Why does the graph of concentration vs. reciprocal time give a straight line?
Because time is inversely proportional to rate, the reciprocal time can be considered an “index” of the rate (not the rate itself, but a directly proportional indicator of the rate). In a first order reaction, the rate (and, consequently, the reciprocal time) are directly proportional to the concentration…the rate is equal to some constant (k) multiplied by the concentration. This gives rise to a straight line for the plot of concentration vs. reciprocal time (“rate index”). Common Errors/Misconceptions: • Many of you took the times that you recorded off of the lab timers, which were in “minutes:seconds” form (20:50 equals twenty minutes and fifty seconds) and treated them as if they were in decimal form (20:50 “equals” 20.50 minutes). This is incorrect, and resulted in your graphs being a little bit off. • Some of you determined that the reaction was second order with respect to IO3-1 because the reciprocal graph was a straight line. This is incorrect for two reasons: • This is not
