Can Temperature Affect the Dissociation of Strong Acids?

[qwerty1232]

Hi Everyone,

I was just wondering if you know of any factors that would affect the dissociation of a strong acid. I know that weak acids are affected by factors such as the common ion effect and temperature, but would this apply to strong acids as well?

I only ask because we added sulfuric acid to a solution and then measured the pH. The [H+] indicated by the pH meter suggests that there are less H+ ions than would be from the concentration of sulfuric acid added. Any suggestions for a cause of this would be greatly appreciated!

Many thanks.

 

[Borek]

1. The only difference between weak and strong acids is the Ka value, very high for strong acids. In practice that means they are usually completely dissociated, but to some extent the same effects as in the case of weak acids can be abserved.

2. Sulfuric acid is strong when it comes to first dissociation step (Ka1 around 1000), but second dissociation step is not that strong - Ka2 = 0.01 (or pKa2=2). That means in 0.01M solution second proton is not fully dissociated (if memory serves me well, it is dissociated in around 75% Edit: more like 40%).

See these pages: calculation of pH of polyprotic acid and simplified methods of calculation of pH of polyprotic acid.

 

[qwerty1232]

Basically what happened is we added approximately 10mL 1.8M sulfuric acid, and the pH was 0.71. This suggests that the [H+] concentration is at least 1.95M. Where did the additional H+ ions go?

 

[Borek]

They don't go anywhere, they are not free in the solution in the first place.

Not to mention fact in such a concentrated solution ionic strength and activity coefficients play very important role, so trying to directly convert pH to concentration is a bad idea.

 

[qwerty1232]

I have two acids in this solution (H2SO4 and HOCl), and want to find out how many of the hydrogen ions detected by the pH metre were contributed by the dissociation of HOCl. I am testing at different temepratures, so can't use standard ka. Do you have any suggestions? I was subtracting the total of number of moles of H+ from the assumed number of moles from H2SO4 to obtain this. However, in some instances, I get negative H+ concentrations.

 

[Borek]

Please elaborate what you are really doing. You add more and more information with each post and each time question becomes slightly different, I would prefer to answer the final one instead of all others.

 

[qwerty1232]

Okay, sorry - here goes:

1. Added 25mL of 10% NaOCl solution (diluted 1:50)
2. Added 10mL of KI (shouldn't affect) and 10mL of approximately 2.0M SUlfuric acid (probably a bit less concentrated, I am checking this).
3. Measured the pH of the solution at different temperatures to determine the extent of dissociation of HOCl (from NaOCl)

However, as I said, the pH of some solutions (colder ones - 18ish degrees C) suggest that the concentration of the H+ ions is less than what should be formed by the dissociation of sulfuric acid (strong acid).

For example;
pH=0.71
therefore, H+ should be at lesat 0.195 (with 0.0088 moles, due to total 45mL solution)
To find the number of moles H+ from HOCl, we substracted this value (total H+) from the number expected from the disociation of sulfuric acid (approx. 0.01878 moles - 10mL of 1.278M).

However, ended up with -0.010005699 moles!

 

[Borek]

I have a feeling you don't understand the experiment you did.

Your measurements are wrong for several reasons.

First - sulfuric acid is not fully dissociated, so even without adding potassium iodide and sodium hypochlorite concentration of H⁺ would not equal twice the acid concentration. I have explained it in the very first post.

Second - after addition of KI you no longer have hypochloric acid in the solution.

Third - in the mixture there is a reaction consuming H⁺.

 

[qwerty1232]

Yeah, that's probably true - I am a bit confused and stressed! What is the reaction consuming H+?

 

[Borek]

Think what reactions are possible. Hint: it is a redox.

 

[qwerty1232]

We haven't really looked at redox reactions yet. i just know that the oxidation of I- by HOCl or OCl- requires the presence of H+ (provided by H2So4).

 

[Borek]

i just know that the oxidation of I- by HOCl or OCl- requires the presence of H+ (provided by H2So4).

That's the reaction I am referring to. Do you know, or can you guess, what are products?

OCl^- + I^- + H⁺ -> ...

 

[qwerty1232]

Is it Cl-, I2 and H20?.

Do you think there is a way of finding just the concentration of HOCl or OCl- by itself, given that I know the pH? Or would it be too hard given all the other reactions occurring?

 

[qwerty1232]

Can I also ask, on a slightly related topic, do you think a weak acid would dissociate fully if it was heated to a large enough temperature? What sort of temperatures would be required?

 

[Borek]

do you think a weak acid would dissociate fully if it was heated to a large enough temperature?

Doubtful.

 

[qwerty1232]

That's what I assumed, but do you know if there's a reason for this?

Do the factors that affect the strength of an acid (eg. stability of conjugate base due to resonance etc., polarity of molecules) contribute to the enthalpy or entropy of the products and reactants. Possibly if they do, I could relate the fact that they do dissociate fully even at higher temperature back to free energy change?

 

[Borek]

My bet is that long before the temperature gets high enough most weak acids will decompose.

Besides, above 100 deg C you have no solution (unless you plan to add high pressure to the problem, making it even more difficult).