Why is the blue flame the hottest on a Bunsen burner?
I thought the outermost part of the flame must be the hottest region because of complete combustion of gases taking place there. Where am I getting wrong?
When you have the intake valve (at the bottom of the bunsen burner) opened, the gas can mix with incoming oxygen. By the time is gets to the top of the tube, you've got a nice continuously-flowing source of well-mixed gas and oxygen. That allows combustion to occur very quickly because the methane and oxygen molecules don't have to travel very far to react, increasing the reaction rate, and therefore the rate at which heat is released.
In contrast, when the intake valve is closed (or when it's open and you're considering the outer non-luminous flame), the oxygen for the combustion reaction comes from the surrounding air, which is not thoroughly mixed with the gas at the time of ignition. That results in a slower reaction, and therefore a slower rate of heat being released.
Periodic videos just did a nice thermal imaging of a bunsen burner with this explanation: https://www.youtube.com/watch?v=in9BGz958jg
The blue flame is indicative of complete combustion. This means more of the energy of the gas is being liberated as thermal energy in the exothermic combustion compared to the yellow portion of the flame.
A yellow flame indicates incomplete combustion. This means products other than carbon dioxide and water result including possibly carbon monoxide. This is why you should be aiming for a blue flame on your gas stove at home too. http://www.elgas.com.au/blog/1585-why-does-a-gas-flame-burn-blue-lpg-gas-natural-propane-methane
I remember doing an experiment where I exposed wooden dowels to the flame at various heights and noted where the wood burned (outer blue flame). That was supposed to be the hottest region, but it's actually just the oxidizing region (wood does not react visibly with the inner, reducing part of the flame). Now I would identify the hottest part of the flame as the boundary between inner and outer flames where the exothermic reaction takes place.