Determination of the standard enthalpy of formation of Ni 2.55P by high-temperature drop calorimetry

Abstract

We have determined ΔfHom(Ni2.55P, cr, 298.15 K) by high-temperature drop calorimetry using two methods. In the first procedure, we measured ΔrHm for 2.55 N i ( c r ) + P ( r e d , a m ) = N i 2.55 P ( m i x ) at 298.15 K by drop calorimetry at a reference temperature of 907 K. The designation “mix” indicates a mixture of Ni3P, Ni2.55P, and Ni12P5 with an overall composition of Ni2.55P. We then used solution drop-calorimetry at a reference temperature of 1181 K to determine ΔrHm for N i 2.55 P ( m i x ) = N i 2.55 P ( c r ) at 298.15 K. These results were combined to give the result for ΔrHm of the reaction: 2.55 N i ( c r ) + P ( r e d , a m ) = N i 2.55 P ( c r ) at 298.15 K of −(188 ± 5) kJ · mol−1. In our second procedure, we used solution drop-calorimetry alone to determine a value for ΔrHm for the reaction: 2.55 N i ( c r ) + P ( r e d , a m ) = N i 2.55 P ( c r ) at 298.15 K of −(181 ± 5) kJ · mol−1. We have taken the mean of these values and applied a correction for ΔfHom(P, red, am, 298.15 K) to obtain a value for ΔfHom(Ni2.55P, cr, 298.15 K) of −(180 ± 8) kJ · mol−1 where the reference state of phosphorus is P(black, orthorhombic).