The work function is a measure of the maximum (reversible) work of all kinds
available in a given isothermal process. The symbol A, is taken from the
German word for work, ‘arbeit’. In Haber’s book this term was referred to as
the ‘reaction energy’. An enthalpy term was referred to as the ‘reaction heat’
and given the symbol Q, so that Q p(T) represents the ‘reaction heat’ at a given
pressure and temperature. Generally, it is unnecessary to give some physical
significance to any of the thermodynamic properties of a system, such as
S and H. They are mathematical quantities important simply because they are
state functions and because they can be related to one another by mathematical expressions. However, students often find their significance easier to
grasp if they are given some kind of physical form, such as entropy in terms of
the order or disorder of the system. This kind of metaphor guided Haber’s
thinking when trying to derive an equation for the total amount of work of all
kinds available from a chemical reaction, ‘energy’ of course, simply being the
capacity to perform work. In natural science Haber argued, we generally
measure forces by opposing them with other quantifiable forces. Mechanical
forces can be measured by opposing them with a balance and weights,
electrical forces by opposite electrical force, but it is not possible to measure
the ‘driving force’ of a chemical reaction with opposing forces of a chemical
nature. In such a case, we have to relate A to other thermodynamic properties.
Haber explained that when a change of state occurs in an isothermal system,
measurable heat is absorbed or evolved. For vapourisation, some energy
becomes ‘latent’, for condensation the same energy appears out of its ‘latent
condition’. Haber then spoke of the ‘latent heat’ of a chemical reaction
progressing at constant temperature and volume, meaning the difference
between the maximum amount of work we can get A, and the corresponding
decrease in the total energy, U. decrease of total energy under const vol for gas
reacts is equal to the heat evolved. And for const pres enthalpy. Defining the
amount of heat ‘used up’, ‘hidden’ or ‘becoming latent’ as –q, Haber derived
his fundamental equation;
A À U ¼ Àq
‘Latent’ of course refers to entropy, and so with S = q/T, he introduced
temperature T into the equation quite simply;
A ¼ U À TS
ð1Þ
and where the reaction energy is now related to the other thermodynamic
properties, S, U and T. Today, A is known as the ‘work function’. Haber’s
main contribution to the thermodynamics of gas reactions was his consideration of heat capacity and entropy. Over small temperature ranges or low
534
Appendix D: The Haber(-Le Rossignol) Equations 1907–1914
available in a given isothermal process. The symbol A, is taken from the
German word for work, ‘arbeit’. In Haber’s book this term was referred to as
the ‘reaction energy’. An enthalpy term was referred to as the ‘reaction heat’
and given the symbol Q, so that Q p(T) represents the ‘reaction heat’ at a given
pressure and temperature. Generally, it is unnecessary to give some physical
significance to any of the thermodynamic properties of a system, such as
S and H. They are mathematical quantities important simply because they are
state functions and because they can be related to one another by mathematical expressions. However, students often find their significance easier to
grasp if they are given some kind of physical form, such as entropy in terms of
the order or disorder of the system. This kind of metaphor guided Haber’s
thinking when trying to derive an equation for the total amount of work of all
kinds available from a chemical reaction, ‘energy’ of course, simply being the
capacity to perform work. In natural science Haber argued, we generally
measure forces by opposing them with other quantifiable forces. Mechanical
forces can be measured by opposing them with a balance and weights,
electrical forces by opposite electrical force, but it is not possible to measure
the ‘driving force’ of a chemical reaction with opposing forces of a chemical
nature. In such a case, we have to relate A to other thermodynamic properties.
Haber explained that when a change of state occurs in an isothermal system,
measurable heat is absorbed or evolved. For vapourisation, some energy
becomes ‘latent’, for condensation the same energy appears out of its ‘latent
condition’. Haber then spoke of the ‘latent heat’ of a chemical reaction
progressing at constant temperature and volume, meaning the difference
between the maximum amount of work we can get A, and the corresponding
decrease in the total energy, U. decrease of total energy under const vol for gas
reacts is equal to the heat evolved. And for const pres enthalpy. Defining the
amount of heat ‘used up’, ‘hidden’ or ‘becoming latent’ as –q, Haber derived
his fundamental equation;
A À U ¼ Àq
‘Latent’ of course refers to entropy, and so with S = q/T, he introduced
temperature T into the equation quite simply;
A ¼ U À TS
ð1Þ
and where the reaction energy is now related to the other thermodynamic
properties, S, U and T. Today, A is known as the ‘work function’. Haber’s
main contribution to the thermodynamics of gas reactions was his consideration of heat capacity and entropy. Over small temperature ranges or low
534
Appendix D: The Haber(-Le Rossignol) Equations 1907–1914