Open an active chat with <Super+n> without taking your hands from keyboard. Fast & simple!
Responding without using mouse!
Tip: You can open Message Tray with <Super+m>, navigate to items with arrows, open them with Enter or Space, close them with Esc. Message Tray doesn’t close with <Super+m>; this is a bug that it will be fixed in 3.8.
Note: You don’t need Empathy Window open to chat, this is just for the demonstration!