Advanced Projects in Chemistry – Chemical Kinetics
The rate of the reaction can be studied for a reaction system involving potassium permanganate and oxalic acid .The chemistry that involves study of reaction rates and the sequence of steps by which reactions occur is called Chemical Kinetics.
Chemical reactions occur at different rates-
a. Very fast or instantaneous reactions –These are reactions that involving ionic species. They occur as soon as reactants are mixed together. These reactions are complete in
10-14 to 10-16 secs.
Precipitation and neutralization reactions occur very fast.
b. Very Slow reactions- Certain reactions take months for completion. It is difficult to study the chemical kinetics of such reactions.
c. Moderate Reactions- These reactions take place at moderate and measurable rates at room temperature. These reactions are molecular in nature.
Reactions that are moderate include decomposition of hydrogen peroxide, hydrolysis of an ester, reaction between acidified potassium permanganate and oxalic acid. The average rate of a reaction is defined as the change in concentration of either reactants or products per unit time. Molecularity of a reaction is the number of reactant molecules taking part in a single step of the reaction.
The minimum number of reacting particles (molecules/atoms/ions) that come together or collide in the rate determining step to form product or products is called molecularity of the reaction. It is a theoretical concept and its value cannot exceed three as not more than three molecules can come together to mutually collide with one another. Consider the following reaction –
2MnO4-+ 16H+ +5C2O42—-> 2Mn2+ + 10CO2 + 8H2O
The molecularity of the above reaction may appear to be greater than 5 however, the reaction is not a simple one step reaction.
It involves a number of steps –
2KMnO4 + 3H2SO4 —-> K2SO4 + 2MnSO4 + 3H2O + 5(O)
H2C2O4 + (O)—.> H2O + 2CO2
The first step is the slow step and the rate determining step while the second step is the fast step. Order of a reaction- is the sum of powers of molar concentrations of the reacting species in the rate equation of the reaction. The order of the reaction for the reaction between oxalic acid and potassium permanganate can be experimentally determined using either of the following methods described.
Graphical Method- The order of a reaction can be determined using the graphical method. If the plot of log (a-x) vs t is a straight line the reaction follows first order kinetics.
If the plot of 1/(a-x) vs ‘t’ is a straight line, the reaction follows second order kinetics.
If the plot of 1/(a-x)2 vs ‘t ‘ is a straight line the reaction follows third order kinetics.
In general the graph of 1/(a-x)n-1 vs t must be a straight line
Rate vs Concentration graphs
Zero order- rate vs conc is a straight line parallel to x-axis
First order- rate vs conc is a straight line passing through the origin.
Second order- The plot of rate vs (conc)2 is a straight line passing through the origin
Third order- The plot of Rate vs (conc)3 is a straight line passing through the origin
Vant Hoff Differential Method-
The rate of a reaction varies as the nth power of concentration of the reactant where ‘n ‘ is the order of the reaction. Thus for two different initial concentrations C1 and C2 equations can be written in the form –
-d(C1)/dt = k(C1)n
-d(C2)/dt = k(C2)n
log(-dC1/dt)= log k + n log C1
log (dC2/dt) = log k +n log C2
n= log (dC2/dt)-log (dC1/dt)/log C1-log C2
Thus the second order of bimolecular reaction between potassium permanganate and oxalic acid was confirmed by both Vant Hoff Differential method as well as Graphical Method. The reaction is second order with respect to both oxalic acid as well as acidified potassium permanganate.