What do we use to determine the enthalpy changes of solution?
Hess's law
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What is involved when an ionic lattice dissolves in water?
Breaking up the bonds in the lattice and forming new bonds between the metal ions and water molecules
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eg ionic equation for dissolving MgCl2
MgCl2(s) + aq= Mg2+(aq) + 2Cl-(aq)
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What must happen when an ionic substance dissolves?
The lattice must be broken up
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What is enthalpy of lattice dissociation equal to?
energy needed to break up the lattice (to gaseous ions)
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What is this step?
Endothermic
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What does size of lattice enthalpy depend on?
Size and charge on ion.
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What makes a strong lattice?
Smaller ion and higher its charge
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Enthalpy of Solution=
Enthalpy of lattice dissociation + sum of enthalpy of hydration
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or Enthalpy of solution=
Enthalpy of lattice formation + sum of enthalpy of hydration
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What is enthalpy of solution?
endothermic
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What are hydration enthalpies and why?
Exothermic as energy is given out as water molecules bond to metal ion
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What are the -ve ions attracted to?
Slightly positive hydrogen on the polar water molecule
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What are the positive ions attracted to?
Slightly negative oxygen on the polar water molecule
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The higher the charge density ...
The greater the hydration enthalpy (eg smaller ions or ions with larger charges)
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Why?
Ions attract the water molecules more strongly
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eg with fluorine and chlorine? Mg and Barium
Fluorine ions have more -ve hydration enthalpies than chloride ions. Mg ions have a more negative hydration enthalpy than barium ion
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What is enthalpy of solution generally not very.?
endothermic or exothermic
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what does this mean?
hydration enthalpy same as lattice enthalpy
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When is a substance more likely to be soluble
if enthalpy of solution is exothermic
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Why is a substance insoluble?
lattice enthalpy is much larger than the hydration enthalpy and isn't energetically favourable to break up the lattice making enthalpy of solution endothermic
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What do we consider to get full picture about solubility?
entropy
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What happens when a solid dissolves into ions
the entropy increases as there is more disorder as solid changes to solution and number of particles increases.
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What can make delta G negative even if solution endothermic ?
positive delta S. especially at high temperatures
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For salts where delta H solution is exothermic the salt will?
always dissolve at all temperatures
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What will delta G be? delta H? delta S?
-ve, -ve, +ve due to the increased disorder as more particles so -TdeltaS always -ve
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For salts where delta H solution is endothermic the salt.
may dissolve depending on whether the -TdeltaS value is more -ve than the delta H value is +Ve
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When will it dissolve?
If delta G is -ve
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What will delta H be? -TdeltaS
+ve, +ve due to inc in disorder as more particles so -TdeltaS always -ve
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What does an inc in temp do?
Makes more likely that delta G will become -ve making reaction feasible and salt will dissolve.
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Other cards in this set
Card 2
Front
What is involved when an ionic lattice dissolves in water?
Back
Breaking up the bonds in the lattice and forming new bonds between the metal ions and water molecules
Card 3
Front
eg ionic equation for dissolving MgCl2
Back
Card 4
Front
What must happen when an ionic substance dissolves?
Back
Card 5
Front
What is enthalpy of lattice dissociation equal to?
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