1. Homogeneous and
Heterogeneous System

2. Difference
HOMOGENEOUS REACTIONS:
• A reaction is homogeneous if it takes place in one phase alone.
HETEROGENEOUS REACTIONS:
• A reaction is heterogeneous if it requires the presence of at least
two phases to proceed at the rate that it does.

3. Cont…
• Homogeneous reaction, any of a class of chemical reactions that occur in a
single phase (gaseous, liquid, or solid), one of two broad classes of
reactions—
• Homogeneous and heterogeneous—based on the physical state of the
substances present.The most important of homogeneous reactions are the
reactions between gases (e.g., the combination of common household gas
and oxygen to produce a flame) and the reactions between liquids or
substances dissolved in liquids (e.g., the reactions between aqueous
solutions of acids and bases).
• From the theoretical standpoint, homogeneous reactions are the simpler of
the two classes of reactions because the chemical changes that take place
are solely dependent on the nature of the interactions of the reacting
substances. Compareheterogeneous reaction.

4. Cont…
• Heterogeneous reaction, any of a class of chemical reactions in which the
reactants are components of two or more phases (solid and gas, solid and
liquid, two immiscible liquids) or in which one or more reactants undergo
chemical change at an interface.
• E.g., on the surface of a solid catalyst.The reaction of metals with acids, the
electrochemical changes that occur in batteries and electrolytic cells, and the
phenomena of corrosion are part of the subject of heterogeneous reactions.
• By far the majority of the researches on heterogeneous reactions are
devoted to heterogeneous catalysis (e.g., the reactions between gases or
liquids accelerated by solids). Heterogeneous reactions are of considerable
practical interest; they are not, however, understood as well as those
reactions that occur in only one phase (homogeneous
reactions). Compare homogeneous reaction.

5. Catalytic vs Non Catalytic
Catalytic reaction:
• Whose rate is altered by materials that are neither reactants nor
products.These foreign materials, called catalysts, need not be
present in large amounts. Catalysts act somehow as go-betweens,
either hindering or accelerating the reaction process while being
modified relatively slowly if at all.
Non Catalytic Reaction:
• The chemical phenomenon completes without involving any
influencing material (i.e. catalyst) is called non catalytic reaction.

6. Classification of Chemical Reactions
Useful in Reactor Design
Homogeneous
Non Catalytic:
• Most gas-phase reactions .
• Fast reactions such as burning of a flame
Catalytic:
• Most liquid-phase reactions .
• Reactions in colloidal systems Enzyme and microbial reactions

7. Cont…
Heterogeneous
Non Catalytic:
• Burning of coal
• Roasting of ores
• Attack of solids by acids
• Gas-liquid absorption with reaction
• Reduction of iron ore to iron and steel

8. Cont…
Catalytic
• Ammonia synthesis
• Oxidation of ammonia to produce nitric acid
• Cracking of crude oil
• Oxidation of SO2 to SO3

9. Variables Affecting the Rate of
Reaction
Many variables may affect the rate of a chemical reaction
In Homogeneous systems:
• Temperature
• pressure, and composition are obvious variables.
In heterogeneous systems:
More than one phase is involved…

10. Cont…
• hence, the problem becomes more complex. Material
may have to move from phase to phase during reaction;
hence, the rate of mass transfer can become important.

11. Examples
• In the burning of a coal briquette the diffusion of oxygen
through the gas film surrounding the particle, and
through the ash layer at the surface of the particle, can
play an important role in limiting the rate of reaction. In
addition, the rate of heat transfer may also become a
factor.

12. Cont…
• Consider, for example, an exothermic reaction taking place at
the interior surfaces of a porous catalyst pellet. If the heat
released by reaction is not removed fast enough, a severe non
uniform temperature distribution can occur within the pellet,
which in turn will result in differing point rates of reaction.
These heat and mass transfer effects become increasingly
important the faster the rate of reaction, and in very fast
reactions, such as burning flames, they become rate
controlling.Thus, heat and mass transfer may play important
roles in determining the rates of heterogeneous reactions.