ch 14 chemical kinetics Flashcards
factors that affect reaction rates4
Greater collisions=greater rate of reaction.
Physical state of reactants: more readily molecules collide with each other (gas and liquid), more rapidly they react. Reactions that involve solids proceed faster if the surface area of the solid is increased (powder is faster than a pill).
Concentration of reactants: As concentration increases, frequency with which reactant molecules collide increases, makes the reaction proceed faster.
Temperature at which reaction occurs: as temp increases, molecules move more rapidly, collide more frequently, and rate of reaction increases. (refrigerate milk)
Presence of a catalyst: catalyst=agent that affect the kinds of collisions in a rxn which increases reaction rates without being used up.
rxn rate
speed of a chemical reaction=change in concentration of reactants/products per unit time. Units=M/s.
Main equation: for the reaction aA+bBcC+dD, rate=-1[A]/at=-1[B]/bt=-1[C]/ct=-1[D]/dt when speaking of a rate without specifying a particular reactant or product
rate law
shows how the rate depends on the concentrations of reactants. for the reaction aA+bBcC+dD, rate=k[A]m[B]n. k=rate constant, magnitude changes with T; m/n=reaction orders and small whole numbers
rxn ordes
if exponent of A is 1, rate is first order in A. Overall reaction order: sum of the orders with respect to each reactant in the rate law. If m and n both =1, overall rxn order=2 and reaction is second order overall.
Reaction orders in a rate law indicate how the rate is affected by the concentration of each reactant. If m/n=1, then doubling [A]/[B] would double the rate. If m=2, doubling [A] would quadruple the rate.
first order reaction stuff
rate depends on the concentration of a single reactant raised to the first power.
Differential rate law for reaction A→ products is rate=-[A]/t=k[A]
Integrated rate law: [A]t : ln[A]t= -kt+ln[A]0
Equation^ can also be used to verify whether a reaction is first order and to determine its rate constant. Graph of ln[A]t vs t gives straight line with slope of -k and y int of ln[A]0.
Half life: (ln1/2/[A]0)/[A]0=-kt1/2-> t1/2=-ln1/2/k=0.693/k. In a first order reaction, the concentration, of reactant decreases by ½ in each regularly spaced time interval: t1/2. Does not depend on starting concentration, so remains constant throughout the reaction.
integrated rate law
relates concentration of A at start of reaction [A]0 to its concentration at any other time . this equation can be used to determine concentration of a reactant remaining at any time, or time required for a given fraction of a sample to react, or time required for reactant concentration to fall a certain level.
second order rxn stuff
rate depends on the reactant concentration of raised to the second power, or the concentrations of two different reactants raised to the first power.
Differential rate law: rate=-[A]/t=k[A]^2
Integrated rate law: 1/[A]t=kt+1/[A]0
If a reaction is second order, a plot of 1/[A]t vs t will yield a straight line with slope k and y intercept 1/[A]0
Half life: t1/2=1/k[A]0. Depends on reactant concentrations and therefore changes as the reaction progresses. Lower initial concentration=longer half life
orientation factor
molecules must be oriented in a certain way during collisions in order for a reaction to occur.
activation energy
minimum energy required to initial a chemical reaction. If molecules are moving too slowly, they only bounce off one another without changing. (you need enough energy to make it over the hill, and then it will release energy going down the hill
There is a barrier (top of the hill), where the molecule is relatively unstable between the initial and final product. Ea is the energy difference between the starting molecule and the top of the hill. Activated complex/transition state: arrangement of atoms at the top of the barrier.
The rate of reaction depends on magnitude of Ea. Lower Ea=faster reaction
fraction of molecules that have Ea or higher equation
Ea: f=e^-Ea/RT (R=gas constant 8.314J/mol-K, T=absolute temp.)
arrhenius equation
Arrhenius found that most reaction-rate data obeyed an equation based on three factors: fraction of molecules possessing and energy of Ea or greater, number of collisions occurring per second, and fraction of collisions that have the appropriate orientation→
Arrhenius equation: k=Ae-Ea/RT (k=rate constant, R=8.314J/mol-K, A=frequency factor, constant as temperature changes.)
As A increases, k decreases because the fraction of molecules that possess the required energy is smaller, thus reaction rates decrease as Ea increases.
to determine Ea graphically
To determine Ea graphically: graph k vs 1/T and use equation ln k=-EaRT+ln A. slope= -Ea/R, y int=ln A
to determine Ea if you know k at two temps
To determine Ea if we know rate constant of a reaction at two temperatures: use equation ln k1k2=EaR(1Ts-1T1)
elementary reactions
Elementary reactions/processes: reactions that occur in a single step. Number of reactant molecules in an elementary reaction is the molecularity of the reaction. Single molecule involved=unimolecular. Collision of two reactant molecules=bimolecular. 3 molecules=termolecular.
intermediate
Intermediate: neither reactant nor product of final reaction, formed in one elementary reaction and consumed in next (on both sides of final added equation, should be cancelled out)
