The common table salt, NaCl, is a representative of an important class of compounds called salts. A salt consists of positive and negative ions, Na+ and Cl- in NaCl. Salts such as KCl, CsCl, CaCl2, CsF, KClO4 NaNO3, and CaSO4 are generally considered ionic compounds, which are composed of (positive) cations and (negative) anions. When they dissolve in water, the solutions contain hydrated positive and negative ions.
A salt is produced in a neutralization reaction or in a direct reaction. For example, NaCl solid can be obtained by
Halide ions are formed when a group 7A element acquires an electron to attain the electronic configuration of a noble gas. Group 6A elements need to acquire two, instead of one, electron to achieve the same, whereas a nitride needs to gain three electrons. Similarly, alkali metals, alkali earth metals, and group 3A elements lose 1, 2, and 3 electrons respectively to attain the same stablilty. The following groups of ions have the same electronic configurations.
|[He] = 1s2||H-||He||Li+||Be2+||B3+|
|[Ne] = 2s22p6||N3-||O2-||F-||Ne||Na+||Mg2+||Al3+|
This list gives you positive and negative ions. Some of them have electronic configurations of some noble gases. Thus, the formation of these ions can be attributed to the tendency of an atom to acquire the electronic configuration of its nearest noble gas on the periodic table of elements.
Transition metal ions also appear frequently, some of the common mono-atomic ions of transition metals are given below with their electronic configurations indicated. The stable cores of noble gases are indicated by , whereas the number of d electrons are indicated by a superscript number after d.
|Sc2+||[Ar] 3d1||Color ion|
|V2+||[Ar] 3d3||V2+ (aq)|
|V3+||[Ar] 3d2||V3+ (aq)|
|Cr3+||[Ar] 3d3||Cr3+ (aq)|
|Mn2+||[Ar] 3d5||Mn2+ (aq)|
|Fe2+||[Ar] 3d6||Fe2+ (aq)|
|Fe3+||[Ar] 3d5||Fe3+ (aq)|
|Co2+||[Ar] 3d7||Co2+ (aq)|
|Ni2+||[Ar] 3d8||Ni2+ (aq)|
|Cu2+||[Ar] 3d9||Cu2+ (aq)|
The ammonium ion and some anions from oxy-acids are typical poly-atomic ions. Some of these are given so that you will be familar with them.
|N(CH3)H3+||Methyl ammonium ion,|
any number of H can be replaced by a methy group
A salt consists of positive and negative ions, and the stoichiometry is determined by balancing the charges so that the salt as a whole is neutral.
The same amount of energy will be released when the ions are condensed from a gaseous state to a solid state. In this view, the released energy is called energy of crystallization, Ecryst. Since energy is released, the sign is negative for such a quantity. Thus, we have
In 1919, M. Born and F. Haber separately devised a method to calculate the lattice energy from known thermodynamic data such as,
|Hsub||Enthalpy of sublimation,|
|Hmelt||Enthalpy of melting,|
|Hvap||Enthalpy of vaporization,|
|Hf||Enthalpy of formation,|
|Ecryst||Energy of crystallization,|
|H||Enthalpy of reaction,|
|D||Bond (dissociation) energy|
|IP||Ionization potential, or ionization energy, IE|
Br2(l) --Hvap ® Br2 (g) --D ® 2 Br(g) --2EA ® 2 Br-(g)
The lattice energy corresponds to the reaction
To put all the information together, we may now draw the cycle:
THE BORN-HABER CYCLE
Note that we have used two moles of NaBr in the above diagram.
This scheme shows that we can calculate the lattice energy of NaBr from
some known thermodynamic data. The same can be calculated from reaction
equations and their associated energies. This is illustrated below
|2 Na(s) + Br2(l) ® 2 NaBr(s)||2 Hf|
|2 Na(g) ® 2 Na(s)|
or 2 Na(s) ¬ 2 Na(g)
|- 2 Hsub|
|2 Na+(g) + 2e ® 2 Na(g)
or 2 Na(g) ¬ 2 Na+ + 2e
|- 2 IP|
or Br2(l) ¬ Br2(g)
|2 Br(g) ® Br2(g)
or Br2(g) ¬ 2 Br(g)
|2 Br-(g) ® 2 Br(g) + 2 e
or 2 Br(g) + 2 e ¬ 2 Br-(g)
|- 2 EA|
|Add all the above equations leading to|
|2 Na+(g) + 2 Br-(g) ® 2NaBr(s)||2 Ecryst|
To solve a problem like this one, you have to know what data are required, and where to find them. A handbook is a good source, but you will have to look them up in several tables. To solve this problem requires the following data:
-----------Na+ + Cl(g)-------- | | |-349 |496+244/2 ¯ | Na+(g) + Cl-(g) | | Na(g) + 0.5Cl2(g) | | |108 | | |Ecryst= -788 Na(s) + 0.5Cl2(g) | | | |-411 | ¯ ¯ -------------- NaCl(s) --------------
U = - Ecryst
= 788 kJ/mol (lattice energy)
The value calculated for U depends on the data used. Data from various sources differ slightly, and so is the result. The lattice energies for NaCl most often quoted in other texts is about 765 kJ/mol.
Compare with the method shown below
|Na(s) + 0.5 Cl2(l) ® NaCl(s)||- 411||Hf|
|Na(g) ® Na(s)||- 108||-Hsub|
|Na+(g) + e ® Na(g)||- 496||-IP|
|Cl(g) ® 0.5 Cl2(g)||- 0.5 * 244||-0.5*D|
|Cl-(g) ® Cl(g) + 2 e||349||-EA|
|Add all the above equations leading to|
|Na+(g) + Cl-(g) ® NaCl(s)||-788 kJ/mol = Ecryst|
There is a another method based on principle of physics to evaluate the lattice energy, and some examples are given in the discussion enthalpy of hydration and lattice energy.
Born-Haber cycle enables us to calculate lattice energies of various compounds. For salts containing polyatomic ions, the Born-Haber cycle is not as useful. Some other means have to be used to evaluate the lattice energy or energy of crystallization.
|Comparison of Lattice Energies (U in kJ/mol) of Some Salts|
Among the mono-valent salts, the lattice energy decrease when the sizes of the ions increase.
Comparing the lattice energy for salts with one divalent ions leads to the same conclusion. The lattice energy decrease when the sizes of the ions increase.
The lattice energy of salts involving a divalent ion are much higher than those of monovalent salts, because much more energy is required to separate these ions.
Discussion: Carbon monoxide is a gas, and C-O bond is predominant covalent. Some covalent character remain in the ionic compound
Identify the electronic configuration of ions
The noble gas beside Cl is argon.
What ions have the same electronic configuration as Ne?
Discussion: Is the process endothermic or exothermic?
Skill: Give appropriate names for various chemical processes and know if the process endothermic or exothermic?
Discussion: EA is the amount of energy released when a atom acquires an electron. Is the process exothermic?
Identify the chemical reactions involving these types of energy:
H_sub, H_melt, H_vap, H_f, H (enthalpy of reaction), D, IP, and EA
Note: Br2(l) -> Br2(g) -> 2Br(g)