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 Form 4 Chemistry: Energy changes in chemical and physical processes lessons

Meaning of Enthalpy of solution,Molar enthalpy of solution,Lattice energy,Hydration (solvation)energy

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Answer Text:
3. Heat of solution/ enthalpy of solution:
- Is the heat change when a given mass (moles) of a substance is dissolved in a stated amount of solvent (water).
Molar enthalpy of solution:
- Is the heat change when one mole of a substance is dissolved in a stated amount of solvent (water).
- Alternatively;
- It is the heat change when one mole of a substance dissolves in water to give an infinitely dilute solution i.e. (a solution which shows no change in
its properties when more water is added).
- Is determined as;
figreactionrates23720201453.JPG
Where,
Lattice energy:
- Is the leaf evolved when one mole of a compound is formed from its separate gaseous ions.
Alternatively;
- It is the energy required to break the ionic bonds within a crystal (solid) lattice. Hydration (solvation) energy:
- Is the heat evolved when one mole of ions are hydrated by water molecules.
Note:
- Water is a good solvent because it has a high negative(-ve) enthalpy of solvation resulting from powerful interaction between polar molecules and
solute ions due to large
dipoles on water molecules.
- Therefore,
- Heat of solution can either be exothermic or endothermic depending on the magnitude of hydration and lattice energies.
- Solution process occurs into two stages.
Example:
- Consider the dissolution of sodium chloride solid.
(i). Energy is taken in to break the crystalline lattice.
#NaCl(s) to Na^(+)+Cl^(-)
DeltaH=+DeltaH _(lat)=+776KJ#
(ii). Heat is evolved when one mole of ions are hydrated by water molecules
#Na^(+)(g)+Cl^(-)(g)overset(H_2O(l))(to)Na^(+)(aq)+Cl^(-)(aq)
DeltaH_(hydration)= -771 KJ#
figreactionrates23720201456.JPG
-The lattice energy is larger than the hydration energy
hence;- ΔH solution = +ve
(b) Energy level for the diagram for an exothermic dissolving process for solid MX(s)
figreactionrates23720201457.JPG
The lattice energy is smaller than hydration energy hence; ΔH solution = -ve.
Note: All gases dissolve with evolution of heat;
Reason:
- There are no intermolecular forces or bonds to break before hydration occurs.
Worked examples:
1. The equation below represents changes in physical states for iron metal.
#Fe(s) to Fe(l); DeltaH=15.4KJMol^-1
Fe(l) to Fe(g) ; DeltaH= 354KJMol^-1
#
2.Calculate the amount of heat required to change 11.2g of solid into iron to gaseous iron. (Fe=56.0)
Solution:
Total heat needed to convert
1 mole of iron from solid to gas = (15.4Kj + 354Kj); = 369.4Kj;
1 mole = 56g; Thus if 56g requires 369.4Kj;
Then 11.2g =?
=#(11.2times 369.4)/56=73.88Kj#
2.The lattice and hydration enthalpies for lithium chloride and potassium chloride are given the the table below:
figreactionrates23720201458.JPG
solution:
Hsolution=#DeltaH_(lat)+DeltaH_(hydration)#
=-(-861)+(-884)
=-23KjMol^(-1)
II) Potassium chloride:
solution;
Hsolution=#DeltaH_(lat)+DeltaH_(hydration)
=-(-719)+(-695)
=719-695
=-24KjMol^(-1)#