Thermodynamic study of disorder in lithium bromide monohydrate

Abstract

The following quantities have been measured: (1) the heat capacity (C_p) of anhydrous lithium bromide from 8 to 402 K, (2)C_p of lithium bromide monohydrate from 17 to 405 K, (3) the integral heats of solution of these two salts in water at 298.15 K. (4) the water vapour dissociation pressure in the equilibrium between the two salts and water vapour from 371 to 423 K. The entropy S(cal) of the α-form of the monohydrate at 298.15 K (the stable modification at this temperature) as derived from the heat capacity measurements is 112.45 J K^–1 mol^–1. This is virtually identical with the value of 112.47 J K^–1 mol^–1 for S(eq), the entropy evaluated from the thermodynamics of the above equilibrium. The estimated uncertainty of the difference between S(cal) and S(eq) is ±1.5 J K^–1 mol^–1. The α-form therefore reaches a completely ordered condition at 0 K, or at least only retains frozen-in disorder to a minor extent.

At 307.0 K the α-form transforms into a β-modification. From previous structural and n.m.r. work it had been concluded that in this form the water molecules are disordered among twelve possible orientations per molecule and the lithium ions positionally disordered among the face-centred sites of the cubic unit cell. However, the entropy gain at the α→β transition is only 6.87 J K^–1 mol^–1, ≈R ln2.3. This fact, together with the conclusion that the α-form is uniquely ordered at 0 K, implies that the β-form (at least just above the transition) is not nearly such a highly disordered structure as the previous studies had suggested.