Acid dissociation constant thermicity

[PPonte]

The acid dissociation constant as an equilibrium constant is affected by temperature. Right? Thus, could someone tell me if the reaction between a general acid and water:

HA(aq) + H₂O(l) ⇌ A^-(aq) + H₃O⁺(aq)

is exothermic or endothermic?
I think it is endothermic by similiarity with water auto-ionization reaction.

 

[Gokul43201]

The thermicity can be determined by whether Ka increases or decreases with temperature.

Practically speaking, I've found with most strong acids, the thermicity of the dilution of the acid is easily noticeable. Whether this is generally true of all acids, I don't know.

 

[PPonte]

Sorry, but I didn't understand if the reaction is exothermic or endothermic. K_a increases or decreases with higher temperatures?

 

[Hootenanny]

Consider which bonds are broken / made

~H

 

[PPonte]

Then, we broke 1 H-A, 2 O-H and made 3 O-H.

If the reaction is exothermic 3 O-H > H-A + 2(O-H) <=> O-H > H-A
If the reaction is endothermic, O-H < H-A

Right?

 

[Hootenanny]

No, you broke a single HA bond and made a single O-H bond, look at your equation.

Edit sorry PPonte, you are right I didn't fully read your post #5, yes the reaction is endothermic if the bond enthalpy of HA is greater than that of O-H.

~H

 

[pkc111]

The reaction is exothermic.

H+(aq) + OH-(aq) ---> H2O(l) /\H = -57kJmol-1



http://www.wbateman.demon.co.uk/asa2sums/sum2.1/sum2.1.htm"

 

[pkc111]

oops sorry
wrong reaction

 

[Hootenanny]

I've corrected my post above, aplogies PPonte.

~H

 

[PPonte]

That's ok, no problem Hootenanny. Thank you for your help.

 

[GCT]

In this case, I'm pretty sure that it's always exothermic

hint: analyze the net ionic equation.