Video Transcript
In this video, we will learn how to
describe and explain the effect catalysts, including enzymes, have on the rate of
chemical reactions.
Some chemical reactions may be very
slow to get started. The reactant molecules do not
combine easily with each other to directly form products. Catalysts allow the reaction to
happen via an easier route. They do this by providing a special
surface where the reactant molecules are concentrated together so that they can
react with each other easily. Some catalysts involve metals or
metal compounds, and the reactants become attached to the surface of the catalyst,
forming an intermediate compound. The catalyst is chemically involved
in the reaction at this point. After a short time, the reactants
have reacted on the catalyst surface and product molecules are formed. Finally, the product molecules must
be released from the catalyst surface for the reaction to be complete.
The pathway for the process
involving the catalyst involves less energy to get started. Since the pathway for this process
is much easier than the direct reaction without the catalyst, the reaction takes
place in less time, so the rate is faster. There is something to note about
the catalyst when its job is complete in this example. Firstly, the catalyst is chemically
unchanged after the reaction. Although the catalyst became
chemically involved in the reaction, its chemical structure and properties are the
same before and after the reaction that it took part in. We can therefore define a catalyst
as a substance that increases the rate of a reaction without undergoing a permanent
chemical change.
As an example of how a catalyst
behaves, we could take one gram of a catalyst substance and add it to a mixture of
reactants. The reactants may be involved in a
very slow reaction, so we might not see much happening there. As soon as the catalyst were added,
the reaction would happen very rapidly and products would be formed. After the reaction, one gram of
catalyst would remain chemically unchanged. The catalyst could be separated out
by filtration. It could be washed and dried and
reused.
Notice that the mass of catalyst at
the start of the reaction is exactly the same as the mass of the catalyst at the end
of the reaction. In reality, only a very small
amount of catalyst is required to speed up a reaction by a large amount. A good example of this is the
reaction for the decomposition of hydrogen peroxide. Hydrogen peroxide has the molecular
formula H2O2. In this decomposition reaction, two
molecules of hydrogen peroxide are decomposed to make two molecules of water. At the same time, one molecule of
oxygen gas is also formed. The decomposition reaction of
hydrogen peroxide to form water and oxygen gas is a very slow reaction without a
catalyst under normal conditions. Hydrogen peroxide is a liquid under
normal conditions. And if we left a beaker of it lying
around, we wouldn’t see many bubbles of oxygen being formed at all.
We could use the decomposition of
hydrogen peroxide to perform a simple experiment to see how a catalyst works. Two test tubes would be clamped
into stands and filled with an equal amount of hydrogen peroxide. Under normal conditions, there will
be no bubbles observed coming from the hydrogen peroxide in either test tube over a
short period of time. When a small amount of manganese
dioxide powder is added to one of the test tubes, the decomposition reaction would
suddenly take place much faster in that test tube. Manganese dioxide takes the form of
a black powder and it’s a catalyst for the decomposition of hydrogen peroxide. Oxygen gas bubbles are produced
rapidly in this test tube and the heat from the reaction would create some water
vapor.
We can tell from these observations
that manganese dioxide is behaving as a catalyst. In the test tube, where no catalyst
was added to the hydrogen peroxide, we wouldn’t see any oxygen gas bubbles over a
short period of time. After the experiment, the catalyst
could be separated out and used over and over again. In this example, the manganese
dioxide is behaving as a positive catalyst as it made the reaction happen
faster.
Some catalysts may have the
opposite effect, and they will slow a reaction down. These are termed negative
catalysts. A negative catalyst is a substance
that slows down the rate of a chemical reaction without being used up or
significantly changed as the reactants turn into products.
In the next section of this video,
we’ll take a look at how enzymes work. Enzymes are biological catalysts
made by certain types of cell. Enzymes are very large molecules,
and they have complex shapes. Amylase is an example of an enzyme
that can break down large starch molecules. Starch is found in the food that we
eat. And because it’s such a large
molecule, it can’t pass easily through the gut wall during digestion. The starch molecule needs to be
broken down into smaller sugar molecules to be absorbed into the body properly. It attaches itself to the enzyme
during this digestion process. At the end of the chemical
reaction, the enzyme is unchanged and the smaller sugar molecules are released.
We’ll now take a look at a simple
experiment to observe the effect of an enzyme. The experiment involves the
decomposition of hydrogen peroxide that we saw earlier. Firstly, two glass beakers would be
filled with a small amount of an identical hydrogen peroxide solution. A small piece of freshly cut sweet
potato is placed into one of the beakers. The other beaker is left alone as a
comparison. After a very short time, bubbles of
oxygen gas are seen to form rapidly in the beaker containing the sweet potato and
the hydrogen peroxide.
This observation provides evidence
that the decomposition of the hydrogen peroxide is occurring rapidly. No bubbles of oxygen gas are
observed in the other beaker, which does not contain the sweet potato. The sweet potato contains a
naturally occurring enzyme called oxidase, which speeds up the decomposition of
hydrogen peroxide compared to normal conditions.
In the next section, we’ll take a
look at the uses of catalysts and why they’re used in certain situations. Catalyst can be used to save time
and energy in chemical processes. We can understand this by looking
at an energy level diagram. If we can save energy in a chemical
process, we’ll be saving money too. The diagram shows that a minimum
amount of energy must be added to the reactants to make them react and turn into
products. If a catalyst is added, less energy
is needed to make the reaction happen. We can see this in the energy
profile diagram for the reaction with a catalyst. In industry, we can save time and
also the cost of energy required to run a chemical process by using the
catalyst.
Catalysts are used in catalytic
converters located in the exhaust pipes of cars. Catalytic converters contain
precious metal catalysts that speed up the removal of toxic gases from the exhaust
stream created by the petrol or diesel engine. Toxic gases leaving the engine from
the combustion process may include carbon monoxide, nitric oxide, nitrogen dioxide,
and even unburned fuel. Since the catalyst metals used are
valuable, they’re spread out over a large honeycombs structure of thin tubes to
increase their surface area. This means only a small amount of
catalyst is needed inside the catalytic converter. Inside the catalytic converter, the
toxic gases produced by the engine are converted into harmless products such as
nitrogen, carbon dioxide, water, and oxygen.
Now, it’s time to look at a
question to test our understanding of catalysts.
Which of the following is not a
characteristic of a catalyst? (A) It decreases the amount of
energy needed for a reaction to proceed. (B) It changes the rate of
reaction, but it does not affect the start or end of the reaction. (C) A large amount of catalyst is
often required to make a reaction occur. (D) It can be bonded to the
reactants during the reaction, but it is separated by the end. (E) It does not change chemically
before or after the reaction.
In this question, we need to
identify a statement that describes something that is not a characteristic of a
catalyst. We’ll investigate each statement in
turn to test its validity. The first statement suggests that
the catalyst decreases the amount of energy needed for the reaction to proceed. We can investigate this using an
energy profile diagram. In the energy profile diagram shown
here, R represents the reactants; P represents the products.
It’s true for any chemical reaction
that we have to raise the energy of the reactants to make them react. We can see from the energy profile
diagram that this amount of energy is higher without a catalyst compared with the
situation when a catalyst is present. So, statement (A) does describe a
characteristic of a catalyst, and it’s not the correct answer.
Catalysts do change the rate of a
chemical reaction. A positive catalyst speeds the
reaction up, and negative catalyst will slow it down. Catalysts also change how the
reaction begins and also how the reaction ends. Catalysts offer a special surface
where reactant molecules are concentrated. Reactant molecules form an
intermediate compound on the catalyst surface. They are combined with the catalyst
at this point in the reaction. At the end of the reaction, the
products are finally released from the catalyst surface. Although this process is different
compared with the direct reaction without the catalyst, the reactants and products
are the same either way. Statement (B) does describe the
characteristics of a catalyst and is therefore not the correct answer.
Statement (C) suggests that we need
to use a large amount of catalyst to get a reaction started. In the decomposition of hydrogen
peroxide into water and oxygen gas, we only need to add a tiny amount of manganese
dioxide catalyst to get the reaction started. As soon as a small amount of
manganese dioxide catalyst, which is a black powder, is added to the hydrogen
peroxide, a large amount of oxygen bubbles are observed. Statement (C) is therefore not a
characteristic of a catalyst, and it could be the correct answer. We’ll check whether the remaining
statements are valid to be sure.
We’ve already seen that reactant
molecules are bonded to the catalyst surface during the reaction in order to
concentrate them. This results in the formation of an
intermediate compound. For the catalyst to work properly,
the products must be released at the end of the reaction, so they must be separated
from the catalyst surface. In fact, the catalyst surface is
left unchanged chemically before and after the reaction. This means that the last two
statements are characteristics of a catalyst and then therefore not the correct
answers.
Statement (C) a large amount of
catalyst is often required to make a reaction happen is the correct answer.
Now, it’s time to review the key
points from this video. Catalysts change the rate of a
chemical reaction without being used up or chemically changed as reactants turn into
products. A catalyst that speeds up a
chemical reaction is known as a positive catalyst, whilst a catalyst that slows down
a reaction is known as a negative catalyst. Enzymes are biological
catalysts. They’re produced by certain types
of specialist cells. Catalysts change the minimum amount
of energy that needs to be added to reactant molecules to make them react. Catalytic converters are used in
cars to reduce emissions of harmful pollutant gases.
