Chemistry: A Molecular Approach (3rd Edition)

If energy flows out of a chemical system and into the surroundings, the sign of $ΔE_{System}$ is negative.
$ΔE_{System}$ = $ΔE_{Final}$ - $ΔE_{Initial}$ = $-$$ΔE_{System}$ If a system loses energy to its surroundings, then $ΔE_{Final}$ $\lt$ $ΔE_{Initial}$ and the end change will be negative.