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Glossary of terms used in MTDATA

The definitions given below are intended to explain the meaning of words as used in MTDATA documentation. They are not necessarily suitable for general use.

Substance

"Substance" is a general word denoting any quantity of material, ranging from an atom or ion upwards, having a definite identity. In MTDATA it is normally used to mean "pure substance" which is restricted to elements, stoichiometric compounds, molecules and ions.

State of matter

For most purposes the states of matter are solid, liquid or gas. Solids are normally crystalline but may also be amorphous or glassy with only short range order. A fourth state of matter, plasma, ie ionised gases, is not treated differently in MTDATA from unionised gases. On the other hand the aqueous phase is modelled in a rather distinct way from other types of phase.

Phase

A phase is chemically and structurally homogeneous and is distinguishable from other phases by its name, structure and properties (mechanical, physical and chemical). Crystalline phases have a definite periodic structure, often with sublattices. In MTDATA two sets of substances with identical structures but radically different chemistry, eg MgO and NiO on the one hand and NaCl and KCl on the other, can be prevented from entering the same phase by tagging their constituents with different names, eg <halite1> and <halite2>. The angular brackets are used to denote phase names. Substances that mix must be described by the same model, qv.

A single phase can unmix into two or more phases with the same name. For example an oxide melt can separate into silica-rich and silica-poor regions, both described by the same name and mathematical model.

Element

An element cannot be broken down to a simpler chemical form by non-nuclear processes. The phase is not defined and the atoms of the element may be present individually or grouped into molecules, eg O2 and As4.

In MTDATA the electron is treated for most purposes as an element with the symbol /-. Positive charge is represented by /+ to avoid negative stoichiometry numbers.

Isotopes

Isotopes of the same element have the same atomic number but differing atomic weights. For a discussion of their representation within MTDATA see information about the ACCESS module.

Atom

An atom is the smallest possible state of division of an element

Molecule

A chemically bonded group of similar or different atoms.

Isomers

Isomers are molecules with the same formula but different structures. In the THERMOTAB and UTILITY modules, in order to distinguish isomers, a tag is appended after the phase name eg <g,cis>. However, when retrieving data from databases and viewing the list of substances, a different convention is used, whereby the tag is appended to the formula by an underscore character eg C2H2Cl2_1,1<g>, C2H2Cl2_trans<g> or C2H2Cl2_cis<g>.

Ion

An ion is an atom or molecule carrying an electric charge.

Species

A species is an atom, ion or molecule. The word is normally used to describe identifiable constituents of phases for example a gaseous molecule or an ion occupying a sublattice.

Compound

A compound is composed of at least two different elements. The phase is not defined. A compound in crystalline form may be made up of individual molecules or it may have extended ionic, covalent or metallic bonding. For example the overall composition of rock salt is governed by the fact that there are equal numbers of Na+ and Cl- ions on two sublattices. The use of the word compound normally implies a stoichiometric composition (ie the amounts of the elements are in simple ratio).

System

A chemical system is defined by a set of chemical entities known as components. In the simplest case these are the elements comprising the system. However, they may also be compounds of these elements, in which case they may be fewer or greater in number than the number of elements. For the example of the three elements C, H and O, the number of components might be 1, ethanol; 2, ethanol-water; 3, C-H-O; or 4, methanol-ethanol-water-benzene, etc.

A common implication of defining a system in terms of components other than the elements is that the only substances present are the named components but this is not the case in MTDATA unless other substances are removed. For example naming a system as 'C2H5OH,H2O' would allow data to be retrieved not only for ethanol and water but also for ethene, C2H4, and diethyl ether, (C2H5)2O, since these can be formed by linear combination of C2H5OH and H2O. On the other hand ethane, C2H6, cannot be formed by a combination of ethanol and water and would therefore be excluded.

The Na-K-Cl system could also be defined as the NaCl-KCl-Cl2 system, the same set of substances would be retrieved from the database. However, substances and overall compositions containing more Na+K than Cl would have negative amounts of Cl2. Negative amounts of components are thus allowed but not negative amounts of elements in the overall composition of the system.

If the system were defined as KCl-NaCl this would impose the constraint that the total amounts of Na+K and Cl were equal. It would also imply that no substances were present containing more or less Na+K than Cl. If the data for KCl and NaCl were modelled by taking account of the existence of ions, it would be necessary to specify the system as 'NaCl,KCl,Cl/-' to allow data for ions, alone or in neutral combination, to be retrieved.

Systems may be closed or open. In closed systems the total amount of the components is fixed, whereas in open systems the composition can adjust to meet some external constraint. In closed systems the amount of electric charge must be zero and hence charge is not a component of the system. Open systems on the other hand are not realistic. For example it is common practice to consider systems in which the pH is low but where there is no explicit anion such as ClO4- to balance the charge. This convention is implicit in many diagrams drawn using COPLOT and can also be adopted in MULTIPHASE.

Component

The meaning of "component" is implied in the definition of a system given above. Different phases in the same system cannot have an independent set of components.

Unary

The word "unary" is used to describe the constituents of a phase to which data can be assigned. For example in a liquid phase the unaries might be H2O<liquid> and C2H5OH<liquid>. The data for these unaries are those of the pure liquids. Unaries are not necessarily experimentally accessible. For example Ni<BCC_A2> is unstable but data for it are required to model the solution of nickel in the bcc phase of steels. Moreover, to meet the requirements of models for ionic phases with sublattices a unary may carry charge. For example the formation of an inverse spinel AB2O4 could be modeled by the mixing of the two unaries A3+(A3+)2O4<spinel> and A3+(B2+)2O4<spinel>, which respectively have charges of +1 and -1 and therefore have no independent existence (see Reference states).

Solution

A solution is a homogeneous mixture within a single phase. Local ordering may be present. A phase that includes a variable proportion of unoccupied sites is also a solution but the vacancies do not constitute a component. The data for solutions are defined by reference to the unaries from which they are made using the model and data describing the ideal and non-ideal mixing between these unaries.

Model

The word "model", as it applies in MTDATA, is given to any mathematical description of the properties of a substance as a function of one or more variables such as temperature, pressure and composition. Within a single phase the same model must be used to describe the mixing between all binary and, if necessary, higher order combinations of unaries. This entails that care must be taken when developing data for a multicomponent system that models and reference states are consistent.

The models used in MTDATA are incorporated in a modular way in the software, making it feasible to add new models. "Model" may, under some circumstances, also have a more restrictive definition in which the parameters in the mathematical description are given definite numerical values.

Reference states

All data normally supplied with MTDATA for unary substances are referred to the enthalpy of the elements from which they are composed at 298.15 K. The symbol for the Gibbs energy is (G-Hser).

Data for charged unaries must be referred to a defined state which is unique for the phase. In the well established case of aqueous solutions, the reference is H+<aqueous>, which by convention is given zero values for G, H and S. This has the effect of defining a notional aqueous electron with the same data as 1/2 H2<gas>.

The phase rule

The phase rule is P + F = C + 2, where P is the number of phases, F is the number of properties of the system that can be varied independently and C is the number of components. Thus in the unary system H2O, at the triple point where ice, liquid water and gaseous water coexist, the intensive properties, namely the pressure and temperature, of the system are completely defined. Only the amounts of the phases and other extensive quantities may vary.

In MULTIPHASE the phase rule is not invoked explicitly. The criterion that the system is at a minimum Gibbs energy is sufficient to ensure that the rule is obeyed.

 

Updated 7 May 2010