

 

Did you want to know anything more than that?

[mississippichem]

My question is; where [from which quarters of the slab's body] did that heat, which the refrigerator sucked out, come from. Where was that heat residing till now.

 

When the glass cools, the vibrations in the [ce] SiO_{2} [/ce] molecules become less intense or slower. Temperature is an expression of the average kinetic energy per molecule or atom. So as the glass cools, and the molecular vibrations slow, you will observe a drop in temperature.

 

So in short, the energy comes from the molecular vibrations.

[Anilkumar]

Thank you hypervalent_iodine,

 

thanks mississippichem,

 

 

You need to look at it from a different angle. What you are describing is a process of heat transfer. The refrigerator isn't sucking anything out at all. The heat is being transferred from the glass, which is of a higher temperature, to the surroundings (the refrigerator), which is of a lower temperature. Simply put, the two systems are trying to reach thermal equilibrium.

 

So to answer your question is no different to how I answered it previously. The heat comes from the glass.

 

Did you want to know anything more than that?

 

I said sucks, because the Refrigerator works to transfer the heat from the slab to the Refrigerator.

 

When the glass cools, the vibrations in the [ce] SiO_{2} [/ce] molecules become less intense or slower. Temperature is an expression of the average kinetic energy per molecule or atom. So as the glass cools, and the molecular vibrations slow, you will observe a drop in temperature.

 

So in short, the energy comes from the molecular vibrations.

 

OK got it,

 

now let us consider an exothermic reaction like combustion of Hydrogen

 

2H₂ + O₂ → 2H₂O

 

Here too, heat is given out. Where did this heat come from?

 

And in an Endothermic reaction like,

 

6CO2+ 6H2O+ energy --->C6H12O6+ 6O2

 

Here, heat is absorbed. Where does that heat fit in the molecule of C6H12O6.

 

Thanks guys.

 

 

 

[hypervalent_iodine]

You should be considering heat as more of a type of energy, rather than simply temperature. The point of exo and endo thermic reactions is that the former releases energy in the form of heat and the latter requires energy to form.

 

It may be useful for you to read this:

 

http://en.wikipedia.org/wiki/Enthalpy

 

I would write a more detailed reply, but I'm afraid my procrastination has to come to an end. I'm sure mississippi can help you with any further questions.

[lemur]

I said sucks, because the Refrigerator works to transfer the heat from the slab to the Refrigerator.

But there's no "sucking." The higher energy/vibration particles are colliding with cooler air particles surrounding the material, transferring motion to those particles, which sends them off as convection to the rest of the refrigerator. The glass is also radiating black body emissions according to its temperature. The only sense in which the cold surroundings are "sucking heat" is that a temperature differential is required for black-body emissions to be absorbed; i.e. a warmer material cannot absorb emissions from a cooler material, or rather it can actually, I think, but the warmer material will just emit more toward the cooler material than vice-versa.

[mississippichem]

OK got it,

 

now let us consider an exothermic reaction like combustion of Hydrogen

 

2H₂ + O₂ → 2H₂O

 

Here too, heat is given out. Where did this heat come from?

 

Alright. So molecules that have stability are stable because the electrons in that bonding configuration are lower in energy than the constituent parts would be. That's why chemical reactions happen at all. If it's not favored by entropy or enthalpy or both, it tends to not happen without catalysis. So the energy coming from your oxygen combustion comes from the fact that the electron configuration of water is a more stable configuration than two hydrogen radicals and an oxygen atom. That energy "surplus" [not over unity, just the difference in energy] comes off as work, heat or light.

 

And in an Endothermic reaction like,

 

6CO2+ 6H2O+ energy --->C6H12O6+ 6O2

 

Here, heat is absorbed. Where does that heat fit in the molecule of C6H12O6.

 

Thanks guys.

 

Alright, this reaction is not spontaneous because [math] \Delta G^{o} > 0 [/math]. So plants and other photosynthetic autotrophs use chlorophyll and photosynthesis [a catalytic process] to carry this out. That energy can come from other exothermic reactions inside a cell. The catalyst just gives the reaction a "kick-start" by lowering the activation energy, the energy "hump" that has to be overcome to get the reaction going.

 

A more simple case is an endothermic reaction happening in a beaker in a lab, life is much easier en vitro. In the lab the heat supplying an endothermic reaction just simply comes from your hot plate or Bunsen burner [people still use those right?]. The hot plate gets it's heat from resistive heating in a wire. The Bunsen burner utilizes the exothermic combustion of propane or butane.

 

Just remember, energy never gets created or destroyed. We just ride the waves of endothermic and exothermic processes. Nature does the dirty work of making hydrocarbons at no expense to us, we supply the initial flash and then harvest the precious Joules that come out.

[Anilkumar]

I have read wiki-ENTHALPY.

 

missisippichem

 

So molecules that have stability are stable because the electrons in that bonding configuration are lower in energy than the constituent parts would be. That's why chemical reactions happen at all.

 

Does it mean that, Molecules tend to reach stability by certain bonding configuration; because they would like to attain a lower energy state? And chemical reactions happen because of that?

 

 

missisippichem

 

If it's not favored by entropy or enthalpy or both, it tends to not happen without catalysis.

 

Does this mean that there would be no chemical reactions if there are no catalysts and favourable entropy/enthalpy?

[Amitash]

Heat from the glass slab might have escaped from anywhere within it but the whole glass slab will always have the same temperature thanks to the uniform distribution of heat. It is impossible for a glass slab in the refrigerator to have different temperatures on different areas on it.

[lemur]

Just remember, energy never gets created or destroyed. We just ride the waves of endothermic and exothermic processes. Nature does the dirty work of making hydrocarbons at no expense to us, we supply the initial flash and then harvest the precious Joules that come out.

I like the way you say this. Hopefully not too much of an aside, but do you know of any non-organic (i.e. non-living) processes that create hydrocarbons? I can't think of any.