Factors Influencing Reactions
Chemical reactions occur when certain physical and
chemical factors make conditions energetically favorable for the reactants to
combine into products . Some factors, such as the
potential energy (stored energy) associated with the reactants, can trigger
a spontaneous chemical reaction.
If the products have a higher level of entropy
(disorder among the particles) than the reactants, this difference can also
initiate a chemical reaction. External factors, such as heat or the presence of
a catalyst (a substance that increases reaction rate without being chemically
changed), can trigger or increase the rate of a reaction
Exothermic and Endothermic Reactions
Chemical reactions can occur spontaneously if the
reactants possess more potential energy (stored energy) than the products. This
type of reaction occurs spontaneously because of the downhill energy path (from
more potential energy to less). These reactions are called exothermic
(heat-producing) reactions, because potential energy is converted to heat as the
reactions proceed. Conversely, endothermic (heat-absorbing) reactions do not
occur spontaneously because of the uphill energy path that exists. The products
of endothermic reactions contain more potential energy than the reactants. As a
result, energy must be added to trigger an endothermic reaction.
Entropy
Entropy is the tendency for matter to become
disordered. Nature requires the input of energy to maintain an ordered state�a
bedroom will become messy if not periodically cleaned; a car will eventually
fall into disrepair if not regularly serviced. Entropy is an important force in
chemistry. If other factors influencing a reaction are held equal, a chemical
reaction will proceed spontaneously if the products have higher entropy (are
more disordered) than the reactants. This law explains why ozone (O3)
gas can spontaneously decompose into molecular oxygen (O2): 2O3(g)
→ 3O2(g). This reaction occurs because the molecular order is
diminished, resulting in a higher level of entropy.
Heat
Applying heat to matter in the solid, liquid, or
gas phase adds energy to the substance, causing the atoms to move faster and
collide into each other with greater force. As a result, heat speeds up a
chemical reaction by bringing atoms into contact with each other with greater
force and frequency.
Catalysts
Catalysts are substances that trigger or speed up
chemical reactions (without chemically altering the catalysts in the process). A
catalyst combines with a reactant to form an intermediate compound that can more
readily react with other reactants. An example of this is the formation of
sulfur trioxide (SO3), which is an important ingredient for producing
sulfuric acid (H2SO4). Without a catalyst, sulfur trioxide
is made by combining sulfur dioxide (SO2) with molecular oxygen: 2SO2
+ O2 → 2SO3. Because this reaction proceeds very slowly,
manufacturers use nitrogen dioxide (NO2) as a catalyst to speed
production of SO3:
Step One: NO2 (catalyst) + SO2
→ NO + SO3 (SO3 is extracted and combined with steam to
produce sulfuric acid)
Step Two: NO (from Step One) + O2 → NO2
(catalyst that is reused in step one)
In the above reactions, nitrogen dioxide (NO2)
acts as a catalyst by combining with sulfur dioxide (SO2) to form
both sulfur trioxide (SO3) and nitrogen monoxide (NO). The sulfur
trioxide is removed from the process (to be used in the production of sulfuric
acid). Nitrogen monoxide (NO) is subsequently combined with molecular oxygen (O2)
to produce the original catalyst, nitrogen dioxide (NO2), which can
be continually reused to catalyze sulfur trioxide (SO3).
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