The rate of a chemical reaction increases as the temperature is increased. A. Dr
ID: 77800 • Letter: T
Question
The rate of a chemical reaction increases as the temperature is increased.
A. Draw a diagram of the Maxwell%u2013Boltzmann distribution of molecular energies at a temperature T1 and at a higher temperature T2
b. Use your diagram and the idea of activation energy to explain why the rate of a chemical reaction increases with increasing temperature.
Explanation / Answer
for the diagram visit following link:http://www.docbrown.info/page03/ASA2rates.htm When the temperature is raised the added 'heat energy' shows itself in the form of increased particle kinetic energy. In the graph above, two distribution curves are shown for a lower/higher temperatures, T1/T2, and it is assumed that the area under the whole curve is the same for both temperatures i.e. the same number/population of molecules. Comparing lower temperature T1 with higher temperature T2, you can see that as the temperature increases, the peak for the most probable KE is reduced, and more significantly with the rest of the KE distribution, moves to the right to higher values so more particles have the highest KE values. Now, if we consider an activation energy Ea, the minimum KE the particles must have to react via e.g. bond breaking, the fraction of the population able to react at T1 is given by the blue area. However, at the higher temperature T2, the fraction with enough KE to react is given by the combined blue area plus the red area. Therefore, because of the shift in the distribution at the higher temperature T2, a greater fraction of particles has the minimum KE to react and hence a greater chance of a fruitful collision happening i.e. reactant molecule bonds breaking en route to product formation. In the diagram, for the sake of argument, a temperature rise from T1 to T2 results in the fraction of particles with a KE of >=Ea being doubled (area blue==>blue+red). For reactions with an activation energy in the range 50-100 kJmol-1 (i.e. most reactions), this results in an approximately doubling of the reaction rate for every 10o rise in temperature i.e. where T2 = T1 + 10, because if you double the number of particles of KE >= Ea, you therefore double the chance of a fruitful collision and hence double the rate of reaction. So, a relatively small change in temperature e.g. 10o rise, can have quite a 'statistically' dramatic effect on the small, but significant population of the highest KE molecules, hence a significant change in reaction rate. doc bThe last point accounts for why a plot of rate versus temperature shows an 'exponential' or 'accelerating' curve upwards. Almost all reaction rates increase by a factor of 1.5 to 4.5 on doubling the temperature, but it does depend on the actual activation energy, so the "10o temperature rise effect" is a very rough rule of thumb when we say it "doubles the rate"! _____________________________________________________________________ 2nd minor factor note: The rise in temperature does lead to an increase in collision frequency, and hence an increase in the possibility of a 'fruitful' collision and so increasing the speed of a reaction. However, this effect on the rate of reaction, is proportionally much smaller by a factor of 100-200x, compared to the increase in reaction speed due to the increase in the proportion of high KE molecules on increasing the temperature as described above.
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