Redox Rxn and gasses

[Chalupa Batman]

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How can you tell which gas is going to evolve from a redox reaction?
Cu + HNO3 -> what gas

NO or H2

How do you tell

 

[circulus vitios]

Nitric acid is a classic example of an oxidizing agent. Oxidizing agents get reduced.

Look at oxidation and reduction from the organic chemistry perspective. Oxidation increases bonds to oxygen or decreases bonds to hydrogen. Reduction decreases bonds to oxygen or increases bonds to hydrogen.

HNO3 --> H2 shows nitric acid losing hydrogen bonds. This means it's oxidized. Not likely because nitric acid is an oxidizing agent.
HNO3 --> NO shows nitric acid losing oxygen bonds. This mean it's reduced. This is the best choice.

 

[polyatomic]

So what you do know automatically is that Cu must be oxidized to Cu+, because this is a redox reaction, and Cu has no other option.
Cu ---> Cu+

From there, you now know something else needs to be reduced (lower in oxidation number). But you have to decide if the N will be reduced (+5 --> +2) or H will be reduced (+1 --> 0).

The only way you can decide between this is by knowing which is more easily reduced. In this case, HNO3 --> NO has a positive reduction potential, while H --> H2 has a 0 reduction potential. It's also helpful to think that Nitrogen is more electronegative than Hydrogen, therefore, N is more likely to be reduced (because electronegative = likes electrons).

 

[Chalupa Batman]

I see
makes a lot more sense
thanks!

 

[MedGrl@2022]

I see
makes a lot more sense
thanks!

for this question were you given a reduction potential chart?

 

[MedGrl@2022]

So what you do know automatically is that Cu must be oxidized to Cu+, because this is a redox reaction, and Cu has no other option.
Cu ---> Cu+

From there, you now know something else needs to be reduced (lower in oxidation number). But you have to decide if the N will be reduced (+5 --> +2) or H will be reduced (+1 --> 0).

The only way you can decide between this is by knowing which is more easily reduced. In this case, HNO3 --> NO has a positive reduction potential, while H --> H2 has a 0 reduction potential. It's also helpful to think that Nitrogen is more electronegative than Hydrogen, therefore, N is more likely to be reduced (because electronegative = likes electrons).

So what would be the result of this equation?

2Cu + HNO3 -> Cu2(O) + NO(g) + OH-