OCR A Chemistry Q on Enthalpy changes of hydration

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Sidd1
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How do you work out a hydration value when you have an energy / hess cycle of for eg CaF2? What are the two different routes? Like for lattice enthalpy I know the enthalpy of formation arrow going down to the ionic compound is one route and the other is all the other enthalpies added together including the lattice enthalpy arrow? How is it for hydration , solution etc cycle?
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ASOC01
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Hi, the hydration enthalpy is the enthalpy change when 1 mol of gaseous ions completely dissolve in water to form a solution, so to work that out you will be most likely provided with the lattice enthalpy of formation, and the enthalpy of solution. (CaF2 (s) to Ca2+ (aq) and 2F+(aq).) Note that the route from gaseous ions to ionic solid than dissolves in solution will have the same enthalpy change as the route from gaseous ions straight to dissolving in solution as stated in Hess Law. Hence, all you need to do is to compare the value of lattice enthalpy of formation+solution to that of hydration enthalpy as they should be the same. So let's say Ca2+ (g) and 2F+ (g) forms ionic solid with enthalpy of formation equaling -1000kJ/mol, and enthalpy of solution -100kJ/mol. Then enthalpy of hydration of Ca2+ = -600kJ/mol, so the enthalpy of hydration of F+ would be -1100 - (-600) all divided by 2 = -250kJ/mol.
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Sidd1
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(Original post by ASOC01)
Hi, the hydration enthalpy is the enthalpy change when 1 mol of gaseous ions completely dissolve in water to form a solution, so to work that out you will be most likely provided with the lattice enthalpy of formation, and the enthalpy of solution. (CaF2 (s) to Ca2+ (aq) and 2F+(aq).) Note that the route from gaseous ions to ionic solid than dissolves in solution will have the same enthalpy change as the route from gaseous ions straight to dissolving in solution as stated in Hess Law. Hence, all you need to do is to compare the value of lattice enthalpy of formation+solution to that of hydration enthalpy as they should be the same. So let's say Ca2+ (g) and 2F+ (g) forms ionic solid with enthalpy of formation equaling -1000kJ/mol, and enthalpy of solution -100kJ/mol. Then enthalpy of hydration of Ca2+ = -600kJ/mol, so the enthalpy of hydration of F+ would be -1100 - (-600) all divided by 2 = -250kJ/mol.
Oh okay that makes sense thank you!
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