Question

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?

Answer 1

@Empty

Answer 2

You didn't provide a graph, but I'm guessing it looks something like this (focus on the reagent line):

http://scienceaid.co.uk/chemistry/physical/images/concentration.png

You can see that early on, the slope of reactant line is quite steep (and negative). This means the concentration of reactants over time decreases rapidly at first. However, over time, the slope of this line becomes less and less steep, indicating that although your reactant concentration is still decreasing, it's doing so at a slower rate.

Why might this be?

COLLISION THEORY. The reaction only proceeds if the reactant molecules collide in precisely the right orientation and with the right energy. When you have many reactant molecules, as you do at the beginning of the reaction, there are many opportunities for successful collisions to occur, and so reactant is rapidly converted to product (reaction rate is high). As the reaction progresses, you have fewer and fewer reactant molecules, meaning there are fewer and fewer opportunities for a successful collision to occur, and reaction rate falls as a result.

Does that make sense?