I'm not sure whether both should be regarded as being correct readings, even if some dictionaries like Jisho.org don't make it very clear that both readings aren't equal. If you were writing a sentence and for some reason chose to write 反応 in hiragana rather than kanji, then I think Japanese readers would regard what you wrote as a typo if you spelt it as はんおう rather than はんのう. I'm not 100% certain but I think the way this word is pronounced is always as はんのう.
I understand that the ON-reading of the kanji 応 individually is おう (not のう. However, due to an aspect of Japanese language called 連声 (れんじょう), in this word [は]ん+お[う] = んの rather than んお.
https://ja.wikipedia.org/wiki/%E9%80%A3%E5%A3%B0
https://en.wikipedia.org/wiki/Japanese_phonology#Renj.C5.8D
Another example is 天皇 (てんのう), which is never written てんおう even though the ON-reading of the second kanji 皇 "emperor" is おう. Spelling it as てんおう would definitely be wrong for this one. And I think the situation might be the same for 反応 too.
And also, if you try to write 反応 on your computer using either Microsoft Japanese IME or Google Japanese Input, はんおう isn't able to convert into the correct kanji but はんのう offers 反応 right at the top of the conversion list. ^^
April 24, 2016 at 12:31am
I'm not sure whether both should be regarded as being correct readings, even if some dictionaries like Jisho.org don't make it very clear that both readings aren't equal. If you were writing a sentence and for some reason chose to write 反応 in hiragana rather than kanji, then I think Japanese readers would regard what you wrote as a typo if you spelt it as はんおう rather than はんのう. I'm not 100% certain but I think the way this word is pronounced is always as はんのう.
Thank you for the explanation. I looked this up in a dictionary and the はんおう translation was displayed first, and afterwards はんのう . I'll make a knot in my handkerchief to this of てんのう whenever I read 反応.