The rate of a reaction can be observed by measuring the change in the amount of a reactant over time, as given in the graph below. how does the rate of a reaction change with time and how does the collision theory explain this change?

Looks like exponential decay. Collision theory is consistent with this model because at higher reactant concentrations we expect the reactants to encounter each other more often (collide) and as the concentration decreases we expect fewer and fewer collision events so the rate of reaction becomes less and less.

14 days is the half-life of phosphorus-32