Low-temperature Molar Heat Capacities Research Articles

Thermodynamic studies of zeolites: clinoptilolite

Calorimetric studies are described of a carefully characterized specimen of clinoptilolite (Malheur County, Oregon, U.S.A.) of composition: S r 0.036 M g 0.124 C a 0.761 M n 0.002 B a 0.062 K 0.543 N a 0.954 A l 3.450 F e 0.017 S i 14.533 O 36.000 ⋅ 10.922 H 2 O . Values are reported of the standard molar enthalpies of formation ΔfHom at 298.15 K and of the standard molar enthalpy increments {Hom(T) − Hom(298.15 K)} for both clinoptilolite and dehydrated clinoptilolite, as well as the low-temperature molar heat capacity Cop,m and, by derivation, the standard molar entropy increment {Som(T) − Som(0)} for clinoptilolite alone. The conventional thermodynamic properties have been calculated for both compounds.

Low-temperature molar heat capacities and entropies of MnO 2 (pyrolusite), Mn 3O 4 (hausmanite), and Mn 2O 3 (bixbyite)

Pyrolusite (MnO 2), hausmanite (Mn 3O 4), and bixbyite (Mn 2O 3), are important ore minerals of manganese and accurate values for their thermodynamic properties are desirable to understand better the { p(O 2), T} conditions of their formation. To provide accurate values for the entropies of these important manganese minerals, we have measured their heat capacities between approximately 5 and 380 K using a fully automatic adiabatically-shielded calorimeter. All three minerals are paramagnetic above 100 K and become antiferromagnetic or ferrimagnetic at lower temperatures. This transition is expressed by a sharp λ-type anomaly in C pm o for each compound with Néel temperatures T N of (92.2±0.2), (43.1±0.2), and (79.45±0.05) K for MnO 2, Mn 3O 4, and Mn 2O 3, respectively. In addition, at T ≈ 308 K, Mn 2O 3 undergoes a crystallographic transition, from orthorhombic (at low temperatures) to cubic. A significant thermal effect is associated with this change. Hausmanite is ferrimagnetic below T N and in addition to the normal λ-shape of the heat-capacity maxima in MnO 2 and Mn 2O 3, it has a second rounded maximum at 40.5 K. The origin of this subsidiary bump in the heat capacity is unknown but may be related to a similar “anomalous bump” in the curve of magnetization against temperature at about 39 K observed by Dwight and Menyuk. (1) At 298.15 K the standard molar entropies of MnO 2, Mn 3O 4, and Mn 2O 3, are (52.75±0.07), (164.1±0.2), and (113.7±0.2) J·K −1·mol −1, respectively. Our value for Mn 3O 4 is greater than that adopted in the National Bureau of Standards tables (2) by 14 per cent.

Low-temperature molar heat capacities and inter-relations of thermodynamic properties of (copper + aluminium) alloys

The heat capacities of six alloys from (copper + aluminium) representing the α, γ 2, δ, ζ 2, η 2, and θ phases which are stable at and below 773 K have been measured between 6 and 300 K and the quantities { S o(298 K)− S o(0)} and {H o(298 K) −H o (0)} T calculated. Assuming that the configurations of the quenched alloys are those present at 773 K, the entropies have been combined with other existing thermodynamic functions to evaluate configurational and vibrational contributions. Inter-relations among standard enthalpies of formation, configurational and vibrational entropies, and volume changes are explored, leading to the conclusion that, although quantitative relations may not be postulated, the results indicate a correspondence between the exothermicity and the ordering and approach of the atomic species on alloying.