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IB SL/HL Chemistry: Kinetics Internal Assessment
This is my HL Chemistry IA. It is under chapter 6 (kinetics), specifically looking at how the rate of urea hydrolysis catalysed by urease changes with temperature.
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How is the initial rate (cm 3 s−1) of the hydrolysis of urea catalysed by urease from soybeans, measured bythe rate at which hydrochloric acid must be added to maintain constant reaction pH, affected by
temperature (40-80°C) and what is the optimum temperature (°C) and activation energy (kJmol−1 ¿ for the
reaction?
Introduction
Urea is an organic compound commonly incorporated into fertilisers, as it is hydrolyzed into ammonia, thus
making its high nitrogen content available to plants. The reaction is catalysed by urease, naturally present in
soil.
Nitrogen is an important element for plants as it is the major component of chlorophyll, the pigment
responsible for the conversion of sunlight to energy required to synthesize sugars from water and carbon
dioxide during photosynthesis. Nitrogen is also a major component in amino acids, nucleic acids, and energy
transfer compounds such as ATP, all of which are vital for a plant’s survival.
Through the process of growing my own plants, I have seen the positive effects of urea fertilizers in increasing
plant growth, and I wondered to what extent temperature would have an effect on the rate of hydrolysis. If
the nitrogen can be made available to plants more quickly, it can positively affect plant growth at a faster rate.
Therefore, I have decided to investigate how temperature affects the rate of hydrolysis of urea, catalysed by
the enzyme urease from soybeans. As the reaction produces ammonia and carbon dioxide, the rate of
reaction can be measured by the rate at which hydrochloric acid must be added to maintain a constant pH. To
extend the investigation into the relationship between reaction rate and temperature, the Arrhenius equation
can be used to estimate the activation energy for the catalysed hydrolysis of urea.
Background information
Hydrolysis is the addition of a molecule of water to a substance, to break the chemical bonds within it.
The reaction equation for the hydrolysis of urea is:
NH 2 CONH 2 + H 2 O C O 2+ 2 NH 3
One mole of water reacts with one mole of urea, to produce two moles of ammonia and one mole of carbon
dioxide. This reaction is catalysed by the enzyme urease, which is naturally abundant in whole soybeans.
An enzyme is a biological catalyst, which increases the rate of a specific reaction. The Lock and Key Hypothesis
states that the shape of an enzyme’s active site is specific to the substrates of the reaction it catalyzes. The
active site of an enzyme (urease) binds to the substrate (urea) and causes a change in the distribution of
electrons in the chemical bonds of the substrate. This can result in the substrate either being built into larger
molecules or broken down, as in the case of the hydrolysis of urea. The enzyme itself remains unchanged, and
can catalyse consecutive molecular collisions.
Figure 1- Lock and Key Model for enzymatic catalysis 1
Several precise mechanisms for the catalysis of urea by urease have been suggested, based on crystallized
enzyme structures, but the exact mode of action of the enzyme is still unclear. When a molecule of urea binds
to the active site of urease, the hydrolytic enzyme attacks the bond between the nitrogen and the carbon in
urea. This enzyme-substrate complex provides an alternative route for the reaction with lower activation
energy- defined as the minimum energy required for a reaction to take place when molecules collide with the
1
Askiitians.com. Enzyme Catalysis - Study Material for IIT JEE | askIITians.: http://www.askiitians.com/iit-jee-
chemistry/physical-chemistry/enzyme-catalysis.aspx
1
, correct orientation. This therefore creates a higher number of particles with sufficient energy to react,
thereby increasing the rate of reaction.
In addition to catalysis, higher temperatures can also increase rate of reaction. As increasing heat energy is
supplied to the reaction mixture, the molecules gain kinetic energy. Molecules therefore collide more
frequently, and more collisions are successful as a higher proportion of molecules have energy higher than
that of the activation energy, which remains constant. Figure 1 shows how at the higher temperature T 2, a
larger proportion of the overall molecules have energy above the activation energy.
Figure 2- Maxwell-Boltzmann Distribution graph showing the effects of temperature on rate of reaction for
2 1
T >T
However, high temperatures can also have the opposite effect on enzyme-controlled reactions such as the
hydrolysis of urea. Enzymes such as urease are globular proteins, formed by interactions between amino acid
chains and structures. The pattern of amino acids in polypeptide chains represent the primary structure of the
protein, which then form regular structures such as alpha helixes and beta pleated sheets through hydrogen
bonding, representing the secondary structure. Each of the 20 amino acids that make up these chains has a
specific ‘R group’, and chemical interactions between ‘R groups’ (e.g. ionic interactions) folds the entire chain,
including the regular structures, into a final whole configuration- the tertiary structure of the protein.
Enzymes are often multi-protein complexes, made up of several protein sub-units, for example, urease
molecules comprise of one large subunit and two smaller ones.
Protein denaturation occurs upon exposure to high thermal energy, when molecules within proteins gain
vibrational energy, which disrupts the hydrogen bonds and non-polar hydrophobic interactions within both
the secondary and tertiary structures of the protein. In enzymes, this disruption in structure eventually leads
to a change in the shape and chemical composition of the active site of the enzyme, meaning it can no longer
catalyse its specific reaction. Consequently, at higher temperatures the rate of urea hydrolysis would be
expected to drop rapidly, as enzymes denature and can no longer serve their catalytic function.
To further explore the relationship between rate of reaction and temperature, the Arrhenius equation can be
used to determine the activation energy of the catalysed hydrolysis of urea, which remains constant as
temperature is manipulated and can be determined graphically. Thus, the minimum energy with which
reactant molecules must possess upon collision in order to successfully react can be calculated.
Ea
k = Ae−( RT )
The Arrhenius Equation:
Where k=rate constant, A=frequency factor (a constant relating to the orientation of colliding molecules), R=
Gas constant (8.314 Jmol−1 K −1),T= Temperature (K) and
Ea= Activation Energy (kJmol−1)
The Arrhenius equation mathematically describes the relationship between the rate constant (k) and
temperature (T). The method by which Ea is calculated using the equation is explained fully in the Data
Processing section.
Methodology
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