Given
#H_2(s) + F_2(g)->2HF(g)color(blue)(.....[1]), DeltaH_1= -537#
#C(s) + 2F_2(g)-> CF_4(g)color(blue)(.....[2]), DeltaH_2= -680#
#2C(s) + 2H_2(g) -> C_2H_4(g)color(blue)(.....[3]), DeltaH_3= +52.3#
To Calculate #DeltaH# for the reaction
#C_2H_4(g) + 6F_2(g)->2CF_4(g) + 4HF(gcolor(blue)(.....[4]),DeltaH_4=?#
#"eq[1]"xx2+"eq[2]"xx2-"eq[3]"# gives
#DeltaH_4=2xxDeltaH_1+2xxDeltaH_2-DeltaH_3#
#=>DeltaH_4=2xx(-537)+2xx(-680)-52.3=-2486.3#