Why is the gas constant in the Arrhenius equation?
Because the universal gas constant is measured in Joules/(Kelvin x Mole)
A is the prefactor, it is different for every reaction.
Ea is the activation energy, measured in Joules
R is the universal gas constant, measured in Joules/(Kelvin x Moles)
T is temperature measured in Kelvin
The Arrhenius equation relates the effect of temperature to reaction rate. The universal gas constant allows us to convert temperature into moles and joules. This is crucial because the activation energy is also a factor. So it is there essentially as a constant to allow us to convert units.