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ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Kinetics > Modeling Reaction Kinetics > Temperature Dependence of Reaction Rates > Changing Reaction Rates with Temperature

Changing Reaction Rates with Temperature

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In the vast majority of cases, we depend on thermal activation, so the major factor we need to consider is what fraction of the molecules possess enough kinetic energy to react at a given temperature. According to kinetic molecular theory, a population of molecules at a given temperature is distributed over a variety of kinetic energies that is described by the Maxwell-Boltzman distribution law.

The two distribution plots shown here are for a lower temperature T1 and a higher temperature T2. The area under each curve represents the total number of molecules whose energies fall within particular range. The shaded regions indicate the number of molecules which are sufficiently energetic to meet the requirements dictated by the two values of Ea that are shown.

It is clear from these plots that the fraction of molecules whose kinetic energy exceeds the activation energy increases quite rapidly as the temperature is raised. This the reason that virtually all chemical reactions (and all elementary reactions) are more rapid at higher temperatures.

 

 

 

Temperature is considered a major factor that affects the rate of a chemical reaction. It is considered a source of energy in order to have a chemical reaction occur. Svante Arrhenius, a Swedish chemist, believed that the reactants in a chemical reaction needed to gain a small amount of energy in order to become products. He called this type of energy the Activation Energy. The amount of energy used in the reaction is known to be greater than the activation energy in the reaction. Arrhenius came up with an equation that demonstrated that rate constants of different kinds of chemical reactions varied with temperature. This equation indicates a rate constant that has a proportional relationship with temperature. For example, as the rate constant increases, the temperature of the chemical reaction generally also increases. As we derive Arrhenius' rate constant expression twice, the result is:

                                   ln k2/k1= Ea/R (1/T1-1/T2)

This equation is known as Arrhenius' equation. T1and T2are temperature variables expressed in Kelvin. T1can be expressed as the initial or lower temperature of the reaction, while T2 is the final or higher temperature of the reaction. Rate constants, k1and k2, are values at T1and T2. Eais the activation energy expressed (Joules/mole)=(J/mol). R is the gas constant expressed as 8.3145 (Joules/mole x Kelvin)=(J/mol*K)

Some may ask how the temperature actually affects the chemical reaction rate. The answer to this is that this phenomenon is related to the collision theory. Molecules will only react if they have a sufficient amount of energy in order for a reaction to take place. When the temperature of a solution increases, the molecular energy levels will also increase which causes the reaction to go faster.

The graph of ln K vs. 1/T is a straight line. This straight line allows us to calculate the activation energy needed for the reaction.

 

                                                 k=Ae-Ea/RT

Some interesting examples:

  1. Notice when you add salt or even food coloring to cold water, room temperature water and hot water. It seems that when the substance blends in with the hot water, the high temperatures allow it become a homogeneous mixture. This is because of the water molecules that are moving faster when the temperature is higher and causing the reaction to go faster.
  2. Another form of energy is light. One example of the effect of temperature on chemical reaction rates is the use of lightsticks or glowsticks. The lightstick goes through a chemical reaction that is called chemiluminescence; but this reaction is not caused or produces heat. It's rate, though, is influenced by temperature. If the lightstick is put in a cold surrounding, the chemical reaction will slow down allowing it to give off light longer. If the lightstick is in a hot surrounding, on the other hand, the reaction will speed up causing the light to wear out faster. -(This example is from the "How Things Work-Lightsticks" from chemistry.about.com)

Outside links

References

  1. Petrucci, Harwood, Madura, Herring. General Chemistry: Principles & Modern Applications, 9th edition., Sec.14-9: The Effect of Temperature on Reaction Rates, p. 594. 
  2. Frost, Arthur A., Kinetics and Mechanism: A Study of Homogeneous Chemical Reactions., Ch.2: Effect of Temperature on Reaction Rate, pgs. 23-24.

Problems

  1. True or false- When the temperature increases in a chemical reaction, the rate also increases.
  2. True or false- Temperatures can be both positive and negative.
  3. A chemical reaction has a rate constant of 5.10x10-9h-1 at 225 K and 6.36x10-3 h-1 at 400 K. What is the activation energy in kJ/mol for this reaction.

Contributors

  • Andrea B.

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External Links

www.purchon.com/chemistry/rates.htm
www.chemistry.about.com/od/howthingsworkfaqs/a/howlightsticks.htm
Posted 18:52, 24 Mar 2014
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