Acids and Bases, Ionization of water

[MedPR]

Three questions!

Autoionization of water is endothermic. Why does the pH decrease as you increase the temperature?

One If you increase the temperature, the equilibrium shifts towards the products, but the products have both OH- and H3O+ in equal molar concentrations.. so why doesn't the pH stay the same?

TwoAlso, insoluble bases (like Mg(OH)2) are obviously very weak bases in aqueous solution, but does the insolubility affect the protonation of Mg(OH)-? For instance, which is a weaker base, Mg(OH)- or ClO-?

Three
Why does the concentration of the acid contribute to determining the percent ionization of the acid?
If you have a strong acid, you assume 100% will ionize in water, why would that vary with increasing or decreasing concentration?

Here's the actual question, EK 1001 Chem #749
"Which of the following factors determine the percent ionization of an acid?"

I. Temperature of solution
II Identity of acid
III [acid]

A. II only
B. I and II only
C. II and III only
D. I, II, and III

I thought it would be I and II only.

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[MT Headed]

1. pH is defined as the negative log of the concentration of H+. As you heat up the water, you get more H+ and you get more OH-. The concentration of H+ went up. So did the Kw, it will be now be larger than 10^-14.

 

[MT Headed]

3. You are correct, strong acids will dissociate 100%. This question must be aimed at weak acids. A weak acid will dissociate until it reaches its equilibrium, known as its Ka, which is defined as [A-][H+]/[HA]. The system will always move towards this equilibrium.

So now let's up the concentration of the original acid. In fact let's keep adding acid until the equilibrium concentration of [HA] is four times its original equilibrium value. What happened to the concentrations of [H+] and [A-]? They each doubled, and equilibrium constant is maintained.

Do you have more acid in solution? yes. Is it more acidic (i.e. is the concentration of H+ higher)? yes. But what about this percent ionization stuff? We have four times as much [HA] floating around. We have only twice as much [A-] floating around. As a percentage, a smaller percentage of the stuff is floating around as [A-] and a larger percentage of the stuff is floating around as [HA]. The percent ionization is smaller.

You can prove this with algebra, and you can even prove this with numbers. Hell you can do an ICE table and see it for yourself. But for the MCAT I prefer to think about these things conceptually.

 

[chiddler]

Whoah wow! I didn't realize that 4x[weak acid] = only 2x protons!

thanks MT headed.

graph form of #1

 

[MedPR]

1. pH is defined as the negative log of the concentration of H+. As you heat up the water, you get more H+ and you get more OH-. The concentration of H+ went up. So did the Kw, it will be now be larger than 10^-14.

Why doesn't the increased H+ and increase OH- cancel out?

 

[MedPR]

3. You are correct, strong acids will dissociate 100%. This question must be aimed at weak acids. A weak acid will dissociate until it reaches its equilibrium, known as its Ka, which is defined as [A-][H+]/[HA]. The system will always move towards this equilibrium.

So now let's up the concentration of the original acid. In fact let's keep adding acid until the equilibrium concentration of [HA] is four times its original equilibrium value. What happened to the concentrations of [H+] and [A-]? They each doubled, and equilibrium constant is maintained.

Do you have more acid in solution? yes. Is it more acidic (i.e. is the concentration of H+ higher)? yes. But what about this percent ionization stuff? We have four times as much [HA] floating around. We have only twice as much [A-] floating around. As a percentage, a smaller percentage of the stuff is floating around as [A-] and a larger percentage of the stuff is floating around as [HA]. The percent ionization is smaller.

You can prove this with algebra, and you can even prove this with numbers. Hell you can do an ICE table and see it for yourself. But for the MCAT I prefer to think about these things conceptually.

Thank you, that makes sense 🙂

 

[chiddler]

Why doesn't the increased H+ and increase OH- cancel out?

pH is defined strictly in terms of H+ concentration regardless of OH- concentration. The difference in this situation from adding an acid to solution is that when adding acid, some of the H+ can react with the OH- and they cancel. When temp increases, both H+ and OH- concentrations increase. Therefore, there is no canceling going on because the increases are from both sides.

 

[MT Headed]

Why doesn't the increased H+ and increase OH- cancel out?

Chiddler answered this question perfectly well, but just to make sure we beat the concept into you:

(1) pH is the negative log of the H+ concentration. OH- doesn't even enter into this equation

(2) the extra H+ and OH- don't cancel into water because it is hot water and the thermal energy is knocking those water molecules apart, making more H+ and more OH- than you'd expect to find at 25 degrees C

(3) didja notice that in hot water, pH + pOH < 14? the pH and the pOH both went down (because H+ and OH- concentrations both went up)

(4) the human body is not at 25 degrees, so you might get a question asking about the pH of pure deionized neutral H2O in a human cyst or something. It is <7 at 37deg.

 

[MedPR]

pH is defined strictly in terms of H+ concentration regardless of OH- concentration. The difference in this situation from adding an acid to solution is that when adding acid, some of the H+ can react with the OH- and they cancel. When temp increases, both H+ and OH- concentrations increase. Therefore, there is no canceling going on because the increases are from both sides.

Chiddler answered this question perfectly well, but just to make sure we beat the concept into you:

(1) pH is the negative log of the H+ concentration. OH- doesn't even enter into this equation

(2) the extra H+ and OH- don't cancel into water because it is hot water and the thermal energy is knocking those water molecules apart, making more H+ and more OH- than you'd expect to find at 25 degrees C

(3) didja notice that in hot water, pH + pOH < 14? the pH and the pOH both went down (because H+ and OH- concentrations both went up)

(4) the human body is not at 25 degrees, so you might get a question asking about the pH of pure deionized neutral H2O in a human cyst or something. It is <7 at 37deg.

Ah I got turned around with [OH]- and pOH. I was thinking that H+ will increase, causing the pH to go down, but OH- would also increase, causing the pOH to go down also.. but that doesn't make sense anyway.

Thank you both.