Chemical Kinetics - Lecture Notes | CHEM 1212, Study notes of Chemistry

Download Chemical Kinetics - Lecture Notes | CHEM 1212 and more Study notes Chemistry in PDF only on Docsity! RF/GCSU Ch 12 Zumdahl 7th Ed.doc Page 1 of 5 CHEM 1212 - Principles of Chemistry II Chapter 12 - Chemical Kinetics  a spontaneous reaction (thermodynamic standpoint) will not necessarily be fast reaction, e.g., the formation of ammonia from nitrogen and hydrogen  the area of chemistry concerned with reaction rates is called chemical kinetics 12.1 Reaction Rates  consider the reaction 2NO2(g) --> 2NO(g) + O2(g)  start with a flask of nitrogen dioxide at 300oC and measure the concentration of nitrogen dioxide, nitric oxide, and oxygen as the nitrogen dioxide decomposes  results summarized in Table 12.1 and plotted in Figure 12.1  chemical kinetic deals with the speed at which changes occur  the speed, or rate, of a process is defined as the change in a given quantity over a specific time  the reaction rate of chemical reaction is defined as the change in concentration of a reactant or product per unit time  rate =) [A]/) t, where A is the reactant or product being considered and the square brackets indicated concentration in mol/L and t is time; the ) indicates change (positive or negative)  can calculate the average rate of decomposition of nitrogen dioxide as a function of time; see Table 12.2  can also calculate and instantaneous rate by computing the slope of line tangent to the curve at that point; see Figure 12.1  the rate of consumption of NO2 = rate of production of NO = 2(rate of production of O2) 12. 2 Rate Laws: An Introduction  chemical reactions are reversible  so far we have only considered the forward reaction, 2NO2(g) --> 2NO(g) + O2(g)  the reverse reaction can also occur as NO and O2 accumulate, they can react to re-form NO2, 2NO(g) + O2(g) – > 2NO2(g)  when gaseous 2NO2 is placed in an otherwise empty container, initially the dominant reaction is 2NO2(g) --> 2NO(g) + O2(g)  in order to measure just the forward reaction rates data are taken just after the reactants are mixed thus the reaction rate will depend only on the reactants  for the decomposition of nitrogen dioxide, we can write, Rate = k[NO2]n  such an expression which shows how the rate depends on the concentration of reactants, is called a rate law RF/GCSU Ch 12 Zumdahl 7th Ed.doc Page 2 of 5  the proportionality constant k, called the rate constant, and n, called the order of the reactant, must both be determined experimentally  note that the concentrations of the products do not appear in the rate law because the reaction rate is being studied under conditions where the reverse reaction does not contribute to the overall reaction Types of Rate Laws  the rate law we have used to this point expresses rate as a function of concentration  a rate law that express how the rate depends on concentration is technically called the differential rate law; simply called the rate law  a second kind of rate law, the integrated rate law, also will be important in our study of kinetics  the integrated rate law expresses how the concentration depends on time  the rate law are related, i.e., that is once we determine experimentally either type of rate law for a reaction, we know the other one  which rate law we choose to determine by experiment depend on what types of data are easiest to collect  a chemist is usually not interested a rate law for its own sake but because of what it reveals about the steps by which a reaction occurs  see text for Rate Laws: A Summary 12.3 Determining the Form of the Rate Law  the first step in understanding how a given chemical reaction occurs is to determine the form of the rate law  need to determine experimentally the power to which each reactant concentration must be raised in the rate law  consider, 2N2O5(soln) --> 4NO2(soln) + O2(g)  see data in text  Rate = k[N2O5]1= k[N2O5], the reaction is first order with respect to N2O5 Method of Initial Rates  the initial rate of a reaction is the instantaneous rate determined just after the reaction begins (just after t = 0)  the idea is to determine the instantaneous rate before the initial concentration of reactants have changed significantly  results are then compared to see how the initial rate depends on the initial concentration  the overall reaction order is the sum of the order of each of the reactants