The warlock's (Pact of the Blade-only) Improved Pact Weapon eldritch invocation (Xanathar's Guide to Everything, p. 57) states that you can use your pact weapon as a spellcasting focus. Using a whip, let's say, would give your weapon a reach of 10 feet.
Using a reach weapon as a spellcasting focus, can you deliver a touch spell through it to a creature 10 feet away from you?
Best Answer
No, a focus does not deliver spells
Page 202 of the Player's Handbook states (emphasis mine):
Normally you would need one empty hand to hold your focus and do somatic components of the spell. Thus, could not hold a weapon and shield while casting. In effect, the Pact of the Blade feature removes this restriction by making your weapon a focus.