If one feels the need to yawn during shemoneh esrei, one should suppress it. If that becomes impossible, one should place his or her hand in front of the mouth before doing so. This applies even when one is alone.
The Zohar, parshas Terumah, writes that one who talks in Shul during davening is guilty of three violations:
- Creating a seperation between K’ll Yisrael and HaShem, as he disturbs the Tefilah of the tzibur;
- Creates a chillul HaShem, as others copy him and think that it is not such a bad thing to do; and
- “ain lo cheilek be’elokei Yisrael” (should need no translation).