Chandra, Flame's Catalyst

You must pay the costs for the spell you wish to cast with Chandra's second ability. If that spell has any alternative costs, you may pay those.

If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, those must be paid to cast the spell.

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.