<?php

$data=['Fe+2HCl'=>'FeCl2+H2', 'Zn0+Cl2'=>'ZnCl2', '2H2+O2'=>'2H2O', '2H2O' => '2H2+O2'];

function foo (array $a) {
	$result = [];
	foreach ($a as $key=>$value) {
		$elems=explode('+',$value);
		foreach ($elems as &$el)
		  $el = preg_replace ('/^\d*/', '', trim($el));
		$result = array_merge ($result, $elems);
	}
	return array_unique ($result);
}

var_dump (foo ($data));