Experiment 8 (chemical kinetics) Flashcards
(14 cards)
Rate (from A + 2B → 3C)
-Rate = -d[A]/dt = -½d[B]/dt
What affects rate (ΔT) of reaction
-1. concentration of the reactants
-2. rate constant (K), which is affected by temperature
-3. the presence of a catalyst can change Ea
Rate Law
-Rate = k[A]^x[B]^y
x and y of Rate Law
-X and y are the order of the reaction with respect to reactants A and B, respectively
—Usually small positive integers or half integers (zero is also an integer)
—X and y are not necessarily the coefficient of the reaction equation (must be obtained experimentally)
—Order of a reaction depends on the mechanism of the reaction
K of Rate Law
-the rate constant in the rate law (depends on the temperature and the presence of a catalyst)
Ea (energy of activation)
-the minimum amount of energy that must be available to reactants for a chemical reaction to occur
-the energy hump that must be overcome to go from reactants to products
-K = Ae^-Ea/RT
-Increasing temperature always increases the rate of a reaction
-When Ea decreases, reaction rate increases
Arrhenius equation
-K = Ae^(-Ea/RT)
-A = Arrhenius frequency factor
-e = 2.718
Determining Rate Law through Initial Rate Method
-For a reaction: aA + bB → cC + dD (Rate = k[A]x[B]y)
-The value of x is determined by investigating how the change of [A] (while [B] is kept at constant) affects the initial rate of the reaction
-The value of y is determined by investigating how the change of [B] (while [A] is kept at constant) affects the initial rate of the reaction
-K = Rate / ([A]^x[B]^y)
-once we know the orders of the reaction (x and y), K can be calculated from the measured rate of reaction
determining rate constants k1 and k2 at temperatures T1 and T2
-k1 / k2= e^ ((Ea/R)(1/T2 - 1/T1)
-Solve this equation to obtain Ea
-A can be determined from knowing Ea and using either equation 1 or equation 2
lnk vs 1/T
-Plotting lnk vs 1/T yields a straight line with slope -EaR and intercept lnA
-lnk = lnA -Ea/RT
Reaction kinetics of persulfate and iodide
-target reaction: S2O8(2-) + 3I- → 2SO4(2-) + I3(-)
-Rate = k[S2O8]x[I-]y
-Target reaction is reversible, but we can manipulate the reactions conditions by adding starch and thiosulfate (S2O3)
Clock reaction (iodine)
-2S2O3(2-) + I3(-) → S4O6(2-) + 3I-
-a chemical reaction where a color change or other observable property change occurs after a predictable time delay, often due to a slow reaction that sets the “clock”
-Also known as sweeper reaction (sweeps up the triiodide to prevent it from reacting and doing a reverse reaction
-Thiosfulate (2S2O3(2-)) reacts with I3(-), removing the product of our target reaction
-Signals an end point by turning blue (signals that all the S2O3(2-) in the solution has been consumed and half as much S2O8(2-) has reacted
-If reaction time is measured, the initial rate of the reaction can be obtained
-S2O3 acts as a reducing agent, and when amount I3- (produced by target reaction) exceeds S2O3, it reacts with starch to turn blue
Rate of target reaction
-Rate of target reaction = 1/2[S2O3(2-)] added / Δt
-rate constant K = Rate of any run / [S2O8]^x[I-]^y
Rate of clock reaction
-Rate = [S2O8(2-)] used up / Δt = [I3-] produced then consumed / Δt = 1/2[S2O38(2-)] initially added / Δt
-Rate of reaction is concentration of S2O8 over Δt
-The concentrations in the above equations are the initial concentrations of each species in the reaction mixture (not the concentrations labeled on the bottles)
-You can determine different values (x, y, k) by comparing trials where one is held constant while the other is a variable and putting them over each other to cancel out (order y = 2^y)
