Determining the Activation Energy

It is widespread knowledge that jiyuushikan.orgical reactions happen even more promptly at better temperatures. Milk turns sour a lot more quickly if stored at room temperature quite than in a refrigerator; butter goes rancid more conveniently in the summer than in the winter; and eggs hard-boil more conveniently at sea level than in the hills. For the same factor, cold-blooded animals such as reptiles and insects tend to be more lethargic on cold days.

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The reason for this is not hard to understand also. Thermal power relates direction to motion at the molecular level. As the temperature rises, molecules relocate much faster and also collide more strongly, greatly boosting the likelihood of bond cleavperiods and rearrangements. Whether it is through the collision theory, transition state theory, or simply prevalent sense, jiyuushikan.orgical reactions are frequently intended to continue quicker at greater temperatures and sreduced at reduced temperatures.


By 1890 it was widespread knowledge that greater temperatures speed up reactions, regularly doubling the price for a 10-degree increase, but the reasons for this were not clear. Finally, in 1899, the Swedish jiyuushikan.orgist Svante Arrhenius (1859-1927) unified the concepts of activation energy and also the Boltzmann distribution legislation into one of the the majority of crucial relationships in physical jiyuushikan.orgistry:


Take a moment to emphasis on the interpretation of this equation, neglecting the A aspect for the time being. First, note that this is one more create of the exponential decay law discussed in the previous section of this series. What is "decaying" below is not the concentration of a reactant as a duty of time, however the magnitude of the price consistent as a role of the exponent –Ea /RT. And what is the definition of this quantity? Recalling that RT is the average kinetic energy, it becomes apparent that the exponent is just the ratio of the activation power Ea to the average kinetic energy. The bigger this ratio, the smaller sized the rate (therefore the negative sign). This suggests that high temperature and low activation energy favor bigger price constants, and for this reason speed up the reaction. Because these terms take place in an exponent, their effects on the price are fairly substantial.

The two plots listed below show the results of the activation power (denoted here by E‡) on the rate consistent. Even a modest activation power of 50 kJ/mol reduces the rate by a factor of 108.


Determining the Activation Energy

The Arrhenius equation,

deserve to be written in a non-exponential create that is frequently more convenient to usage and also to analyze graphically.Taking the logarithms of both sides and separating the exponential and pre-exponential terms yields

<eginalign ln k &= ln left(Ae^-E_a/RT ight) \<4pt> &= ln A + ln left(e^-E_a/RT ight) label2 \<4pt> &= left(dfrac-E_aR ight) left(dfrac1T ight) + ln A label3 endalign>

Equation ef3 is in the develop of (y = mx + b) -the equation of a straight line.

< ln k=ln A - dfracE_aRT >

wbelow temperature is the independent variable and the price consistent is the dependent variable. So if one were given a file collection of miscellaneous values of (k), the rate continuous of a specific jiyuushikan.orgical reaction at varying temperature (T), one might graph (ln (k)) versus (1/T). From the graph, one can then determine the slope of the line and realize that this worth is equal to (-E_a/R). One deserve to then fix for the activation energy by multiplying through by -R, wbelow R is the gas consistent.

This affords a simple means of determining the activation power from worths of k observed at various temperatures, by plotting (ln k) as a duty of (1/T).

Example (PageIndex1): Isomerization of Cyclopropane

For the isomerization of cyclopropane to propene,

api/deki/files/17850/cyclopropisom_plot.png?revision=1" />

T, °C 1/T, K–1 × 103 k, s–1 ln k
477 523 577 623
1.33 1.25 1.18 1.11
0.00018 0.0027 0.030 0.26
–8.62 –5.92 –3.51 –1.35

Example (PageIndex3)

It takes about 3.0 minutes to cook a hard-boiled egg in Los Angeles, but at the greater altitude of Denver, wbelow water boils at 92°C, the cooking time is 4.5 minutes. Use this indevelopment to estimate the activation power for the coagulation of egg albumin protein.


The proportion of the rate constants at the elevations of Los Angeles and Denver is 4.5/3.0 = 1.5, and the particular temperatures are (373 ; mK ) and (365; mK). With the subscripts 2 and 1 referring to Los Angeles and also Denver respectively:

<eginalign* E_a &= dfrac(8.314)(ln 1.5)dfrac1365; mK – dfrac1373 ; mK \<4pt> &= dfrac(8.314)(0.405)0.00274 ; mK^-1 – 0.00268 ; mK^-1 \ &= dfrac(3.37; mJ; mol^–1 K^–1)5.87 imes 10^-5; mK^–1\<4pt> &= 57,400; m J; mol^–1 \<4pt> &= 57.4; mkJ ;mol^–1 endalign*>

Comment: This low worth seems reasonable bereason thermal denaturation of proteins mostly entails the disruption of relatively weak hydrogen bonds; no covalent bonds are broken (although disulfide bonds can interfere through this interpretation).

The Pre-exponential Factor

Up to this point, the pre-exponential term, (A) in the Arrhenius equation (Equation ef1), has been ignored bereason it is not straight associated in relating temperature and also activation power, which is the primary handy use of the equation.


However before, because(A) multiplies the exponential term, its worth clearly contributes to the value of the price consistent and also thus of the rate. Respeak to that the exponential component of the Arrhenius equation expresses the fractivity of reactant molecules that possess enough kinetic power to react, as governed by the Maxwell-Boltzmann law. This fractivity deserve to run from zero to practically unity, relying on the magnitudes of (E_a) and also of the temperature.

If this fractivity were 0, the Arrhenius law would certainly reduce to

In various other words, (A) is the fraction of molecules that would certainly react if either the activation power were zero, or if the kinetic power of all molecules exceeded (E_a) — admittedly, an unprevalent scenario (although barriermuch less reactions have actually been characterized).

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What would certainly limit the rate consistent if tbelow were no activation energy requirements? The the majority of obvious element would be the rate at which reactant molecules come right into contact. This have the right to be calculated from kinetic molecular theory and also is known as the frequency- or collision factor, (Z).