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InterviewSolution
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Magnitude of crystal lattice enthalpies generallyA. increase with increasing charge and size of ionsB. increase with decreasing charge and size of ionsC. increase with increasing charge and decreasing size of ionsD. increase with decreasing charge anf increasing size of ions |
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Answer» Correct Answer - 3 The sizes of the lattice enthalphy increase as the ionic charge densities increase and therefore as the strength of electrostatic attractions within the crystal increase. The charge//radius ratio is the ionic charge divided by the ionic radius in angstroms. This is a measure of the charge density around the ion. Hydration enthalpies vary in the same order. Crystal lattice enthalpies and hydration enthalpies are generally much smaller in magnitude for molecular solids than for ionic solids. Hydration and the effects of attractions in a crystal oppose each other in the dissolution process. Hydration enthalpies and lattice anthalpies are usually of about the same magniyude for low-charge species, so they often nearly cancel each other. As a result, the dissolution process is slightly endothermic for many ionic substances. Ammonium nitrate `(NH_(4)NO_(3))` is an example of a solid salt that dissolves in water in a very endothermic process, absorbing heat from its surroundings. This property is used in "instant cold packs" for early treatment of injuries, such as sprains and bruises to minimize swelling. For this purpose `NH_(4)NO_(3)` and `H_(2)O` are packaged in a plastic bag in which they are kept separate by a partition that is easily broken when squeezed. As the `NH_(4)NO_(3)` dissolves in the `H_(2)O`, the mixture absorbs heat from its surroundings and the bag becomes cold to the touch. Some ionic solids dissolve with the release of heat. Examples are anhydrous sodium sulphate `(Na_(2)SO_(4))`, calcium acetate, `Ca(CH_(3)COO)_(2)`, calcium chloride, `CaCl_(2)`, and lithium sulphate hydrate, `Li_(2)SO_(4). H_(2)O`. As the charge-to-size ratio (charge density) increases for ions in ionic solids, the magnitude of the crystal lattice anthalpy usually increases more than the hydration enthalphy. This makes dissolution of solids that contain highly charged ions (such as `AlF_(3), MgO` and `Cr_(2)O_(3))` very endothermic. As a result, these compounds are not very soluble in water. |
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