Conservation Law, Crank Nicholson Scheme

Question

Use the integral form of the conservation law to show that the Crank-Nicolson scheme is $O(∆x^2)+O(∆t^2)$ for the heat equation with a source term $$ \dfrac{\partial U}{\partial t} = \nu \dfrac{\partial^2 U}{\partial x^2} +F(x,t),$$ when uniform, vertex center grid is used.