Timing, exact rates and weather conditions are extremely important. A good option is to hire a professional lawn care specialist to apply chemicals. Spot spraying with a selective broadleaf herbicide (weed killer for lawns) is a good way to kill your weeds without damaging your lawn. I spot spray (as opposed to applying herbicides to the entire lawn) most of the lawns that I manage because it uses less chemicals and is the responsible approach to take.
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones.
Convection heat transfer happens when a fluid such as air or water gets heated by a hot object that touches the air or water. Forced air furnaces are great examples of convection heat as the air moving over the heat exchanger gets warm and then passes that warmth to objects it touches once it is in the room.