chem. termpaper 1 .rk6004b41

8/8/2019 Chem. Termpaper 1 .RK6004B41

1/13

Term paper

CHEMISTRY

CHE 101

Topic: Catalysts

DOA:2 nd sep 2010

DOS: 10 th November 2010

Submitted to: Submitted by:

Ms. Vibhuti Walia Ms.Surbhi Ghai

Deptt. Of Chemistry Roll. No. : RK6004B41

Reg.No: 11004145

Class: K6004


2/13

ACKNOWLEDGEMENTS

Thanks giving are a sacred Indian culture. Although I owe a lot to many, due to space constraint, I amexplicitly expressing my thanks to only few.

I express my thanks to,

Respected authors of the books from where I had taken reference, our course instructor Ms.Vibhuti Waliawho helped me with my difficulties, my friends who had helped me a lot to clear my queries related to thetopic.

My parents Mr.Sanjeev Ghai and Mrs.Usha Ghai deserves special thanks for there continued patience, care,support and encouragement all through this project.

Finally my sincere thanks to my father Mr.Sanjeev Ghai for his moral support.


3/13

ABSTRACT

The catalysts are the chemical substance that affects the rate of reaction, it can either increase the rate of reaction or it can decrease it depending upon various conditions like weather it is a positive catalyst or it is anegative catalyst. Positive catalysts are defined as the chemical substance which enhances the rate of reaction similarly the negative catalysts are the chemical substance that retards the rate of reaction. Catalyst

provides an alternate mechanism for the reaction which tells weather the products are formed more fastly or slowly under some particular conditions like temperature etc.

There are some characteristics of catalysts like, catalysts remain unchanged inmass and chemical composition at the end of the reaction, a small quantity of catalyst is enough to catalyze

no. of reactions, catalysts cannot initiate the reaction, and catalysts are generally specific in nature.

There are many ways by which catalysts can be prepared, it can either be artificially or can be foundnaturally. In many reactions they are generated artificially i.e. created within the reaction there are lot manyways to create catalysts artificially and it is very beneficial for industries etc. Enzymes are the mostcommonest and efficient form of catalysts found in nature. Catalysts occur in nature in many forms, they can

be found in living organisms and from many more sources.


4/13

CONTENTS

About Catalysts

Affect of catalysts on the rate of reaction

Types of catalysts

Characteristics of catalysts

Preparation of catalysts

Occurrence of catalysts.


5/13

INTRODUCTION

The topic with which I am dealing is catalyst which is basically based on the following points

Affect of catalyst

The catalysts are the substance that alters the rate of reaction. It can either increase or decrease the rate of

reaction depending on its nature.

Types of catalysts

There are mainly two types of catalysts positive catalysts or negative catalysts. Positive catalysts increase

the rate of reaction and negative catalysts decrease the rate of reaction.

Characteristics of catalysts

Catalysts remain unchanged in mass and chemical composition at the end of the reaction, a small quantity of catalyst is enough to catalyze no. of reactions, catalysts cannot initiate the reaction, and catalysts aregenerally specific in nature.

Preparation of catalysts

In many reactions they are generated artificially i.e. created within the reaction there are lots many ways to

create catalysts artificially.

Occurrence of catalysts

Catalysts occur in nature in many forms, they can be found in living organisms and from many moresources.


6/13

CATALYST

AFFECT OF CATALYSTS

A catalyst is a chemical substance that affects the rate of a chemical reaction by altering the activationenergy required for the reaction to proceed. This is called catalysis. A catalyst is not consumed by thereaction and it may participate in multiple reactions at a time. The only difference between a catalyzedreaction and an uncatalyzed reaction is that the activation energy is different. There is no effect on theenergy of the reactants or the products. The H for the reactions is the same.

TYPES OF CATALYSTS

Usually when someone refers to a catalyst, they mean a positive catalyst , which is a catalyst whichspeeds up the rate of a chemical reaction by lowering its activation energy. There are also negative catalystsor inhibitors, which slow the rate of a chemical reaction or make it less likely to occur. A promoter is asubstance that increases the activity of catalyst. A catalytic poison is a substance that inactivates a catalyst.There are many more types of catalysts. The catalysts divided into various types according to their

behaviour and pattern of action. Positive catalyst Negative catalyst Auto catalyst


7/13

Induced catalyst

Positive catalyst:

A catalyst which enhances the speed of the reaction is called positive catalyst and the event is known as

positive catalysis. It decreases the energy requirement in the reaction.

Negative Catalyst: Negative Catalyst is definite substances which, when added to the reaction mixture, retard the reaction rateinstead of increasing it. These are called negative catalysts or inhibitors and the event is known as negativecatalysis.

Auto catalyst: In certain reactions, it is observed that one of the products formed during the reaction acts as a catalyst for that reaction. This type of catalyst is called auto catalyst and the event is known as auto catalysis .

Induced Catalyst: When one reactant influences the speed of other reaction, which does not happen under ordinary conditions,

this process is known as induced catalysis. Sodium arsenite element is not oxidised by air. If, however, air istravelled through a mixture of the solution of sodium arsenite and sodium sulphite, both of them go throughsimultaneous oxidation. Thus sulphite has induced the arsenite and hence is called induced catalyst.

CHARACTERISTICS OF CATALYSTS

The following are the characteristics which are common to must of catalytic reactions.

(1) A catalyst remains unchanged in mass and chemical composition at the end of the reaction.

(2) A small quantity of the catalyst is generally sufficient to catalyses almost unlimited reactions

For example,

(i) In the decomposition of hydrogen peroxide, one gram of colloidal platinum can catalyses 10 8litres of hydrogen peroxide.

(ii) In Friedel crafts reaction, anhydrous aluminium chloride is required in relatively large amount to theextent of 30% of the mass of benzene,


8/13

C6H6+C 2H5Cl AlCl 3C 6H5C2H5 + HCl

(3) The catalyst cannot initiate the reaction:

The function of the catalyst is to alter the speed of the reaction rather than to start it. (4) The catalyst is generally specific in nature:

A substance, which acts as a catalyst for a particular reaction, fails to catalyse the other reaction, differentcatalysts for the same reactant may for different products.

Example:

(5) The catalyst cannot change the position of equilibrium :

The catalyst catalyse both forward and backward reactions to the same extent in a reversible reaction andthus have no effect on the equilibrium constant.

(6) Catalytic promoters :

Substances which themselves are not catalysts, but when mixed in small quantities with the catalystsincrease their efficiency are called as promoters or activators . (i) For example, in Habers process for the synthesis of ammonia, traces of molybdenum increases theactivity of finely divided iron which acts as a catalyst. (ii) In the manufacture of methyl alcohol from water gas (CO + H 2), chromic oxide (Cr 2O3) is used as a

promoter with the catalyst zinc oxide (ZnO).


9/13

(7) Catalytic poisons :

Substances which destroy the activity of the catalyst by their presence are known as catalytic poisons . (i) For example, the presence of traces of arsenious oxide (As 2O3) in the reacting gases reduces the activityof platinized asbestos which is used as catalyst in contact process for the manufacture of sulphuric acid. (ii) The activity of iron catalyst is destroyed by the presence of H 2S or CO in the synthesis of ammonia byHabers process. (iii) The platinum catalyst used in the oxidation of hydrogen is poisoned by CO.

(8) Change of temperature alters the rate of catalytic reaction as it does for the same reaction inabsence of catalyst :

By increasing the temperature, there is an increase in the catalytic power of a catalyst but after a certaintemperature its power begins to decrease. A catalyst has thus, a particular temperature at which its catalyticactivity is maximum. This temperature is termed as optimum temperature .

(9) A positive catalyst lowers the activation energy

(i) According to the collision theory, a reaction occurs on account of effective collisions between thereacting molecules.

(ii) For effective collision, it is necessary that the molecules must possess a minimum amount of energyknown as activation energy ( E a).

(iii) After the collision molecules form an activated complex which dissociate to yield the productmolecules. (iv) The catalyst provides a new pathway involving lower amount of activation energy. Thus,


10/13

larger number of effective collisions occur in the presence of a catalyst in comparison to effective collisionsat the same temperature in absence of a catalyst. Hence the presence of a catalyst makes the reaction to gofaster.

(v) Figure shows taht activation energy E a, in absence of a catalyst is higher than the activation energy E a, in presence of a catalyst.

(vi) E R and E P represent the average energies of reactants and products. The difference gives the value of ?G,i.e., ?G = E R E P


11/13

WORKING OF CATALYSTS:

Catalysts work by changing the activation energy for a reaction, i.e., the minimum energy needed for thereaction to occur. This is accomplished by providing a new mechanism or reaction path through which thereaction can proceed. When the new reaction path has a lower activation energy, the reaction rate isincreased and the reaction is said to be catalyzed. If the activation energy for the new path is higher, thereaction rate is decreased and the reaction is said to be inhibited.

PREPARATION OF CATALYSTS:

Catalysts are mostly generated within the reaction. Catalysis is also important in chemical laboratories andin industry. Some reactions occur faster in the presence of a small amount of an acid or base and are said to

be acid catalyzed or base catalyzed. For example, the hydrolysis of esters is catalyzed by the presence of asmall amount of base. In this reaction, it is the hydroxide ion, OH -, that reacts with the ester, and theconcentration of the hydroxide ion is greatly increased over that of pure water by the presence of the base.Although some of the hydroxide ions provided by the base are used up in the first part of the reaction, theyare regenerated in a later step from water molecules; the net amount of hydroxide ion present is the same atthe beginning and end of the reaction, so the base is thought of as a catalyst and not as a reactant .

OCCURANCE OF CATALYSTS:

Enzymes are the commonest and most efficient of the catalysts found in nature. Most of the chemicalreactions that occur in the human body and in other living things are high-energy reactions that would occur slowly, if at all, without the catalysis provided by enzymes. For example, in the absence of catalysis, it takesseveral weeks for starch to hydrolyze to glucose; a trace of the enzyme ptyalin, found in human saliva,accelerates the reaction so that starches can be digested. Some enzymes increase reaction rates by a factor of one billion or more.


12/13

APPLICATIONS AND FUTURE PERSPECTIVE OF CATALYSTS:

Its most recent use is

Improved nanotechnology catalysts bring clean energy applications closer( Nanowerk Spotlight ) It has been almost 200 years since chemists stumbled across the fact that certain chemicalscan speed up a chemical reaction a process, now known as catalysis, that has become the foundation of themodern chemical industry. By some estimates 90% of all commercially produced chemical products involvecatalysts at some stage in the process of their manufacture. Catalysis is the acceleration of a chemical reaction bymeans of a substance, called a catalyst, which is itself not consumed by the overall reaction.The most effective catalysts are usually transition metals or transition metal complexes. The increasing role thatnanotechnology is playing in modern catalysis has to do with two main effects of catalysts that have onlynanoscale dimensions. On one hand, the increasing surface-to-volume ratio with decreasing particle size stronglyincreases the specific catalytic activity, while on the other hand quantum confinement effects can completelyalter the chemical characteristics of sufficiently small nanoparticles. Researchers have also shown that the atomiccharacteristics of metallic nanoparticles, including particle size and surface composition, are critical to catalyticactivity and selectivity.

Nanotechnology catalytical techniques are having a profound impact on clean energy research and development,ranging from hydrogen and liquid fuel production to clean combustion technologies. In this area, catalyststability is paramount for technical application, and remains a major challenge, even for many conventionalcatalysts.Thermal stability in particular is a challenge across many currently discussed technical applications and anobstacle for many nanocatalyst-enabled devices, from sensors to fuel production. In particular fuel processingtechnologies (hydrogen and/or liquid fuel production from fossil and renewable resources, clean combustion)

typically proceed at particularly severe conditions (high temperatures, high through-put, contaminated fuelstreams, etc) and hence require particular attention to catalyst stabilization, but even many processes at muchlower temperature conditions, such as fuel cells, are still looking for catalysts with improved stability.In essence, the poor thermal stability of many nanomaterials currently constitutes probably one of the mainchallenges on the way to technical application of many nanomaterials.Researchers have now overcame a major hurdle in developing more efficient nanoparticle catalysts bydemonstrating high-temperature stability in metallic nanoparticles.


13/13

REFERENCES

WEBSTES:

http://www.searchengineguide.com

http://science.ratnasagar.co.in

http://www.tifr.res.in/~base/links/websites.html

http://www.hep.fsu.edu/~wahl/Quarknet/links.html

http://www.ncsu.edu/imse/4/elementaryEd.htm

Books:

R.chang
http://www.searchengineguide.com/http://science.ratnasagar.co.in/http://www.tifr.res.in/~base/links/websites.htmlhttp://www.hep.fsu.edu/~wahl/Quarknet/links.htmlhttp://www.ncsu.edu/imse/4/elementaryEd.htmhttp://www.searchengineguide.com/http://science.ratnasagar.co.in/http://www.tifr.res.in/~base/links/websites.htmlhttp://www.hep.fsu.edu/~wahl/Quarknet/links.htmlhttp://www.ncsu.edu/imse/4/elementaryEd.htm

chem. termpaper 1 .rk6004b41

Documents