IB Chemistry syllabus and notes

16.3 - Activation energy

16.3.1: Describe qualitatively the relationship between the rate constant (k) and temperature (T).

Effect of temperature on the reaction rate

Direct observations make it clear that increasing the temperature increases the rate of a chemical reaction. In approximate terms, most reactions double in rate for a ten degree increase in temperature.

This effect was first quantified by Arrhenius who produced the equation:

rate constant k = Ae^-Ea/RT

quantity	description	meaning
A -	the Arrhenius constant	This is a measure of the proportion of molecules that collide with enough energy to react AND which have the correct orientation for successful collision.
e -	the natural number on which the natural logarithms is based	2.303
Ea -	the activation energy	This is the minimum energy that a molecular collision must have before it can be successful and lead to reaction - units kJ mol^-1
R -	the universal gas constant	8.314 in SI units
T -	the Absolute temperature in Kelvin (K)	Equal to the temperature in Celsius + 273

Using the Arrhenius equation

Although it is not easy to see the relationship between the rate constant and the absolute temperature from the equation, if we break it down into steps perhaps it will help.

The temperature appears in the term Ea/RT
If T increases then the term Ea/RT gets smaller
However in the Arrhenius equation Ea/RT has a negative value, therefore as T increases Ea/RT gets LESS negative.
So as -Ea/RT is the power to which 'e' is raised then the term e^-Ea/RT gets larger (as the power gets less negative) as the temperature increases.
The rate constant is directly proportional to the term e^-Ea/RT and so the rate constant gets larger as T gets larger.

Example

Calculate the rate constant when T = 300K (A = 0.3, Ea = 50kJ mol^-1)

k = Ae^-Ea/RT

Ea/RT = 50000/(8.314 x 300) = 20.05

e^-Ea/RT = 1.97 x 10^-9

k = Ae^-Ea/RT

k = 5.90 x 10^-10

16.3.2 Describe how the Arrhenius equation can be used to determine the activation energy and the Arrhenius constant (A). Arrhenius equation: A relates to the geometric requirements of the collisions (see 7.2). Direct substitution using simultaneous equations and a graphical method can be used. The logarithmic form of the Arrhenius equation is: Both methods should be explained, but actual calculations are not needed.

16.3.3 Draw and explain enthalpy level diagrams for reactions wih and without catalysts. t

16.3.4 Distinguish between homogeneous catalysts and heterogeneous catalysts. Homogeneous catalyst- reactants and catalyst are in the same phase.
Heterogeneous catalyst- reactants and catalyst are in different phases.

16.3.5 Outline the use of homogeneous and heterogeneous catalysts. Examples include hydrogenation using metals (see 13.2.7) and acid-catalysed formation of esters.

Resource

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