Using iPad2 Charger to Charge iPhone5S
As you can see from Apple's product page for the 12W USB Power Adapter, it's compatible with iPhone 5S. The output voltage is the same in both, except the 12W adapter gives a higher amperage (2.4A instead of 1A), but your phone can regulate it's intake.

I use my iPad Air's charger occasionally with my iPhone 5S with no problems.