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:
C, start subscript, 6, end subscript, H, start subscript, 12, end subscript, O, start subscript, 6, end subscript, left parenthesis, s, right parenthesis, plus, 6, O, start subscript, 2, end subscript, left parenthesis, g, right parenthesis, right arrow, 6, C, O, start subscript, 2, end subscript, left parenthesis, g, right parenthesis, plus, 6, H, start subscript, 2, end subscript, O, left parenthesis, l, right parenthesis, plus, h, e, a, t, space, space, delta, G, degree, space, a, t, space, 25, space, degree, C, equals, minus, 2885, space, start fraction, k, J, divided by, m, o, l, end fraction
Without this reaction, learning chemistry would be much harder. Luckily, the oxidation reaction is thermodynamically favored at 25, space, degree, C since delta, G, degree, is less than, 0.
a slice of a dark grape, about 5 mm thin and seen against a window
Did you know that glucose was first isolated from raisins? Image from Wikimedia Commons, public domain
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, space, degree, C, 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
  1. Lowering the energy of the transition state, thus lowering the activation energy, and/or
  2. 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, C, O, start subscript, 2, end subscript, right parenthesis and water left parenthesis, H, start subscript, 2, end subscript, O, right parenthesis to form carbonic acid. When the concentration of C, O, start subscript, 2, end subscript in the body is too high, carbonic anhydrase catalyzes the following reaction:
C, O, start subscript, 2, end subscript, plus, H, start subscript, 2, end subscript, O, right arrow, H, start subscript, 2, end subscript, C, O, start subscript, 3, end subscript
By regulating the concentration of carbonic acid in the blood and tissues, the enzyme is able to keep the p, H balanced in the body.
Ribbon diagram of human carbonic anhydrase II. The zinc ion is visible at the protein's center as a dark grey sphere.
A ribbon diagram of human carbonic anhydrase II. Isn't chemistry beautiful? The grey sphere in the center of the protein is a zinc ion. Image from Wikimedia Commons, public domain
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.
Diagram of a catalytic reaction (specifically, that catalysed by carbonic anhydrase in the presence of high carbon dioxide concentrations) showing difference in activation energy in uncatalysed and catalysed reaction. The starting materials and products have the same energy for the reactions with and without enzyme, so the overall change in energy for the system does not change.
Diagram of energy for reaction between carbon dioxide and water to form carbonic acid. The addition of catalyst (blue line) lowers the energy of the transition state, but does not change delta, H, start subscript, r, x, n, end subscript compared to the uncatalyzed reaction (red line). Image from Wikimedia Commons, CC BY-SA 3.0
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.
Great question! The Arrhenius equation describes the relationship between the rate constant k and the activation energy, E, start subscript, a, end subscript:
k, equals, A, e, start superscript, minus, E, start subscript, a, end subscript, slash, R, T, end superscript
where A is called the pre-exponential factor, R is the gas constant, and T is the temperature in kelvin.
To learn more about the different parts of the equation and how it is used, check out the Arrhenius equation video.
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, H, start subscript, r, x, n, 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 H, start superscript, plus, end superscript ion. In base catalysis, the catalyst is usually an O, H, 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:
Sucrose reversibly reacts with a hydrogen proton, H+, to form protonated sucrose where the oxygen that connects the glucose and fructose molecules gets protonated. The protonated sucrose reversibly reacts with water to form one molecule of glucose, one molecule of fructose, and H+.
The acid-catalyzed reaction to form glucose and fructose from sucrose, which is also known as table sugar
In the first step, sucrose reversibly reacts with H, start superscript, plus, end superscript (in red), to form protonated sucrose. The protonated sucrose reversibly reacts with water (in blue) to give H, start superscript, plus, end superscript, one molecule of glucose, and one molecule of fructose. The overall reaction can be written as:
Since the H, 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.
A catalyst that is in the same phase as the reactants is called a homogeneous catalyst. Homogeneous catalysts are important, too! Enzymes are an example of homogeneous catalysts, and acids can also be homogeneous catalysts.
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.
Cross section of metal tube showing solid tan honey-comb like porous material, the solid-state catalyst.
The solid phase catalyst inside a catalytic converter reduces emissions of toxic gases, unburned fuel, and particulate matter. The solid support is designed to have a high surface area to increase the surface area of catalyst available to react with the exhaust stream. Image from Oak Ridge National Laboratory on flickr, CC BY-NC-ND 2.0
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.
X-ray showing a right hip (left of image) has been replaced, with the ball of the ball-and-socket joint replaced by a metal head that is set in the femur and the socket replaced by a white plastic cup (clear in this X-ray).
Polyethylene is also used for artificial joints! The metal ball-joint in this artificial hip fits into a polyethylene socket, which appears clear in the X-ray. Image from Wikimedia Commons, public domain
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!

შინაარსი

  • 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.