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Topic: Balancing Cr with H+  (Read 1143 times)

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Offline dogman

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Balancing Cr with H+
« on: April 30, 2022, 11:37:20 AM »
Hello ya'll! First time here =)
Feel like this is a pretty noob question, but honestly, I am tripping over this for days.
I was given very little information:

"The metal chromium (Cr) reacts with an acid to produce Cr+3 and hydrogen gas. Write the balanced reaction."

Yeah, that's really it. I was also given the tip that the acid is related to H+, and the reaction may produce water-vapor.

Any ideas? :) Someone suggested to me to think about "half-reactions" when considering this topic, but I didn't really manage to get what they meant.

EDIT:
I apologize, I didn't explain my issue here well enough: My problem with the question is that I have no idea with which reagents I am working with. The way the question is phrased, makes it look like the equation is just : "Cr + H+", and that can't be right, and a follow-up question highly suggests there is water involved somehow.
Anyone have any idea what else might be at play here, that was intentionally left out by my lecturer?
« Last Edit: April 30, 2022, 03:24:39 PM by dogman »

Offline Orcio_87

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Re: Balancing Cr with H+
« Reply #1 on: April 30, 2022, 12:25:39 PM »
Quote
"The metal chromium (Cr) reacts with an acid to produce Cr+3 and hydrogen gas. Write the balanced reaction."

Yeah, that's really it. I was also given the tip that the acid is related to H+, and the reaction may produce water-vapor.
Water can by the side-product if Cr2O3 is used instead of the Cr. But here it is clear - Cr and acid products Cr3+ and hydrogen gas.

Offline Corribus

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Re: Balancing Cr with H+
« Reply #2 on: April 30, 2022, 01:22:29 PM »
It's a little vague but for sake of simplicity let's assume the acid is dissociated. In that case, the counterion - call it A- - is inconsequential (a spectator) and you only have to worry about the H+. But, your post indicates you may not yet know what how to describe an acid in reaction formalism so ask for clarification if you find that confusing.

So why not start with the basics? Do you know how to write a(ny) chemical reaction? I.e., what goes on the left and what goes on the right. And do you know how to balance chemical reactions? If the answer is "yes", then you just have to put the reactants and products where they go and make sure all the charges and atoms are equal on both sides.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline dogman

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Re: Balancing Cr with H+
« Reply #3 on: April 30, 2022, 02:20:23 PM »
Hey, cheers spending some time on this :p
I feel like I failed to properly elaborate what my problem with the question was, because I do have the basics down, but the question itself was defined so vaguely, probably on purpose, that I just need some help on clearing out the fog...

And that's to say, with which reagents am I working here? I mean, is my equation truly just:
"Cr + H+"? That can't be right, can it? I'm sure there's more going on, that's not explicitly written. :s It's implied that there is water involved in a follow-up question, but it's not explicit.

EDIT: A friend of mine noted that "water can't be part of the equation, metals and acid produce "salt" and hydrogen gas. The 'water' can only be the source of the acid. I assume you need to add an equation of the water's-breakdown to H3O+ and OH-"
I am translating from another language, so the terms are most likely incorrect, sorry about that. <:
Any thoughts on how this might lead me to the initial equation, to this incredibly vague question?
« Last Edit: April 30, 2022, 03:25:25 PM by dogman »

Offline Orcio_87

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Re: Balancing Cr with H+
« Reply #4 on: April 30, 2022, 03:31:48 PM »
Quote
And that's to say, with which reagents am I working here? I mean, is my equation truly just:
"Cr + H+"? That can't be right, can it? I'm sure there's more going on, that's not explicitly written.
Did you heard about the oxidation-reduction reactions ?

Here H+ is an oxidant and the Cr is the reductant. You don't need anything else.

Offline dogman

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Re: Balancing Cr with H+
« Reply #5 on: April 30, 2022, 04:40:01 PM »
    Quote
    And that's to say, with which reagents am I working here? I mean, is my equation truly just:
    "Cr + H+"? That can't be right, can it? I'm sure there's more going on, that's not explicitly written.
    Did you heard about the oxidation-reduction reactions ?

    Here H+ is an oxidant and the Cr is the reductant. You don't need anything else.

    Hmm...So you claim I truly need absolutely nothing but Cr and H+ to end up with Cr^+3 and hydrogen gas? Am I getting you right?
    I posted this question in another forum, and received this reply, which showcases the use of water? (while I did encourage posters to somehow find a way to add water to solving the problem, I am asserting that I wanted clarification over what is necessary in the process, since it was heavily implied that water is somehow related by follow-up questions...)

    And in an attempt to not be as vague and confusing as the lecturer who came up with this ambiguous question, I will also update the post to feature the entirety of the question, just so some context may be added, perhaps...

    EDIT:
    Well, seems like I can't edit my OP, so:
    The metal chromium (Cr) reacts with an acid to produce Cr+3 and hydrogen gas.


    • Write the balanced reaction.
    • with the reaction of the metal with an excess of acid, 94.7 ml of hydrogen gas is created.The gas is picked up over water with pressure of 754 mmHg and temperature of 20C.
      Calculate the mass of the metal that reacted. Show your calculations. (water vapor in 20C is 24 mmHg)
    • Explain how the volume of the gas will change, if Cr{+2} would be produced (and not Cr{+3}).
    • Calculate the number of present water molecules in the volume of the gas-phase (sorry, kind of confused how to translate this one ><).
    • Calculate the ratio between the avg speed of the hydrogen gas molecules and between the avg speed of the water-vapor molecules that are present in the same temperature.
    [/list]
    « Last Edit: April 30, 2022, 04:53:46 PM by dogman »

    Offline Borek

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    Re: Balancing Cr with H+
    « Reply #6 on: April 30, 2022, 05:33:55 PM »
    Putting aside some details which can make the thing quite confusing, reactions between metals and acids all follow the same scheme (this is just a skeletal reaction, not intended to be balanced):

    Me + H+ + counteranion  :rarrow: Men+ + H2 + counteranion

    where H+ + counteranion is a lousy way of writing "dissociated acid".

    Note that in the counteranion doesn't change, so we can safely ignore it and write so called "net ionic equation":

    Me + H+ :rarrow: Men+ + H2

    (again, not balanced and not intended to be balanced, just a scheme).

    This can easily get tricky when you have to deal with the acid dissociation (not every acid is fully dissociated), when the metal is less reactive (so needs something stronger than H+ to be oxidized), plus technically there is no "naked" H+ in water solutions, we typically assume they are present as H3O+ (H+ and a water molecule) - which is not entirely true either.

    The part about water in your question has nothing to do with the reaction itself, gas collected over water is always saturated with water vapor. You are given partial pressure of the water vapor so you should use it to in your calculations.
    ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

    Offline Orcio_87

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    Re: Balancing Cr with H+
    « Reply #7 on: May 01, 2022, 04:05:43 AM »
    Quote
    Hmm...So you claim I truly need absolutely nothing but Cr and H+ to end up with Cr^+3 and hydrogen gas? Am I getting you right?
    Right. Look at the table:

    https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)

    Cr ----> Cr3+ + 3 e-
    H+ + e- ----> 1/2 H2

    All you have to do is multiply second equation and add them together to make electrons will cancel out.

    Offline dogman

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    Re: Balancing Cr with H+
    « Reply #8 on: May 01, 2022, 12:11:43 PM »
    Hmm...Okay.
    From what I understand, is that I am missing a buncha study subjects. I'll have to address this to my lecturer. :s
    Thanks everyone! =)

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