The tall opener struck three sixes in his maiden ODI hundred and shared century partnerships with Jason Roy (54) and Joe Root (63) as England amassed 283 for five after winning the toss.
Eoin Morgan's men laid the platform to post a score in excess of 300 but lost three wickets in the last five overs, Pakistan fast bowler Wahab Riaz finishing with three for 43 following a clever late spell.
England fast bowler David Willey then claimed two early wickets and Chris Woakes ripped through the middle order as Pakistan collapsed to 50 for five.
Sarfraz Ahmed hit 64 in 76 balls but it proved in vain as Woakes ended with four for 33 and Willey took three for 25.
The third match in the four-game series is on Tuesday in Sharjah.
Image: England's Alex Hales bats during the second One Day-International against Pakistan at the Zayed Cricket Stadium in Abu Dhabi on Friday.
Photograph: Gareth Copley/Getty Images