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### Course: AP®︎/College Chemistry>Unit 9

Lesson 2: Relationship between reaction concentrations and time

# 2015 AP Chemistry free response 5

Kinetics of bleaching food coloring. From 2015 AP Chemistry free response 5.

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• 630 nm is actually red light, if the solution is blue it absorbs all wavelengths except blue, no?
• Yes. The spectrophotometer is measuring the how the molecule absorbs that red light.
• If the reaction had been endothermic , wouldn't increase of temperature ..slow the reaction ?
• Increasing the temperature will always speed up the reaction, whether it is exothermic or endothermic.
• Why increasing the concentration of the blue coloring would increase the time for the mixture to reach 0 absorbance? If the reaction is first order for the coloring wouldn't the increase in concentration cause a proportional increase in the reaction rate? so at the end it will take the same time to reach 0 absorbance?
• If you plot the ln graphs for two different starting concentrations of blue colouring, you will get two parallel lines, not two converging lines, which is what you are suggesting. Therefore, increasing the concentration of blue colouring means that it takes longer for the absorbance to reach near zero.
• How can we know from graph 3 that it's not a second order reaction?
• We can tell the order of the reaction based on its graphs, here are the rules:

-If the graph of [reactant] is a straight line, the order is ZERO
-If the graph of ln[reactant] is a straight line, the order is ONE.
-If the graph of 1/[reactant] is a straight line, then the order is TWO.

In the problem, we see that the graph of ln(absorbance) is a straight line, which tells us that it must be first order.

Note: brackets [] represent "concentration"