It is because say water boils at 212 F, if it goes higher at 213 it would get so much heat and energy it turns into a gas, so it cannot stay a liquid with 213 because at that point it would be gas, thus when water reaches 212 it's max if it goes any higher it will be gas
Hrxn = Q reaction / mol of reaction
mol of reaction = M * V = 10 * 1 = 10 mmol = 0.01 mol
Q water = m * C * (Tf - Ti)
= (10 + 10) (4.184) (26-20) = 502.08 J
Q reaction = - Q water = -502.08 J
Hrxn = -502.08 / (0.01) = - 50208 J = - 50.21 kJ/mol
