misc stuff (1 Viewer)

[mojako]

Mmm.. probably someone could give definite answers....

In HSC exam, can we use the term "dinitrogen monoxide" instead of "nitrous oxide"? (just in case I suddenly forget that its called nitrous..)

I know that primary standard means a substance (solid) of high purity and stability which can be mixed with water used to make a standard solution (definition in conquering chemistry). What is a "secondary standard"? I found that term in a summary handout, but it says that primary standard is the standard solution from the Conquering Chemistry's "primary standard" (although in the next paragraph it implies that primary standard is the solid..), while secondary standard is the solution with previously-unknown-concentration which has now been known after titration. So maybe secondary standard actually refers to that handout's primary standard... do you understand my question?

Thank you

 

[CM_Tutor]

Yes, you can use the name dinitrogen monoxide for N₂O - it is its systematic name, after all.

As for the difference between primary and secondary standards:

A primary standard is a substance of sufficiently high stability and purity that a standard (ie a solution of known concentration) can be prepared directly from the solid - for the HSC, usually by dissolution. Sodium carbonate decahydrate, Na₂CO₃.10H₂O, is a primary standard, because I can dissolve x g into 0.5000 L and produce a solution with concentration x / [M(Na₂CO₃.10H₂O) * 0.5000]. So, the concentration can be determined by recording the mass dissolved.

By contrast, a secondary standard cannot be determined in this way. Certainly you can prepare the solution, but its concentration cannot be accurately determined from its mass, wither because of impurities in the solution or because of changes in the solution. NaOH is an example of a secondary standard - it absorbs water from the air (ie. it is hygrascopic), and so any known mass of NaOH contains an unknown amount of H₂O. This cannot be adequately removed by drying, and even if it could it wouldn't matter, as in addition NaOH absorbs CO₂ from the air via:

NaOH_(s) + CO_{2 (g)} ---> NaHCO_{3 (s)}

and so the solid contains an unknown amount of NaHCO₃ impurity as well. For this reason, to use NaOH as a standard, the solution is prepared roughly, and then determined by a method such as titration. To complicate the situation for NaOH, the solution also absorbs CO₂ via the reaction:

OH^-_(aq) + CO_{2 (g)} ---> HCO₃^-_(aq)

and so it must be used as a standard immediately after it is determined, as the concentration drifts with time, and so must be redetermined before use if it has been allowed to stand for any substantial period of time.

Proper analysis often requires both a primary and secondary standard. For example, suppose you needed to determine the concentration of an unknown solution of KOH. You could determine it by titration against a suitable strong acid such as HCl. Unfortunately, HCl is not a primary standard, and so it must be determined as well. So, you'd

1. Prepare a standard solution of a primary standard, such as sodium carbonate.

2. Prepare / obtain a solution of a secondary standard, such as HCl.

3. Use the known concentration of the primary standard, Na₂CO_{3 (aq)} to determine (via titration) the unknown concentration of the secondary standard, HCl_(aq).

4. Since the secondary standard has now been standardised (ie its concentration has been determined), it can be used in a titration to determine the concentration of the unknown KOH that you were interested in in the first place.

Does all this make sense?

 

[mojako]

>> Does all this make sense? <<
Of course it does
Thank you...
But can I (correctly) call the solution of Na2CO3 a primary standard? (not the solid but the solid mixed with liquid water)

Also, another question, equivalence point occurs at pH = 7, right? Some texts say it's at the middle (midpoint) of the almost-vertical part of the titration curve (which is not always at pH=7 depending on the strengths of the acid and base).

Thanks once again.

 

[CM_Tutor]

Originally posted by mojako
>> Does all this make sense? <<
Of course it does
Thank you...
But can I (correctly) call the solution of Na2CO3 a primary standard? (not the solid but the solid mixed with liquid water)

Yes, in my opinion, you can, as a standard is a solution of known concentration. Also, sodium carbonate solution retains its concentration over time - that is, it does not drift due to absorption of atmospheric gases, etc., and so it remains a standard. This is one of those areas where terminology is used somewhat carelessly, and so you must determine for yourself (from context) whether the solid or the solution is being described in any given situation.

Also, another question, equivalence point occurs at pH = 7, right? Some texts say it's at the middle (midpoint) of the almost-vertical part of the titration curve (which is not always at pH=7 depending on the strengths of the acid and base).

Thanks once again.

NO - equivalence point is NOT (that is NOT) necessarily at pH = 7. I have posted in another thread on this subject, so you might want to go have a look at the other titration threads.

THIS IS A COMMON STUDENT MISTAKE - It is vital that you understand the distinction between equivalence point and end point, and that you know that the word neutral or pH = 7, or whatever, should not (generally) be used with these terms, and if they must be used, do so with great caution.

Now, let's consider the equivalence point. It is the point in the titration when the two substances (the analyte and the titrant, or in the HSC, the acid and the base) are present in their stoichiometric ratio. That is, there is exactly enough of each for complete reaction, with neither being in excess. So, what is present? Well, if we ignore the indicator, the only substances present are the salt product and water. Thus, the pH at the equivalence will be solely dependant on the nature of the salt. If the salt is acidic (like NH₄Cl, for example), then the pH at the equivalence point will be acidic. If the salt is neutral - neither acidic nor basic - (like NaCl, for example), then the pH at the equivalence point will be neutral. If the salt is basic (like NaCH₃CO₂, for example), then the pH at the equivalence point will be basic.

So, we have established that the equivalence point may be acidic, basic or neutral. How do we know which? Well, we could look at the pH curve - the inflection (at the midpoint of the near-vertical section) represents the equivalence point, both in terms of pH and volume. Alternately, we must examine the behaviour in water of the cation and anion that make up the salt - do they undergo hydrolysis - reaction with water - and if so, what is the effect of this? This is a major reason that the syllabus covers the concept of pH's of salts.

Now, what about the end point? Well, the end point is the point in a titration where the first permanent colour change of the indicator occurs. IF the indicator is approriate, the end point should be just after the equivalence point. How do we choose an appropriate indicator? We need an indicator whose pH range includes the pH of the equivalence point. Since this has different values for different titrations, we need a variety of possible indicators - which is why we use indicators such as methyl red, methyl orange, bromothymol blue, bromocresol green, and phenolphtalein. If the equivalence point were always at the same pH, then we'd only need one indicator.

 

[mojako]

I see now.
BTW CM_Tutor, did you type this from the start or did you cut and paste it somewhere? I just want to say that u don't actually need to explain this much. And it would certainly waste your time. The only bit that I need is:
"Now, let's consider the equivalence point. It is the point in the titration when the two substances (the analyte and the titrant, or in the HSC, the acid and the base) are present in their stoichiometric ratio. That is, there is exactly enough of each for complete reaction, with neither being in excess. So, what is present? Well, if we ignore the indicator, the only substances present are the salt product and water. Thus, the pH at the equivalence will be solely dependant on the nature of the salt. If the salt is acidic (like NH4Cl, for example), then the pH at the equivalence point will be acidic."
I can of course ask you again if I need more elaboration...

Thanks a lot, though

 

[mojako]

So.. acid tastes sour.
But does [H3O+] (or pH) correspond to the degree of sourness we get if we put the substance on our tongue?? (the lower the pH, the more sour it is)

 

[Xayma]

I think it does, as if you dilute the juice from a lemon (and hence lowering the citric acid concentration) it tastes less sour.

 

[mojako]

Originally posted by Xayma
I think it does, as if you dilute the juice from a lemon (and hence lowering the citric acid concentration) it tastes less sour.

What if we are talking about different acids, eg HCL (can't imagine drinking HCl lol) and citric acid.
Does the sour taste depend on the [H+] only or on the structure of the acid as well?

 

[CM_Tutor]

Originally posted by mojako
I see now.
BTW CM_Tutor, did you type this from the start or did you cut and paste it somewhere? I just want to say that u don't actually need to explain this much.

Thanks for your concern, but I wasn't just posting an answer for you - others will read it, and this is a point that even first year Uni students constantly mess up. It is (IMO) worth posting a full explanation when it's an issue that causes problems for many.

As for your question about sourness / pH, Xayma's explanation sounds reasonable to me, but i suspect it's a question to which we can't give a definite answer - since we don't taste chemicals for safety reasons, I doubt much data exists. Certainly taste depends on structure, and even on geometry, so the taste of citric acid and hydrochloric acid would certainly be different, but I don't know whether the sourness would be different.

 

[speersy]

Originally posted by CM_Tutor
the inflection (at the midpoint of the near-vertical section) represents the equivalence point

The near vertical section, is this an accurate way of determining the equivalence point, could this be a question in an exam?

 

[Xayma]

Yeah the point of inflection is the equivalence point, however any indicator that changes pH over that near vertical range will be sufficient for use it doesnt need to change exactly at the equivalence point (since all change over a range) just very close to it.