Oxidation of primary alcohols: reaction enthalpy
Potassium dichromate (but also potassium permanganate) oxidises primary alcohols in acid conditions to their respective carboxylic acids in a fairly
exothermic reaction. For ethanol the overall reaction can be described as:
K2Cr2O7 + 3/2 C2H5OH + 4 H2SO4 === > Cr2(SO4)3 + K2SO4 + 3/2 CH3COOH + 11/2 H2O
So 1 mol of dichromate oxidises 1.5 mol of alcohol.
Using an electrically compensated calorimeter, the enthalpy of reaction was estimated.
Approx. (but accurately weighed) 3.0 g of K2Cr2O7 (about 0.01 mol) was combined with 10 ml of 50 v/v % H2SO4 and 50 ml of water in the calorimeter and
mixed and temperature read. 4 ml (a large excess) of denaturated ethanol (denaturated with methanol - so called methylated spirits) was then added and
after reaction another 100 ml of water of known temperature was added and temperature then closely followed in the closed calorimeter.
It was clear that at these concentrations the reaction speed was really small, with flask temperature creeping up 0.1 C by 0.1 C over the course of
about 1 h, after which temperature was finally stable (ΔT was 10.7 K). The total heat evolved was then measured with electrical compensation and
ΔH could thus be estimated to be around - 464 kJ/mol, in reasonable agreement with a prior measurement made here:
http://www.sciencemadness.org/talk/viewthread.php?tid=12832&...
A value of - 464 kJ/mol of reaction enthalpy for a reaction that involves no lattice energies is really quite high. For instance for Classic Thermite
® (Fe2O3 + Al) the reaction enthalpy is about - 851 kJ/mol of Fe2O3, not even twice this value!
The high value is reflected also by the high heat generated by oxidising diols (with nitric acid) to synth. oxalic acid, malonic acid etc. And also in
by the fact that mixtures of 100 % glycerine (a triol) and potassium permanganate evolve enough heat to light up thermite reactions…
Next I might have a try with glycol of propan-1,3-diol and with secondary alcohols (oxidation to ketones)…
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