W

Published:17 May 2024
Concepts in Physical Chemistry, Royal Society of Chemistry, 2nd edn, 2024, pp. 352356.
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Physical chemistry is the part of chemistry that seeks to account for the properties and transformations of matter in terms of concepts, principles, and laws drawn from physics. This glossary is a compilation of definitions, descriptions, formulae, and illustrations of concepts that are encountered throughout the subject. This section describes the concepts that begin with the letter W; where appropriate, the entries also describe subsidiary but related concepts. Refer to the Directory for a full list of all the concepts treated.
Walden’s Rule
Walden’s rule states that the product ηΛ_{m}, where η is the solvent viscosity and Λ_{m} the molar conductivity, is approximately constant for the same ions in different solvents.
Wavefunction
The groundstate wavefunction of a system has no nodes (locations where a function passes through zero). Regions of high curvature contribute high kinetic energy to the total energy (Figure W.2). Spatial wavefunctions may be complex or real: complex wavefunction correspond to a specific direction of travel; real wavefunctions do not. (See translational motion.) Oneelectron wavefunctions in atoms and molecules are known as orbitals.
Wavelength
For electromagnetic radiation in a vacuum, c_{medium} is the speed of light, c. Note that an electromagnetic wave retains its frequency when entering a medium but its wavelength changes on account of the change in its speed of propagation. See refractive index.
Wavenumber
A wavenumber (which is commonly expressed in reciprocal centimetres, cm^{−1}) can be pictured as the number of wavelengths of the radiation per centimetre.
Work
In thermodynamics, work is a process equivalent to (in the sense that it can in principle be adapted to) achieving the raising of a weight in the surroundings. It is one of the ways in which energy may be transferred between a system and its surroundings and therefore bring about a change in the internal energy, U, of a system. The sign convention normally adopted in chemical thermodynamics (but not in engineering thermodynamics, where the opposite is commonly adopted), is that w < 0 if energy leaves the system as work.
Nonexpansion work (or additional work), work that does not involve expansion, includes driving an electric current through an external circuit. See Table W.1.
Type of work .  dw .  Comments .  Units ^{b} . 

Expansion  $\u2212 p ex dV$  p_{ex} is the external pressure  Pa 
dV is the change in volume  $ m 3 $  
Surface expansion  $\gamma d\sigma $  γ is the surface tension  $N m \u2212 1 $ 
dσ is the change in area  $ m 2 $  
Extension  $fdl$  f is the tension  N 
dl is the change in length  m  
Electrical  $\varphi dQ$  ϕ is the electric potential  V 
dQ is the change in charge  C  
$Qd\varphi $  dϕ is the potential difference  V  
Q is the charge transferred  C 
Type of work .  dw .  Comments .  Units ^{b} . 

Expansion  $\u2212 p ex dV$  p_{ex} is the external pressure  Pa 
dV is the change in volume  $ m 3 $  
Surface expansion  $\gamma d\sigma $  γ is the surface tension  $N m \u2212 1 $ 
dσ is the change in area  $ m 2 $  
Extension  $fdl$  f is the tension  N 
dl is the change in length  m  
Electrical  $\varphi dQ$  ϕ is the electric potential  V 
dQ is the change in charge  C  
$Qd\varphi $  dϕ is the potential difference  V  
Q is the charge transferred  C 
In general, the work done on a system can be expressed in the form $dw=\u2212Fdz$ , where $F$ is the magnitude of a ‘generalized force’ and dz is a ‘generalized displacement’.
For work in joules (J). Note that 1 N m = 1 J and 1 V C = 1 J.
In molecular terms, work is the transfer of energy that makes use of the uniform motion of atoms in the surroundings (Figure W.4); for instance, the uniform upward motion of the atoms of a weight.