 Temperature
 Temperature Jump Technique
 Term
 Theoretical Plate
 Thermal Conduction
 Thermal Motion
 Thermal Wavelength
 Thermochemistry
 Thermodynamic Force
 Thermodynamics
 Third Law of Thermodynamics
 Tie Line
 torr (the unit)
 Transition Dipole Moment
 Translational Motion
 Transport Number
 Transport Property
 Triangular Phase Diagram
 Triple Point
 Trouton’s Rule
 Tunnelling
 Twodimensional NMR
 Twolevel System
T

Published:17 May 2024
Concepts in Physical Chemistry, Royal Society of Chemistry, 2nd edn, 2024, pp. 325338.
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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 T; where appropriate, the entries also describe subsidiary but related concepts. Refer to the Directory for a full list of all the concepts treated.
Temperature
Temperature Jump Technique
In the temperature jump technique, a reaction mixture is subjected to a sharp rise in temperature and its return to the new equilibrium composition is monitored. The relaxation time is interpreted in terms of the rate constants (at the new temperature) of the forward and reverse reactions.
Term
The notation continues G, H,…. The values of S, L, and J are obtained for the configuration by coupling the angular momenta in accord with the Russell–Saunders coupling scheme.
Theoretical Plate
A theoretical plate is a tie line in a binary liquid/vapour phase diagram linking the liquid and vapour phase compositions that are in equilibrium at any given overall composition of the mixture. The term is used in connection with fractional distillation, with the number of theoretical plates for a succession of vaporizations and condensations between an initial composition and a final composition (Figure T.1). That number is a measure of the efficiency of the separation process.
Thermal Conduction
Thermal Motion
Thermal motion is chaotic molecular motion; the higher the temperature, the more energetic is the thermal motion.
Thermal Wavelength
Many translational states are occupied when V ≫ Λ ^{3}.
Thermochemistry
Thermochemistry is the branch of thermodynamics that deals with the energy released or required as heat by a chemical reaction. See enthalpy of reaction and calorimeter.
Thermodynamic Force
The units of thermodynamic force, a molar quantity, are newtons per mole (N mol^{−1}).
Thermodynamics
Thermodynamics is the branch of science concerned with the transformations of energy. It is expressed in terms of four laws. See the entry for each law.
Third Law of Thermodynamics
The phenomenological (observationbased) statement of the Third law is:

It is impossible to reach T = 0 in a finite number of cyclic steps.
The most succinct statement in terms of the entropy (a property introduced by the Second law), which implies the phenomenological statement, is:

The entropy of a pure, perfectly crystalline substance is zero at T = 0.
The experimental foundation of the law is the Nernst heat theorem. Its importance lies in the implication that it permits the identification of the thermodynamic and statistical definitions of entropy.
Tie Line
A tie line in a phase diagram is a straight line connecting two phases that are in equilibrium for a given overall composition of the system (Figure T.2). See lever rule.
torr (the unit)
The torr (symbol: Torr) is a nonSI unit used to report pressure and defined such that 760 Torr = 1 atm exactly. It follows that 1 Torr ≈ 133.32 Pa. See mmHg.
Transition Dipole Moment
It is a measure of the dipolar character of the shift in charge that occurs during the transition. The transition is allowed if the transition moment is nonzero and forbidden if it is zero. The conditions under which the transition moment is nonzero are expressed by the selection rules. The transition dipole moment is a vector with three components: the nonvanishing components express the polarization of the electric vector in the electromagnetic radiation that is absorbed or generated in the transition.
Translational Motion
Transport Number
Transport Property
Four examples are
Property transported .  Gradient .  Name .  Coefficient . 

Matter  concentration  diffusion  D 
Energy as heat  temperature  thermal conduction  κ 
Momentum  velocity  viscosity  η 
Charge  electric potential  electrical conduction 
Property transported .  Gradient .  Name .  Coefficient . 

Matter  concentration  diffusion  D 
Energy as heat  temperature  thermal conduction  κ 
Momentum  velocity  viscosity  η 
Charge  electric potential  electrical conduction 
The kinetic theory of gases can be used to derive expressions for the corresponding coefficients in the first three cases and the individual entries should be consulted.
Triangular Phase Diagram
A triangular phase diagram is used to depict the phase equilibria of ternary (threecomponent) systems at constant pressure and temperature. Its interpretation is illustrated in Figure T.4. Note that for any point in the interior of the triangle it follows from the geometry of an equilateral triangle that x_{A} + x_{B} + x_{C} = 1 and that all points on a straight line terminating at the corner A have B and C in the same mole ratio, and likewise for the other two corners.
Triple Point
A triple point in a phase diagram is where three phases are in mutual equilibrium (Figure T.5). For a system with a single component (C = 1), it is an invariant point (F = C − P + 2= 0; that is, the pressure and temperature are unchangeable). One triple point of water, the one involving iceI (T_{3} = 273.16 K, p_{3} = 611.657 Pa), was used to define the size of the kelvin, but it is now an experimentally determined point.
Trouton’s Rule
Trouton’s rule states that for most liquids, Δ_{vap}H ^{⦵}/T_{b} is a constant and equal to about 85 J K^{−1} mol^{−1}. The exceptions include liquids in which there is a significant amount of structural ordering (by hydrogen bonding, for instance). The explanation of the rule is that Δ_{vap}H ^{⦵}/T_{b} is the standard entropy of vaporization, and there is a similar change in the degree of disorder whenever an unstructured liquid forms a vapour.
Tunnelling
Tunnelling is a quantum mechanical phenomenon in which a particle is found in or passing through regions from which it is excluded according to classical mechanics (where its kinetic energy would, formally, be negative). The effect arises from the fact that a wavefunction does not fall abruptly to zero at the walls of a container unless the potential energy there is infinite, but decays in the region. If the potential energy falls back to a low but nonzero level after a finite distance, the wavefunction will have a nonzero amplitude and therefore a nonzero probability of being found beyond the barrier (Figure T.6).
In this case, the tunnelling probability decreases exponentially with the height of the barrier and with the mass of the particle. Tunnelling is very important for electrons, moderately important for protons (which accounts for the rapid equilibration of proton transfer reactions), less important for deuterons, and not important in general for heavier particles. See scanning tunnelling microscopy for one application.
Twodimensional NMR
In twodimensional NMR, the spectrum is displayed along using two axes to facilitate its interpretation. For example, in correlation spectroscopy (COSY) the pulse sequence 90°(x)[delay t_{1}]90°(x)[acquisition t_{2}] is applied, followed by a double Fourier transform with respect to the two time delays t_{1} and t_{2}.
Twolevel System
These functions are plotted in Figure T.8−T.12, at two scales of temperature.