PREPARED BY BURAK COBAN
In this experiment we will study the rate of decomposition of hydrogen peroxide to form oxygen according to the net equation:
2H2O2 (aq) 2H2O(l) + O2
by measuring the rate at which oxygen evolved, we will investigate how the rate changes with varying initial concentrations of hydrogen peroxide and iodide catalyst. After we will study the affect of changing its concentration the rate oxygen evolution. At the end of experiment we will summarize our results by attempting to write a rate law for the reaction, showing the defences on the concentrations of H2O2 and I.
Chemical reactions can be fast (think of any explosion ) or slow . It is very important to understand what is affecting the rate of the reaction and what is the mechanism of the reaction with such knowledge, we can often control a reaction to proceed at just the speed we need. we can thus avoid an explosion or speed up a reaction that seems too slow.
In this chapter we start out by discussing rates of reactions and the rate law. The rate law indicates the affect that the concentration of the reactants has on the reaction rate. In general, adding more of a reactant speeds things up (rather like pushing the gas pedal to put more gas into the car engine ). But how much faster is the reaction if say the concentration of a reactant is doubled ? the rate law will help us answer such equations.
Another way to affect the rate of a reaction is to change the temperature. We refrigerate food to slow the rate of bacterial metabolism that can cause food to spoil. If we want food to cook faster, we increase the heat. We also look at why some reactions seem to need a “kick start,” although once started, they continue to react. We will examine the activation barrier to reactions and its implications.
Rate laws and kinetics experiments also tell us a lot about the mechanism by which a reaction occurs. Understanding the mechanism gives us another way to control the reaction. This knowledge enables us to design better catalyst or to create new compounds (such as new pharmaceuticals) that will be more effective.
Differential Rate Laws:
In many reactions, the rate of reactions changes as the reaction progresses. Initially the rate of reaction is relatively large, while at very long times the rate of reaction decreases to zero. In order to characterize the kinetic behaviour of a reaction, it is desirable to determine how the rate of reaction varies as the reaction progresses.
A rate law is a mathematical equation that describes the progress of the reaction. There are two forms of a rate law for chemical kinetics: the differential rate law and the integrated rate law.
The differential rate law relates the rate of the reaction to the concentrations of the various species in the system.
Differential rate laws can take on many different forms, especially for complicated chemical reaction. However, most chemical reactions obey one of three differential rate laws. Each rate law contains a constant, k, called rate constant. The units for the rate constant depend upon the rate law, because the rate always has units of mole L-1 sec-1 and the concentration always has units of mole L-1.
Zero – Order Reaction:
For a zero order reaction, the rate of reaction is a constant. When the limiting reactant is completely consumed, the reaction abrupt stops.
Differential rate law: R=k
The rate constant., k , has units of mole L-1 sec-1.
First - Order Reaction:
For first order reaction, the rate of reaction is directly proportional to the concentration of ane of the reactants.
Differential rate law: R = k[A]
The rate constant, k, has units of sec-1.
Second – Order Reaction:
For a second...