oxidation states

[Medgen]

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******WARNING: THE FOLLOWING CONTAINS INFORMATION THAT DISCUSSES CONTENT ON AN AAMC PRACTICE EXAM******

In one of the AAMC exams, it required us to know the oxidation state of Zn. I incorrectly assumed that it would have a +1 charge, but there was no indication in the question as to what its oxidation state could be. It was a limiting reagent problem that required that we first come up with a balanced equation.

Do you think the actual MCAT exam will require us to know oxidation states of some important transition metals? Or if there is any general trend in oxidation states among transition metals? Thanks!

 

[dsoz]

In one of the AAMC exams, it required us to know the oxidation state of Zn. I incorrectly assumed that it would have a +1 charge, but there was no indication in the question as to what its oxidation state could be. It was a limiting reagent problem that required that we first come up with a balanced equation.

Do you think the actual MCAT exam will require us to know oxidation states of some important transition metals? Or if there is any general trend in oxidation states among transition metals? Thanks!

Zn in a compound is always +2 and Ag in a compound is always +1. All the other transition metals have variable oxidation states. Was there anything with the zinc like chloride or nitrate to indicate possible ion charge? For example was it ZnCl2 or Zn(NO3)2?

Dsoz

 

[Medgen]

Zn in a compound is always +2 and Ag in a compound is always +1. All the other transition metals have variable oxidation states. Was there anything with the zinc like chloride or nitrate to indicate possible ion charge? For example was it ZnCl2 or Zn(NO3)2?

Dsoz

****** WARNING: THE FOLLOWING CONTAINS SPOILER INFORMATION REGARDING AN AAMC PRACTICE EXAM*****

There was nothing to indicate it. They told me that solid zinc reacted with HCl and the calculation was relatively simple but I balanced the reaction incorrectly- as I thought that Zn was +1. But I'll commit Zn and Ag to memory. I understand the question and was trying to word my post in a way that wouldn't give away what exactly the question was or which AAMC exam it belonged too. I just wanted clarification on what we would be expected to know for the MCAT. I don't find the information that AAMC puts out all that helpful. Thanks!

 

[whiteshadodw]

There was nothing to indicate it. They told me that solid zinc reacted with HCl and the calculation was relatively simple but I balanced the reaction incorrectly- as I thought that Zn was +1. But I'll commit Zn and Ag to memory. Thanks!

*please edit your post and say there is a spoiler* this could have ruined an AAMC exam for other people.

you shouldn't have to. if you balanced it correctly you'd get:

Zn (aq) + HCl (aq) --> H+ + ZnCl2 which would mean it has to be +2...

however, here is the caveat you would HAVE to know that Zn takes on a +2 charge. so in order to not memorize this and learn something else, take a look at the periodic table.

Zn electronic configuration = 4s2 - 3d10, the s electrons have low ionization energy, meaning its easy to lose the s electrons and its easy to lose BOTH of them because they are so far away from the nucleus.

 

[Medgen]

*please edit your post and say there is a spoiler* this could have ruined an AAMC exam for other people.

you shouldn't have to. if you balanced it correctly you'd get:

Zn (aq) + HCl (aq) --> H+ + ZnCl2 which would mean it has to be +2...

however, here is the caveat you would HAVE to know that Zn takes on a +2 charge. so in order to not memorize this and learn something else, take a look at the periodic table.

Zn electronic configuration = 4s2 - 3d10, the s electrons have low ionization energy, meaning its easy to lose the s electrons and its easy to lose BOTH of them because they are so far away from the nucleus.

That makes sense- but not all transition metals behave this way and all transition metals have a filled s orbital... Is there anyway that I can differentiate between those that behave this way and those that don't? I mean- I can sort of see why Ag would only lose one S electron; so the remaining electron would drop down into the d orbital and complete it.

Thanks! I made the correction to the above posts. I didn't mean to give away information that might ruin the test for other people and I was trying to be as vague as possible because I know that AAMC doesn't allow people to post their exact questions. I don't think that they would ask a very complicated question involving this and your explanation makes perfect sense.

 

[whiteshadodw]

That makes sense- but not all transition metals behave this way and all transition metals have a filled s orbital... Is there anyway that I can differentiate between those that behave this way and those that don't? I mean- I can sort of see why Ag would only lose one S electron; so the remaining electron would drop down into the d orbital and complete it.

Thanks! I made the correction to the above posts. I didn't mean to give away information that might ruin the test for other people and I was trying to be as vague as possible because I know that AAMC doesn't allow people to post their exact questions. I don't think that they would ask a very complicated question involving this and your explanation makes perfect sense.

i think this is something you pretty much have to know based on the ionization energy patterns for the d metals. chromium for example is going to be +1 and the electronic configuration of such ion is:

3s2 3p6 3d5 (loss of 1 4s electron and the other electron that would have been in 4s went to the 3d to half fill it). however for metals like iron, it can be +2 or +3 so they would really have to give that to you. but that question was basically like two questions in one. for example, you're expect to know Li, K, and Na all have +1 charge when ionized. Mg, Ca, etc are +2.

 

[movax]

******WARNING: THE FOLLOWING CONTAINS INFORMATION THAT DISCUSSES CONTENT ON AN AAMC PRACTICE EXAM******

In one of the AAMC exams, it required us to know the oxidation state of Zn. I incorrectly assumed that it would have a +1 charge, but there was no indication in the question as to what its oxidation state could be. It was a limiting reagent problem that required that we first come up with a balanced equation.

This sounds like CBT11.

I just remembered that Zn typically has 2+, and when you consider how you have to balance the reaction to elicit zinc chloride + diatomic hydrogen gas, you need 2 moles of HCl that results in ZnCl2 + HCl.

Then set up stoichiometric ratio, and you find you need 0.005g H2(g). If I remember the problem correctly, you have 50.0mL of a .1M HCl solution, giving you .005 mol of HCl. Multiply by 1 mol H2 / 2 mol HCl and then 2g H2 / 1 mol H2 and you get 0.005g. I think the information about Zinc is simply provided to indicate it is not a limiting reagent. (Again, going by memory, I believe you end up with .1 mol of Zn, since they give you 6.54g).

For metals, its good to know they're generally cations, and IIRC, free metal ions would much rather get oxidized than reduced.

e: further going off memory, I believe this is a discrete, not a passage-related question. Discretes should make you dig in your brain a bit, but should not seem to be obnoxiously difficult. A Zn with a 1+ charge just wouldn't make sense here once you balance the equation: Zn2Cl should immediately look "off", and then your brain would realize it's ZnCl2.