oxidation-reduction question, help!

[orangeblossom]

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it is question #866 from the EK 1001 chemistry book.

I dont understand why this is NOT a redox reaction:

CaH2 + 2BH3 --> 2BH4- + Ca2+

I just cant figure out how to assign oxidation states to 2BH4- ?? Does this have a total charge of -1 or -2 (because of the coefficient in the front)?

 

[Postictal Raiden]

it is question #866 from the EK 1001 chemistry book.

I dont understand why this is NOT a redox reaction:

CaH2 + 2BH3 --> 2BH4- + Ca2+

I just cant figure out how to assign oxidation states to 2BH4- ?? Does this have a total charge of -1 or -2 (because of the coefficient in the front)?

Hydrogen can have either +1 or -1 formal charge. When it bonds with metals, it takes the -1 formal, but when it bonds to nonmetals it has +1 charge.

Looking at the reaction, nothing is oxidized or reduced. Calcium started and ended with an OS of +2. Boron started and ended with +3. Hydrogen started and ended with an OS of -1.

For 2BH4-, ignore the coefficient because it's distributed to both Boron and hydrogen. There is one Boron (+3 ) and four Hydrogen (-1). 3+4(-1) = -1.

 

[orangeblossom]

oh okay, thank you!
I was getting confused because I was distributing the 2 to B and H (in 2BH4-) so I was getting (-8) for H and only (+6) for B. I guess i shouldn't do that?

 

[Postictal Raiden]

oh okay, thank you!
I was getting confused because I was distributing the 2 to B and H (in 2BH4-) so I was getting (-8) for H and only (+6) for B. I guess i shouldn't do that?

Well, you still can do that, but remember to also multiply the overall charge by two.