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Author Topic: How do acids and bases work?  (Read 8501 times)

Offline Tronix

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How do acids and bases work?
« on: 03/05/2005 16:12:30 »
These question has been bugging me for a while. How do acids and bases work? How does an acid corrode things like the cement of a metal tool, but wont melt plastic? Does a base do the same thing? How do they react to eachother? thanks for your expertise in advance

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"If i cannot have company whose minds are clearly free, I would prefer to go alone."                  -Dr. Gideon Lincecum

The BPRD rejected my application becuase their brain-controled by Cthulhu Rip-offs. And im sure "Sparky" is sleeping with them too, kinky little firecracker she is...
« Last Edit: 25/05/2016 13:40:50 by chris »


 

Offline rosy

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Re: Acid! (the legal kind)
« Reply #1 on: 04/05/2005 20:22:09 »
I'm not sure I can do a first-principles explanation of acids and bases in text, but here goes...

OK, going with the conventional (Bronstead) definition of acids/bases, and looking at systems where the only solvent is water...
Basically an acid is anything which can donate protons, and a base anything which can accept them (protons are hydrogen nuclei which have lost their one electron, they can be held by a molecule by interacting with a pair of electrons not otherwise involved in holding the molecule together).

In water (H2O), there is an equilibrium by which some of the H2O molecules react to give H3O+ (an acid, known as a hydronium ion) and OH- (a base, known as a hydoxide ion).
The hydronium and hydroxide are in equilibrium so that if you multiply the concentration of OH- in water by that of H3O+ you always get the same number (for a given temperature).
This means that if we have some other acid which wants to put H+ into the system (such as sulphuric acid H2SO4) it donates H+ to water to give more H3O+ and the concentration of OH- goes down.
Conversely, if a base is added (either simply hydroxide in the form of sodium hydroxide or another base such as ammonia, which takes protons from the water to leave extra OH-) we'll see the concentration of H3O+ go down.
You've probably come across the idea of pH to measure acidity... it's a (logarithmic) scale of H3O+ concentration.

Right, so now why do acids react with some materials and not others?
quote:
How does an acid corrode things like the cement of a metal tool, but wont melt plastic?

I'm going to have to leave the cement to one side as I don't know what you're talking about and so can't guess at what it's made of.
Acids corrode metals because the extra protons on the H3O+ take electrons from the metal (they tend "not to be held very strongly" into the metal system) and turns itself into hydrogen gas (H2) which has two electrons and two protons. This pulls some of the metal into the solution as metal ions (the classic example being magnesium:
Mg(s) + (2H3O+)(aq) --> (Mg2+)(aq) + 2H20(l) + H2(g)
They also corrode carbonate minerals such as limestone and marble (made of calcium carbonate) by donating a proton to the (basic) carbonate ion (CO3 2-) turning it into soluble hydrogen carbonate.
(H3O+)(aq)+ CaCO3 (s) --> (Ca2+)(aq) + (HCO3-)(aq) + H2O(l)

Stronger (lower pH, higher H3O+) acids will take carbonate right down to carbon dioxide.
2(H3O+)(aq)+ CaCO3 (s) --> (Ca2+)(aq) + CO2(g) + 3H2O(l)
This is the classic GCSE marble chips experiment...

Plastics... well, many plastics just don't have a chemical makeup that allows them to interact with acids and bases. This is certainly true of poly(ethene)- polythene- but I'm not convinced about poly(ester)s... hydrolysis of ester linkages is catalysed by both acids and bases (tho' not simultaneously, they'd neutralise each other first!)

I suspect that's about as clear as mud. If you can make enough sense of it to figure out which bits you don't understand let me know and I'll have another shot!
 

Offline Tronix

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Re: Acid! (the legal kind)
« Reply #2 on: 05/05/2005 15:53:31 »
Thank you for replying, and i have had a bit of self-education in chemistry. i think understood a nice chunk of it. So basically, an acid, like H3O gives one of its protons with no electron assigned to it to the base, like -OH, which in turn takes the proton, becoming H2O.

And acid turns the limestone in cement (i meant concrete, sorry about the confusion) and turns it into hydrogen gas. Plastic molucules are carbon chains, which simply dont have the chemistry to interact with them. Am i right?

So its basically a give and take of protons and their charges, but only from hydrogen atoms, right?

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"If i cannot have company whose minds are clearly free, I would prefer to go alone."                  -Dr. Gideon Lincecum

The BPRD rejected my application becuase their brain-controled by Cthulhu Rip-offs. And im sure "Sparky" is sleeping with them too, kinky little firecracker she is...
 

Offline rosy

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Re: Acid! (the legal kind)
« Reply #3 on: 05/05/2005 16:11:09 »
That's pretty much it.
I should probably have been a bit more clear about the proton donation thing... basically a hydrogen atom (not in a molecule or anything) *is* just a proton and an electron, so donation of a proton is just the same as donating any other single-plus ion, only smaller as there are no inner electron shells.
Acid will really only release hydrogen gas if it reacts with metals... if there's a base there it'll just donate protons to neutralise that base. Reaction with carbonate releases CO2 and the H+ reacts with the extra O from (CO3 2-) to give H2O.
 

Offline Tronix

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Re: Acid! (the legal kind)
« Reply #4 on: 09/05/2005 20:29:27 »
ah that makes alot of sense, since soil acidity has alot to do with soil health, because the acid in the soil breaks down carbon (dead plant matterial) and turns it into water and CO2.

Well thanks for clarifying what acid was rosy. tank you mucho

--------------------------------------------
"If i cannot have company whose minds are clearly free, I would prefer to go alone."                  -Dr. Gideon Lincecum

The BPRD rejected my application becuase their brain-controled by Cthulhu Rip-offs. And im sure "Sparky" is sleeping with them too, kinky little firecracker she is...
 

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Re: Acid! (the legal kind)
« Reply #4 on: 09/05/2005 20:29:27 »

 

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