the reaction equation depends on the antacid used. Stomach acid is HCl, not H2SO4. Common equations would include: Mg(OH)2 + 2HCl --> MgCl2 + 2H2O Al(OH)3 + 3HCl --> AlCl3 + 3H2O NaHCO3 + HCl...
Since, an antacid tablet neutralizes the acid, hence it must be a base. Now, the pH of the tablet depends on its strength and varies. But since it is basic, the pH must lie somewhere between 10-14