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melvados · Oct 1, 2009, 11:08 PM

Originally Posted by Unknown008

Wait, I hope you have seen that zinc does not react with sodium carbonate solution to give sodium and zinc carbonate!

But isn't that the (actual) chemical equation for it?
Zinc + sodium carbonate will gives sodium + zinc carbonate?

By the way, the surface of the zinc looks dull/dark, is it due to the zinc carbonate?

Unknown008 · Oct 2, 2009, 09:53 AM

That reaction is not possible! Perito already told you that. What really happens is that your zinc will react with the water and carbonic acid in the sodium carbonate solution.

You cannot have a less reactive metal displacing a more reactive one.

If that was the case, the sodium would immediately react with water in the solution, and forming sodium hydroxide.

Perito · Oct 2, 2009, 12:21 PM

1. Since you say that the pits show the locations of the zinc corroding/corroded, do you mean to say it's the metal itself?
(Its not the oxide layer or anything right?)

2. Does the occurrence of the pits happens in one direction or (there are straight/lamellar? Lines in the structure), or do they simply expand itself from the initial pits' location

1. The dark pits are probably finely-divided metal shavings coated with hydroxide/oxide and metal hydroxide. In aqueous solution, it's unlikely that you'll form much oxide. Most of it will be zinc hydroxide. Finely-divided metal appears black. Pure zinc oxide is white.

2. Pits can be oriented in one direction, or they can be perfectly round. Corrosion is more prominent in metal that is under stress and when metal is "worked", it will often stress in one direction. Corrosion pits will then be elongated. In annealed zinc, it would be more likely to see fairly round pits.

Pitting and corrosion can also be a sign of impurities in the metal. Metallurgists spend a lot of time examining failed pieces to determine what caused the failure. Impurities ar often found as causes of failure. Impurities can weaken metal and they can accelerate corrosion.

melvados · Oct 2, 2009, 10:52 PM

Okay. Let me summarize to see if I am correct so far. Say zinc + sodium carbonate reaction, is it okay if I write out the following reactions as of below:

1. Na2CO3 + H2O --> HCO3 + OH + 2Na (Not very sure about this)
(Reaction of sodium carbonate with water I presume; As I am using sodium carbonate solution - 'implies there is presence of water'? forms hydrates?)

2. Zn + 2H2O --> Zn(OH)2 + H2
(Since there is water in the reagents, another chemical equation can be occurred, thus forming zinc hyroxide)

3. H2O + CO2 --> H2CO3
(water in the solution reacts with the carbon dioxide in the air, forms carbonic aci

4. H2CO3 + Zn --> ZnCO3 + H2
(Formation of carbonic acid reacts with zinc, forms zinc carbonate and hydrogen gas)
* Do I need to write out another equation about carbonic acid reacting with the zinc oxide if I assume there's an oxide layer?

Then supposedly, if the equation I wrote in step 1 is correct, will it be okay if I write this equation:
ZnCO3 + 2Na --> Zn + Na2CO3 ?
(To show that it will be more realistic, rather than I wrote Zn + Na2CO3 --> ZnCO3 + 2Na, as there is no way that zinc will produce sodium)

Perito · Oct 3, 2009, 08:36 AM

Originally Posted by melvados

Okay. Let me summarize to see if I am correct so far. Say zinc + sodium carbonate reaction, is it okay if i write out the following reactions as of below:

1. Na2CO3 + H2O --> HCO3 + OH + 2Na (Not very sure about this)
(Reaction of sodium carbonate with water I presume; As I am using sodium carbonate solution - 'implies there is presence of water'?, forms hydrates?)

2. Zn + 2H2O --> Zn(OH)2 + H2
(Since there is water in the reagents, another chemical equation can be occurred, thus forming zinc hyroxide)

3. H2O + CO2 --> H2CO3
(water in the solution reacts with the carbon dioxide in the air, forms carbonic aci

4. H2CO3 + Zn --> ZnCO3 + H2
(Formation of carbonic acid reacts with zinc, forms zinc carbonate and hydrogen gas)
* Do I need to write out another equation about carbonic acid reacting with the zinc oxide if I assume there's an oxide layer?

Then supposedly, if the equation i wrote in step 1 is correct, will it be okay if i write this equation:
ZnCO3 + 2Na --> Zn + Na2CO3 ?
(To show that it will be more realistic, rather than I wrote Zn + Na2CO3 --> ZnCO3 + 2Na, as there is no way that zinc will produce sodium)

1. There are many separate equilibria involved. The pH of the solution and the concentration of the other solutes will determine exactly which species exist.

$Na_2CO_3 + H_2O \rightleftharpoons Na^+ + NaCO_3^- \rightleftharpoons 2Na^+ + CO_3^{-2}$

$CO_3^{-2} + H_2O \rightleftharpoons HCO_3^- + OH^-$

$HCO_3^- + H_2O \rightleftharpoons H_2CO_3 + OH^-$

$H_2CO_3 \rightleftharpoons H_2O + CO_2$

2. This equation will be driven to the right because the hydrogen that is formed will bubble out of the mixture.

$Zn + 2H_2O \rightarrow Zn(OH)_2 + H_2(g) \uparrow$

3. This also occurs. The pH of the solution determines how far to the right it will go. If the solution is basic, a lot of CO2 will be absorbed. The hydroxide ion (OH-) will react with H2CO3 to form H2O and HCO3- and CO3(-2). As in #1, above, all of the species will exist, but the exact amounts of them will depend on the solution pH and the amount of solute that is dissolved in the solution.

If you leave absolutely pure water in the air, the pH will drop from 7 to below 6 because CO2 is absorbed from the air.

$H_2O + CO_2 \rightleftharpoons H_2CO_3$

4. Remember that the hydrogen ion (H+) is the actual species that reacts with zinc.

$2H^+ + Zn \rightarrow Zn^{+2} + H_2$

$Zn^{+2} + CO_3^_{-2} \rightleftharpoons ZnCO_3$

The important thing to understand is that in solution, salts will dissociate into their respective ions. Sometimes the ions will react with each other to form insoluble species (that will drive the reaction; possibly to completion), and some species will leave the solution (that may also drive the reaction to completion).

Gases won't always leave the solution. There is always a finite solubility of a gas in a solution. Oxygen, for example, dissolves in water. If this weren't true, fish would die.

melvados · Oct 3, 2009, 08:13 PM

Originally Posted by Perito

1. There are many separate equilibria involved. The pH of the solution and the concentration of the other solutes will determine exactly which species exist.

$Na_2CO_3 + H_2O \rightleftharpoons Na^+ + NaCO_3^- \rightleftharpoons 2Na^+ + CO_3^{-2}$

Thanks Perito, but mind explaining to me again for the above equation in the quote? Not quite able to understand it. Sorry for the inconvenience caused.

By the way, CO3 is carbonate right, just to confirm? And what is HCO3?

Unknown008 · Oct 4, 2009, 06:58 AM

For the equation you quoted, the same thing applies. You have all these simultaneously. $Na_2CO_3$ dissociating into $Na^+$ and $NaCO_3\,^-$ , and the $NaCO_3\,^-$ dissociating into $CO_3\,^{2-}$ and $Na^+$ .

Yes, $CO_3\,^{2-}$ is carbonate.

$HCO_3\,^-$ is hydrogen carbonate, an ion between $H_2CO_3$ and $CO_3\,^{2-}$ if you want.

It's the same as for the sulfuric acid, hydrogen sulfate and sulfate ion: $H_2SO_4,\, HSO_{4}\,^-,\, SO_{4}\, ^{2-}$

The carbonic acid dissociates into the $H^+$ , $HCO_3\,^-$ and $CO_{3}\, ^2-$

melvados · Oct 11, 2009, 08:52 PM

Thanks guys. Sorry fot the late reply. Finally I have understand how the equation works.

By the way just like to know something.
Metals reacts well with cold acid/alkaline or with hot acid/alkaline? If so why?

melvados · Oct 11, 2009, 08:53 PM

Originally Posted by Perito

1. The dark pits are probably finely-divided metal shavings coated with hydroxide/oxide and metal hydroxide. In aqueous solution, it's unlikely that you'll form much oxide. Most of it will be zinc hydroxide. Finely-divided metal appears black. Pure zinc oxide is white.

2. Pits can be oriented in one direction, or they can be perfectly round. Corrosion is more prominent in metal that is under stress and when metal is "worked", it will often stress in one direction. Corrosion pits will then be elongated. In annealed zinc, it would be more likely to see fairly round pits.

Pitting and corrosion can also be a sign of impurities in the metal. Metallurgists spend a lot of time examining failed pieces to determine what caused the failure. Impurities ar often found as causes of failure. Impurities can weaken metal and they can accelerate corrosion.

Then Perito, for the darks pits you have said in the first paragraph, can I say that they are actually the signs of hyroxides/oxides, then other the rest of the structure seen will actually be the metal itself (w/o the darks pits color) as shown in the following picture?