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 + H2
2NaBr + 3H2
O + SO2
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
= +1.36V and for the reduction of Br2
= +1.07V. Both of these values are more positive than the electrode potential for the reduction of SO42-
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
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!!!