Energy and the Formation of Ions
Formation of an ion pair by transfer of an electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. from an Li atom to an H atom results in an overall lowering of the total energyA system's capacity to do work. of the two nuclei and four electrons involved. How and why this occurs is best seen if we break ion-pair formation into three simpler steps and consider the energy change involved in each. The three steps are
1 Removal of the 2s electron from an Li atom to form an Li+ ion.
2 Addition of that same electron to an H atom to form an H– ion.
3 The coming together of the two ions to form an ion pair.
The energy required in step 1 to remove an electron completely from an isolated atom is called the ionization energyThe quantity of energy required to remove an electron from a neutral atom or molecule or from a positive ion.. The ionizationA process in which an atom, molecule, or negative ion loses an electron; a process in which a covalent molecule reacts with a solvent to form positive and negative ions; for example, a weak acid reacting with water to form its conjugate base (an anion) and a hydrogen (hydronium) ion. energy of lithium is 520 kJ mol–1. In other words, 520 kJ of energy is needed to remove a moleThat chemical amount of a substance containing the same number of units as 12 g of carbon-12. of 2s electrons from a mole of isolated lithium atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. in order to form a mole of isolated lithium ions. Alternatively we can say that 520 kJ is needed to ionize a mole of lithium atoms. While energy is needed to accomplish step 1, we find that energy is released in step 2 when a hydrogen atom accepts an electron and becomes a hydride ion. The reason for this can be seen in the Formation of LiH ion pair dot densityThe ratio of the mass of a sample of a material to its volume. diagram. The second electron acquired by the hydrogen atom can pair up with the electron already in the 1s orbitalA mathematically defined region of electron density around one or more atoms; a wave function that defines the properties of a particular electron in an atom or molecule. without contradicting the Pauli exclusion principleThe statement that no two electrons in an atom can have the same set of four quantum numbers; the principle leads to the rule that only two electrons (having opposite spin) can occupy an atomic orbital.. As a result, the new electron can move in close enough to the hydrogen nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass. to be held fairly firmly, lowering its energy significantly. Although the paired electrons repel each other somewhat, this is not enough to offset the attraction of the nucleus for both. Since the energy of the electron is lowered, the law of conservation of energy requires that the same quantity of energy must be released when a hydrogen atom is transformed into a hydride ion. The energy released when an electron is acquired by an atom is called the electron affinityThe energy change that occurs as an atom or negative ion accepts an electron. The first electron affinity applies to a neutral atom combining with an electron; the second electron affinity applies to a minus-one ion accepting an electron; etc. Sometimes defined as negative when the negative ion is more stable than the neutral atom and sometimes defined as positive for the same circumstance; check the definition in any source of data.. The electron affinity of hydrogen is 73 kJ mol–1 indicating that 73 kJ of energy is released when 1 mol of isolated hydrogen atoms each accepts an electron and is converted into a hydride ion. Since 520 kJ mol–1 is required to remove an electron from a lithium atom, while 73 kJ mol–1 is released when the electron is donated to a hydrogen atom, it follows that transfer of an electron from a lithium to a hydrogen atom requires (520 – 73) kJ mol–1 = 447 kJ mol–1. At room temperatureA physical property that indicates whether one object can transfer thermal energy to another object. processes which require such a large quantity of energy are extremely unlikely. Indeed the transfer of the electron would be impossible if it were not for step 3, the close approach of the two ions. When oppositely charged particles move closer to each other, their potential energy decreases and they release energy. The energy released when lithium ions and hydride ions come together to 160 pm under the influence of their mutual attraction is 690 kJ mol–1, more than enough to offset the 447 kJ mol–1 needed to transfer the electron. Thus there is a net release of (690 – 447) kJ mol–1 = 243 kJ mol–1 from the overall process. The transfer of the electron from lithium to hydrogen and the formation of an Li+H– ion pair results in an overall lowering of energy.
In the above figure, the energy change in each step and the overall change are illustrated diagrammatically. As in the case of atomic structure, where electrons occupy orbitals having the lowest allowable energy, a collection of atoms tends to rearrange its constituent electrons so as to minimize its total energy. Formation of a lithium hydride ion pair is energetically “downhill” and therefore favored.
A more complete picture of the energies involved is provided by the "Born-Haber CycleA description of the relationship among the lattice energy, ionization energy, electron affinity, and heats of atomization of ionic compounds; used to calculate any one of these quantities from the others." diagram below. It emphasizes that the EA and IP apply to single atoms in the gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. phase, and that other energies may be more important in determining the exothermicity of formation of ionic compounds. The greatest energy released is not the EA, but the lattice energyThe heat energy tranfer into a system as gaseous ions come together to form an ionic compound. Different textbooks define the sign of this quantity differently.. Other energy costs involve formation of single atoms in the gas phase by vaporizationThe formation of a vapor from a liquid; evaporation or boiling. of Li (s) (ΔHvap) to give Li (g), and the dissociationThe breaking apart of one species into two or more smaller species; often applied to ions in a crystal lattice, which dissociate when the ionic solid dissolves in water. Dissociation refers to separation of particles that already exist; ionization refers to the formation of ions from neutral species, as in the ionization of a weak acid in aqueous solutoin. energy of F2 (ΔH dissociation) to give 2F. The sum of all the energies is the energy of formation of the ionic compound, according to Hess' LawThe generalization that if a chemical reaction can be written as the sum of two or more other chemical reactions, then the heat of the overall reaction is the sum of the heats of reaction for the other reactions. A similar rule applies to enthalpy and Gibbs energy..