Sodium chloride standard enthalpy

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

Sodium Chlorite. The standard enthalpy, Gibbs free energy of formation, and standard entropy for aqueous chlorite ions ate AH° = —66.5 kJ/mol ( — 15.9 kcal/mol), AG = 17.2 kJ/mol (4.1 kcal/mol), and S° = 0.1883 kJ/(molK) (0.045 kcal/(molK)), respectively (107). The thermal decomposition products of NaClO, in the 175—200°C temperature range ate sodium chlorate and sodium chloride (102,109) ... 

As the enthalpy of the dissolved sodium chloride in its standard state according to Henry s law is that of the infinitely dilute solution, A// , for the reaction in Equation (20.74) is... 

Q Calculate the standard enthalpies of formation for the compounds (i) sodium chloride and (ii) potassium iodide. The interionic distances in the compounds are 282 and 353 pm, respectively. Compare your answers with the accepted experimental values for these quantities, which are -411 and -327.6 kJ mol1, respectively. 

The atomisation enthalpy of elemental sodium Afl%tom, the first ionisation energy of atomic sodium Iu the dissociation enthalpy D of gaseous chlorine, the electron attachment energy Ex of atomic chlorine and the enthalpy of formation A//)1 of crystalline sodium chloride can all be taken from standard tabulations of experimental data. An experimental lattice energy UL is thus given by ... 

The standard change of enthalpy for the formation of sodium chloride in an infinitely dilute aqueous solution is thus given by the sum of the standard changes of enthalpy for the last two changes of state. Therefore,... 

The second step shown here combines the terms AHZ (for expanding the solvent) and AH3 (for solvent-solute interactions) and is called the enthalpy (heat) of hydration (AH d). This term represents the enthalpy change associated with the dispersal of a gaseous solute in water. Thus the standard enthalpy of solution for dissolving sodium chloride is the sum of AH and AH yd-... 

The standard lattice enthalpy is the energy absorbed when one mole of gaseous sodium ions and one mole of gaseous chloride ions form one mole of crystalline sodium chloride. It has a negative value. 

According to Hess s law, the standard enthalpy of formation of sodium chloride is equal to the sum of the enthalpy changes in the various steps ... 

The value of AH is an approximate measure of the stability of a substance relative to the elements from which it is made. The standard enthalpies of formation of graphite, diamond, water, ethyne (acetylene, C2H2), ammonia and sodium chloride are shown in Fig. 13.6. The reference states of elements define an energy baseline or sea level . Compounds such as ethyne, for which AHf is positive, and which therefore possess a greater enthalpy than their constituent elements, appear above sea level and are called endothermic compounds. Compounds such as water, ammonia and sodium chloride, for which AHf is negative and which therefore possess a lower enthalpy than their constituent elements, appear below sea level and are called exothermic compounds. 

When strong bases neutralize strong acids in solutions that have molar concentrations of 1 mol dm-3, the enthalpy of the reaction is observed to be -55.83 kJ mol - irrespective of the counter ions (e.g. the chloride ion derivable from HC1 and the sodium ion contained in NaOH) present. For example, when a standard solution (1 mol dm-3) of hydrochloric acid is neutralized by a standard solution (1 mol dm-3) of sodium hydroxide, the change in enthalpy of the reaction is -55.83 kJ mol-1. Because the strong acid HCI and the strong base NaOH are 100% dissociated in aqueous solution, theucutruli/atiun reaction may be written as ... 

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