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Topic: Acid dissociation constant vs enthalpy of neutralisation  (Read 249 times)

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Offline kaizer64

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Acid dissociation constant vs enthalpy of neutralisation
« on: November 21, 2019, 04:21:18 AM »
Hello,
I have performed an experiment in which I used calorimetry to determine the enthalpy of neutralisation between several week acids (Ethanoic, Methanoic, Citric and Ascorbic) and Sodium Hydroxide to determine how the enthalpy of neutralisation is dependant on pKa.

I am however struggling to figure out the theory behind it, I initially assumed that the stronger the acid, the higher the enthalpy of neutralisation, due to a higher degree of ionisation, yet my results do not support this.

I have found little concerning this on the internet, so I am wondering if anybody here is able to enlighten me regarding this.



Offline AWK

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Offline Enthalpy

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Re: Acid dissociation constant vs enthalpy of neutralisation
« Reply #2 on: November 21, 2019, 05:26:52 AM »
Welcome, kaizer64! My two cents:

H++OH- :rarrow: H2O, should produce the same heat per mole independently of the nature of the acid and base at reasonable concentration.

While you neutralize the weak acid with NaOH, the acid ionizes more and more, and the heat amount produced by H++OH- depends only on the amount of NaOH used. It doesn't relate with the amount of H+ present before the neutralization.

Much of the ionisation of the acid happens during the measure of heat. It should absorb heat.
 
NaOH shouldn't be too concentrated as its dilution produces significant heat. The heat of dilution of OH- doesn't matter because it gets neutralized, but the heat of dilution of Na+ matters.

Offline kaizer64

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Re: Acid dissociation constant vs enthalpy of neutralisation
« Reply #3 on: November 21, 2019, 06:02:34 AM »
Thank you for your answer.
Would then the pKa of the acid determine the degree of ionisation it undergoes?
How would you then calculate the amount of energy absorbed? (and by effect the total enthalpy change)

Offline mjc123

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Re: Acid dissociation constant vs enthalpy of neutralisation
« Reply #4 on: November 21, 2019, 06:51:59 AM »
The pKa determines the degree of ionisation, but for weak acids this degree is very small, except at very low concentrations. Almost all of the acid will be in the undissociated form. So what happens can be regarded as the sum of two reactions:
HA + H2:rarrow: H3O+ + A-
H3O+ + OH-  :rarrow: 2H2O
The enthalpy for the total reaction will be the sum of the enthalpy for the two reactions. Reaction 2 is the same for all acids. The enthalpy for reaction 1 will depend on the acid concerned. You can't make a direct correlation between ΔH and pKa because pKa depends directly on ΔG, not ΔH. (ΔH1 will also need to be multiplied by (1-α), where α is the degree of dissociation, but for a weak acid where α≈0.01, say, the difference will be small.)

Offline kaizer64

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Re: Acid dissociation constant vs enthalpy of neutralisation
« Reply #5 on: November 21, 2019, 08:40:51 AM »
So, if the concentration I used (1 mol / L) is not low enough, the enthalpy change should be roughly equal for each acid, despite them having different pKa values?
Are there other factors regarding the acids that would result in a varying enthalpy change (given that they are all weak acids of the same concentration and volume reacting with the same concentration and volume of sodium hydroxide) or are variations in my results likely due to inaccuracies.

Offline Enthalpy

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Re: Acid dissociation constant vs enthalpy of neutralisation
« Reply #6 on: November 22, 2019, 12:06:18 PM »
The dissociation changes with the pH. You must evaluate it before and after the neutralisation.

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