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IB Chemistry IA, level 7
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IB chem IA level 7 example on rate of reaction
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The effect of concentration of H2O2 on the rate of its decomposition with 0.050gof a MnO2 catalyst with comparison of its rate equation to the one of H2O2
decomposition using Aspergillus niger catalase.
1. Exploration and Personal Engagement
1.1 INTRODUCTION
Background Information
Hydrogen peroxide, H2O2 , is an unstable colourless liquid at room temperature used in many cleaning and
personal care products such as bathroom cleaners, toothpaste, bleach or hair dyes, as well as an antiseptic in cleaning wounds
(Chemical Safety Facts). This peroxide naturally slowly decomposes to oxygen and water with the release of heat
(PubChem), through having a weak and unstable oxygen-oxygen single bond. This reaction can be sped up using catalysts
alike KI, MnO2 , FeCl3 or catalase (Bell-Young). Catalysts provide alternate pathways for a reaction to occur which have
lower activation energies that enable the reaction to go faster. This is due to the fact that lower kinetic energy is needed for
the reaction to occur, resulting in an increased frequency of successful collisions. The experiment conducted in this report
utilized the manganese dioxide (MnO2) catalyst.
MnO2 is an inorganic black to brown compound in the form of a dry powder. It is used in zinc-carbon and alkaline
batteries and is currently being researched to have the potential to work as an efficient cathode in rechargeable magnesium
batteries (Ling and Zhang). Catalase is an enzyme that can prevent damage to cellular organelles and tissues by peroxide and
is mostly found in the liver of mammals (The Editors of Encyclopaedia Britannica). This specific investigation involved
Aspergillus niger catalase where Aspergillus niger is a fungus and this particular catalase was chosen due to it being stable
and active over a wide pH range of 1-12 (Merck). Along MnO2 it also catalyzes the decomposition of H2O2 , in which both
catalysts bind to H2O2 to break it down into water and oxygen. However, since Aspergillus niger catalase is a biocatalyst, it
possesses a more specific active binding site, thus a more selective affinity for its substrates (Bergtrom). Furthermore, catalase
is a homogeneous catalyst in the decomposition of H2O2 because it is aqueous alike H2O2 , whilst MnO2 is a heterogeneous
catalyst due to being a solid, which is in a different phase to H2O2 . Typically, homogeneous catalysts are more efficient as
they can interact with the substrates more conveniently, being able to be mixed with the substrates well unlike when catalysts
and the substrate are different phases.
The rate equation of the decomposition of H2O2 using 30 units/mL of Aspergillus niger catalase was found to be:
Rate = 2981mol-1dm3s-1 [H2O2]2 in a study by Miłek. As seen through the rate constant units of mol-1dm3s-1 and squaring of
the concentration of H2O2 , this reaction is second order with respect to the concentration of H2O2 . Hence, the rate of
reaction is directly proportional to the square of the concentration of H2O2 . Nonetheless, Do et al. found that the order of
the reaction is first order in the decomposition of H2O2 with the MnO2 catalyst. Therefore, the rate of decomposition of
H2O2 with this chemical catalyst is directly proportional to the concentration of H2O2 .
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, This investigation aims to differentiate between the rate equations of the decomposition of H2O2 when different
types of catalysts are utilized, specifically a homogeneous biocatalyst, Aspergillus niger catalase, and an heterogeneous
inorganic catalyst, MnO2 . It should be noted that different order reactions don’t indicate a difference in speed of reactions,
rather, it demonstrates the extent of the effect of changing concentrations has on the rate of reaction. Through a range of
concentrations of H2O2 , the rate of gas loss with the MnO2 catalyst was graphed and the order of the reaction and rate
constant were determined, which allowed a rate expression to be formed. Therefore, comparison of this rate expression to
using Aspergillus niger catalase in the decomposition of H2 O2 was conducted in order to recognize how different types and
different phases of catalysts could result in different rate expressions from the same reaction. This could further indicate how
drastic the effect of changing the concentration has on the rate of reaction, differs among dissimilar catalysts.
Research Question
How does the rate equation of H2O2 decomposition catalyzed by the inorganic heterogeneous catalyst MnO2 ,
measured by the rate of gas loss at different concentrations from 0.010- 0.050 mol dm-3, differ from the known rate equation
of 2981 [H2O2]2 when using the homogeneous biocatalyst of Aspergillus Niger catalase?
Predicted Effect
Increasing H2O2 concentration allows the rate of reaction to be faster through the increasing frequency of
successful collisions of sufficient energy and the correct orientation (Dillon), therefore as concentration increases from 0.010
to 0.050 mol dm-3, the rate of gas loss will also increase. The rate equation of H2O2 decomposition using MnO2 may
showcase a first-order reaction, found by Do et al., as compared to catalase from Aspergillus niger which is a second-order
reaction (Miłek). The activation energy of the decomposition of H2O2 using Aspergillus niger catalase is 12.9±0.7 kJ/mol
(Miłek), whilst the chemical catalyst is 50kJ/mol (Jildeh et al.), which can enable the reaction mechanism of H2O2
decomposition to differ among them, in which catalase will allow changes in concentration to drastically influence the rate
of reaction compared to with MnO2 . This is due to MnO2 having a higher activation energy, requiring more energy for
successful collisions, making it less likely for large effects on the rate of reaction to occur when concentration changes. In
addition, since the attraction to substrates is greater than the chemical catalysts, Aspergillus niger catalase may be more
effective as a biocatalyst in allowing more drastic changes to the rate of reaction when concentration differs since it may bind
to H2O2 more frequently. Also, since the Aspergillus niger catalase is homogeneous, it may indicate that catalase can interact
with H2O2 more easily, influencing a larger effect on rate of reaction when concentration changes, than the MnO2 catalyst.
Personal Interest & Justification
Personally, I was interested in comparing chemical catalysts to biocatalysts/enzymes since biocatalysts have been
discussed to be more efficient from deriving from more renewable sources and producing lower amounts of waste products.
Thus, it led me to investigate the difference in rate equations to determine which catalyst could be more efficient in the
decomposition of H2O2 through enabling larger effects on reaction rates when there are small changes in concentration.
This can instigate further research in differentiating the rate of other chemical reactions using enzymes and chemical
catalysts, or even homogeneous and heterogeneous catalysts. This could also be applied in finding which catalysts are most
effective for specific reactions and circumstances (concentration, temperature, etc). This can be beneficial to search for the
most efficient methods in chemical processes that occur faster and are cheaper, requiring less material.
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