Question

Lactic acid, CH3CH(OH)COOH, is a solid at 298.15 K. The standard reaction enthalpy for the combustion of lactic acid,cH°, is-1344 kJ mol-1. What is thestandard enthalpy of formation of lactic acid? As part of your answer, write the reaction to which the standard reaction enthalpy of combustion of lactic acid refers.

Answer 1

CH3CH(OH)COOH can shortly be written as C₃H₆O₃.

Writing Combustion reaction as

C₃H₆O₃ + 3O₂   3CO₂ + 3H₂O  H_r = -1344 kJ/mol

Now, For calculating standard enthalpy of formation of lactic acid, we have to use the standard enthalpy of formation of CO₂ and H₂O.

Standard enthalpy of formation of CO₂ = -393 kJ/mol

and, Standard enthalpy of formation of H₂O = -285 kJ/mol

So, we can write for the given reaction that

H_r = 3Hf_CO2 + 3H_^fH2O - H_^fC3H6O3

So, -1344 = 3(-393) + 3(-285) - H_^fC3H6O3

So,   H_^fC3H6O3 = -1344 + 1179 + 855

So,   H_^fC3H6O3 = +690 kJ/mol