Good Q on acidic environment (1 Viewer)

[lfc_reds2003]

i am struggling to get a sufficient answer which would be worth the full 4 marks....


thanks u guys

 

[lfc_reds2003]

just t show i am not just blindly asking a q...

obviously the ph increases because [H3O+] is decreasing...

but i dun know how to flesh out the answer....

 

[Will Hunting]

No worries, man.

I'm guessing you probably get the basic idea of all this, but you're talking about trying to get the full four marks from it, so here's what I'd do with it:
You might start by jotting down the major terms that the examiners want to be seeing in your answer. These are, diprotic acid, amphiprotic, buffer solution and, maybe, hydrolysis, if you want to showboat

1. Define potassium metabisulphate - It is a diprotic acid because it donates two protons to ionise completely (note that it doesn't matter that this is a salt, and not really an acid, because it still qualifies as a Bronsted-Lowry acid!)

2. What does the first reaction do to pH? Firstly, what is the first reaction? The salt is undergoing hydrolysis (a reaction with water) to form H2SO3-, a weak acid. Note that the salt acts as a BL base and water as a BL acid because the salt "accepts" a proton from the water to form H2SO3- (which is the salt's conjugate acid). The first reaction lowers the pH of the wine since the [H30+] has increased.

3. What about the second reaction? Firstly, define it. H2SO3- acts as a BL base and accepts a proton from water, which acts as a BL acid. Then, SO2 is the conjugate acid and water the conjugate base on the LHS. But, in step 2, H2SO3- was a BL acid and water a BL base, therefore H2SO3- and H20 are amphiprotic. At this point, the pH level increases since the [H30+] is decreasing.

4. Buffer solution!?!? Note that the second reaction is an equilibrium reaction. This is because H2SO3- is a weak acid. This means that the wine now has comparable amounts of H2SO3- and its conjugate base, SO2, i.e. the H2SO3-, H2O eqm mixture is a buffer solution. Noting that a buffer solution is able to maintain a constant pH even when strong acids and bases are added to it (i.e. the wine is at risk of oxidation), the addition of potassium metabisulfate ultimately leads to the stabilisation of the pH level of wine.

For the real thing, man, you wouldn't be expected to write as much as I have, so don't stress about that. I just wrote alot to explain it to you as well as answer the question. If you've got the basic ideas down and you're able to structure your response the way I've done (there's probably large room for improvement there, though ), you'll be set! Make sure you link the points you make back to the question as much as you can, while still demonstrating the knowledge you need to

 

[Casmira]

You can start by talking about that when somethings' an acid it release a H+, that H+, attachesi tself to a water molecule making a Hydronium ion (H30+), you explain that acids contain this H30+ and the more of it, the lower the pH, then you explain how by adding Potassium metabisulphate neutrilises the Hydronium ion back into the water molecule, and as the Hydronium ion concetration decreases, the pH increases ..

 

[lfc_reds2003]

o ritey

sounds good

ty

 

[Casmira]

no worries, i know in short its a short answer but 4 mark questions ask you to relate everything to the question, those sort of questions test if people can mark relationships between topics

 

[lfc_reds2003]

yeh its not just a matter af answering the q but doin a it more

 

[illin]

Just remember to answer the question
will hunting has done all the hard work but did not answer the question specifically
He said that the water acts as a buffer in stabilising the pH of the wine. However, the question asks how the addition of the substance affects the pH. (important word is how)
I had no idea about this question when i first saw it but just remember to answer the question and it will all flow

 

[Will Hunting]

will hunting has done all the hard work but did not answer the question specifically

"The first reaction lowers the pH of the wine since the [H30+] has increased...
At this point, the pH level increases since the [H30+] is decreasing...
The addition of potassium metabisulfate ultimately leads to the stabilisation of the pH level of wine"

The "how" you're after came into points 2,3 and 4, dude

Btw, my purpose wasn't to answer the question myself, but rather to give aural_sax a hand in learning how to go about answering it himself.

 

[lfc_reds2003]

and he did that thanks Will HUntng... u should be a benefactor

 

[Casmira]

Sif wheres my recognition

 

[lfc_reds2003]

sry cas


...


here it is then:

i love you i love you, thankyou so much for clearing up my problems, i was soo stuck and you came and rescued me


lol

 

[illin]

Yeah sry, just pointing it out though, u got the answer, u own chem, ive seen some of ur other posts
will hunting = awesome