Sodium fluoride, lattice

Figure 7.6 An enthalpy (kJ mol-1) diagram for the formation of the sodium fluoride lattice...

Potassium fluoride crystallizes in a sodium chloride lattice. The length of the edge of the unit cell (sometimes called the cell or lattice constant) has the value 267 pm for KF. 

Sodium fluoride, NaF, is a favorable choice for X-ray analysis of the lattice energy of an ionic crystal. Both Na and F are relatively light atoms, and the Na 3s-radial distribution, though diffuse, is not quite as spread out as the Li 2s shell (single-C values are 0.8358 and 0.6396 au-1, respectively see appendix F), and therefore contributes to a larger number of reflections. 

This calculation is still hypothetical, in that the actual substance formed when sodium metal reacts with difluorine is solid sodium fluoride, and the standard enthalpy of its formation is -569 kJ mol-1. The actual substance is 311 kJ mol-1 more stable than the hypothetical substance consisting of ion pairs, Na+F (g), described above. The added stability of the observed solid compound arises from the long-range interactions of all the positive Na+ ions and negative F ions in the solid lattice which forms the structure of crystalline sodium fluoride. The ionic arrangement is shown in Figure 7.5. Each Na+ ion is octahedrally surrounded (i.e. coordinated) by six fluoride ions, and the fluoride ions are similarly coordinated by six sodium ions. The coordination numbers of both kinds of ion are six. 

The series 6 - 12/21/2 + 8/31/2 - 6/41/2 + 24/51/2 -. .. eventually becomes convergent and gives the value for the Madelung constant for the sodium chloride lattice (the standard description of lattices which have the same form as that adopted by sodium fluoride). The values of Madelung constants for some common crystal lattices are given in Table 7.5. 

The electrostatic contribution to the lattice energy, L, for the sodium fluoride arrangement (the energy required to form gas phase ions from the solid crystalline lattice) is the value of the change in internal energy (i.e. A U) for the reaction ... 

Properties of the alkali fluorides.—The anhydrous alkali fluorides crystallize in the cubic system.8 Lithium fluoride forms regular optohedrons and nacreous plates sodium fluoride crystallizes in cubes, but in presence of sodium carbonate, the crystals are octohedrons. The cubic crystals are frequently en tremies. H. Schwendenwein has discussed the space lattice of the alkali fluorides, and K. Fajans and H. Grimm estimated the distance of the atoms apart in sodium and potassium fluorides to be respectively 2 34 X 10 8 and 2 67 X10-8 cm. and the respective lattice energies to be 210-4 and 192 2 Cals, per mol. The taste of potassium fluoride is acrid and salty. 

Sodium fluoride and sodium chloride both crystallize into the same type of structure. Which do you predict to have the higher lattice energy, NaF or NaCl ... 

The lattice energy of an ionic crystal may be defined as the energy emitted when the correct number of ions emerge from distant locations and station themselves in their appropriate places in the crystal lattice. For a mole of sodium fluoride, for example, one may obtain such a lattice energy by multiplying the potential energy (as given in Equation... 

A fluoride electrode, in which the membrane is a single crystal of lanthanum fluoride doped with europium to increase the conductivity, is one of the best ion-selective electrodes available. Conduction through the membrane is facilitated by the movement of F" ions between anionic lattice sites which in turn is influenced by the F ion activities on each side of the membrane. If the electrode is filled with a standard solution of sodium fluoride, the membrane potential is a function of the fluoride activity in the sample solution only. Thus,... 

Sodium fluoride adopts the NaO type lattice. Estimate the lattice euergy of NaF usiug au electrostatic model. 

Cubic or tetragonal crystals (NaCl lattice), d 2.78. mp 993 . bp 1704 , Poisonous Soly in water (g/100 ml) 4.0 (l5°) 4,3 (25°) 5.0 (10(f). Insol in ale. Aq solns have an alkaline reaction caused by partial hydrolysis. pH of freshly prepd satd soln 7.4. Aq solns etch glass, but the dry crystals or powder may be kept in glass bottles. Sodium fluoride sold as household insecticide must be tinted Nile Blue. LDW orally in rats 0.18 g/kg (Smyth). 

Sodium fluoride adopts the NaCl structure type. Estimate the lattice energy of NaF using an electrostatic model. 

Use the concept of lattice energy to rationahze why sodium fluoride dissolves in water, whereas calcium fluoride does not. Extending this reasoning, would you e q>ect magnesium fluoride to be soluble ... 

As a result of this transfer of one electron, both sodium and fluorine form ions that have the same electron configuration as neon, the noble gas nearest each in atomic number. The attraction between ions is what permits ionic salts such as sodium fluoride to form a strong crystal lattice and gives them a high melting point. 

Hence, if one brings together a good electron donor (such as a sodium atom) and a good electron acceptor (such as a fluorine atom), one might expect a chemical reaction to occur. Electrons are transferred and an ionic compound (sodium fluoride, NaF) is produced. A three-dimensional solid lattice of sodium and fluoride ions is created, where each sodium ion is surrounded by fluoride ions, and each fluoride ion is in turn surrounded by sodium ions. Another very important aspect of such a relation is the fact that energy is released as the product is formed. This release of energy associated with the formation of the reaction products becomes very important in the consideration of the chemistry of pyrotechnics and explosives. 

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