# Acid dissociation constant thermicity

1. Apr 29, 2006

### 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.

2. Apr 29, 2006

### Gokul43201

Staff Emeritus
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.

3. Apr 30, 2006

### PPonte

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

4. Apr 30, 2006

### Hootenanny

Staff Emeritus
Consider which bonds are broken / made

~H

Last edited: Apr 30, 2006
5. Apr 30, 2006

### 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?

6. Apr 30, 2006

### Hootenanny

Staff Emeritus
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

Last edited: Apr 30, 2006
7. Apr 30, 2006

### 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" [Broken]

Last edited by a moderator: May 2, 2017
8. Apr 30, 2006

### pkc111

oops sorry
wrong reaction

9. Apr 30, 2006

### Hootenanny

Staff Emeritus
I've corrected my post above, aplogies PPonte.

~H

10. Apr 30, 2006

### PPonte

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

11. Apr 30, 2006

### GCT

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

hint: analyze the net ionic equation.