Activation Energy Section 16.3 (AHL)

Introduction All chemical reactions require minimum energy (activation energy, E a ) to occur At higher temperatures, a greater proportion of the molecules have an energy greater than the activation energy This is the major reason why reactions occur more rapidly at higher temperatures

Don’t forget! The Maxwell-Boltzmann Distribution

Arrhenius Equation The effect of temperature on the rate constant for many reactions is found by the expression: This is called the Arrhenius equation

Meaning E a is the activation energy T is the absolute temperature in Kelvin R is the gas constant (8.314 J K -1 mol -1 ) A is called the Arrhenius constant The Arrhenius constant is dependent on the collision rate and steric factors

Simplified Arrhenius Equation If you take logarithms to the base e and simplify you get: This is the equation of a straight line The rate can be found by calculating the gradient of the line

Example Problem The rate constant, k, was determined for a reaction at various temperatures. Here are the results: k in mol -1 dm 3 s -1 Temperature in °C 6.81 x x x x

Problem Continued First draw up a table showing T -1 and ln k values, T must be in absolute temp. (K) Plot a graph of ln k against T -1 Take the ln k axis from -12 to -8 and the T -1 axis from to (Use graph paper provided)

ln k mol -1 dm 3 s -1 T -1 (K-1) x x x x10 -3 Table of lnk and T -1 to graph

Problem Continued Calculate the gradient of your Arrhenius plot and use it to determine a value for E a in kJ mol -1 y-intercept = about Gradient is about So y = -6098x

Problem Continued Gradient = -E a /R = E a = (-6098 K) (8.314 J mol -1 K -1 ) E a = J mol -1 E a = 50.7 kJ mol -1

Problem Continued Calculate an approximate value for the Arrhenius constant, A, using the Arrhenius plot ln A is the y-intercept, so ln A = A = 2.54 x mol -1 dm 3 s -1 A reaction with a steeper gradient will have a higher E a

Checking with Excel

Catalysts There are two types of catalysts: Heterogeneous catalyst Homogeneous catalyst

Heterogeneous Catalyst This is when the catalyst and the reactants are in different phases Often this involves the reaction of gas molecules on the surface of a solid

Homogeneous Catalyst This is when the catalyst and the reactants are in the same phase Usually involves the formation of an intermediate during the reaction