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Activation Energy
In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (''E''a) of a reaction is measured in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol). Activation energy can be thought of as a magnitude of the potential barrier (sometimes called the energy barrier) separating minima of the potential energy surface pertaining to the initial and final thermodynamic state. For a chemical reaction to proceed at a reasonable rate, the temperature of the system should be high enough such that there exists an appreciable number of molecules with translational energy equal to or greater than the activation energy. The term "activation energy" was introduced in 1889 by the Swedish scientist Svante Arrhenius. Other uses Although less commonly used, activation energy also applies to nuclear reactions and various other physical phenomena. Temperature ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Incandescence
Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electronic, molecular, and lattice oscillations in a material. Kinetic energy is converted to electromagnetism due to Larmor formula, charge-acceleration or dipole oscillation. At room temperature, most of the emission is in the infrared (IR) spectrum, though above around 525 °C (977 °F) enough of it becomes Visible spectrum, visible for the matter to visibly glow. This visible glow is called incandescence. Thermal radiation is one of the fundamental mechanisms of heat transfer, along with Thermal conduction, conduction and Convection (heat transfer), convection. The primary method by which the Sun transfers heat to the Earth is thermal radiation. This energy is partially absorbed and scattered in the atmosphere, the latter process bei ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Catalyst
Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst. The rate increase occurs because the catalyst allows the reaction to occur by an alternative mechanism which may be much faster than the noncatalyzed mechanism. However the noncatalyzed mechanism does remain possible, so that the total rate (catalyzed plus noncatalyzed) can only increase in the presence of the catalyst and never decrease. Catalysis may be classified as either homogeneous, whose components are dispersed in the same phase (usual ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Exergonic Reaction
In chemical thermodynamics, an exergonic reaction is a chemical reaction where the change in the free energy is negative (there is a net release of free energy). This indicates a spontaneous reaction if the system is closed and initial and final temperatures are the same. For processes that take place in a closed system at constant pressure and temperature, the Gibbs free energy is used, whereas the Helmholtz energy is relevant for processes that take place at constant volume and temperature. Any reaction occurring at constant temperature without input of electrical or photon energy is exergonic, according to the second law of thermodynamics. An example is cellular respiration. Symbolically, the release of free energy, , in an exergonic reaction (at constant pressure and temperature) is denoted as :\Delta G=G_-G_<0.\, Although exergonic reactions are said to occur ''spontaneously'', this does not imply that the reaction will take place at an observable [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Standard State
The standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions. A degree sign (°) or a superscript ⦵ symbol (⦵) is used to designate a thermodynamic quantity in the standard state, such as change in enthalpy (Δ''H''°), change in entropy (Δ''S''°), or change in Gibbs free energy (Δ''G''°). The degree symbol has become widespread, although the Plimsoll is recommended in standards, see discussion about typesetting below. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry (IUPAC) recommends a conventional set of standard states for general use. The standard state should not be confused with standard temperature and pressure (STP) for gases, nor with the standard solutions used in analytical chemistry. STP is commonly used for calculations involving gases that approximate an ideal gas, whereas standard state condit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Entropy Of Activation
In chemical kinetics, the entropy of activation of a reaction is one of the two parameters (along with the enthalpy of activation) that are typically obtained from the temperature dependence of a reaction rate constant, when these data are analyzed using the Eyring equation of the transition state theory. The standard entropy of activation is symbolized and equals the change in entropy when the reactants change from their initial state to the activated complex or transition state ( = change, = entropy, = activation). Importance Entropy of activation determines the preexponential factor of the Arrhenius equation for temperature dependence of reaction rates. The relationship depends on the molecularity of the reaction: * for reactions in solution and unimolecular gas reactions *: , * while for bimolecular gas reactions *: . In these equations is the base of natural logarithms, is the Planck constant, is the Boltzmann constant and the absolute temperature. is the ideal gas ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Entropy
Entropy is a scientific concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the microscopic description of nature in statistical physics, and to the principles of information theory. It has found far-ranging applications in chemistry and physics, in biological systems and their relation to life, in cosmology, economics, sociology, weather science, climate change and information systems including the transmission of information in telecommunication. Entropy is central to the second law of thermodynamics, which states that the entropy of an isolated system left to spontaneous evolution cannot decrease with time. As a result, isolated systems evolve toward thermodynamic equilibrium, where the entropy is highest. A consequence of the second law of thermodynamics is that certain processes are irreversible. The thermodynami ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gibbs Energy
In thermodynamics, the Gibbs free energy (or Gibbs energy as the recommended name; symbol is a thermodynamic potential that can be used to calculate the maximum amount of work, other than pressure–volume work, that may be performed by a thermodynamically closed system at constant temperature and pressure. It also provides a necessary condition for processes such as chemical reactions that may occur under these conditions. The Gibbs free energy is expressed as G(p,T) = U + pV - TS = H - TS where: * U is the internal energy of the system * H is the enthalpy of the system * S is the entropy of the system * T is the temperature of the system * V is the volume of the system * p is the pressure of the system (which must be equal to that of the surroundings for mechanical equilibrium). The Gibbs free energy change (, measured in joules in SI) is the ''maximum'' amount of non-volume expansion work that can be extracted from a closed system (one that can exchange heat and work with i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Eyring Equation
The Eyring equation (occasionally also known as Eyring–Polanyi equation) is an equation used in chemical kinetics to describe changes in the rate of a chemical reaction against temperature. It was developed almost simultaneously in 1935 by Henry Eyring, Meredith Gwynne Evans and Michael Polanyi. The equation follows from the transition state theory, also known as activated-complex theory. If one assumes a constant enthalpy of activation and constant entropy of activation, the Eyring equation is similar to the empirical Arrhenius equation, despite the Arrhenius equation being empirical and the Eyring equation based on statistical mechanical justification. General form The general form of the Eyring–Polanyi equation somewhat resembles the Arrhenius equation: \ k = \frac e^ where k is the rate constant, \Delta G^\ddagger is the Gibbs energy of activation, \kappa is the transmission coefficient, k_\mathrm is the Boltzmann constant, T is the temperature, and h is the Pl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Transition State
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked with the double dagger (‡) symbol. As an example, the transition state shown below occurs during the SN2 reaction of bromoethane with a hydroxide anion: The activated complex of a reaction can refer to either the transition state or to other states along the reaction coordinate between reactants and products, especially those close to the transition state. Peter Atkins and Julio de Paula, ''Physical Chemistry'' (8th ed., W.H. Freeman 2006), p.809 According to the transition state theory, once the reactants have passed through the transition state configuration, they always continue to form products. History of concept The concept of a transition state has been important in many theories of the rates at which chemical re ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Reaction Coordinate
In chemistry, a reaction coordinate is an abstract one-dimensional coordinate chosen to represent progress along a reaction pathway. Where possible it is usually a geometric parameter that changes during the conversion of one or more molecular entity, molecular entities, such as bond length or bond angle. For example, in the homolysis (chemistry), homolytic dissociation of molecular hydrogen, an apt choice would be the coordinate corresponding to the bond length. Non-geometric parameters such as Bond order#Non-integer bond orders, bond order are also used, but such direct representation of the reaction process can be difficult, especially for more complex reactions. In computer simulations collective variables are employed for a target-oriented sampling approach. Plain simulations fail to capture so called rare events, because they are not feasible to occur in realistic computation times. This often stems from to high energy barriers separating the reactants from products, or ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Van Der Waals Force
In molecular physics and chemistry, the van der Waals force (sometimes van der Waals' force) is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and therefore more susceptible to disturbance. The van der Waals force quickly vanishes at longer distances between interacting molecules. Named after Dutch physicist Johannes Diderik van der Waals, the van der Waals force plays a fundamental role in fields as diverse as supramolecular chemistry, structural biology, polymer science, nanotechnology, surface science, and condensed matter physics. It also underlies many properties of organic compounds and molecular solids, including their solubility in polar and non-polar media. If no other force is present, the distance between atoms at which the force becomes repulsive rather than attractive as the atoms approach one another is called the van der ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hydrogen Bond
In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently bonded to a more Electronegativity, electronegative donor atom or group (Dn), interacts with another electronegative atom bearing a lone pair of electrons—the hydrogen bond acceptor (Ac). Unlike simple Dipole–dipole attraction, dipole–dipole interactions, hydrogen bonding arises from charge transfer (nB → σ*AH), Atomic orbital, orbital interactions, and quantum mechanical Delocalized electron, delocalization, making it a resonance-assisted interaction rather than a mere electrostatic attraction. The general notation for hydrogen bonding is Dn−H···Ac, where the solid line represents a polar covalent bond, and the dotted or dashed line indicates the hydrogen bond. The most frequent donor and acceptor atoms are nitrogen (N), oxyg ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
