Oxidation and Combustion

[IceQube]

I combust glucose. I get 2870 kJ of heat per mole of glucose. (Note: 2870 kJ is the actual amount of heat).

Will I get the same amount of heat if I oxidize glucose completely?

[beowulff]

Explain the difference.

[IceQube]

There’s a difference :dubious:?

Edit: Uh … oxidation occurs at a lower temperature than combustion, right?

[beowulff]

@IceQube

There’s a difference :dubious:?

Edit: Uh … oxidation occurs at a lower temperature than combustion, right?

Not necessarily.
Combustion is just rapid oxidation that occurs at temperatures high enough to produce luminescence.

So, I’m not sure what you are asking.

[IceQube]

Here are my questions. I should have worded my OP better.

1. Does the oxidation of glucose occur at a lower temperature than the combustion of glucose. I’m fairly certain the answer is yes but confirmation is needed.

2. Does the oxidation of glucose release less energy than the combustion of glucose?

[Iggy]

It makes no difference what temperature it is carried out at.

6O2 + C6H12O6 —> 6CO2 + 6H2O either way. It releases the same amount of heat, in the net.

[IceQube]

All right. One question down, one more to go. Thank you.

[johnpost]

living things oxidize glucose at lower than combustion temperature.

[IceQube]

@johnpost

living things oxidize glucose at lower than combustion temperature.

That’s what I suspected. Not to be cheeky, but could you provide a cite? I couldn’t find one via Google so that’s why I asked.

[Blake]

@Iggy

It makes no difference what temperature it is carried out at.

6O2 + C6H12O6 —> 6CO2 + 6H2O either way. It releases the same amount of heat, in the net.

Only technically.

Realistically combustion is a long way from ideal. You can prove this yourself by heating a tablespoon of sugar over a cigarette lighter an then igniting it. Combusting glucose sure don’t end up with water and carbon dioxide as the only end products. You end up with a lot of soot, smoke and charred, carmelised crap as well.

So in the real world, the combustion of glucose will produce appreciably less energy than complete oxidation.

[njtt]

@IceQube

Here are my questions. I should have worded my OP better.

1. Does the oxidation of glucose occur at a lower temperature than the combustion of glucose. I’m fairly certain the answer is yes but confirmation is needed.

2. Does the oxidation of glucose release less energy than the combustion of glucose?

Combustion is a form of oxidation, but there are ways of oxidizing glucose that can occur more slowly, and at lower temperatures than combustion. Indeed, oxidation of glucose is happening in pretty much every cell of your body right now.

As Iggy has already told you, if the glucose is completely consumed (turned into CO2 and water) the amount of energy released by slow or fast oxidation is the same. However, this energy need not all go into heat (not straight away, anyway). In your cells, some of it will go into other forms of chemical energy, such as forming ATP, and may then be used in building yet further molecules, such as proteins, or enabling you to contract your muscles, and so move, etc. etc.

[johnpost]

anything that can consume sugar as a fuel will do that. look up aerobic respiration for one type of using glucose.

[njtt]

@Blake

Only technically.

Realistically combustion is a long way from ideal. You can prove this yourself by heating a tablespoon of sugar over a cigarette lighter an then igniting it. Combusting glucose sure don’t end up with water and carbon dioxide as the only end products. You end up with a lot of soot, smoke and charred, carmelised crap as well.

So in the real world, the combustion of glucose will produce appreciably less energy than complete oxidation.

It depends how hot you get it. Sure, if you put a match to some sugar it will not burn completely, because the early stages of the burning do not themselves get the temperature high enough for the early combustion products to be burned themselves . However, if you do get it hot enough it will burn away completely.

[njtt]

@IceQube

That’s what I suspected. Not to be cheeky, but could you provide a cite? I couldn’t find one via Google so that’s why I asked.

Your cite is that you are not as hot as fire.

[Blake]

@njtt

Your cite is that you are not as hot as fire.

That doesn’t prove anything because the oxidation in the body is very dispersed throughout a huge heat sink coupled to a very good cooling mechanism.

By this argument, wood burned in fireplaces oxidise at a lower temperature than wood burned in forests. After all, every house in this town has a wood fire, but the town is not as hot as a burning forest. Ergo the fireplaces oxidize wood at lower than combustion temperature.

I’m not saying that the premise is wrong. I suspect that living things do oxidise glucose at much lower temperatures, but the *mean *temperature of the entire *volume *of a living thing is not evidence of that claim.

[Blake]

@njtt

It depends how hot you get it. Sure, if you put a match to some sugar it will not burn completely, because the early stages of the burning do not themselves get the temperature high enough for the early combustion products to be burned themselves . However, if you do get it hot enough it will burn away completely.

IOW it depends on what the OP means when he talks about “combustion”. Under most real-world circumstances the combustion of glucose will release less heat than total oxidation.

[IceQube]

@njtt

Combustion is a form of oxidation, but there are ways of oxidizing glucose that can occur more slowly, and at lower temperatures than combustion. Indeed, oxidation of glucose is happening in pretty much every cell of your body right now.

As Iggy has already told you, if the glucose is completely consumed (turned into CO2 and water) the amount of energy released by slow or fast oxidation is the same. However, this energy need not all go into heat (not straight away, anyway). In your cells, some of it will go into other forms of chemical energy, such as forming ATP, and may then be used in building yet further molecules, such as proteins, or enabling you to contract your muscles, and so move, etc. etc.

So combustion is fast oxidation?

[Blake]

@IceQube

So combustion is fast oxidation?

For your purposes, yes.

[IceQube]

So I conclude that

1. Oxidation releases the same amount of energy as does combustion. If the two processes are carried out perfectly (completely), that is.

2. Oxidation occurs at lower temperatures than does combustion.

[beowulff]

@IceQube

So I conclude that

1. Oxidation releases the same amount of energy as does combustion. If the two processes are carried out perfectly (completely), that is.

2. Oxidation occurs at lower temperatures than does combustion.

I still don’t agree with #2.

It’s all a matter of definition. Oxidation is just slow combustion. Each time a glucose molecule is completely combined with Oxygen, the same amount of energy is released. What you call combustion is just this process happening rapidly. So naturally, the temperatures are higher. But, burning is still just oxidation.