Regla del pulgar

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  • Publicado : 7 de febrero de 2012
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In many textbooks one can find an empirical rule for how to evaluate quickly an increase in the rate of a chemical reaction when temperature changes.For example: “according to an old rule of thumb, the rate of a reaction approximately doubles with each 10c temperature rise.
Different authors sometimes add extra information to this rule. Thus, J. B.Russel, among others pointed out that “unfortunately, the rule is so approximate as to be of limited value”. It has been suggested that a reaction mist proceed in a homogeneous system (2,3) or insolution (4,5), or “at ordinary temperature” (5), at room temperature (6). Even mention of a reaction rate could be found: “this rule… applies only to reaction that last longer than a second or two”Other authors specify the temperature coefficient itself. “the specific rate is usually increased by a factor of about two or three for every 10c rise of temperature” (3). “In reality, the factor isusually in the range from 1.5 to 4”. (7). “as a rough working rule a 5 per cent to 10 per cent increase in rate per degree centigrade rice in temperature many be assumed” (6). This gives a factor from1.0510=1.63 to 1.1010=2.6 for a 100 increase in temperature.
On the other hand, it is now widely accepted that over a limited range of temperature the majority of the reactions studied can be adequatelydescribed with the Arrhenius equation
Rate= A exp(-E/RT)
Where the pre-exponential factor A and activation energy E are assumed to be constant (it should be noted that in special circumstances,experimental data, especially those obtained with improved precision and over a wide range of temperature, may not fit into the Arrhenius equation [8,9] moreover , there are many reactions with negativetemperature coefficients (10). Assuming the validity of this equation we can see that the increase in a rate depends on both activation energy and absolute temperature. Thus, for the gas-phase...
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