LYCOS RETRIEVER 
Activation Energy
built 656 days ago
An activation energy diagram for an endothermic reaction is illustrated here. The potential energy for products is higher than the energy for reactants. The magenta curve is the reaction pathway for an uncatalyzed reaction. The cyan (blue) curve shows what a catalyst does to the energy for the reaction path. The reaction is faster when the catalyst is present. The activation energy is lower with the catalyst present.
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The Arrhenius equation relates temperature, rate constant, and activation energy. Solving these equations requires measuring the rate constant at more than one temperature. Activation energy can be determined from temperature (T) and rate constant (k) by graphing ln k versus 1/T. The slope is equal to –Ea/R, where R is the gas constant. If the gas constant value of 8.314 J/mol•K is used, the units of activation energy will be in joules.
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The graphs below depict the activation energy for the process of migrating an oxygen ion from a lattice site to a vacancy. The vacancy studied here is at the same lattice site for all cases; it's the starting positions of the migrating ions that are different. The pictures on the left hand side show a small section of the starting configurations; the pink balls are titania ions, the blue are calcium and the red, oxygen. The white ball is the migrating oxygen ion. By placing the mouse cursor on these pictures you should see the final configuration after the ionic migration has been completed.
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For reasons that are beyond the scope of any course at this level, this expression counts the fraction of the molecules present in a gas which have energies equal to or in excess of activation energy at a particular temperature. You will find a simple calculation associated with this further down the page.
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It would seem from Figure 1 that B, and therefore the activation energy, is negative. This is due to the fact that the x-axis is actually a reciprocal scale. The x-axis is plotted on a reciprocal scale simply for practical purposes. It is just easier to read the life at 404 than at 0.00248 (the reciprocal of 404). Figure 1 actually illustrates that B is positive. As the reciprocal of the stress increases, the stress decreases.
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[B]y plotting ln(s/t) against (1/T), for a given value of N, the activation energy Q can be found from the gradient (-Q/R).The photos below are taken after creeping for one minute. The spacings between the 11th and 12th coils (in mm) are marked.
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Activation Energy