can you charge an iphone 5s with the 12 watt ipad air charger?

According to Apple's product page for the 12 Volt charger you can. If you go to the page and scroll down just a little bit you will see the list of compatible devices, every model of iPhone and iPad to date is listed as compatible.

http://store.apple.com/us/product/MD836LL/A/apple-12w-usb-power-adapter

Hope it helps!

Yes, you can. I've been using iPad chargers for my iPhones for years. As noted, the separate chargers Apple sells are for both devices.

Best of luck.