Yacoubb's Chemistry 3/4 Thread

[lzxnl]

Solid NaOH is hygroscopic (i.e. it absorbs water from the atmosphere) and so is unsuitable for use as a primary standard (recall the definition of a primary standard). Anhydrous sodium carbonate, on the other hand, has all the properties of a primary standard, and so is infinitely more suitable.

I would like to add that sodium hydroxide (solid OR aqueous) reacts with carbon dioxide in the atmosphere, another reason why it's no good as a primary standard. Anhydrous sodium carbonate doesn't really react with much in comparison.

[Yacoubb]

I would like to add that sodium hydroxide (solid OR aqueous) reacts with carbon dioxide in the atmosphere, another reason why it's no good as a primary standard. Anhydrous sodium carbonate doesn't really react with much in comparison.

Yeah I thought so; sodium hydroxide reacts with carbon dioxide in the air and is also deliquescent (reacts with water in the atmosphere). Thus sodium carbonate would be better for a primary standard, which doesn't deteriorate or react with the atmosphere.

[Yacoubb]

I've just finished with Chapter 5 of the Heinemann Chemistry 2 textbook, and I just have a few queries in regards to Redox titrations. I know that redox titrations involve reactions between reductants and oxidants, but my questions are:
(a) How can we identify that we are dealing with a redox titration?
(b) When we are given an equation of a reaction, must we find the ionic equation, find the reduction and oxidation equations, formulate the balanced redox equation and then use that as our basis?
(c) Is there anything that must be dealt with in redox titrations that varies from basic stoichiometric calculations in acid-base titrations/volumetric analysis?

Also, if we leave a standard solution over-night in a glass burette, would it be feasible to say that because the solution reacts with the silica of the glass burette, the glassware can provide inaccurate results, hence why we shouldn't have the standard solution in the glassware over-night.


Thank you

[psyxwar]

(a) How can we identify that we are dealing with a redox titration?

(a) when a redox reaction is occuring (change in oxidation numbers?). Alternatively if no acids or bases are involved, then isn't it pretty obviously a redox titration?

[Yacoubb]

(a) when a redox reaction is occuring (change in oxidation numbers?). Alternatively if no acids or bases are involved, then isn't it pretty obviously a redox titration?

But I came across a redox titration question that involved nitric acid, so I don't think acid or base involvement renders a titration not redox. Is there a way of figuring it out without having to use oxidation numbers?

Thanks for that though psyxwar

[lzxnl]

(a) when a redox reaction is occuring (change in oxidation numbers?). Alternatively if no acids or bases are involved, then isn't it pretty obviously a redox titration?

When sodium hydride is added to water, the resulting reaction is both acid-base and redox.

2NaH(s) + H₂O(l) => 2NaOH(aq) + H₂(g)

But I came across a redox titration question that involved nitric acid, so I don't think acid or base involvement renders a titration not redox. Is there a way of figuring it out without having to use oxidation numbers?

Thanks for that though psyxwar

You have to check oxidation numbers in some shape or form. Over time, with practise, you'll be able to spot the species undergoing oxidation or reduction quite quickly.

[Yacoubb]

Is deliquescent an appropriate term to describe a solution that reacts with gases (e.g. CO2) and water in the atmosphere? Thanks

[scribble]

No, deliquescent materials absorb H2O(g) from the air, dissolving into solution. They are often salts, eg NaOH(s).
efflorescence is the opposite, so when a solution loses water to the atmosphere and the salt crystallises out. 

[Yacoubb]

No, deliquescent materials absorb H2O(g) from the air, dissolving into solution. They are often salts, eg NaOH(s).
efflorescence is the opposite, so when a solution loses water to the atmosphere and the salt crystallises out.

Thank you. So some standard solutions (i.e. not primary ones), are deliquescent, deliquescent meaning they absorb water from the atmosphere which dissolves in the solution.

[fadzsta1]

Thank you. So some standard solutions (i.e. not primary ones), are deliquescent, deliquescent meaning they absorb water from the atmosphere which dissolves in the solution.

Well... technically a standard  solution is a solution whose concentration is known accurately. In order for that to happen, the solute of the solution must be stable ie. not reacting with H₂O and CO₂ in the atmosphere - thus rendering all standard solutions to be primary standard solutions. Otherwise if standard solutions were deliquescent, they wouldn't have a known concentration. So to be precise, I think what you mean to say is that some solutions are deliquescent. Not standard solutions...   

[lzxnl]

Actually, you can make standards of sodium hydroxide solution; their concentrations can be determined by titration, and indeed a preparatory titration with a known concentration of acid is often needed before titrating with sodium hydroxide.
It's just that these aren't PRIMARY standards, which are made from dissolving a known mass of pure solute in a solvent.

[Yacoubb]

Actually, you can make standards of sodium hydroxide solution; their concentrations can be determined by titration, and indeed a preparatory titration with a known concentration of acid is often needed before titrating with sodium hydroxide.
It's just that these aren't PRIMARY standards, which are made from dissolving a known mass of pure solute in a solvent.

Well... technically a standard  solution is a solution whose concentration is known accurately. In order for that to happen, the solute of the solution must be stable ie. not reacting with H₂O and CO₂ in the atmosphere - thus rendering all standard solutions to be primary standard solutions. Otherwise if standard solutions were deliquescent, they wouldn't have a known concentration. So to be precise, I think what you mean to say is that some solutions are deliquescent. Not standard solutions...   

Not all standard solutions are primary standard solutions. A standard solution is any solution with an accurately known concentration. As lzxnl mentioned, an example of a standard solution that is deliquescent is sodium hydroxide. To obtain accurate results, the standard solution must be prepared just prior to use to ensure it does not absorb water from the atmosphere, or react with carbon dioxide in the atmosphere. To say that any standard solution is a primary standard is wrong, because one specific characteristic of primary solutions is that they are not deliquescent/react with gases in the atmosphere, which would be responsible for affecting the originally (known) concentration of the solution. Just clarifying what you have mentioned. But thanks for the input! Much appreciated.

Actually, you can make standards of sodium hydroxide solution; their concentrations can be determined by titration, and indeed a preparatory titration with a known concentration of acid is often needed before titrating with sodium hydroxide.
It's just that these aren't PRIMARY standards, which are made from dissolving a known mass of pure solute in a solvent.

Thanks

[fadzsta1]

Oh jeez sorry for the invalid input.... just to clarify my own chemistry, what you guys are saying is:
- primary standard solutions have to have a known molarity but the solutes of secondary standard solutions don't have to be stable ie NaOH...
- you can have deliquescent standard solutions ie. NaOH(aq) by preparing it to not absolve water or CO2 from the atmosphere.... (ps. how is that done in this case? clearly haven't done this area in depth sorry haha)

If you guys can clarify these statements, that would be appreciated. Sorry for the crappy response though Yacoubb! 

[Yacoubb]

Oh jeez sorry for the invalid input.... just to clarify my own chemistry, what you guys are saying is:
- primary standard solutions have to have a known molarity but the solutes of secondary standard solutions don't have to be stable ie NaOH...
- you can have deliquescent standard solutions ie. NaOH(aq) by preparing it to not absolve water or CO2 from the atmosphere.... (ps. how is that done in this case? clearly haven't done this area in depth sorry haha)

If you guys can clarify these statements, that would be appreciated. Sorry for the crappy response though Yacoubb! 

It's fine!
- I'd say that the primary standard solution must be stable, whilst standard solutions which are not primary are not necessarily stable.
- The way to prepare a standard solution:
• Add a measured amount of solute and add it to the volumetric flask.
• Add water to the half-way point of the flask; shake the flask to ensure the solute dissolves in the water.
• Add water to calibration line and then shake again to ensure the solute has fully dissolved. You now have a standard solution which you should use as the titrant or analyte immediately, to avoid the concentration of the solution being affected.
- A primary standard solution is not deliquescent; standard solutions can be deliquescent.

Hope this is clarified !

[Yacoubb]

Hi guys trying to resurrect my thread!

To those who have completed Chemistry and have done ample practice exams, what patterns are seen in VCAA exams in terms of questions about biofuels/alternative sources of energy? I'm trying to construct a summary sheet where I can collate all these questions. Thanks!