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$$\ce{Cr2O7^2-(aq) + 14H+(aq) + 6Cl-(aq) -> 2Cr^3+(aq) + 3I2(aq) + 7H2O(l)}$$

Two chromium atoms gain 3 electrons each. They go from $0$ to $3+$

Similarily, 6 iodine atoms go from $-1$ to $0$, thus gaining one electron. However, my textbook says that iodine has lost electrons. How?

It went from negative to positive, i.e. from $-1$ to $0$, just like chromium from $0$ to $+3$.

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  • $\begingroup$ 1) Are you sure those chromium charges are correct? 2) If you take a neutral species and add an electron, what is its resulting charge? $\endgroup$ – Nicolau Saker Neto Feb 15 '15 at 2:18
  • $\begingroup$ 1) yes 2) +1?.. $\endgroup$ – neutron123 Feb 15 '15 at 2:35
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    $\begingroup$ 1) Check that $\ce{Cr2O7^{2-}}$ ion again. What are the charges on the chromium and the oxygen atoms? Remember the charges of all atoms in an ion must sum to the ion's net charge. 2) Electrons have negative charge. If you add a negatively charged electron to a neutral species, is the resulting charge positive or negative? $\endgroup$ – Nicolau Saker Neto Feb 15 '15 at 3:02
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The mistake here is understanding what happens to the dichromate ion. Chromium has gone from the +6 to the +3 oxidation state. Its oxidation state has decreased by three therefore it has gained three electrons in the process: $$\ce{Cr2O7^{2-} + 14H+ + 6e- -> 2Cr^{3+} + 7H2O}$$

Iodine has gone from the -1 to the 0 oxidation state. Its oxidation state has increased by one, losing one electron in the process. $$\ce{2I- -> I2 + 2e-}$$

Putting these two half equations together gives you the overall equation which you started with. $$\ce{Cr2O7^{2-} + 14H+ + 6I- -> 2Cr^{3+} + 7H2O + 3I2}$$

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Two points before we begin:

  1. + charge means deficiency in electrons.
  2. - charge means excess of electrons.

Now lets come to the reaction you are talking about.

Two chromium atoms gain 3 electrons each. They go from 0 to 3+

Moving from neutral to +3 charge means it just lost 3 electrons. (Electrons have negative charge.)

Similarily, 6 iodine atoms go from -1 to 0, thus gaining one electron. However my textbook says that Iodine has lost electrons. How?

Your textbook is right and you're wrong here. Moving from -1 charge to 0 means that an atom just lost an electron (in other words, it lost the excess negative charge it had and now it is neutral).

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