Chemicals {catalyst}| can increase reaction rate, but chemical reaction does not alter them.
amount
Reaction needs only small catalyst amount, because reaction reuses catalyst. However, catalysts can break down, have dirt or product coatings, or have surface damage.
processes
Catalysts reduce energy needed to start reaction. Catalysts allow transition state with lower activation energy, make molecule easier to attack, allow leaving group to leave easier, make attacking group attack better, orient molecules for optimum bond stretching, provide functional groups for forces or transfer, or line up reactant molecules.
types
Enzymes are protein catalysts.
Acids and bases are catalysts {homogeneous catalyst}. Basic catalysts cause isomerization, halogenation, or condensation. Acid catalysts cause tautomerism, solvolysis, or inversion. Neutral catalysts polarize solvent.
types: solid
Solid catalysts {heterogeneous catalyst} provide structured surfaces. Ceramic or metal catalysts are for industrial processes. Surface chemistry is for catalysis, corrosion, membranes, surface tension, and electrodes.
If molecule collision energy with surface is same as surface thermal-vibration energy, surface can absorb molecule and collision energy. Molecule-absorption rate depends on collision energy. Electrode surfaces have an ion layer, covered by an opposite-charge ion layer.
Catalytic surfaces must not bind too strongly or too weakly. Collision rate is not important, because absorption surface is large. Activation energy is small and not determining factor for surface catalysts.
As atoms bind to catalyst, catalyst surfaces orient molecules and dissociate molecular bonds. Then new bond can form by collision or reorientation. Molecules on catalysts can move depending on impurities, defects, and crystal planes. Movement allows reaction atom transfer.
types: gas and metals
Gas molecules chemisorb on metals, because metal absorption area is much greater than gas collision area, so entropy decreases. How saturated surface is affects absorption. If concentration is high or time on surface is long, absorption is less. Because neighboring sites move, they affect absorption sites.
Metals bind oxygen strongest, then acetylene, ethylene, carbon monoxide, hydrogen, carbon dioxide, and nitrogen. Platinum, iron, vanadium, and chromium can adsorb all these substances. Manganese and copper can adsorb some. Magnesium and lithium only absorb oxygen. Iron, nickel, platinum, and silver surfaces are catalysts for hydrogenations and dehydrogenations.
Nickel oxide, zinc oxide, and magnesium oxide are catalysts for oxidations and dehydrogenations, because they are semiconducting. Metal sulfides are catalysts for desulfurations, because they are semiconducting. Aluminum oxide, silicon oxide, and magnesium oxide are catalysts for dehydrations, because they are insulators. Phosphoric acid and sulfuric acid are catalysts for polymerizations, isomerizations, alkylations, and dealkylations {cracking, petroleum}.
Physical Sciences>Chemistry>Inorganic>Chemical Reaction>Energy
5-Chemistry-Inorganic-Chemical Reaction-Energy
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Date Modified: 2022.0224