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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
What is a catalyst? Includes examples of enzymes, acid-base catalysis, and heterogeneous (or surface) catalysis.
Key points
- A catalyst is a substance that can be added to a reaction to increase the reaction rate without getting consumed in the process.
- Catalysts typically speed up a reaction by reducing the activation energy or changing the reaction mechanism.
- Enzymes are proteins that act as catalysts in biochemical reactions.
- Common types of catalysts include enzymes, acid-base catalysts, and heterogeneous (or surface) catalysts.
Introduction: A kinetics thought experiment
Your brain is powered by the oxidation of glucose. The oxidation of glucose can be represented as the following balanced chemical reaction:
Without this reaction, learning chemistry would be much harder. Luckily, the oxidation reaction is thermodynamically favored at 25, degrees, start text, C, end text since delta, start text, G, end text, degrees, is less than, 0.
Why don't we give it a try? Find some food that is nice and sugary, such as a raisin. Add some oxygen gas (i.e. hold it out in the air). What happens?
Do you notice a release of heat energy? The formation of water and a nice explosive poof of carbon dioxide gas?
Chances are, the raisin doesn't do much besides maybe dry out a little bit more. Even though the oxidation of glucose is a thermodynamically favorable reaction, it turns out that the reaction rate is really really really slow.
The rate of a reaction depends on factors such as:
- Activation energy
- Temperature: if you heat up the raisin to a high enough temperature, it will probably catch on fire and oxidize
These two factors are closely related: increasing the reaction temperature of the reaction increases the kinetic energy of the reactant molecules. This increases the likelihood that they will have enough energy to get over the activation barrier.
How does your body solve this problem for the oxidation of glucose? After all, your body temperature isn't much higher than 25, degrees, start text, C, end text, so how is this reaction happening continuously in your body?
Biological systems use catalysts to increase the rate of the oxidation reaction so that it can occur at a faster rate at lower temperatures. in this article, we will talk more about what a catalyst is, and the different types of catalysts.
What is a catalyst?
Catalysts are substances that can be added to a reaction to increase the reaction rate without getting consumed in the process. They usually work by
- Lowering the energy of the transition state, thus lowering the activation energy, and/or
- Changing the mechanism of the reaction. This also changes the nature (and energy) of the transition state.
Catalysts are everywhere! Many biochemical processes, such as the oxidation of glucose, are heavily dependent on enzymes, proteins that behave as catalysts.
Other common kinds of catalysts include acid-base catalysts and heterogeneous (or surface) catalysts.
Example: Carbonic anhydrase
The enzyme carbonic anhydrase catalyzes the reversible reaction of carbon dioxide left parenthesis, start text, C, O, end text, start subscript, 2, end subscript, right parenthesis and water left parenthesis, start text, H, end text, start subscript, 2, end subscript, start text, O, end text, right parenthesis to form carbonic acid. When the concentration of
start text, C, O, end text, start subscript, 2, end subscript in the body is too high, carbonic anhydrase catalyzes the following reaction:
By regulating the concentration of carbonic acid in the blood and tissues, the enzyme is able to keep the start text, p, H, end text balanced in the body.
Carbonic anhydrase is one of the fastest known enzymes, with reaction rates between 10, start superscript, 4, end superscript and 10, start superscript, 6, end superscript reactions per second. This is even more amazing compared to the uncatalyzed reaction, which has a rate of ~0, point, 2 reactions per second. That is a ~10, start superscript, 5, end superscript, minus, 10, start superscript, 7, end superscript increase in rate!!
The following diagram shows an energy diagram for the reaction between carbon dioxide and water to form carbonic acid. The reaction with catalyst is indicated with a blue line, and the uncatalyzed reaction is indicated with a red line.
The catalyst lowers the energy of the transition state for the reaction. Since the activation energy is the difference between the transition state energy and the reactant energy, lowering the transition state energy also lowers the activation energy.
Notice that the energies of the reactants and products are the same for the catalyzed and uncatalyzed reaction. Therefore, the overall energy released during the reaction, delta, start text, H, end text, start subscript, start text, r, x, n, end text, end subscript, does not change when you add the enzyme. This emphasizes a very important point: the kinetics of a reaction, i.e. reaction rate, is not directly related to the thermodynamics of the reaction.
Acid-base catalysis
In acid catalysis, the catalyst is usually a start text, H, end text, start superscript, plus, end superscript ion. In base catalysis, the catalyst is usually an start text, O, H, end text, start superscript, minus, end superscript ion.
An example of a reaction that can be catalyzed by acid is the hydrolysis of sucrose, also known as table sugar. Sucrose is a combination of two simpler sugars (or monosaccharides), glucose and fructose. With the addition of acid or an enzyme such as sucrase, sucrose can be broken down into glucose and fructose as shown by the following series of reactions:
In the first step, sucrose reversibly reacts with start text, H, end text, start superscript, plus, end superscript (in red), to form protonated sucrose. The protonated sucrose reversibly reacts with water (in blue) to give start text, H, end text, start superscript, plus, end superscript, one molecule of glucose, and one molecule of fructose. The overall reaction can be written as:
Since the start text, H, end text, start superscript, plus, end superscript appears as both a reactant and a product in equal amounts, it is not consumed during the course of the reaction. Therefore, the catalyst does not appear on the reactant or product side of the overall reaction.
Heterogeneous and surface catalysis
Heterogeneous catalysts are catalysts that are in a different phase than the reactants. For example, the catalyst might be in the solid phase while the reactants are in a liquid or gas phase.
One example of a heterogeneous catalyst is the catalytic converter in gasoline or diesel-fueled cars. Catalytic converters contain transition metal catalysts embedded on a solid phase support. The solid-phase catalyst comes into contact with gases from the car's exhaust stream, increasing the rate of reactions to form less toxic products from pollutants in the exhaust stream such as carbon monoxide and unburnt fuel.
The catalytic converter is also an example of surface catalysis, where the reactant molecules are adsorbed onto a solid surface before they react with the catalyst to form the product. The rate of a surface-catalyzed reaction increases with the surface area of catalyst in contact with the reactants. Therefore, the solid support inside of a catalytic converter is designed to have a very high surface area, hence the porous, honeycomb-like appearance.
Another example of heterogeneous and surface catalysis is the process used to make common plastics (or polymers) such as polyethylene. These catalysts are called Ziegler-Natta catalysts, and they are used to make everything from plastic wrap to yogurt cups. Transition metal catalysts are embedded on a solid support before reacting them with the starting materials (also called monomers) in the gas or solution phase.
Even though the reactants are in the gas phase, the product polymer is usually a solid. I imagine this reaction being analogous to making popcorn: the unpopped corn kernel is the catalyst on the solid support. The gaseous monomers react to form layers of solid product polymer that build up on the surface of the catalyst, which eventually becomes a polymer "popcorn" bead. Chemistryminusit's like magic!
Summary
- A catalyst is a substance that can be added to a reaction to increase the reaction rate without getting consumed in the process.
- Catalysts typically speed up a reaction by reducing the activation energy or changing the reaction mechanism.
- Enzymes are proteins that act as catalysts in biochemical reactions.
- Common types of catalysts include enzymes, acid-base catalysts, and heterogeneous (or surface) catalysts.
Want to join the conversation?
What are some common catalysts to reactions in a school lab, for example?(8 votes)
Acid and base catalysts are extremely common! If you have ever used super glue, the reaction that makes the glue become tough is actually catalyzed by trace amounts of acid on the surfaces of things (like your finger, if you are unlucky).(24 votes)
- Can a catalyst change the product of a reaction,? I meant ,if we get any product without using catalyst could be different from that ,we get from the reaction using catalyst?(4 votes)
Yes, that can happen. When two reactants are mixed in the absence of a catalyst, there may be a major route to Product A and a minor route to Product B, each route involving different reaction mechanisms. Hence Product A will predominate in this situation.
However, in the presence of a catalyst, it is possible that the minor route could be speeded up but not the major route. This would then lead to a predominance of Product B.(8 votes)
Can you further explain what a heterogeneous catalyst is? What does "a different phase mean"? An how would you test for a heterogeneous catalyst?(2 votes)
Phase refers to solid, liquid, gas, or aqueous. A catalyst is heterogeneous when it is a different phase from the reactants whose reaction it is catalyzing. So if you have a platinum metal catalyst (solid) catalyzing the reaction of H2 and ethene (gases) then you would consider the platinum to be a heterogeneous catalyst.(6 votes)
- If the catalyst will not be consumed, will it work until all reactants are turned into products? Also how to determine how much of a catalyst is needed? is it by stoichiometry (as if it's a reactant)?(1 vote)
- Since a catalyst is not used up in a reaction, you only need a small amount (a lot less than stoichiometric ammounts).
Catalysts speed up the rate of reaction, but they do not alter the position of equilibrium of a reaction. If without the catalyst your reaction would go to completion(all to products), even if very slowly, then yes, in the presence of catalysts, the reactants will all be turned into products. But if the reaction you are catalysing doesn't go to completion anyway, then the catalyst won't change that. This is because a catalyst increases both the rate of the forward and back reaction.
Whilst catalysts don't get used up in the reactions they catalyse, they may stop working for other reasons. For example catalytic converters in cars can be 'poisoned' when certain chemicals cover their surfaces.(4 votes)
how do catalyst work on a particle level?(1 vote)
If we go by collision theory,
positive catalysts may either:
(1) Reduce the activation energy required for the reactant particles to become an activated complex.
(2) Force the particles to attain multiple or altered 'transition states' in such a way that the reaction takes place faster or with less energy.
BTW, positive catalyst is a term for a catalyst that speeds up a reaction. Negative catalysts slow down, or make it harder for a reaction to occur.
Please note: This is a very simple explanation for an extremely complex topic and I cannot do it full justice. I suggest researching enzymes in the body to get a better idea of how intricate this subject is.(3 votes)
- How do catalysts use adsorption process?(1 vote)
- For gas phase reactions, one or more of the gases are adsorbed onto the surface of the catalyst. Exactly what happens will depend on the reaction in question, but adsorption may, for example, weaken the bonds in the reactant molecules which facilitates the chemical reaction.
This diagram shows the sort of thing that might happen - http://www.docbrown.info/page07/SSquestions/heterocatalysistrans1.gif. The intermolecular forces shown as dotted red lines are responsible for weakening the H-H and C=C bonds, which promotes the reaction.(2 votes)
- What is activation energy?(1 vote)
- Activation energy is explained in this video - https://www.khanacademy.org/science/chemistry/chem-kinetics/arrhenius-equation/v/collision-theory.(2 votes)
What are the catalysts that is a factor is in a chemical reaction involving four or more reactants? and what are some chemical equation for that?(1 vote)- This sounds like a homework question so what are your thoughts?(2 votes)
- Catalysts are useful for many things in the past few videos and this article. Even thought catalysts are great, should they or are they always useful? And can they "mess up" an experiment?(1 vote)
We usually think of catalysts as something we would add to aid a reaction, in which case, we would never add a thing which can mess-up. But generally speaking, there may be undesired catalysts in nature, as in some kinds of rust is catalyzed by atmospheric components, and some catalysts might be too potent to have a controllable reaction.(2 votes)
How to know which catalysts to use in a chemical reaction or when the mechanism of such is given?(1 vote)- There is really no way of predicting what will catalyse any particular reaction. This has to be determined experimentally.
If you are given an overall equation for a chemical reaction, if a catalyst is involved, its chemical formula should be given above the arrow in the equation (such as shown above in the sucrose example). If it's not, and you are just given a sequence of elementary steps for the overall reaction, then you need to identify a component that gets involved in the reaction but is then regenerated (ie, something that is not consumed so is there at the start and at the end).
Be careful to distinguish between catalysts and intermediates. Catalysts are present at the start and at the end, while intermediates get formed part way through and then get consumed.(2 votes)
