Chem 1/2 thread!

[psyxwar]

This is a question you can only do with some numbers, and a bit of dodgy chemistry.

The acidity constant for the first dissociation of carbonic acid is around 4.29*10^-7, while the acidity constant for the second dissociation of carbonic acid is around 4.8*10^-11. Let's denote the first constant as K1 and the second constant as K2.

Write out the equilibrium expressions for both dissociations. We have K1 =
and K2 =

Multiplying the two K values together, we get K1*K2 =
Assuming the concentrations of carbonate and carbonic acid are roughly the same, an assumption which often does work out in solutions that are not too dilute, we have
Using the known values of K1 and K2, we have the square root equals 4.53*10^-9, which is less than 10^-7. Therefore, the concentration of H+ is lower than OH- and we have a basic solution.

Alternatively, note that the Ka of HCO3- is 4.8*10^-11, while the Kb (basicity constant) of HCO3- is 2.33*10^-8. You can see that the hydrogen carbonate ion is a stronger base than it is an acid.

mate this is the 1/2 thread!

[Kuroyuki]

This is a question you can only do with some numbers, and a bit of dodgy chemistry.

The acidity constant for the first dissociation of carbonic acid is around 4.29*10^-7, while the acidity constant for the second dissociation of carbonic acid is around 4.8*10^-11. Let's denote the first constant as K1 and the second constant as K2.

Write out the equilibrium expressions for both dissociations. We have K1 =
and K2 =

Multiplying the two K values together, we get K1*K2 =
Assuming the concentrations of carbonate and carbonic acid are roughly the same, an assumption which often does work out in solutions that are not too dilute, we have
Using the known values of K1 and K2, we have the square root equals 4.53*10^-9, which is less than 10^-7. Therefore, the concentration of H+ is lower than OH- and we have a basic solution.

Alternatively, note that the Ka of HCO3- is 4.8*10^-11, while the Kb (basicity constant) of HCO3- is 2.33*10^-8. You can see that the hydrogen carbonate ion is a stronger base than it is an acid.

I was just thinking to myself man I have missed out on a lot of acid base chem. Then I read equilibrium and thought ahh 3/4 stuff.

[lzxnl]

mate this is the 1/2 thread!

How else do you want me to explain the question? What, just by stating that the hydrogen carbonate ion is a stronger base than acid?

[psyxwar]

How else do you want me to explain the question? What, just by stating that the hydrogen carbonate ion is a stronger base than acid?

It's a question from a 1/2 book; explaining it at a higher level using foreign concepts isn't what the question is asking for, nor does it really help explain the question if he isn't familiar with equilibrium.

[Sanguinne]

dw guys, il just ask my chem teacher on what is needed for this question

thanks for the help tho 

[lzxnl]

You can hardly state that the HCO3- ion is a stronger base than acid, as it's an assertion that is not backed up by anything in the course.

[Sanguinne]

When a solution of sodium hydroxide is slowly added to a solution of ethanoic acid, the PH of the mixture gradually increaes.

Correct me if I am wrong, but is this because sodium hydroxide is a base and hence has a pH >7 and when its added to ethanoic acid which has a ph<7 its pH would be forced to increase??

[SocialRhubarb]

Kind of. Your method of thinking is correct, in that the resultant pH of the solution will always be between the pH of sodium hydroxide and the pH of ethanoic acid.

But the pH of a solution is determined by the H+ concentration. By adding NaOH in solution, we add hydroxide ions which neutralise the H+ ions. This leads to a decrease in concentration of H+ ions and an increase in pH.

Thinking of it in terms of two solutions with different pH's isn't 'wrong', but pH isn't a property of a solution in and of itself, it's something determined by hydrogen ion concentration.

[Sanguinne]

thank you, i understand where i went wrong

can anyone explain this?
The pH of a 1M solution of hydrochloric acid is greater than the ph of a 1M solution of sulfuric acid.

I tried to find the pH of the hydrochloric acid but I ended up with 0 which i think is completely wrong.

[SocialRhubarb]

You can have a pH of 0, or even negative pH.

[lzxnl]

thank you, i understand where i went wrong

can anyone explain this?
The pH of a 1M solution of hydrochloric acid is greater than the ph of a 1M solution of sulfuric acid.

I tried to find the pH of the hydrochloric acid but I ended up with 0 which i think is completely wrong.

Sulfuric acid and hydrochloric acid are both strong acids in that HCl will dissociate essentially completely and H2SO4 will dissociate almost completely to form H+ and HSO4-. However, HSO4- is a relatively strong acid, around 1000 times stronger than ethanoic acid, so that will also partially contribute to the [H+]. Hence a 1 M solution of HCl will have less H+ than a 1 M solution of H2SO4 and so the former's pH will be higher.

[psyxwar]

You can hardly state that the HCO3- ion is a stronger base than acid, as it's an assertion that is not backed up by anything in the course.

Pretty sure 1/2 chem teaches you that generally weak acids yield strong conjugate bases and vice versa. I know that a lot of it might be overly simplistic/ not a real explanation for anything, but that seems to be the case in science education where every year you find out what you learnt previously was pretty much wrong.

[lzxnl]

Pretty sure 1/2 chem teaches you that generally weak acids yield strong conjugate bases and vice versa. I know that a lot of it might be overly simplistic/ not a real explanation for anything, but that seems to be the case in science education where every year you find out what you learnt previously was pretty much wrong.

Seriously? Oh the counterexamples to that...the ethanoate ion is a shoddy base, the ammonium ion is a very weak acid, the fluoride ion is a very weak base...