Video Transcript
In a chemical reaction, A and B
react to form C: A plus B produces C. The changes in the concentration of
A and C during the reaction can be seen in the following graph. Which statement correctly describes
what has occurred when the two lines crossed, as indicated by the black arrow? (A) B and C begin to react. (B) The reaction has finished. (C) The reaction has run out of
substance B. Or (D) A and C have equal
concentrations.
Scientists monitor the changes in
concentration of a reactant or product to determine the rate of a chemical
reaction. As reactant particles quickly
collide to react, their concentrations will drop. They react to form products, whose
concentrations will increase. The graph shows a chemical reaction
and plots the changing concentrations of reactant A and product C. The blue line represents the
concentration over time of reactant A as it reacts with reactant B. The red line plots the changing
concentration of product C as it is formed over time from the particle collisions
between A and B.
We can see that this reaction
occurred quickly, as the lines are both initially steep. And thus the concentrations changed
quickly in the first 20 or so seconds. We can deduce that reactant A
rapidly reacted with reactant B to form product C.
The graph has concentration in
moles per liter on the 𝑦-axis as a function of time in seconds. The arrow describes a point in time
when the red and blue lines cross each other. This point represents a time in
seconds when reactant A and product C have the same value on the 𝑦-axis. This means at this moment in the
reaction both substances had the same concentrations. We can see that this is described
by answer choice (D).
Therefore, the statement that
correctly describes what occurred when the two lines crossed, as indicated by the
black arrow, is answer choice (D); A and C have equal concentrations.
