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- Catalyst, in chemistry, any substance that increases the rate of a reaction without itself being consumed. Enzymes are naturally occurring catalysts responsible for many essential biochemical reactions. In general, catalytic action is a chemical reaction between the catalyst and a reactant.
www.britannica.com/science/catalyst
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What is Catalysis in chemistry?
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Why are catalysts important in a chemical reaction?
Catalyst, in chemistry, any substance that increases the rate of a reaction without itself being consumed. Enzymes are naturally occurring catalysts responsible for many essential biochemical reactions. In general, catalytic action is a chemical reaction between the catalyst and a reactant.
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2017年2月27日 · A catalyst is some material that speeds up chemical reactions. With a helping hand from a catalyst, molecules that might take years to interact can now do so in seconds. Factories rely on catalysts to make everything from plastic to drugs. Catalysts help process petroleum and coal into liquid fuels.
Because a catalyst decreases the height of the energy barrier, its presence increases the reaction rates of both the forward and the reverse reactions by the same amount. In this section, we will examine the three major classes of catalysts: heterogeneous catalysts, homogeneous catalysts, and enzymes.
- Overview
- Key points
- Introduction: A kinetics thought experiment
- What is a catalyst?
- Example: Carbonic anhydrase
- Acid-base catalysis
- Heterogeneous and surface catalysis
- Summary
What is a catalyst? Includes examples of enzymes, acid-base catalysis, and heterogeneous (or surface) catalysis.
•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.
Your brain is powered by the oxidation of glucose. The oxidation of glucose can be represented as the following balanced chemical reaction:
C6H12O6(s)+6O2(g)→6CO2(g)+6H2O(l)+heatΔG∘at 25∘C=−2885kJmol
Without this reaction, learning chemistry would be much harder. Luckily, the oxidation reaction is thermodynamically favored at 25∘C since ΔG∘<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.
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.
The enzyme carbonic anhydrase catalyzes the reversible reaction of carbon dioxide (CO2) and water (H2O) to form carbonic acid. When the concentration of CO2 in the body is too high, carbonic anhydrase catalyzes the following reaction:
CO2+H2O→H2CO3
By regulating the concentration of carbonic acid in the blood and tissues, the enzyme is able to keep the pH balanced in the body.
Carbonic anhydrase is one of the fastest known enzymes, with reaction rates between 104 and 106 reactions per second. This is even more amazing compared to the uncatalyzed reaction, which has a rate of ~0.2 reactions per second. That is a ~105−107 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.
In acid catalysis, the catalyst is usually a H+ ion. In base catalysis, the catalyst is usually an OH− 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 H+ (in red), to form protonated sucrose. The protonated sucrose reversibly reacts with water (in blue) to give H+ , one molecule of glucose, and one molecule of fructose. The overall reaction can be written as:
Sucrose+H2O→acid catalystGlucose+Fructose
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.
[What do you call a catalyst in the same phase as the reactants?]
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. Chemistry− it's like magic!
•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.
The well known catalysts are Pt and Rh, but other technology such as memberanes are also used for syngas productions. Selection of a catalyst is important in industrial productions. For example, using rhodium or platinum as catalysts have shown to give very
Catalysts are substances that speed up a reaction but which are not consumed by it and do not appear in the net reaction equation. Also — and this is very important — catalysts affect the forward and reverse rates equally; this means that catalysts have no.
2022年9月21日 · In chemistry and biology, a catalyst is a substance the increases the rate of a chemical reaction without being consumed by it. Catalysis is the process of speeding up a reaction using a catalyst. The word “catalyst” comes from the Greek word kataluein, which means to loosen or untie.
