Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: amyrosylily on October 18, 2005, 12:41:22 PM
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hi! this is my first time to be here.
would anyone be kind enough to explain to me why a weak acid is usually less negative than for strong acids and bases? My answer is: because weak acids are not completely ionized in the solution, energy has to be used to break down the bond between the H+ ions and A- ions. Hence, the amount of energy released in the reaction, i.e. the heats of neutralization, becomes smaller. Am I getting into the wrong way?
Besides, I don't understand why experimental results for hydrochloric acid are usually a little less negative than -57.6 kJmol-1, while the others are not. (That's what my teacher told me, without explanation.) Should they be the same?
Please *delete me* Thank you!
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a negative enthalpy change indicates that energy is being "released" by the reaction. Weak acids do not dissociate in water because they have STRONG bonds holding the hydrogen and acid together as you hinted at. therefore if this followed suit in order to break these bonds you are in need to "absorb" energy to break the bonds and therefore have a positive enthalpy change...but im not 100% certain of this
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the magnitude for enthalpy of neutralisation for a weak acid seem to be less because less H+ are available per mole of acid.
the enthalpy change for H+ + OH- -> H2O is always the same value at the same reaction conditions, eg. 1atm, 30C. However, you have to use more moles of weak acid to achieve the same extent of neutralisation compared to using a strong acid, assuming both acids are of the the same basicity. Hence, for a weak acid, the enthalpy change per mole of weak acid is less because you used more weak acid than strong acid to achieve the same extent of neutralisation.
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the magnitude for enthalpy of neutralisation for a weak acid seem to be less because less H+ are available per mole of acid.
Are you telling us that 1 mole of HCl used for neutralization of 1 mole NaOH contains more H+ than 1 mole of acetic acid used for the same purpose?
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not exactly.
1mol of H+ is available for 1mol of HCl solution.
however, 1mol of H+ is not necessary available from 1mole of acetic acid solution. This is because of partial ionisation. Unless the neutralisation reaction proceed to completion (which normally don't happen), only a fraction of the 1mole of acetic acid ionises to produce H+ for neutralisation.
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Unless the neutralisation reaction proceed to completion (which normally don't happen), only a fraction of the 1mole of acetic acid ionises to produce H+ for neutralisation.
If you mix equimolar amounts of NaOH and acetic acid ALL acetic acid will be neutralized - for 0.1M solution only 8e-6M will be in the AcH form. That's below 0.01%!
Even for very weak acids the difference will be neglectable - for HCN pKa=9.3 (still 0.1M solutions) 1.5% of acid is not neutralised. That doesn't explain smaller entalpy change - unless it is smaller by 1.5% only.