Will global warming expand the tropical zone
It depends on how you define the tropics. Strictly speaking, the tropics can be defined as the area between the tropic of capricorn and the tropic of cancer (23.5$^\circ$N/S respectively). This is because of the earth's axial tilt of 23.5$^\circ$. Since I don't think that the climate change will affect the axial tilt too much, I doubt it.
However, if you define the tropics in terms of climate, probably. Since the Hadley cell is expected to expand with more greenhouse gases, it follows that the tropical weather that accompanies the Hadley cell should expand with it.