How to Find Activation Energy

Written by charles ezeani
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How to Find Activation Energy
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Activation energy, abbreviated Ea, can best be described as the minimum energy required before a chemical reaction can proceed. It is like a barrier that has to be overcome in order for your chemical reaction to take place. Activation energy can be calculated in two ways: by using the rates of reaction at two different temperatures or by using the Arrhenius equation.

Skill level:
Moderate

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Things you need

  • Paper
  • Pencil
  • Graphing calculator

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Instructions

    By reaction rate at two different temperatures method

  1. 1

    Write down the equation for finding activation energy.

    Activation energy, Ea = ((T1 x T2 x R) / (T2 - T1)) x In(k2 / k1)

    "T1" is the first temperature and "T2" is the second temperature. R is the universal gas constant, which equals 8.314 J (K mol). "k1" is the rate of reaction at "T1" and "k2" is the rate of reaction at "T2."

  2. 2

    Plug in the values for the temperatures, reaction rates and universal gas constant. As an instance, take a reaction that has a reaction rate of 0.0139 at 285K and a rate of reaction of 0.0493 at 350K. The resulting equation is:

    Ea = (((285K x 350K x 8.314 J/(K mol)) / (350K - 285K))) x In(0.0493 / 0.00139).

  3. 3

    Use your calculator to solve the equation and obtain the activation energy. In this case:

    Ea = (829321 / 65) * In(35.47) = 45532.11 J/mol = 45.53 kJ/mol.

    By experimental method using the Arrhenius equation

  1. 1

    Take the natural logarithm of both sides of the Arrhenius equation. The Arrhenius equation is:

    k = A e ^ -Ea/RT.

    "k" is the rate of reaction constant, A is the pre-exponential factor which is assumed to be independent of temperature change, Ea is the activation energy itself, R is the universal gas constant and T the temperature in Kelvin (K).

    This gives ln k = ln A - Ea/(RT).

  2. 2

    Rearrange the equation into the formula of a straight line in the form y = mx + c. This yields:

    ln k = -Ea/(RT) + constant or ln k = -(Ea/R)(1/T) + constant.

  3. 3

    Use the second option to plot a graph of In k vs. 1/T with a graphing calculator to get (- Ea/R) as your slope as the second option is already in the form y = mx + c.

  4. 4

    Divide the calculated slope by the universal gas constant to isolate the value (- Ea) from (- Ea/R).

  5. 5

    Multiply (- Ea) by -1 to get Ea, the activation energy of your reaction. This forms the basis of the experimental determination of activation energy.

Tips and warnings

  • Always cross-reference your answer/final result with published values in laboratory data log books, chemistry textbooks or on online databases for accuracy and correct units.

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