Lab 2 Flashcards
what is kinetics
the study of rates and mechanisms of chemical reactions
rate of a chemical reactions describes what
how fast a rxn occurs
factors such as concentration of _____ affect rates of rxn
- conc of reactants
- temperature
- presence or absence of a catalyst
mechanisms
- rxn pathways
- describe the series of elementary steps a given rxn takes going from reactants to products
What rxn is given as an example
- initial rate of rxn in terms of change in conc of reactant B
2A + B = C
-d[B]/2dt
What might the reaction form
- intermediates
- which react with each other or react with more reactant to form the product
- contributes to overall rate of rxn and may be rate-controlling
rate-limiting step
A rate-limiting step is the slowest step in a biochemical pathway or process, determining the overall speed of the entire process.
- addition of more reactant has little to no effect
objective of chemical kinetics
determining rate law
rate law
statement about how the rate of a reaction depends on the concentration of a participating species
- determining by analyzing 1+ reactant or product as a function of time as reaction proceeds
general form of rate law
rate = k[A]^x[B]^y
- [A] and [B] represent reactant concentrations
- exponents x and y are the order of the reaction with respect to each reactant
- k is the rate constant for that specific reactions at a given temperature
Overall Order: sum of exponents
how are the variables in the rate law (gen. form) determined
- experimentally and are a reflection of the reaction mechanism, not
exponents
generally are 0,1,2
no effect= 0
doubling of conc causes doubling of rate = 1
doubling of conc causes quadruple of rate = 2
decomposition kinetic of iodide-catalyzed decomposition of H2O2
H2O2 (aq) + I- (aq) = IO- (aq) + H2O (l)
+
H2O2 (aq) + IO- (aq) = I- (aq) + H2O (l) + O2 (g)
2 H2O2 (aq) = O2 (g) + 2 H2O (l)
measure of initial rates
- what we will be using to determine the rate of rxn
- used to measure initial rate of O2 (g) produced in rxn by monitoring pressure changed in a sealed vessel over time for several concentrations of reactants: H2O2 and I- (from KI)
equation:
initial rate = d[H2O2]/2dt = k[H2O2] ^x[I-]^y
- if [I-] is constant, then plot of ln(rate) versus ln[H2O2] has a slope equal to order of reaction (x) and y-intercept equal to ln(k).
- you can also hypothesize the order
Hypothesizing order
I- conc is fixed
H2O2 conc is varied
BASED ON H2O2:
if x=1, plot is linear (H2O2 VS H2O2)
if x=2, plot is linear (H2O2 VS H2O2^2)
= only 1 will be linear
BASED ON I-:
if y=1, plot is linear (H2O2 VS I-)
if y=2, plot is linear (H2O2 VS I-^2)
= only 1 will be linear, slope=k
Rate law for this experiment
k[H2O2]^x[I^-]^y
closer the coefficient is to 1
The more linear it is
