Chem 1008 Lab Name 1. Given the following table of salts answer the questions be

Question

Chem 1008 Lab Name 1. Given the following table of salts answer the questions below Lattice Energy (k/mole MP Salt LiF LiCl LiBr Lil MgCh MgO 1017 828 787 732 2527 3890 845 610 550 450 714 2800 A. Justify the observed melting points of LiF versus LiCl using Coulomb's Law. B. Compare LiCI versus MgCl2 using Coulomb's Law. C. Compare MgCl2 versus MgO using Coulomb's Law. The dipole shifts as more chlorine's are added to the structure, with no net dipole resulting in carbon tetrachloride. Given the 5 following structures and net dipoles listed below place the compounds in order of melting point, from highest to lowest. Briefly justify your answer. 2. Net dipole "-1.920 Net dipole -1.600 Net dipole -1.04D -00 No net Eipole No net digale CH2Clg CHO

Explanation / Answer

1. A The strength of the ionic bond depends on Coulomb's law for the force acting between two charged particles where larger force means stronger ionic bond. According to Coulomb's law,

F=kq1q2 / r2  

k is a constant; q1q1 and q2q2 are the charges on the ions and r is the distance between the ions.

So the larger the charge the greater the force, and the smaller the radius the smaller the distance between the ions (smaller r in the equation) again leading to a stronger force.

In LiCl and LiF cation Li+ is common for both the salts so on comparing F- and Cl- , fluorine ions are smaller than chlorine ions. So, LiF will have higher bond strength than LiCl which means it will have higher melting point since stronger bonds need more energy to break.

B. The melting point of MgCl2 is considerably higher, because the charges on the magnesium ions are larger than those on the lithium ions.
The electrostatic force of attraction between the ions depend on charge of ions and the distance between them according to the equation above so, Mg2+ has a higher charge and a smaller size than Li+. Higher charge and smaller size results in a stronger ionic bond between Mg2+ and Cl- because of the larger coulombic interactions between them ,so more energy is needed to separate them during melting.

Lattice energy is the energy required to convert one mole of ionic solid to it's constituent ions in the gas phase. It depends on the magnitudes of the charges of the ions and on the distance between them. Greater is the charge on the ion and lesser is the distance between the ions more is the lattice energy of the ionic solid. Therefore, MgCl2  has considerably high lattice energy than LiF.

C.  In MgO the distance between Mg2+ and O2- is less than the distance between Mg2+ and Cl- in MgCl2. Moreover, higher charge of O2- than Cl- creates larger coulombic interactions between them ,so more energy is needed to separate them during melting and for this reason, melting point of MgO is very high than MgCl2and for the same reason lattice energy of MgO is greater than MgCl2.

2. Dipole moment determines the percentage of the ionic character of a covalent compound. Higher the difference in the electronegativity among the elements present in the compound, higher will be the polarity of the compound and thus the percentage of ionic character will be more, followed by higher bond strength. Thus bond strength directly depends on dipole moment. We know already that melting and boiling points also depends on bond strength as stronger the bonds would be, higher the energy will be required to break it thus the melting and boiling points would also be high. Hence, melting and boiling points are proportional to the dipole moment.

So, according to the concept mentioned above the order of melting pont is :

CH3Cl > CH2Cl2 > CHCl3 > CCl4 > CH4

CCl4 and CH4 both has zero dipole moment but melting point of CCl4 is greater than CH4 because CCl4 contains many more electrons which can generate an instantaneous dipole moment more readily than CH4.

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