LESSON 8 Activation Energy

RECAP 1.Two species, P and Q, react together according to the following equation. P + Q → R The accepted mechanism for this reaction is P + P → P2 fast P2 + Q → R + P slow What is the order with respect to P and Q? PQ A.11 B.12 C.21 D.22

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LESSON 8: ACTIVATION ENERGY  Objectives:  Understand the term activation energy

 The common relationship between rate and temperature is that for every 10 degrees Celsius you are essentially doubling the rate.  This is not a law, it is generally used as a rule of thumb.  We have learned that the rate of a reaction depend on two things.  k, the rate constant  Concentration of reactants  Since increasing the temp doesn’t effect the concentration, its effect goes to k.  k is a general measure of the rate of a reaction at a given temperature.

ACTIVATION ENERGY  Activation energy is the minimum energy to colliding particles need in order to react  You can think of it as:  The energy required to begin breaking bonds  The energy that particles need to overcome the mutual repulsion of their electron shells.  Can you think of an analogy?

THE ARRHENIUS EQUATION  We met the rate constant, k, a couple of lessons ago  The Arrhenius Equation tells us how k is related to a variety of factors: Where: k is the rate constant E a is the activation energy T is the temperature measured in Kelvins R is the gas constant, J mol -1 K - 1. e is Euler’s number A is the ‘frequency factor’

REARRANGING ARRHENIUS  If we take logs of both sides, we can re-express the Arrhenius equation as follows:  This may not look like it, but is actually an equation in the form y = mx + c Where: ‘y’ is ln k ‘m’ is -E a /R ‘x’ is 1/T ‘c’ is ln A

TO DETERMINE E A EXPERIMENTALLY: (ASSUMING WE KNOW THE RATE EQUATION)  Measure the rate of reaction at various different temperatures.  Keeping all concentrations the same  Calculate the rate constant, k, at each temperature.  Plot a graph of ln k (y-axis) vs 1/T (x-axis)  The gradient of this graph is equal to ‘-E a /R’, this can be rearranged to calculate E a.

RECAP  Activation energy can be determined by the gradient of a graph of ln k vs 1/T