Determination of rate constant for catalytic decomposition reaction of hydrogen peroxide

1、 Experimental purpose

1. Determine the rate constant of hydrogen peroxide catalytic decomposition reaction;

2. Master the method of tracking reaction system concentration by measuring the volume of the reaction system to study reaction rate.

2、 Experimental principle

The decomposition reaction of hydrogen peroxide proceeds very slowly in the absence of a catalyst. Adding catalysts can increase the decomposition rate. The stoichiometric formula for the decomposition reaction of hydrogen peroxide is as follows:

H2O2(l) = H2O(l) + 1/2O2(g)

IfKIAs a catalyst, inKIThe catalytic decomposition steps under action are:

KI(l) + H2O2(l) = KIO + H2O(l)(Slow)

KIO = KI(l) + 1/2O2(g)

Due to the much slower rate of step * compared to the second step, step * is the controlling step of the reaction. Therefore, it can be assumed that the rate equation of its reaction is:

-dcA/dt =k’cKIcA

In the equation,cATo react in the reaction systemtmomentH2O2Concentration, due toKIDuring the reaction process, the concentration remains constant, so the above equation can be simplified as

- dIcA/dt = kIcA（14.1）

in the formulak=k’cKI,Integrate the separated variables in the above equation:

whent=0When,CA=C0；t=tWhen,CA=C t;

The definite integral formula is:

AACCtAAkdtcdc00 （14.2）

Integral result:

0lnlncktct???（14.3）

The formula istct~lnThe linear equation. During the reaction process, measure different time pointstThe concentration of oxygen in the reaction systemctObtain several groupsctThetAfter obtaining the datalncttotDraw a line to indicate that the reaction is a first-order reaction (quasi first-order reaction), with a slope of-k.

The research method of physical chemistry is to use physical methods to determine the concentration of a certain component in a reaction system. The so-called physical method is to use the characteristics of certain physical properties (such as volume, pressure, electromotive force, refractive index, optical rotation, etc.) of a certain component or each component in the reaction system that have a definite single value function relationship with it, and indirectly measure the concentration change by measuring the change of this physical property in the system. The biggest advantage of this physical chemistry experimental method is that it can track the changes in the physical properties of a certain component or group of components in the system, so that the concentration of a certain component or group of components in the reaction system can be measured at any time without terminating the reaction.

InH2O2During the catalytic decomposition processtmomentH2O2The concentration can be determined by measuring the volume of oxygen released during the corresponding time period. The volume of released oxygen and the decomposed oxygenH2O2The quantity is proportional, and its proportionality coefficient is a constant value. orderV∞ representsH2O2The volume of oxygen released by complete decomposition,V texpressH2O2IntIf the volume of oxygen released during decomposition is constant, then:

co∝V∞，ct∝ （V∞－Vt）

Substitute the above relationship into（14.3）Formula, obtained:

ln（V∞－Vt）= - kt + ln V∞(14.4)

if based onln（V∞－Vt）Yes, that's righttDraw a straight line to verify that it is a first-order reaction; By the slope of the straight linemCan calculate the rate constantk，m = - k.(14.4)forln（V∞－Vt）～-tThe linear equation, whereV∞ isH2O2The volume of oxygen released from complete decomposition, reaction temperature, andKIWhen the concentration is constant, it does not change over time. During the experiment, only the different moments of the reaction need to be measuredttimeH2O2Volume of oxygen released during decompositiont(Several pieces of data)At the end of the reactionH2O2Volume of oxygen released from complete decompositionV∞ (one data), toln（V∞－Vt）Yes, that's righttDraw a straight line with a slope of -kThe reaction rate constant can be calculated using the graphical methodk.

V∞ can also be calculated using the method introduced in "Data Processing" without measurement. Since the data is recorded at equal time intervals, according to Lagrange's mean value theorem,

Thus madeln(dVt/dt)—tThe graph shows a strong linear pattern in the data results, and the slope of the line can be calculatedmThe reaction rate constant can be obtainedk/min-1=－m.