Chemistry Lab C

[jaah5211]

Oopss totally forgot to ask the next question...

Which proton in ascorbic acid is the most acidic?


Why is that so?

[Cathy-TJ]

Oopss totally forgot to ask the next question...

Which proton in ascorbic acid is the most acidic?


Why is that so?

Answer (Click to Show Hidden Text)

The most acidic proton is going to be the one that makes the most stable conjugate base. In this case, removal of the proton on the 4th carbon of the furan ring results in three possible resonance structures, while removing the others results in two. 
[img]https://i.stack.imgur.com/KVCSd.png[/img]
This is a good illustration of the three resonance structures.

[whythelongface]

Answer (Click to Show Hidden Text)

The most acidic proton is going to be the one that makes the most stable conjugate base. In this case, removal of the proton on the 4th carbon of the furan ring results in three possible resonance structures, while removing the others results in two. 
[img]https://i.stack.imgur.com/KVCSd.png[/img]
This is a good illustration of the three resonance structures.

I don't like the second of the three resonance structures. Even stabilized, carbanions are inherently unstable (a lot more so than oxyanions) and I don't think it contributes to the stability of the ascorbate ion in any major way.

[Cathy-TJ]

Answer (Click to Show Hidden Text)

The most acidic proton is going to be the one that makes the most stable conjugate base. In this case, removal of the proton on the 4th carbon of the furan ring results in three possible resonance structures, while removing the others results in two. 
[img]https://i.stack.imgur.com/KVCSd.png[/img]
This is a good illustration of the three resonance structures.

I don't like the second of the three resonance structures. Even stabilized, carbanions are inherently unstable (a lot more so than oxyanions) and I don't think it contributes to the stability of the ascorbate ion in any major way.

That's a good point.
Looking over the possible structures again, I still think that proton is the most acidic, since it has two stable resonance structures, while the other proton on the OH of the third carbon in the ring only has one. Correct me if I'm wrong.
Thanks for catching that!
[/quote]

[whythelongface]

That's a good point.
Looking over the possible structures again, I still think that proton is the most acidic, since it has two stable resonance structures, while the other proton on the OH of the third carbon in the ring only has one. Correct me if I'm wrong.
Thanks for catching that!

Pretty sure the conclusion is correct! Google confirms this. Anyway, you should ask the next question!

[Cathy-TJ]

a) What two substances mixed together make aqua regia? In what relative concentrations?
b) Describe how aqua regia dissolves gold, and write a balanced equation for the reaction.

[wec01]

a) What two substances mixed together make aqua regia? In what relative concentrations?
b) Describe how aqua regia dissolves gold, and write a balanced equation for the reaction.

Answers (Click to Show Hidden Text)

a) 3 parts hydrochloric acid, 1 part nitric acid
b) The nitric acid oxidizes the gold while the hydrochloric acid provides chloride ions.
[math]Au + HNO_3 + 4 HCl \rightarrow [AuCl_4]^- + [NO] + [H_3O]^+ + H_2O[/math]

[Cathy-TJ]

a) What two substances mixed together make aqua regia? In what relative concentrations?
b) Describe how aqua regia dissolves gold, and write a balanced equation for the reaction.

Answers (Click to Show Hidden Text)

a) 3 parts hydrochloric acid, 1 part nitric acid
b) The nitric acid oxidizes the gold while the hydrochloric acid provides chloride ions.
[math]Au + HNO_3 + 4 HCl \rightarrow [AuCl_4]^- + [NO] + [H_3O]^+ + H_2O[/math]

Looks good! You have the next question.

[wec01]

1. Write a balanced chemical equation for the reaction between calcium carbonate and hydrochloric acid.
2. Is this reaction exothermic or endothermic?
3. What molecule is produced that most greatly contributes to the answer in part 2?

[BrownieInMotion]

1. Write a balanced chemical equation for the reaction between calcium carbonate and hydrochloric acid.
2. Is this reaction exothermic or endothermic?
3. What molecule is produced that most greatly contributes to the answer in part 2?

My attempt (Click to Show Hidden Text)

1. [math]CaCO_3 + 2HCl \rightarrow H_2O + CO_2 + CaCl_2[/math]
2. Exothermic.
3. CaCl2? Forming ionic bonds is usually very exothermic so CaCl2 should have a very negative Hf.