C2H4 + CO + H2O -> C3H6O2 (Propanoic acid)
Can anyone explain to me how this reaction works on molecular basis ? The product will be Propioacid. In what order does the reaction take place ? Google couldn't help me yet, hope here's someone who knows this.

In industry, propanoic acid is mainly produced by the hydrocarboxylation of ethylene using nickel carbonyl as the catalyst.

H2C=CH2 + H2O + CO → CH3CH2CO2H

From Wikipedia!

What else is there to explain? Did you want the full molecular equation with drawn out arrows?