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# annoying chemistry question!! watch

1. Hi everyone,

basically ive got a question which i am finding it hard to understand the reasoning behind.http://www.ocr.org.uk/Images/65361-q...d-elements.pdf

question 6D.

i dont understand the last bit of the answer, which in the mark scheme says, that the enthalpy change of solution is affected more by lattice enthalpy, than enthalpy change of hydration.

2. anybody?
3. (Original post by mabz123)
anybody?
The enthalpy of solution is a balance between the lattice enthalpy and the hydration enthalpy of the ions...

The process that a salt has to go through when it dissolves is:

step1: KCl(s) --> K+ + Cl-

step 2: K+ + Cl- + (aq) --> K+(aq) + Cl-(aq)

Step 1 is the lattice enthalpy and is endothermic, while step 2 is the sum of the hydration enthalpies and is exothermic.

Hence the overall value is the sum of the two steps ...
4. but how does lattice enthalpy have a greater effect, i mean why isnt it enthalpy change of hydration?
5. (Original post by mabz123)
but how does lattice enthalpy have a greater effect, i mean why isnt it enthalpy change of hydration?
LE is positive, hydration enthalpy is negative. If the value change is positive then LE has the greater effect on solution enthalpy

Solution enthalpy = lattice enthalpy + hydration enthalpy

Solution enthalpy becomes more positive therefore lattice enthalpy is more important than hydration enthalpy in the comparison ...
6. (Original post by mabz123)
but how does lattice enthalpy have a greater effect, i mean why isnt it enthalpy change of hydration?
They both have the same influence, they are are of similar orders of magnitude. The importance is the balance between the two. The hydration enthalpy is more exothermic than the lattice enthalpy is endothermic making to process of dissolution exothermic.
The reason the lattice enthalpy appears to dominate is because there is more of a difference between the values for the two compounds than for the hydration enthalpy. It is the difference between the enthalpy changes for the two processes which is ultimately what is important, not their absolute value.

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Updated: April 4, 2013
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