[abadir16]

The following gas-phase elementary reaction:

CH4(g) + F•(g) = HF(g) + CH3•(g)
has an activation energy (Ef) equal to 5.1 kJ mol−1 and for the methyl radical dHf(298 K) = 145.7 kJ mol−1

the enthalpy of formation of HF is -271.1 kJ mol−1

calculate the activation energy for the reverse of this reaction (Er)

answer choices are: 134.7, -5.1, 284.3, 55.7, -124.5 kJ mol−1

my attempt:

Adding the methyl radical and HF = -125.4 kJ mol−1

ANSWER= 5.1+125.4= 130.5         have done something wrong? thx for your help

[abadir16]

after harder thinking i found out my mistake and hence found the solution.