Solution Enthalpy Change

To form a solution, there are three processes:

  1. Separation of solute molecules, overcoming the attraction forces between solvent molecules, this requires energy input. (Enthalpy greater than zero)
  2. Separation of solvent molecules, overcoming the attraction forces between solvent molecules, this also requires energy input. (Therefore, enthalpy greater than zero.)
  3. Formation of nteractions between solute molecules and solvent molecules

Let’s give symbols for the energy changes of the each step mention above:

ΔH1 – for separation of solute molecules (ΔH1 > 0, absorbs heat/energy)

ΔH2 – for separation of solvent molecules (ΔH2 > 0, absorbs heat/energy)

ΔH3 – for formation of interactions between solute molecules and solvent molecules (ΔH3 < 0, releases heat/energy)

So the total enthalpy change is: ΔH = ΔH1 + ΔH2 + ΔH3

 

When will addition of solute to solvent be accompanied by cooling then?

Answer is, when ΔH1 + ΔH2 + ΔH3 is > 0, because cooling means total enthalpy > 0. Therefore Absolute value of ΔH < Absolute value of (ΔH1 + ΔH2) (remember ΔH1 + ΔH2    is always positive, so if the solution formation is ended up with cooling, total enthalpy is greater than zero for the reason of having greater absolute value of (ΔH1 + ΔH2 ).

When will addition of solute to solvent be accompanied by getting hotter then?

Answer is, when ΔH1 + ΔH2 + ΔH3 is < 0, because hotter means total enthalpy < 0. Therefore Absolute value of ΔH > Absolute value of (ΔH1 + ΔH2) (remember ΔH3 is always negative, so if the solution formation is ended up with getting hotter, total enthalpy is less than zero for the reason of having greater absolute value of ΔH3.

 

  • Subject : Science
  • Topic : Chemistry
  • Posted By : Jason
  • Created on : 02-18-2011

Watch Our Demo

As featured in the