catalytic reaction

Solid Catalyzed Reactions:Rate Equations

by mujettegodmalin

With many reactions, the rates are affected by

materials which are neither reactants nor products.

So what are

they?

They are catalysts.

Catalystssubstance that affects the rate of reaction but

emerges from the process unchanged.

“a substance which alters the rate of a chemical reaction but is chemically unchanged

at the end of the reaction.”

This means that there is just as much catalyst at the end of a reaction as there

was at the beginning. The catalyst is used over and over again. Because catalysts work so rapidly and are used again and

again, it is only necessary to have very small quantities of catalyst present to make a

chemical reaction go faster.

"..... but is chemically unchanged at the end of the reaction."

This means that it is some kind of chemical

substance! It could be a pure element; e.g. Platinum, Nickel; or it

could be a pure compound, e.g. Manganese Dioxide, Silica, Vanadium V Oxide, Iron III

Oxide; it could be dissolved ions, e.g. Copper ions, Cobalt II ions; or it could be a mixture, e.g. Iron-Molybdenum, or it could

be a much more complicated compound

such as protein

"A catalyst is a substance ....."

Catalysts can speed a reaction by a factor of a million or much more, or they may slow a reaction

(negative catalyst).

Normally when we talk about a catalyst, we mean one that

speeds up a reaction, although strictly speaking, a catalyst can either accelerate

or slow the formation of a particular product species.

2 broad classes:

Biochemical catalysts (enzymes)Man-made catalysts

THINGS YOU OUGHT TO KNOW

Our bodies have hundreds of different enzymes and are other catalysts that are busily at work all the time, keeping us alive.Man-made catalysts play an important role in many industrial processes. Over 50% of all the chemical products produced today are made with the use of catalysts.Catalysts select.

Although a catalyst can easily speed the rate of reactions a thousandfold or millionfold, still, when a variety of reactions encountered, the most important characteristic of a catalyst

is its selectivity.

SelectivityIt only changes the rates of certain reactions,

often a single reaction, leaving the rest unaffected.

How catalysts work?

You already know that when two different molecules bump into each other, they might react to make new chemicals. We usually talk about "collisions" between molecules, it would be much simpler to say that the

molecules bumped into each other. How fast a chemical reaction is depends upon how frequently the molecules collide. You have probably been told about the "kinetic theory" which is all about heat and how

fast molecules move around. What catalysts are doing when they make a chemical reaction go faster is to increase the chance of molecules colliding.

The first method is by "adsorption", the second method is by the formation of intermediate

compounds.

There are two ways in which catalysts work.

AdsorptionThis occurs when a molecule sticks onto the

surface of a catalyst.

“For a catalytic reaction to occur, at least one and frequently all of the reactants must become

attached to the surface.”

Intermediate Compounds

the chemicals involved in the reaction combine with the catalyst making an intermediate compound, but

this new compound is very unstable. When the intermediate compound breaks down, it releases the

new compounds and the original catalyst.

Representation of the action of a catalyst.

A catalyst changes a reaction rate by promoting a different molecular path

(“mechanism”)for the reaction.

Once a reactant has been adsorbed onto the surface, it is capable of reacting in a number of ways to form the reaction

product. There are 3 ways:

Single-site Mechanism

Only the site in which the reactant is adsorbed is involved in the reaction.

A · S B · S

Dual-site Mechanism

Adsorbed reactant interact with another site to form product.

A · S + S B · S + S

Eley-Rideal Mechanism

Between an adsorbed molecule and a molecule in the gas-phase:

A · S + B (g) C · S + D (g)

The RATE EQUATION

…reaction takes place on an active site on the surface of the catalyst. Three

steps are viewed to occur successively at the surface.

Step 1: A molecule is adsorbed onto the surface an is attached to an active site. (adsorption)

Step 2:It then reacts either with another molecule on an adjacent site (dual site mechanism), with one coming from the main gas stream (single-site mechanism), or it simply decomposes while on the site.(surface reaction)

Step 3:Products are desorbed from the surface, which then free the site. (desorption)

STEPS IN A CATALYTIC REACTION

All species of molecules, free reactants, and free products as well as site-attached

reactants, intermediates, and products taking part in these three processes are assumed to

be in equilibrium.

Rate expressions derived from various postulated mechanisms are all of the form

Rate of reaction =(kinetic term)(driving force or displacement from equilibrium)

(resistance term)

For example, for the reaction

occurring in the presence of inert carrier material U, the rate expression when adsorption of A controls is

When reaction between adjacent moleculesof A and B controls, the rate expression is

whereas for desorption of R, controlling it becomes

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