IA - Decomposition of Hydrogen Peroxide (H2O2)

[Ali1415]

Hi,

For my Chemistry IA, I want to compare the activation energy of the decomposition of hydrogen peroxide catalayzed by potassium iodide (KI) versus the activation energy of the decomposition of hydrogen peroxide catalyzed by a solution of catalase enzyme, to determine which one is the more efficient catalyst (i.e. the one that yields a lower activation energy for the reaction). For both catalyzed reactions, I'm going to measure the volume of oxygen gas evolved per second using a gas syringe, at 5 different temperatures (30°C, 35°C, 40°C, 45°C and 50°C). Then, I'm going to create an Arrhenius plot for each catalyzed reaction, and determine the activation energy from the plot.

There are a few things that I'm confused about:

1. How do I determine the appropriate concentrations and volumes for each chemical?

2. How do I calculate the rate constant based on the data I collect? (In order to create the Arrhenius plot, I would need to plot ln(rate constant) on the y-axis and 1/Temperature on the x-axis).

Alternatively, I found an article that says that I can simply plot ln(initial rate) over 1/Temperature. (https://eic.rsc.org/feature/investigating-activation-energies/2020172.article) This article says the following:

The variation of reaction rate with temperature is given by the Arrhenius equation, which in its integrated form is:

k = Ae-Ea /RT

where A is a constant, the frequency factor, Ea is the activation energy, R is the universal gas constant (8.314 J mol-1K-1), and T  is the absolute temperature. Since initial rate = ck, the initial rate can be found by:

initial rate = cAe-Ea /RT

Taking logarithms:

ln (initial rate) = lncA - Ea/RT

or

log (initial rate) = logcA - Ea/2.303RT

Thus, we can plot log (initial rate) against 1/T  to obtain a straight line. The slope is multiplied by -2.303R to get Ea. The rates can be expressed in volume of oxygen per second or in arbitrary units because the slope of the straight line will not be affected.

Would this work as well? I figure it would be easier to plot ln(initial rate), because then I wouldn't need to calculate the values for the rate constant at each temperature.

Edited March 4, 2018 by Ali1415

[kw0573]

If you can find the volume of oxygen or water produced then you can use to find the rate. H2O2 --> H2O + O2, r = -d[H2O2]/dt = d[H2O]/dt = d[O2]/dt
One downside of using initial rate is that it introduces more uncertainty but it can be calculated easily. Regardless, you should find k using only reactions at same instantaneous concentrations. So you cannot do something like plotting rate of every 5 seconds because the concentrations at those times would not be the same across different temperatures.

[Ali1415]

So, should I measure the volume of oxygen that is produced in a certain period of time at each temperature? (e.g. volume of oxygen produced in the first 10 seconds of the reaction, at each temperature.)

Or should I measure how long it takes to reach a certain volume of oxygen at each temperature? (e.g. amount of time it takes for the reaction to produce 5 cm^3 of oxygen, at each temperature.)

Also, to calculate k, would the following equations be correct?

k = rate of reaction / [H2O2][I-]  (for decomposition catalyzed by potassium iodide)

and 

k = rate of reaction / [H2O2][catalase]  (for decomposition catalyzed by catalase)

Thanks!

[kw0573]

I like finding volume produced over a short period of time instead of time to reach a certain volume. Theoretically speaking it shouldn't make a difference.

You should look up the mechanism with and without the catalase, and write the rate law of the rate determining step.

[Ali1415]

I ended up collecting data on pressure over time (instead of volume over time) using a gas pressure sensor and test tubes. Apparently, for each trial, I need to do a calculation to account for the pressure of the air that was already in the test tube before the oxygen from the reaction began to evolve, but I am unsure about how to proceed. Any tips?