[thermodynamics]
how much energy needs to be added to react 20.0 mol cl2 with excess ch4?

45.6 kj is req. to react with 2 moles of cl2

therefore,

for 1 moles energy req.=45.6/2

for 20 moles nrg req=45.6×20/2

=456 kj

increased stability in the completion of fluorine's outer shell.