Consider the lattice energies of the following Group 2Acompounds: BeH2, 3205 kJ/mol; MgH2, 2791 kJ/mol;CaH2, 2410 kJ/mol; SrH2, 2250 kJ/mol; BaH2, 2121 kJ/mol.(a) What is the oxidation number of H in these compounds?(b) Assuming that all of these compounds have the samethree-dimensional arrangement of ions in the solid, whichof these compounds has the shortest cation–anion distance?(c) Consider BeH2. Does it require 3205 kJ of energyto break one mole of the solid into its ions, or does breakingup one mole of solid into its ions release 3205 kJ of energy?(d) The lattice energy of ZnH2 is 2870 kJ/mol. Consideringthe trend in lattice enthalpies in the Group 2A compounds,predict which Group 2A element is most similar in ionic radiusto the Zn2+ ion.
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
Consider the lattice energies of the following Group 2A
compounds: BeH2, 3205 kJ/mol; MgH2, 2791 kJ/mol;
CaH2, 2410 kJ/mol; SrH2, 2250 kJ/mol; BaH2, 2121 kJ/mol.
(a) What is the oxidation number of H in these compounds?
(b) Assuming that all of these compounds have the same
three-dimensional arrangement of ions in the solid, which
of these compounds has the shortest cation–anion distance?
(c) Consider BeH2. Does it require 3205 kJ of energy
to break one mole of the solid into its ions, or does breaking
up one mole of solid into its ions release 3205 kJ of energy?
(d) The lattice energy of ZnH2 is 2870 kJ/mol. Considering
the trend in lattice enthalpies in the Group 2A compounds,
predict which Group 2A element is most similar in ionic radius
to the Zn2+ ion.
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