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Topic: Sodium chloride and concentrated sulphuric acid  (Read 59561 times)

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Offline cliverlong

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Re: Sodium chloride and concentrated sulphuric acid
« Reply #15 on: May 20, 2008, 06:36:38 AM »
If you go deep enough and you use all acid/base definitions, almost each reaction can be classified as acid/base.
Which supports my (not well thought out) opinion, that the Lewis definition of acids and bases is so broad that it doesn't really help classify or categorise or distinguish the interaction of various species.

Why categorise reactions? Because you can then give a general description of a type of reaction. e.g. acid as a proton donor reacting with metal carbonates. Then maybe (just maybe) predict what the result of adding chemical A to chemical B might produce. Rash thoughts, I realise.

Also reaction mechanisms are all the rage in EdExcel A-Level chemistry and this acid/salt reaction was another one to add to the collection.

I have an aversion to learning pages of reactions which may be due to laziness. I recognise that High School / A-Level chemistry probably focuses on elements that exhibit reasonably "regular" behaviour and that "most" chemistry may have to dealt with on a case-by-case basis. I'm just trying to contain the amount of "unconnected" learning I have to do.


Offline ptryon

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Re: Sodium chloride and concentrated sulphuric acid
« Reply #16 on: January 06, 2012, 01:25:02 AM »
Classifying the reactions is not an explanation of why the reaction occurs. I agree with Clive that classification is a tool to help us predict what reactions will occur. In the case of these reactions I would argue prediction is far more difficult than rote memorization. Read on...

A question about the reactions of concentrated sulphuric acid with sodium halide salts cropped up in one of the CIE A2 exams. This question expected students to use the electrochemical series to predict what would happen in each reaction.

According to the mark scheme the reactions are as follows:

Eqtn1: NaCl + H2SO4  :rarrow: NaHSO4 + HCl
Eqtn2: 2NaBr + 3H2SO4  :rarrow: 2NaHSO4 + 2H2O + SO2 + Br2 

Eqtn1 is not redox, but eqtn2 is because Br- (-1) ions are oxidized to Br2(g) (0) and sulphate ion (+6) are reduced to sulphur dioxide (+4)

Students are supposed to use the electrochemical series to work this out, however it doesnt work. The standard electrode potential for the reduction of Cl2 to Cl- = +1.36V and for the reduction of Br2 to Br- = +1.07V. Both of these values are more positive than the electrode potential for the reduction of SO42- to SO2 which = +0.17V. The electrode potentials would lead students to predict that neither Br- ions or Cl- ions present in the salts would be capable of reducing the sulphate ion! I puzzled over this until I realized that neither of the reactions are under standard conditions. The extremely high concentration of sulphate ions make them more subseptible to reduction- however, it is only the Br- ions that are capable of reducing them since Br2/Br- has the less positve standard electrode potential than Cl2/Cl-.

I don't understand how anyone could predict that the concentration is high enough to make the reduction using bromide ions thermodynamically favourable but not chloride ions. I suspect you just have to memorize the reactions then pretend that you used the electrochemical series!!!  :'(

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