Why does the forward reaction slow down over time

As reactions proceed, they will approach equilibrium, meaning they are completely balanced. As this occurs, there will be fewer products to react. Eventually, after all products are reacted, the reaction will completely stop. Since this doesn't happen instantaneously (usually), the reaction will start quickly and slow as time progresses. hope this helps you hope i am brainliest i need it 

The reaction does not stop at equilibrium.  Equilibrium is the point at which the rate of the forward reaction equals the rate of the reverse reaction which makes it so that there is no net change (that means that it looks like the reaction has stopped but the forward and reverse reaction is still happening).  

The forward reaction slows down over time due to the fact that the concentrations of reactants gets lower over time which makes it so that there are less collisions between reactants per unit of time.  Since reactions occur when the reactants collide in the right way, when the number of collisions lowers, the rate of the reaction will also lower.  This can be seen through the fact that rate of the forward reaction is rate=k[reactants].  As [reactants] gets lower, the rate of the forward reaction will get lower as well.  (rate=k[reactants] is the rate law for the forward reaction with [reactants] being the concentration of reactants and k being the rate constant).

I hope this helps.  Let me know if anything is unclear.

The gas would begin to expand, and eventually the can would burst or explode.