Question

Vapor pressure of a pure liquid lab experiment

Why is the value of the heat of vaporization for sucrose greater than water? comment on the effect of a non-electrolyte such as sucrose on the vapor pressure and delta vap H.

What can be the possible systematic source of error in this experiment?

Answer 1

Note: Vapour pressure is basically the tendency of a material to escape to the environments via formation of gas. From thermodynamics, entropy increases when we move from liquid to gas phase.

Explanation: 1. If we are considering the pure sucrose and water. Sucrose has to move from solid Liquid Gas phase. However water has to move from Liquid to Gas form only. So the more amount of internal energy is needed to break the barrier in case of sucrose. Hence we need to provide more internal energy. Heat (enthalpy) of vaporization is more for sucrose.

2. Considering the sucrose-water solution and pure water. The same will be observed. Heat of vaporization for water-sucrose is higher compare to pure water. Because sucrose-water is a non-electrolyte and non-volatile solution mixture. Where sucrose interacts with Water molecule through H-bonding and occupies the surface of solvent (water) and lowers the tendency of water to escape (lesses entropy for sucrose-water to pure water). which results in decrease of vapour pressure.

3. lnP_vap = -H_vap/R (1/T) + C

We can plot a graph between lnP_vap and (1/T) to obtain a graph and -H_vap/R be the slope of graph.

4. The error which can appeared mathematically will be

a. If we plot the P vs T, the increase in P will higher compare to T and its difficult to obtain a standard value of H_vap. Which can be avoid by using above equation.