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“I performed an experiment using egg shells (calcium carbonate) to represent teeth and reacted the calcium carbonate with acids. When measuring the rate of reaction citric acid had a higher rate of reaction and from my research i saw that citric acid forms a more stable compound with calcium hence it chelates the calcium more than ethanoic acid.
In the mouth this would remove calcium ions from
Ca10(PO4)6(OH)2(s) + 2H+(aq) ⇌ 3Ca3(PO4)2(s) + Ca2+(aq) +2H2O(l)
that equation hence the position of equilibrium shifts to the right so more tooth decay.
But with egg shells:
CaCO3(s)+ 2H+ (aq) __> Ca+2(aq) +CO2(g)+H2O(l)
there is no equilibrium so if more calcium ions are chelated why does this increase the rate of reaction?
In the mouth this would remove calcium ions from
Ca10(PO4)6(OH)2(s) + 2H+(aq) ⇌ 3Ca3(PO4)2(s) + Ca2+(aq) +2H2O(l)
that equation hence the position of equilibrium shifts to the right so more tooth decay.
But with egg shells:
CaCO3(s)+ 2H+ (aq) __> Ca+2(aq) +CO2(g)+H2O(l)
there is no equilibrium so if more calcium ions are chelated why does this increase the rate of reaction?
8 years ago
Dentistry Question asked by Salesh
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1 Answer
Hi Salesh, your question is not clear. Were you looking at yield or rates? If you were looking at rates did you mean "When measuring the rate of reaction citric acid had a higher rate of reaction... than an ethanoic acid solution of the same concentration?" Rates has nothing to do with equilibrium (yield has). The rate of a reaction depends solely on Temperature, surface area, catalyst and concentration of reactants (pressure for gases). The reason why an equimolar citric acid solution has a higher rate than ethanoic acid is most likely because it is a triprotic acid (3H+ per mole) while ethanoic acid is monoprotic. Also the first pKa of citric acid is lower than that of ethanoic acid, making it a stronger acid. All these factors result in a higher concentration of H+ ions (one of the reactants) in this acid-base reaction. The chelating effect should have less effect on the rate than the proton concentration, but could increase surface area by dissolving the CaCO3 and increasing the dissolved carbonate concentration (base -> other reactant of this acid-base reaction). More information about rates can be found on the flashcard `Kinetics - rate of reaction` on my website: www.alevelchemistryrevision. Hope this answer helps. Christoph
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