Question

One of the substances (ethylene glycol) appeared to increase in temperature as it evaporated (Change in temperature upon complete evaporation: +1.4). How can this be the case? What alteration to the experiment could have been done to test and see if this evaporation could be exothermic or did that liquid even evaporate at all?

Answer 1

Ethylene glycol has a strong network of hydrogen bonds and also non specific interactions which all need to be overcome during evaporation. This means that ethylene glycol will absorb heat from surrounding and evaporate, which means that the temperature of the system will reduce due to endothermic nature of the process. While, condensation will be just the reverse, ie, exothermic since the excess heat will be released when the vapor form of the substance liquifies back. Thus if the liquid first evaporates and then condenses back we can expect the temperature of the substance to rise.

If the process is endothermic we would expect higher evaporation at higher temperature. While if the process if exothermic then lower temperature would give higher rate of evaporation. To see which one is correct, take equal amount of ethylene glycol in graduated beaker. Keep one beaker in refrigerator for one hour at 2-6 degree Celsius and the other outside at room temperature. Cover both the beakers. Then place both the beakers on window sill or on a table at room temperature for one hour. Remove the cover of the beakers. Come back and check to see which beaker shows greater reduction in volume of ethylene glycol. You will find that the beaker kept at room temperature from beginning will show greater reduction in level of ethylene glycol, indicating that evaporation is favored at higher temperature. This in turn shows that evaporation is endothermic. Also the reduction in level of ethylene glycol in the beaker shows that evaporation has actually taken place.