· Chemical
Kinetic: The study concerned with the rates of chemical
reactions and the factors that affect the rates of chemical reaction, is known
as chemical kinetics.
· The
Concept of Reaction Rate: During a chemical reaction, reactants are being
converted to products. The reaction rate tells us how fast the reaction is
taking place by indicating how much of a reactant is consumed or how much of a
product forms in a given time, Hence;
Reaction rate = change in concentration
of a substance / time taken
In practice, it is usual to plot a graph of the
concentration or amount of a particular substance against time. Then, reaction
rate can be obtained at particular times by drawing tangents to the resulting
curve.
FACTORS AFFECTING THE RATE OF A
· The
Availability of Reactants and Their Surface Area: Anyone
who has camped knows that it is easier to start a fire using sticks rather than
tree trunks. Similarly powdered calcium carbonate reacts with HCl more rapidly
than marble chips. In general, the smaller the size of reacting particles, the
greater is the total surface area exposed for reaction and consequently, the
faster the reaction.
In the cause of heterogeneous
system, in which the reactants are in different states, area of contact between
the reacting substance will influence the reaction rate considerably.
· The
Concentration Of Reactants: Increasing the concentration of a
reactant normally causes an increase in the rate of reaction. Furthermore, the
different reactants can affect the rate of a
particular reaction in different ways. For example, when NO reacts with oxygen:
2NO(g)
+ O2(g) → 2NO2(g)
The reaction rate
doubles when the oxygen concentration
doubles. But doubling the concentration of NO quadruples the rate of
reaction.
Temperature: As the temperature is
raised, rate of reaction increases. As the temperature increase, the velocity
of molecules increases and so there is an increase in the collision frequency.
Due to an increase in temperature, kinetic energy of
each molecule increase. It has been found that by raising the temperature by
10K, the fraction of molecules possessing the threshold or activation energy
becomes double. As a result, number of effective collisions is also double and
therefore, rate is doubled.
Light: Photosynthesis
and photography both involve light-sensitive reaction. The leave of plants
contain a green pigment called chlorophyll. This can absorb radiation in the
visible region of the electromagnetic spectrum and use this energy to
synthesize chemicals and provide food for plant.
During photosynthesis,
plants transform carbon dioxide and water into oxygen and sugars such as
glucose:
6CO2(g) + 6H2O(1)
hv→ C6 H12
O6 (aq) + 6O2(g)
∆H = 2820 KJ
In the absence of
sunlight, energy is no longer provided and photosynthesis ceases.
White silver chloride
turns purple and finally dark grey when it is exposed to sunlight. Sunlight
provides the energy required to decomposed silver chloride.
AgCl(s) →
Ag(s) + 1/2 Cl2(g)
The use of silver salts
in photography depends on photosensitivity of this kind.
The reactions of
halogens with hydrogen and with alkanes and further examples of photochemical
reaction. Thus, chlorine reacts slowly with hydrogen or methane in diffused day
light. When the reaction is exposed to intense ultraviolet radiation it becomes
explosive. The effect of sunlight is believed to result from its ability to
split chlorine molecules into highly single atoms. These are known as radicals.
They contain an
unpaired electron.
Catalyst:
Catalysts are the substances which alter the rate of chemical reactions without
undergoing any overall chemical change themselves.
Positive Catalyst:
It increases the rate of reaction. It provides an alternate path to reaction.
The activation energy of this alternative path is lower, so more reactant
molecules possess the energy required for a successful collision. The number of
effective collision per unit time increases and so rate of reaction increase.
Inhibitor:
It decreases the rate of reaction. Negative catalyst do not lower the energy of
activation but they combine with reactant molecules, thus decreasing the number
of colliding reactant molecules. This decrease the effective collisions and so
rate lower.
Consider the reactant;
2H2O2 → 2H2 + 2O2
In the above reaction
of decomposition of H2O2 MnO2 is positive
catalyst and glycerine acts as negative catalyst.
Autocatalyst: In
some of the reactions, a product formed act as catalyst. This phenomenon is
known as auto-catalysis.
For example, the
reaction of oxalic acid with acidified KMnO4 is slow at the
beginning but after sometime MnSO4 produced in the reaction makes it
faster.
Hydrolysis of
ethylacetate generates acetic acid, which acts as a catalyst for further
reaction.
Enzyme Catalysis: Enzymes
are the complex protein molecules and catalyst the organic reactions in the
living cells. Enzyme are unique in their efficiency and have a high degree of
specificity. For example urease catalysis the hydrolysis of urea only and it
cannot hydrolysis any other amide even methyl urea.
Enzyme catalytic
reactions have the maximum rates at an optimum temperature.
The pH of the medium
also controls the rates of enzyme catalyzed reactions and rate passes through a
maximum at a particular pH, known as an optimum pH. The catalytic activity of
enzymes is greatly enhanced by thr presence of a co-enzyme or activator.
Instantaneous and
Average Rate: The rate of reaction between two specific time intervals is known
as average rate of reaction. The rateat any one instant during the interval is
called the instanyaneous rate.
Unit of Rate of
Reaction: It is expressed in mole dm-3, sec-1. In the
case of gaseous reactant, the may be expressed in unit of atm. Sec-1.
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