Dusclops
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Hydroxide ions in alkaline solutions - reactants or products?
In many redoxreactions that take place in acidic and alkaline solutions, protons or hydroxide ions are reactants - which makes intuitive sense. In a
question of my textbook however, there is a redoxreaction that takes place in a weak alkaline solution where hydroxide ions are products. Shouldn't
hydroxide ions be reactants if the reaction takes place in a (weak) alkaline solution?
The redoxreaction is between nitrate and methanol, which should give the products nitrogen gas, water and carbon dioxide. However, in order for the
formula to be balanced, hydroxide ions must also be among the products (according to the textbook).
My question is: Is there any way (other than through balancing) to determine whether hydroxide ions or protons are reactants or products in a
reaction?
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hkparker
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a pH test during (or before and after) the reaction should offer some insight.
I tried balancing it the full way by separating out the oxidation and reduction reactions and came to this:
<s>6NO<sub>3</sub><sup>-</sup> + 3CH<sub>3</sub>OH --> 5N<sub>2</sub> +
21H<sub>2</sub>O + 5CO<sub>2</sub> + 6OH<sup>-</sup></s>
I might have very well made some mistakes though so let me know if you arrive at a different conclusion and I will try again.
EDIT:
I am wrong.
[Edited on 1-9-2011 by hkparker]
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"Nothing is too wonderful to be true if it be consistent with the laws of nature." -Michael Faraday
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Magpie
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6NO3- + 5CH3OH ---> 3N2 + 5CO2 +7H2O + 6OH-
This is a redox equation as the nitrogen is reduced from 5 to 0 and the carbon is oxidized by 6.
The tricky part is determining the change in oxidation state of the carbon. Reducing the number of hydrogens on the carbon increases the state by one
for each hydrogen. Then by adding 3 C-O bonds the increase due to oxygen is 3. So 3+3=6. This is taught in 1st qtr (semester) organic IIRC.
Once you know this use the standard method for doing redox equations.
---------------------------------------------
This does not , however, answer the question of the OP. I'll try: Apparently the author of the question in your text has assumed that conditions are
such (high enough temperature) that full oxidation of the methanol occurs by the oxidant NO3-. In this case then the above products result. It
doesn't have anything to do with the presence or lack thereof of protons and/or hydroxide ions.
[Edited on 1-9-2011 by Magpie]
[Edited on 1-9-2011 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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hkparker
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Yes, I did not have fun trying to find that out, I think its where I wen't wrong. Thank you Magpie!
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"Nothing is too wonderful to be true if it be consistent with the laws of nature." -Michael Faraday
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kmno4
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You cannot have both CO2 and OH ions as products, it is (formally) impossible.
It is because CO2 and OH react further ( -> HCO3 or CO3 ions).
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Dusclops
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Magpie: Thanks.
kmno4: How should the reactionformula be written in that case? And what criteria are there for formal reactions?
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