1 THERMODYNAMICS
Worked example KEY IDEAS
Calculate the enthalpy of solution of magnesium chloride given:
›› lattice enthalpy of MgCl2(s) = 2526 kJ mol−1 ›› enthalpy of hydration of Mg2+(g) = −1891 kJ mol−1 ›› enthalpy of hydration of Cl−(g) = −381 kJ mol−1 Equation for dissolution: MgCl2(s) + aq → Mg2+(aq) + 2Cl−(aq) Enthalpy cycle:
›› Water molecules are polar and are attracted to cations and anions.
›› The hydration enthalpy of an ion is greater for an ion with a higher charge-to-size ratio.
›› The enthalpy of hydration, ΔhydHϴ, is the en-
thalpy change when one mole of gaseous ions is completely surrounded by water molecules, i.e. infinite dilution.
›› If ΔhydHϴ is greater than ΔlattHϴ, energy will
Mg2+(g) + 2CI–(g)
be released when ions are separated to form a solution.
–
Enthalpy
∆lattH [MgCI2(s)]
–
–
∆hydH [Mg2+(g)] + 2 x ∆hydH [CI–(g)]
1.4 ORDER, DISORDER AND ENTROPY
MgCI2(s) + aq
–
∆solH [MgCI2(s)]
Mg2+(aq) + 2CI–(aq)
Using Hess’s law, ΔsolHϴ[MgCl2(s)] = ΔlattHϴ[MgCl2(s)] + ΔhydHϴ[Mg2+(g)] + (2 × ΔhydHϴ[Cl−(g)]) ΔlattHϴ[MgCl2(s)] = 2526 kJ mol−1 ΔhydHϴ[Mg2+(g)] = −1891 kJ mol−1 ΔhydHϴ[Cl−(g)] = −381 kJ mol−1 Therefore, ΔsolHϴ[MgCl2(s)] = 2526 + (−1891) + (2 × −381) = −127 kJ mol−1 The dissolution of magnesium chloride in water is exothermic.
QUESTIONS 16. Which of the enthalpies of solution of KCl, RbCl, MgCl2 and SrCl2 are: a. negative (exothermic)? b. positive (endothermic)? (See Table 7).
Some systems change spontaneously, and some change if they are triggered in some way. Some never appear to change. What is the explanation? What determines whether or not a change (physical or chemical) takes place? Enthalpy changes are important – the more exothermic a change is, the more likely it is to happen. However, some endothermic changes also happen spontaneously. You have just seen, for example, that the dissolution of an ionic solid may be exothermic or endothermic. So there must be another factor to consider. That factor is entropy.
Entropy Order and disorder are important concepts in chemistry. In all natural physical and chemical changes, the system that is changing and its surroundings spontaneously go from an ordered state to a less ordered state. This happens regardless of whether the change is exothermic or endothermic. The degree of disorder in a system is called its entropy. As the disorder increases, so does the entropy value. Some examples of changes that increase disorder are as follows.
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