two questions with redox reactions

[yoyohomieg5432]

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these are 2 questions from AAMC test 5.
1) my first question is with #24. the solution says the answer is A and the redox equation is NO3- + Cu ---> NO + CuO2 . I am confused on how to know the NO is the gas? and how to get this equation? The molecules present would be H+, NO3-, and Cu. No idea how we are supposed to know that this forms NO and CuO2.

2) in #26, again they give an equation Cu + 2AgNO3 ---> Cu(NO3)2 + 2Ag

Again, I don't really understand how to get this equation.

Also, the solution mentions "since the initial starting product is Ag+ and not Ag metal...." is Ag+ not considered a metal? what is it then?

 

[mehc012]

1) my first question is with #24. the solution says the answer is A and the redox equation is NO3- + Cu ---> NO + CuO2 . I am confused on how to know the NO is the gas? and how to get this equation? The molecules present would be H+, NO3-, and Cu. No idea how we are supposed to know that this forms NO and CuO2.

2) in #26, again they give an equation Cu + 2AgNO3 ---> Cu(NO3)2 + 2Ag

Again, I don't really understand how to get this equation.

Also, the solution mentions "since the initial starting product is Ag+ and not Ag metal...." is Ag+ not considered a metal? what is it then?

Ag metal implies Ag(s). Ag⁺ is a metal ion.

As for the rest...honestly, if you want detailed answers, you're far better off posting the questions in their entirety.

 

[HotHamH2O]

Without seeing the question in it's entirety, I only answer the question based on intuition. As far as knowing that Cu + NO3 forms Cu02 + NO, you have to know that copper is a money metal and money metals on the MCAT are almost always reduced. You should know that the reduction potential of Cu is preferable over reduction of N. Knowing that Cu exists as a 2+ ion in the reactant side and that it is reduced leads you to come up with the products from above.

 

[BerkReviewTeach]

Without seeing the question in it's entirety, I only answer the question based on intuition. As far as knowing that Cu + NO3 forms Cu02 + NO, you have to know that copper is a money metal and money metals on the MCAT are almost always reduced. You should know that the reduction potential of Cu is preferable over reduction of N. Knowing that Cu exists as a 2+ ion in the reactant side and that it is reduced leads you to come up with the products from above.

Huh? You might want to check your definition of reduction and oxidation here. Cu metal is being oxidized into a cation if you read the reaction from left to right. So the reality is that the reduction potential of N (in a +5 oxidation state) is greater than copper in the oxidation state listed in the question (which is not a possible oxidation state, but that will be discussed later). That's the problem you run into when you blindly memorize things. The "money metal" trick might be good for high school chemistry (it's a common mnemonic amongst high school teachers), but they write their own exam and will likely make all of the examples fit their rule. With the MCAT, there are so many different passage authors that generalizations like that can get you in trouble. You need to be very careful making bold statements about what the MCAT will ask, unless you've seen every MCAT since 2007. What was true on the one MCAT you took might not be true on other MCATs.

1) my first question is with #24. the solution says the answer is A and the redox equation is NO3- + Cu ---> NO + CuO2 . I am confused on how to know the NO is the gas? and how to get this equation? The molecules present would be H+, NO3-, and Cu. No idea how we are supposed to know that this forms NO and CuO2.

If you drop a penny into nitric acid, the solution will bubble and a brown gas will form above the solution. That means that the reaction is:

NO₃^- + Cu ---> (brown gas) + oxidized copper of some sort​

This is where multiple choice intuition is so critical. You need to eliminate choices that don't fit that general observation and then make an educated guess as to the identity of the products. For what it's worth, you should double check the question, because Cu cannot take on a +4 oxidation state (cannot be CuO₂ under those conditions. Double check to see if they put a 2- charge on that molecule.

You should also list the source of your questions so that people reading this thread don't get a question spoiled from their future homework passages.

Lastly, did the passage give you any information about the reaction? That might be helpful in identifiying the gas. You need to fish for everything you can in passages. This is very important in your practice.

 

[HotHamH2O]

Huh? You might want to check your definition of reduction and oxidation here. Cu metal is being oxidized into a cation if you read the reaction from left to right. So the reality is that the reduction potential of N (in a +5 oxidation state) is greater than copper in the oxidation state listed in the question (which is not a possible oxidation state, but that will be discussed later). That's the problem you run into when you blindly memorize things. The "money metal" trick might be good for high school chemistry (it's a common mnemonic amongst high school teachers), but they write their own exam and will likely make all of the examples fit their rule. With the MCAT, there are so many different passage authors that generalizations like that can get you in trouble. You need to be very careful making bold statements about what the MCAT will ask, unless you've seen every MCAT since 2007. What was true on the one MCAT you took might not be true on other MCATs.



If you drop a penny into nitric acid, the solution will bubble and a brown gas will form above the solution. That means that the reaction is:

NO₃^- + Cu ---> (brown gas) + oxidized copper of some sort​

This is where multiple choice intuition is so critical. You need to eliminate choices that don't fit that general observation and then make an educated guess as to the identity of the products. For what it's worth, you should double check the question, because Cu cannot take on a +4 oxidation state (cannot be CuO₂ under those conditions. Double check to see if they put a 2- charge on that molecule.

You should also list the source of your questions so that people reading this thread don't get a question spoiled from their future homework passages.

Lastly, did the passage give you any information about the reaction? That might be helpful in identifiying the gas. You need to fish for everything you can in passages. This is very important in your practice.

My bad. I was responding to questions #1. I didn't have the full question and was assuming that the reactant was Cu+2 being reduced to Cu02. Since molecular oxygen has an oxidation state of 0 wouldn't that be a gain of electrons for Cu?

 

[BerkReviewTeach]

My bad. I was responding to questions #1. I didn't have the full question and was assuming that the reactant was Cu+2 being reduced to Cu02. Since molecular oxygen has an oxidation state of 0 wouldn't that be a gain of electrons for Cu?

CuO₂ is not molecular oxygen just as CO₂ is not molecular oxygen. Oxygen assumes a -2 oxidation state when it is the most electronegative element in the compound (unless it is bonded to itself). In CuO₂ you have two Os at -2 each, making Cu +4. In the reaction listed by the OP, Cu is the reactant and Cu²⁺ is not involved whatsoever. Ignoring the fact that the actual reaction should be the oxidation of Cu(s) into Cu²⁺, the question feeds you the reactant and product (or so it seems from the post).

Because oxidation-reduction chemistry is prevalent in so many areas (biological pathways, electrochemical cells, and carbonyl reactions to name a few), this is an area that should be understood at a deep level.

 

[yoyohomieg5432]

my bad guys.. totally forgot to post the questions even though i had them screenshotted. sorry i updated the first post

i'd really apprciate some help with these now that i've got the pictures up 🙂

 

[HotHamH2O]

CuO[sub2[/sub] is not molecular oxygen just as CO[sub2[/sub] is not molecular oxygen. Oxygen assumes a -2 oxidation state when it is the most electronegative element in the compound (unless it is bonded to itself). In CuO[sub2[/sub] you have two Os at -2 each, making Cu +4. In the reaction listed by the OP, Cu is the reactant and Cu²⁺ is not involved whatsoever. Ignoring the fact that the actual reaction should be the oxidation of Cu(s) into Cu²⁺, the question feeds you the reactant and product (or so it seems from the post).

Because oxidation-reduction chemistry is prevalent in so many areas (biological pathways, electrochemical cells, and carbonyl reactions to name a few), this is an area that should be understood at a deep level.

After seeing the question and reading back through your explanation, I can see how off I was. You are right, I was thinking "money metals get reduced" and didn't bother to really pay attention to what was happening. Good lesson indeed!

 

[sdm33]

these are 2 questions from AAMC test 5.
1) my first question is with #24. the solution says the answer is A and the redox equation is NO3- + Cu ---> NO + CuO2 . I am confused on how to know the NO is the gas? and how to get this equation? The molecules present would be H+, NO3-, and Cu. No idea how we are supposed to know that this forms NO and CuO2.

2) in #26, again they give an equation Cu + 2AgNO3 ---> Cu(NO3)2 + 2Ag

Again, I don't really understand how to get this equation.

Also, the solution mentions "since the initial starting product is Ag+ and not Ag metal...." is Ag+ not considered a metal? what is it then?

Q 24
In my opinion this is a hard Q, so the first thing one should do is POE, we know that O3 is not going to happen, and CO2 is out because C is not a reactant.
So we are left with H2 and NO, it is tempting to pick H2 on the first look, because metals like to give up e-s , however, money metals are an exception (Gold, Silver , Cu) they are stable.

As of the other question, a similar approach can be taken, silver is a money metal as well and it is more stable than Cu so Cu is going to get oxidized and Ag+ reduced, also please note that it is much easier to reduce Ag + vs trying to reduce Ag(s). Furthermore, if the question used iron instead of silver then you know iron is going to get oxidized. Besides knowing the chemistry, it is helpful to look at the logic behind it, and the big picture to know the physical properties of metals.

 

[BerkReviewTeach]

my bad guys.. totally forgot to post the questions even though i had them screenshotted. sorry i updated the first post

i'd really apprciate some help with these now that i've got the pictures up 🙂

Thanks for putting the source in the post, because anyone who has yet to take that AAMC exam will know to avoid looking at the thread. I once again want to go back to what I typed before. If you drop a penny into nitric acid, the solution will bubble and a brown gas will form above the solution. That means that the reaction is:

HNO₃ + Cu ---> (brown gas) + oxidized copper of some sort​

They give you four choices for the identity of the gas. O starts with an oxidation state of -2 in NO₃^-, and it is not going to give up electrons (get oxidized) to form O₃, where it has an oxidation state of 0. So O₃ cannot be right. They put CO₂ as choice hoping to fool people who have seen it as answer to "identify the gas" questions in the past (where a carbonate or bicarbonate salt is treated with strong acid to form carbonic acid, which then decomposes). Carbon is not involved in the reaction, so CO₂ is out.

To decide between H₂ and NO, you can tap into your life experiences. H₂ gas has no color, and the question says it's a brown gas. On the other hand, smog is a brown gas that results from the partial oxidation of N₂ in combustion engines. It is reasonable that NO is a brown gas.

The other way to solve this question would have been to use the reduction potentials from the passage. Reduction of H⁺ into H₂ releases 0 volts (by definition) while the reduction of Cu²⁺ into Cu(s) releases 0.34 volts, so it is easier to reduce copper cation than hydronium ation. The formation of H₂ coupled with the oxidation of Cu(s) cannot occur naturally. The answer cannot be H₂, so NO is the only choice that remains standing.

 

[sdm33]

EK1001 Chem has the same Qs look at Q929

 

[yoyohomieg5432]

Thanks for putting the source in the post, because anyone who has yet to take that AAMC exam will know to avoid looking at the thread. I once again want to go back to what I typed before. If you drop a penny into nitric acid, the solution will bubble and a brown gas will form above the solution. That means that the reaction is:

HNO₃ + Cu ---> (brown gas) + oxidized copper of some sort​

They give you four choices for the identity of the gas. O starts with an oxidation state of -2 in NO₃^-, and it is not going to give up electrons (get oxidized) to form O₃, where it has an oxidation state of 0. So O₃ cannot be right. They put CO₂ as choice hoping to fool people who have seen it as answer to "identify the gas" questions in the past (where a carbonate or bicarbonate salt is treated with strong acid to form carbonic acid, which then decomposes). Carbon is not involved in the reaction, so CO₂ is out.

To decide between H₂ and NO, you can tap into your life experiences. H₂ gas has no color, and the question says it's a brown gas. On the other hand, smog is a brown gas that results from the partial oxidation of N₂ in combustion engines. It is reasonable that NO is a brown gas.

The other way to solve this question would have been to use the reduction potentials from the passage. Reduction of H⁺ into H₂ releases 0 volts (by definition) while the reduction of Cu²⁺ into Cu(s) releases 0.34 volts, so it is easier to reduce copper cation than hydronium ation. The formation of H₂ coupled with the oxidation of Cu(s) cannot occur naturally. The answer cannot be H₂, so NO is the only choice that remains standing.

how do you know that NO3 oxygen would not oxidize to O3?