I have read the test for chloride ions (chromyl chloride test). The reaction is: $$\ce{K2Cr2O7 + 4NaCl + 6H2SO4 -> 2CrO2Cl2 + 2KHSO4 + 4NaHSO4 +3H2O}$$ My doubt here is if chlorine gas is evolved in a side reaction. Is it evolved in detectable amount? The side reaction is: $$\ce{6Cl- + (Cr2O7)^{2-} + 14 H+ → 3Cl2 + 2Cr^{3+} + 7H2O}$$ Can someone please confirm this?
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asked Oct 24, 2016 at 5:53
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1$\begingroup$ Yes, in a very good old book about preparative chemistry it is stated that chlorine is a side product in the synthesis of chromyl chloride. $\endgroup$– vapidCommented Oct 24, 2016 at 13:28
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$\begingroup$ Can you give the reference of the book? I am interested to check it out, assuming it's still in print. $\endgroup$– Black Jack 21Commented Oct 24, 2016 at 16:37
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$\begingroup$ Sure, I can give you the reference, but it is in Polish... $\endgroup$– vapidCommented Oct 25, 2016 at 7:06
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1$\begingroup$ In short, potassium dichromate (VI) is not strong enough to oxidse chloride ions. However, if potassium permanganate (VII) is used, the chloride ions will be oxidse because potassium permanganate (VII) is strong enough to oxidse it. For more explanation, I write my explanation in the answer section below. $\endgroup$– Simon-Nail-ItCommented Oct 25, 2016 at 11:38
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$\begingroup$ Chlorine gas will be evolved if there is water in the reaction mixture. In completely dry condition, the amount of chlorine evolved will probably be very small. $\endgroup$– S R MaitiCommented Jan 28, 2018 at 9:09
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1 Answer
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If you write the ionic equation, it seems that chlorine gas will produce. However, we see their half equation and standard potential.
$$\ce{ Cr2O7^2- + 14H+ + 6e- -> 2Cr^3+ + 7H2O}$$ Standard potential: $E^\circ = \pu{+1.33V}$
$$\ce{ 6Cl- -> 3Cl2 + 6e-}$$ Standard potential: $E^\circ = \pu{-1.36V}$
Thus, the standard potential cell for this ionic you say is equal to $$\pu{+1.33V} + (\pu{-1.36V}) = \pu{-0.03V}$$
Because the standard potential cell is negative, so, it is call not energetically feasible or this reaction will not happen.
P.S.: The explanation only work under standard conditions.
Mathew Mahindaratne
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answered Oct 25, 2016 at 11:34
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1$\begingroup$ As you noted the difference in potentials is very small, therefore some of the chloride can get oxidized as practice shows (some sources give 1.36 V value for both chlorine and Cr(VI) reduction). $\endgroup$– vapidCommented Oct 25, 2016 at 14:46
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$\begingroup$ This is not correct. Potassium dichromate does oxidise chloride ion in solution in presence of acid. Unless someone specifically measures the reactants, they are not in standard condition( 1 molar) so, the general method of adding potentials does not work. $\endgroup$ Commented Jan 28, 2018 at 9:07