reaction kinetics
TRANSCRIPT
CONTENTS:
KINETICS ORDER OF REACTION FACTORS
INFLUENCING RATE OF REACTION (PHYSICAL & CHEMICAL)
COMPLEXATION
KINETICS Kinetics is the study of the
rate of reactions, or how fast they occur under different conditions.
It usually includes a study of the mechanisms of reactions, which is a look at how the reacting molecules break apart and then form the new molecules.
This knowledge allows chemists to control reactions and/or design new or better ways to produce the desired products.
ORDER OF REACTION: In chemical kinetics, the order of reaction with respect
to certain reactant is defined as the index, or exponent, to which itsconcentration term in the rate equation is raised.
For example, given a chemical reaction with a rate equation,
2A + B → Cr = k[A]2[B]1
The reaction order with respect to A in this case is 2 and with respect to B in this case is 1; the overall reaction order is 2 + 1 = 3.
It is not necessary that the order of a reaction be a whole number – zero and fractional values of order are possible – but they tend to be integers.
First-Order Reactions A first-order reaction is a reaction that
proceeds at a rate that depends linearly only on one reactant concentration.
Rate law:
Rate is the reaction rate and k is the reaction rate coefficient.
In first order reactions, the units of k are 1/s. However, the units can vary with other order
reactions.
Graphing First-order Reactions
The following graph represents concentration of reactants versus time for a first-order reaction.
Pseudo-First-Order Reactions
In a pseudo-1st-order reaction, we can manipulate the initial concentrations of the reactants. One of the reactants, A, for example, would have a significantly high concentration, while the other reactant, B, would have a significantly low concentration. We can then assume that reactant A concentration effectively remains constant during the reaction because its consumption is so small that the change in concentration becomes negligible. Because of this assumption, we can multiply the reaction rate, k′, with the reactant with assumed constant concentration, A, to create a new rate constant (k=k′[A]) that will be used in the new rate equation,
Rate=k′[B]as the new rate constant so we can treat the 2nd order reaction as a 1st order
reaction. One way to create a pseudo-1st-order reaction is to manipulate the
physical amounts of the reactants
Graphing Pseudo-First-Order Reactions
Second-Order Reactions In a second-order reaction, the sum of the exponents in the rate
law is equal to two. The two most common forms of second-order reactions are
following: Case 1: Two of the same reactant (A) combine in a single
elementary step.
The reaction rate for this step can be written as
where k is a second order rate constant with units of M-1min-1 or M-1s-1.
Second-Order ReactionsCase 2: Two different reactants (A and B)
combine in a single elementary step. The reaction rate for this step can be
written as where the reaction order with respect to
each reactant is 1.
Graphing Second-Order Reactions
For a second-order reaction,
the rate of reaction increases with the square of the concentration,
producing an upward curving line in the rate-concentration plot.
Zero-Order Reactions A zero-order reaction is a reaction that proceeds at
a rate that is independent of reactant concentration.
In other words, increasing or decreasing the concentration of reactants will not speed up or slow down the reaction, respectively.
This means that the rate of the reaction is equal to the rate constant, k, of that reaction.
Graphing Zero-Order Reactions
If we plot rate as a function of time, we obtain the graph below.
this only describes a narrow region of time.
The slope of the graph is equal to k, the rate constant.
Therefore, k is constant with time. In addition, we can see that the reaction rate is completely independent of how much reactant you put in.
Rate vs. time of a zero-order reaction.
The rate constant, k, has units of mole L-1 sec-1.
FACTORS INFLUENCING RATE OF REACTION
Physical FactorsTemperatureIonic strength PhU.V lightAcid-Base catalysis
Chemical FactorsHydrolysisOxidation-Reduction Reaction
Temperature Endothermic
Reaction: If we increase the temperature
in the endothermic reaction the rate of reaction will increase and vise versa.
Exothermic Reaction: If we increase the temperature
in the exothermic reaction the rate of reaction will decrease and vise versa.
Ionic strength The De-bye Huckel equation may be used to
demonstrate that increased ionic strength would be expected to decrease the rate of reaction between oppositely charged ions and increase the rate of reaction between similarly charged ions .
Thus, the hydrogen ions catalyzed hydrolysis of sulphate esters is inhibited by increasing electrolyte concentration.
ROSO3 + H2O ROH + HSO4-
PH Enzymes are affected by changes
in pH. The most favorable pH value - the
point where the enzyme is most active - is known as the optimum pH.
Extremely high or low pH values generally result in complete loss of activity for most enzymes.
pH is also a factor in the stability of enzymes. As with activity, for each enzyme there is also a region of pH optimal stability.
Enzyme pH Optimum
Lipase (pancreas) 8.0
Lipase (stomach) 4.0 - 5.0
Lipase (castor oil) 4.7
Pepsin 1.5 - 1.6
Trypsin 7.8 - 8.7
Urease 7.0
Invertase 4.5
Maltase 6.1 - 6.8
Amylase (pancreas)
6.7 - 7.0
Amylase (malt) 4.6 - 5.2
Catalase 7.0
U.V Light Light energy may be aborbed by certain molecules
which then sufficiently activated for participation in a reaction.
Only frequencies in the visible and ultra-violet region can provide sufficint energy to cause photochemical reaction.
Since the for enery for activation is provided by light in these reaction the rates of the latter are independent of temperature.
Photochemical reaction involve the absorption of definite wavelength
Acid-Base Catalysis In acid catalysis and base catalysis a
chemical reaction is catalyzed by an acid or a base. The acid is often the proton and the base is often a hydroxyl ion. Typical reactions catalyzed by proton transfer are esterfications and aldol reactions. In these reactions the conjugate acid of the carbonyl group is a better electrophile than the neutral carbonyl group itself. Catalysis by either acid or base can occur in two different ways: specific catalysis and general catalysis.
Specific catalysis: In specific acid catalysis taking place in solvent S, the reaction rate is
proportional to the concentration of the protonated solvent molecules SH+.The acid catalyst itself (AH) only contributes to the rate acceleration by shifting the chemical equilibrium between solvent S and AH in favor of the SH+ species.
S + AH → SH+ + A-
For example in an aqueous buffer solution the reaction rate for reactants R depends on the pH of the system but not on the concentrations of different acids.
This type of chemical kinetics is observed when reactant R1 is in a fast
equilibrium with its conjugate acid R1H+ which proceeds to react slowly with R2 to the reaction product; for example, in the acid catalysed aldol reaction.
General catalysis: In general acid catalysis all species capable of
donating protons contribute to reaction rate acceleration.[2] The strongest acids are most effective. Reactions in which proton transfer is rate-determining exhibit general acid catalysis, for example diazonium coupling reactions.
When keeping the pH at a constant level but changing the buffer concentration a change in rate signals a general acid catalysis. A constant rate is evidence for a specific acid catalyst.
Hydrolysis Hydrolysis is a reaction involving the
breaking of a bond in a molecule using water. The reaction mainly occurs between an ion and water molecules and often changes the pH of a solution. In chemistry, there are three main types of hydrolysis: salt hydrolysis, acid hydrolysis, and base hydrolysis.
Oxidation-Reduction Reaction
An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron. Redox reactions are common and vital to some of the basic functions of life, including photosynthesis, respiration, combustion, and corrosion or rusting.
COMPLEXATION A chemical reaction that takes
place between a metal ion and a molecular or ionic entity known as a ligand that contains at least one atom with an unshared pair of electrons.
This is a sample reaction coordinate of a complex reaction.
Note that it involves an intermediate and multiple transition states.
A complex reaction can be explained in terms of elementary reactions.
