Use the Madelung constant for the NaCl lattice (1.74756) for CaF. and the Madelu

Question

Use the Madelung constant for the NaCl lattice (1.74756) for CaF. and the Madelung constant for the CaF_2 lattice (2.1939) for KF_2 Using these estimates and the data from question I, construct thermodynamic cycles to estimate DeltaH^+, for the formation of CaF and KF_2 from the elements in their standard states (This is in some way, the reverse of what we did for NaCl in class. There, we took the DeltaH^+ and derived an experimental lattice enthalpy; here, we start with a theoretical lattice enthalpy. and figure out what the theoretical DeltaH^+ would be for a hypothetical compound)

Explanation / Answer

3. CaF and KF2 are not observed in nature.

Because in CaF, Ca has +1 charge (group IIA elements exhibit stable +2 charge) and is unstable. In KF2, K has +2 charge (group IA elements exhibit stable +1 charge) which is also unstable. The compounds CaF and KF2 are unstable. Therefore they are not observed in nature, not sold by chemical companies.

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