Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: postitme on January 08, 2020, 05:39:29 AM
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Hello all,
I am trying to work out if the thermal decomposition of propylene glycol to its major product , lactic acid, is exothermic.
I haven't done any of this sorta thing since school, which was a long time ago, but have worked out that I need to calculate all the bond enthalpy values on each side of the equation. Im stuck balancing the equation though.
Can anyone help out with this, or know generally whether the decomposition reaction of propylene glycol gives off heat?
Many thanks in advance for any help on this.
Ben
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One of the first rules of the Forum:
whenever asking questions show you have already tried to solve them on your own
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Sorry, I have done some of my own research, which I can show here.
I know I need to compare the bond enthalpies on both sides of the equations. I have calculated those for propylene glycol and lactic acid, but Im not sure how to balance the equation for what else is involved (possibly water?)
CH3-CHOH-CH2OH ---> CH3-CHOH-COOH
So where Im stuck is Ive lost two hydrogen atoms from polypropylene glycol, where did they go? Did the extra O atom come from breaking a water molecule, or O2 dissolved in the solution?
Thanks
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At the risk of possibly going off on a tangent, are you interested in what happens in vivo, or in some other situation?
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Under "thermal decomposition of X" it's usually understood that the starting compound X is alone.
Then, as oxygen and hydrogen tell, 1 molecule of propylene glycol can't make 1 molecule of lactic acid. It needs more glycol, and other molecules are produced too, common ones being H2O and CO2 or CO, but there are many more.
Problem: due to the many expected products, I suppose a simple balanced equation can't be written. Without additional information, I wouldn't try to predict the heat of reaction.
As Babcock_Hall pointed out, other compounds or cells at the glycol would change the products.
Is this really a thermal decomposition, or rather an oxidation?
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Good points, I should have mentioned this is not in vivo, its an aqueous solution in hot copper pipes which leaked out onto insulation material.
You are of course correct and there are other products formed (oxalic acid, formic acid etc), so a complete balanced equation would be very tough given the variables, but Im just trying to get a ball park result for the major product formed from the degradation of propylene glycol (the lactic acid).
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Try simple oxidation with O2.
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Then it's an oxidation rather than a decomposition. Yes it is exothermic, because it creates C=O and O-H bonds from O=O and C-H bonds.
To evaluate the heat of reaction by summing bond energies, don't forget the O=O in the reactants and the O-H in the products. But you can neglect the many bonds that stay "identical" from propylene glycol to lactic acid.
Alternately, you can find in a table the enthalpies of formation of propylene glycol (-501kJ/mol), lactic acid (-694kJ/mol), water (-286kJ/mol as a liquid) and molecular oxygen (zero per convention). The difference is the heat of reaction. By convention, heat received when forming a compound is positive, so do it on a clear head day, and compare the sign with well-known compounds or reactions, like usual combustions.
The situations where this heat of reaction matters in the wild must not be common. If the oxidation stops at the acid, it's probably slow, and then the heat has time to escape.