2.5 mole O2 should react (2 * 2.5) = 5.0 mole H2 but there is 6.0 mole H2. H2 remain in excess. O2 is the limiting reactant. mole of H2O formed = 2 * 2.5 = 5.0 mole.
The reaction has a negative rate law; i.e., Rate = - ΔConcentration / ΔTime which is graphically a negative slope for the plot of Rate as a function of reactant concentration. => Rate ∝ f(Reactant Concentration). However, by raising the temperature, an increase the probability of reaction occurs and the formation of more product.