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Chemistry library
Course: Chemistry library > Unit 17
Lesson 3: Arrhenius equation and reaction mechanisms- Collision theory
- The Arrhenius equation
- Forms of the Arrhenius equation
- Using the Arrhenius equation
- Collision theory and the Maxwell–Boltzmann distribution
- Elementary reactions
- Reaction mechanism and rate law
- Reaction mechanism and rate law
- The pre-equilibrium approximation
- Multistep reaction energy profiles
- Catalysts
- Types of catalysts
- Types of catalysts
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Types of catalysts
Catalysts can be categorized as homogeneous, heterogeneous, or enzymatic. Homogeneous catalysts exist in the same phase as the reactants, whereas heterogeneous catalysts exist in a different phase than the reactants. Enzymes are biological catalysts that speed up biochemical reactions in living organisms, often by many orders of magnitude. Created by Jay.
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- So the hydronium ion donate a proton to the ether linkage to give the O a formal charge of 1+ that in further steps results in the split of glucose and fructose. A catalyst is not consumed in the reaction so the proton is given up to form hydronium again?(3 votes)
- Yep, that's exactly correct. Good job.(3 votes)
Video transcript
- [Instructor] A catalyst
speeds up a reaction by lowering the activation energy. And there are many types of catalysts. And first we're going to look at enzymes which are biological catalysts. Let's say that this represents our enzyme, and the place where the reaction occurs is called the active site of the enzyme. So right in here, let's say
this is our active site. And then the substance that
reacts at the active site is called the substrate. So this little picture here
with two triangles together, that's the substrate for our reaction. In the next step, the substrate binds to the
enzyme at the active site. And when the substrate binds, the substrate can produce
changes in the shape of the active site, that
allow for better binding. So on the left here, we
can see how the shape of the active site changes slightly when the substrate binds to it. This formation of the
enzyme substrate complex is called the induced fit model. The substrate interacts with the enzyme through non-covalent
interactions in the active site. So things like hydrogen bonding or dipole-dipole interactions. Perhaps some of these
non-covalent interactions cause a shift in electron density which make it easier to reach the transition
state for the reaction. Therefore lowering the activation energy and speeding up the overall reaction. Next let's say the bond between
the two triangles breaks and we get our two products here. So two individual triangles. And the enzyme active site
goes back to the original shape and it's ready to
catalyze another reaction. Next, let's talk about
a homogeneous catalyst, which is a catalyst that's present in the same phase as the
reactants in a reaction mixture. So let's look at the hydrolysis of sucrose to turn into glucose and fructose. This reaction can be catalyzed by the hydronium ion H30+. And since sucrose, our reactant
is an aqueous solution, and so as the hydronium ion, we can say that the hydronium
ion is a homogeneous catalyst. And it's a source of protons to catalyze this hydrolysis reaction. This is a drawing of the sucrose molecule, which is a disaccharide
composed of two monosaccharides. Glucose is over here on the left and fructose is over here on the right. And these two monosaccharides are joined by an ether linkage. So we can see, we can see this, this connection here, alright? This oxygen in between
our two monosaccharides isn't ether linkage. And ethers are fairly nonreactive. Since ethers are generally unreactive, the hydrolysis of sucrose
is a pretty slow reaction. And to speed it up, we need
to add an acid catalyst. So if we add an acid catalyst and we have hydronium
ions in aqueous solution, a lone pair of electrons
on the oxygen on the ether, will take this proton and these electrons move in to form water. Protonation of the oxygen, gives the oxygen a plus one formal charge. And allows an acid catalyzed
mechanism to proceed. And there are more steps to the mechanism but ultimately sucrose is
broken down to form glucose and fructose in this acid
catalyzed hydrolysis of sucrose. Honey bees actually have the
enzyme to convert sucrose which is table sugar,
into glucose and fructose. And since fructose is
sweeter than sucrose, honey is sweeter than table sugar. A heterogeneous catalyst is
a catalyst that's present in a different phase from the reactants in a reaction mixture. As an example, let's look
at a hydrogenation reaction. And this reaction ethene
reacts with hydrogen on a surface of platinum to form ethane. Now, since the platinum
is in the solid form and our reactants are
in the gaseous state, the platinum is an example
of a heterogeneous catalyst. So here in our picture, we have
our piece of platinum metal and both the ethene molecule
and hydrogen are adsorbed to the surface of the platinum metal. Next the bond between the
two hydrogen atoms breaks and we get the two individual
hydrogen atoms bonded to the surface of the platinum metal. Eventually these two hydrogens add across the double bond of ethene and form the ethane molecules C2H6. So the hydrogenation of
ethene to form ethane is catalyzed by the presence
of the platinum metal.