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The troublesome nature of this post, is that I was given very little information to work with:

"The metal chromium (Cr) reacts with an acid, to create Cr^+3 ions, and hydrogen gas. Show the balanced reaction." (paraphrasing)

Addendum:

  • It is highly implied that there is water involved somehow, in a follow-up question, there is mention of water-vapor that "carries the hydrogen gas".

Any ideas? :)

EDIT: I failed to mention that the core problem of this question, is that I am entirely unsure with what I am entirely working with here.

"Cr + H^+" doesn't seem right, surely there's more to it?

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    $\begingroup$ Are you familiar with half-reactions? $\endgroup$
    – TheRelentlessNucleophile
    Commented Apr 30, 2022 at 14:09
  • $\begingroup$ @TheRelentlessNucleophile Yes, I am actually! I just fail to see how is it related? :s $\endgroup$
    – dogman
    Commented Apr 30, 2022 at 14:56
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    $\begingroup$ Well, then you should be able to tell that chromium undergoes oxidation accompanied by the reduction of hydrogen ions to hydrogen gas. The addendum is (probably) wrong, I don't see how any water vapor could emerge. You can write the global equation without referring to half-reactions, but I figured intermediary steps might make everything clearer. How many electrons does an atom of metallic chromium lose to ionize as the trivalent ion? How many electrons do hydrogen ions accept to become diatomic hydrogen gas? What is the least common multiple of the results obtained above? $\endgroup$
    – TheRelentlessNucleophile
    Commented Apr 30, 2022 at 16:56
  • $\begingroup$ @TheRelentlessNucleophile I'll try to be a tad more elaborate, I feel like we're not quite on the same page: I am confused regarding with which materials am I working with here? You noted that the by-product is (probably) wrong, that there's no vapor being produced. However, do I maybe have H2O, or some other type of reagent in the base step? Right now I am working with "Cr + H^+" and I don't feel like that adds up. Thanks! >< $\endgroup$
    – dogman
    Commented Apr 30, 2022 at 18:14
  • $\begingroup$ Can you do the problem of aluminum metal reacting with acid to give trivalent aluminum ions and hydrogen gas? If so, then you are essentially done. $\endgroup$
    – Ed V
    Commented Apr 30, 2022 at 19:12

1 Answer 1

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The half-equations are : $$\ce{Cr -> Cr^{3+} + 3 e-}$$ $$\ce{2 H+ + 2 e- -> H2}$$ So doubling the first and multiplying the second by $3$ and adding the results gives the final equation : $$\ce{2 Cr + 6 H+ -> 2 Cr^{3+} + 3 H2}$$ Water is not taking part to this reaction, except if your teacher wants you to replace the ion $\ce{H+}$ by $\ce{H3O+}$. In this case, the second half-equation becomes : $$\ce{2 H3O+ + 2 e- -> H2 + 2 H2O}$$ And the final equation becomes : $$\ce{2 Cr + 6 H3O+ -> 2 Cr^{3+} + 3 H2 + 6 H2O}$$ But there is nowhere any water vapor, except if you admit hat the hydrogen gas is a little wet and contains here and there some $\ce{H2O}$ molecules.

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  • $\begingroup$ I tried to help the OP by using Al instead of Cr, but they really wanted that water! ;-) $\endgroup$
    – Ed V
    Commented Apr 30, 2022 at 19:50
  • $\begingroup$ Absolute legend, this should be it! =) @EdV yeah, my lecturer kept pointing at it...Heh. I didn't know what I could or couldn't use. $\endgroup$
    – dogman
    Commented May 1, 2022 at 6:30

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