Got another one for you...

[ironizer]

Ok this question is the real BS:

 

The blood alcohol (C2H5OH) level can be determined by titrating a sample of blood plasma with an acidic potassium dichromate solution, resulting in the production of Cr3+(aq) and carbon dioxide. The reaction can be monitored because the dichromate ion (Cr2O72-) is orange in solution, and the Cr3+ ion is green. The unbalanced redox equation is shown below.

Cr2O72-(aq) + C2H5OH(aq) → Cr3+(aq) + CO2(g)

If 31.05 mL of 0.0600 M potassium dichromate solution is required to titrate 30.0 g of blood plasma, determine the mass percent of alcohol in the blood.

 

I have no clue where to even start. I don't know how to balance that equation, as I dont even know how the atoms move around.I dont see any poly ions in the alcohol or the dichromate.

 

So hits please.

[UC]

Let's make this simple.

 

What oxidation state does hydrogen usually take in compounds? (not including hydrides)

What oxidation state does oxygen usually take in compounds? (not including peroxides)

Since ethanol and CO2 are neutral compounds, the sum of the oxidation states of their atoms must equal 0. The two carbons in ethanol have different oxidation states. Pay attention to what other atoms each is connected to. Both become CO2 in the end. The hydrogens and oxygens are just along for the ride. It's the carbon you need to focus on.

 

Write your half equations for each carbon seperately. Google redox equations and pay better attention in class.

[John Cuthber]

He can pay as much attention as he likes in class. That equation is just plain wrong. The product of the reaction is acetic acid, so googling it might not help.

There's another way of balancing equations that might help here.

Pretend that the Cr(VI) is present as CrO3 (not a million miles out for acid dichromate) and that it's converted to Cr2O3.

4 CrO3 --> 2 Cr2O3 + 3O2

Then write out the oxidation as if its being burned in oxygen

C2H5OH + (however many) O2 --> CO2 + H2O

Each O2 requires 4/3 CrO3

From that you can get the stoiciometry of the reaction.

[hermanntrude]

OK what you need to do is google "reaction of dichromate and ethanol". You'll be able to find a balanced equation which might explain things better.

 

After that, all you have to do is realise this a volumetric analysis. Volumetric analysis questions usually follow the form of a c=n/v calculation, followed by a stoichiometry calculation, followed by another c=n/v.

[ironizer]

The problem with paying attention in class is that he didn't go over Redox reactions until today, so I guess I wasn't supposed to know this until now.

 

And I did pay attention to the Redox lecture, but I don't really understand the meaning of the Oxidation Numbers. What are they? How are they useful?

[hermanntrude]

ironizer, google is your friend. I could tell you what oxidation numbers are but there are twenty websites out there that can tell you better. And your instructor ought to be able to tell you too. sometimes face to face is easier than pure text.

 

and of course don't forget your textbook.

[UC]

He can pay as much attention as he likes in class. That equation is just plain wrong. The product of the reaction is acetic acid, so googling it might not help.

 

Yes, but this is rather clearly a homework question. Regardless of how correct the equation in the homework is (which it isn't in this case) the answer is going to reflect the unbalanced equation he was given. Conceptually, it is a fine problem, although the chemistry is wrong.

 

Calculating for O2 and converting to dichromate is extremely roundabout. The two half reactions are a shorter and cleaner way to calculate this.

 

Oxidation states/numbers are vital for understanding redox chemistry. They get muddled a bit since inorganic compounds are more defined in terms of oxidation states than organic molecules, but the assumptions you make for them can correctly predict stoichiometries.

 

Basically, look at how many valence electrons an element has. Then look at the electronegativities of it and the atoms bonded to it. Oxidation number is a crude predictor of the electron density around a nucleus in a compound.

 

First, recognize that a bond represents one electron from each atom coming together.

 

Two of the same element are the same electronegativity and neither steals the other one's electrons away completely. There is perfect sharing and the average density around each nucleus is not going to change. We don't change the count on the oxidation number for this. Something that is all one element, therefore has an oxidation number of 0.

 

If one of the two elements is more electronegative, we pretend that it takes both electrons to itself (stretching the truth except for purely ionic bonds) An electron has a -1 charge, so the more electronegative element gets a -1 for that bond and the less electronegative element gets a +1 for that bond. The oxidation number can never be more than the number of valence electrons, nor less than it's negative: example: oxygen can never be more than +2 and never less than -2.

 

Take methane for example. CH4. Carbon has a higher electronegativity than hydrogen. So, we say that each hydrogen is +1 and the carbon, since it has 4 +1s attached to it must be -4. This is true because methanol doesn't have any charge, so the +s and -s must balance.

 

Now, let's look at methanol. The carbon has three hydrogens and a single bond to an oxygen (which is itself bonded to a hydrogen). the least electronegative thing here is the hydrogen, so they will all give their electrons up and be +1s, while the oxygen being the most electronegative must take electrons from both bonds that it forms. Oxygen is therefore a -2. Now for the carbon. The oxygen is going to take one electron from the bond, making the carbon +1. However, the carbon has those three hydrogens giving 1 electron each for a total of -3. Put together, the overall oxidation number of the carbon in methanol is -2.

 

Redox reactions are a simple balancing of oxidation numbers. If you want to make methanol go to CO2, you need to make the carbon go from -2 to +4. This is a 6 number increase. So, you need something to drop 6 numbers. Lets use Cr2O7(2-). Using the method above, you will find that chromium is in a +6 state and likes to drop down to +3. This is a drop of 3. So, you need 2 times the drop in 3 to make up for the increase in 6. Since there are two Cr(VI) in Cr2O7(2-), one mole of dichromate will oxidize one mole of methanol to CO2.

 

Use google to look up the half reactions method if you didn't learn it in class. This is a way of keeping track of all the other atoms involved while figuring out how to balance the reaction. It will also tell you if the reaction is promoted by acidic or alkaline reaction conditions.

 

Well, Im done with my little essay. If you need better explanation, just ask. I tend to be convoluted sometimes.

[ironizer]

Ok so I was reading up some more on this, but nobody talks about why this oxidation stuff makes sense. Maybe it just doesn't, I don't know. To me it seems like somebody made all these rules up. Book doesn't explain why things are like that, example: Fluorine is always assigned -1, so SF6 would be -6 fluorine and +6 sulfur. Why?

 

This is exactly why I hate chemistry, nothing makes logical sense. You can't just "think about it" and understand it. In physics and math you can figure things out on your own, in chemistry you just have to memorize crap and take someone's word for it, and not even be given an explanation. I can't wait until the end of the quarter so I wouldn't have to put up with this BS.

 

And why is it called oxydation where there isn't necessarily any oxygen involved?

[UC]

The short answer is that atoms like to have 8 (or 0)valence electrons surrounding them. This is an extremely stable configuration. Carbon has 4 electrons. By forming 4 bonds, it has a total of 8 (2 in each bond) surrounding it. Oxygen has 6 surrounding it. by forming two bonds, it adds 2 to it's collection and has 8. etc. etc. Electronegativities are experimentally derived, but follow a relatively simple pattern.

 

If you get into higher chemistry all these things slip into place. Fluorine gets a -1 because one more electron (with a -1 charge) completes the set of electrons that occupy its 2p atomic orbitals (a 3-dimensional region of probability for the position of an electron around the nucleus defined by quantum mechanics). This probability can be written as an equation on paper, but this is advanced college physics and beyond me even though I understand the basic concepts. The thing that makes this part of chemistry inaccessible and restrains it to memorization in lower level classes is how incredibly complex the equations are. The rules resulting from them are basic principles for chemistry though. The easy way to get around trying to explain such a complex concept early on is to just confine it to memorization.

 

Chemistry walks the fine line, in my opinion, between the impracticality and inapplicability of math and physics to everyday life (beyond basic stuff), and the overly-specific analyses of biology that also lack applicability.

 

It's called oxidation because of it's history. Oxygen being a rather accessible and common oxidant is where the reaction was first observed and only later on was the concept extended to other reactions as well as the mechanism for it.

 

If you continue to complain and call chemistry BS in all your posts I don't think I'm going to take the time to respond anymore. I take it quite seriously.

[John Cuthber]

Ironiser,

Trust me, it does make sense, and you can just think about it. The problem is that you need a lot of background knowledge before you can do that- not least you need to understand a lot of quantum mechanics.

In the meantime, have fun trying to live your life without having to remember stuff.