SUMMARY / RELATED TOPICS

Activation energy

In chemistry and physics, Energy of Activation is the energy which must be provided to a chemical or nuclear system with potential reactants to result in: a chemical reaction, nuclear reaction, or various other physical phenomena. The activation energy of a reaction is measured in joules, kilojoules per mole or kilocalories per mole. Activation energy can be thought of as the magnitude of the potential barrier separating minima of the potential energy surface pertaining to the initial and final thermodynamic state. For a chemical reaction, or division 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. The Arrhenius equation gives the quantitative basis of the relationship between the activation energy and the rate at which a reaction proceeds.

From the equation, the activation energy can be found through the relation k = A e − E a / where A is the pre-exponential factor for the reaction, R is the universal gas constant, T is the absolute temperature, k is the reaction rate coefficient. Without knowing A, Ea can be evaluated from the variation in reaction rate coefficients as a function of temperature. At a more advanced level, the net Arrhenius activation energy term from the Arrhenius equation is best regarded as an experimentally determined parameter that indicates the sensitivity of the reaction rate to temperature. There are two objections to associating this activation energy with the threshold barrier for an elementary reaction. First, it is unclear as to whether or not reaction does proceed in one step. Second if the reaction being studied is elementary, a spectrum of individual collisions contributes to rate constants obtained from bulk experiments involving billions of molecules, with many different reactant collision geometries and angles, different translational and vibrational energies—all of which may lead to different microscopic reaction rates.

A substance that modifies the transition state to lower the activation energy is termed a catalyst. A catalyst increases the rate of reaction without being consumed in the reaction. In addition, the catalyst lowers the activation energy, but it does not change the energies of the original reactants or products, so does not change equilibrium. Rather, the reactant energy and the product energy remain the same and only the activation energy is altered. A catalyst is able to reduce the activation energy by forming a transition state in a more favorable manner. Catalysts, by nature, create a more "comfortable" fit for the substrate of a reaction to progress to a transition state; this is possible due to a release of energy that occurs when the substrate binds to the active site of a catalyst. This energy is known as Binding Energy. Upon binding to a catalyst, substrates partake in numerous stabilizing forces while within the active site. Specific and favorable bonding occurs within the active site until the substrate forms to become the high-energy transition state.

Forming the transition state is more favorable with the catalyst because the favorable stabilizing interactions within the active site release energy. A chemical reaction is able to manufacture a high-energy transition state molecule more when there is a stabilizing fit within the active site of a catalyst; the binding energy of a reaction is this energy released when favorable interactions between substrate and catalyst occur. The binding energy released assists in achieving the unstable transition state. Reactions otherwise without catalysts need a higher input of energy to achieve the transition state. Non-catalyzed reactions do not have free energy available from active site stabilizing interactions, such as catalytic enzyme reactions. In the Arrhenius equation, the term activation energy is used to describe the energy required to reach the transition state, the exponential relationship k = A exp holds. In transition state theory, a more sophisticated model of the relationship between reaction rates and the transition state, a superficially similar mathematical relationship, the Eyring equation, is used to describe the rate of a reaction: k = exp.

However, instead of modeling the temperature dependence of reaction rate phenomenologically, the Eyring equation models individual elementary step of a reaction. Thus, for a multistep process, there is no straightforward relationship between the two models; the functional forms of the Arrhenius and Eyring equations are similar, for a one-step process and chemically meaningful correspondences can be drawn between Arrhenius and Eyring parameters. Instead of using Ea, the Eyring equation uses the concept of Gibbs energy and the symbol ΔG‡ to denote the Gibbs energy of activation to achieve the transition state. In the equation, kB and h are the Planck constants, respectively. Although the equations look similar, it is important to note that the Gibbs energy contains an entropic term in addition to the enthalpic one. In the Arrhenius equation

Miles Lewis

Professor Miles Lewis is an Australian academic serving as a Professor in the Faculty of Architecture, Building & Planning, at the University of Melbourne, Australia. He is one of Australia's most notable Architectural historians, a member of the Order of Australia, he is a Fellow of the Australian Academy of the Humanities, a former President of Australia ICOMOS, of the Society of Architectural Historians Australia and New Zealand and of the Council for the Historic Environment. He is an immediate past President of the Town and Country Planning Association, current Vice-President of the Comité International d’Architecture Vernaculaire, he is a former member of the Administrative Appeals Tribunal, Victoria and a former Auckland University Foundation Fellow. Professor Lewis has been a consultant to the Getty Institute, he participated in China. He has many research interests include urban conservation, urban renewal, building history, vernacular architecture, urban policy. A well-known and forthright architectural historian and commentator on planning issues in the media, Professor Lewis has a number of useful databases online relating to architectural history and the history of building construction in Australia which are essential sources for others in the profession.

2013 Professor Miles Lewis was awarded Honorary Life Membership of the National Trust of Australia. Lewis MB. 1999. Suburban backlash: the battle for the world's most liveable city. Melbourne: Bloomings Books. Lewis MB. 1994. Melbourne: the city's history and development. Melbourne: City of Melbourne. Lewis MB. 1991. Victorian churches. Melbourne: National Trust of Australia. Lewis MB. 1988. Two hundred years of concrete in Australia. North Sydney: Concrete Institute of Australia. Lewis MB. 1983. The essential Maldon. Richmond, Vic: Greenhouse in association with the National Trust of Australia, Lewis MB. 1977. Don John of Balaclava. Melbourne: Brian Atkins. Lewis MB. 1977. Victorian primitive. Carlton, Vic: Greenhouse Publications. Staff webpage at the University of Melbourne Personal webpage Link to Miles Lewis data base page Who invented the Hill's hoist? ABC TV 22/8/04 Universities demanding foreign students be passed, ABC TV 7:30 Report, 20/2/07 Protesters get tunnel vision as Smith St becomes a test case, 23/6/04 Collingwood Action Group report, edited by Miles Lewis, 17/8/04 Boulton, M, 26/8/05, The Age, What would it take to make Melbourne a more liveable city

Darius Vase

The Darius Vase is a famous vase painted by an anonymous Magna Graecia Apulian vase painter called the Darius Painter, the most eminent representative at the end of the "Ornate Style" in South Italian red-figure vase painting. The vase was produced between 340 and 320 BCE in a large factory-like workshop in the Greek city of Taranto, Magna Graecia, well before the fall of Taranto to the Romans in 272 BCE, it is an important part of Apulian vase painting. The "Darius Vase" was discovered in 1851 near Canosa di Puglia and is now on display at the Museo Archaeologico Nazionale, Naples; this work, a volute krater is of large dimensions. It is 1.93 meters in circumference. The vase contains several inscriptions, such as naming individual figures, but there are thematical names. To some extent these inscriptions can be seen as "titles". All available space on the vase is used for figural depictions, arranged in three registers; some individual zones are structured by opulent ornamental friezes. The Darius Painter is considered the first painter to have exploited the possibilities of large-format vase painting.

His drawing style is reputed to be good, particular as regards faces, which he depicts in a three-quarter profile. The neck of the vase shows combat scenes between the Greeks and the Persians, it is thought that these scenes represents the combats between Alexander the Great and Darius III, rather than the earlier combats of the troops of Darius I during his First Persian invasion of Greece. Above Darius stands a line of Greek Gods: Artemis riding a stag, Apollo seated holding a swan, Aphrodite together with Eros, Zeus holding a winged thunderbolt, Hellas standing, Athena holding a shield, Apate holding two torches, Asia seated on an altar, next to a pillar holding a head. Darius I is shown seated, wearing a long, sleeved robe and a high Persian hat. A body guard stands behind him, as Darius is listening to an allegory of the Persian people, enjoining him not to attack the Greeks. Darius could be listening to a messenger. Xerxes I, still a Prince, is said to be represented, second from right.

The scene of the audience given by an Achaemenid ruler seems to have been quite conventional, appears in a similar fashion in the frieze on the tomb of Lycian ruler Arbinas. A tax collector, the Royal Treasurer, is seen receiving payments by various conquered nations, whose representatives crouch before him. On a table lays a calculating table, with a number of small pebbles or counters in front of the Greek numerals for calculating large numbers; the symbol "O" appears in a Boetian symbol for the obol, or small unit. The use of pebbles on a board to make calculations is illustrated down to modern times by the fact that calx is "pebble" in Latin, the etymological root for the word "calculation"; the board contains the letters Ψ, H, Δ and. White pebbles are added next to each letter. Next to them appear the former symbols used to represent the Greek coins: Obol, half an Obol and a quarter of an Obol; these symbols resemble. The number here shown is 1741 and 4/6 Drachms; the tax collector holds an open diptych in which can be read the letters TAΛNTA:H meaning tal'anta hekaton'.

The Darius vase may have represented a scene from a Greek drama. The depiction of Darius on his name-vase is derived in its details from the Persae of Phrynikos, C. Anti concluded in 1952, Schmidt 1960 follows him; however Oliver Taplin notes in Pots and Plays, 2007, p. 235-7, the only strong indications of tragic reference are Darius himself and the old man in paidagogos outfit on the plinth inscribed ΠΕΡΣΑΙ, who might be performing the messenger role. Taplin speculates that the iconography of tragedy "could be assimilated into other contexts without danger of confusion", op. cit. p. 237. Apulian vase painting Margot Schmidt. Der Dareiosmaler und sein Umkreis: Untersuchen zur Spätapulischen Vasenmalerei, Munich: Aschendorff, 1960. Jean-Marc Moret. L'Ilioupersis dans la céramique italiote, les mythes et leur expression figurée au IVe siècle, Institut Suisse de Rome, 1975. Thomas Morard, Horizontalité et verticalité. Le bandeau humain et le bandeau divin chez le Peintre de Darius, von Zabern, 2009.

Alexandre Cambitoglou, Arthur Dale Trendall. The Red-figured Vases of Apulia, II, Late Apulian, Oxford, 1982: p. 482-522. Bibliography. Christian Aellen, Alexandre Cambitoglou, Jacques Chamay. Le peintre de Darius et son milieu, Vases grecs d'Italie Méridionale, Hellas et Roma, Genf 1986. Arthur Dale Trendall. Rotfigurige Vasen aus Unteritalien und Sizilien. Ein Handbuch. Von Zabern, Mainz 1991, ISBN 3-8053-1111-7. Françoise-Hélène Massa-Pairault. Le Peintre de Darius et l'actualité. De la Macédoine à la Grande Grèce, in L'incidenza dell'Antico II: studi in memore di Ettore Lepore, Napoli, 1996. Rolf Hurschmann. Dareios-Maler, in Der Neue Pauly Vol. 3, col. 324. Claude Pouzadoux, Guerre et paix en Peucétie à l'époque d'Alexandre le Molosse, in Le Canal d'Otrante et la Méditerranée antique et médiévale, colloque organisée à l'Université de Paris X - Nanterre, Bari, 2005