CHEM 112 - MIDTERM II Flashcards
Reaction with a faster rate will have more _____.
products formed
Rate expression
rate = -1/a [A]/ t = -1/b [B]/ t =- 1/c [C]/ t
Average rate vs. instantaneous rates
Average rate: rate of the reaction over a certain period of time (takes the two time points from initial to final to find the change in concentration)
Instantaneous rate: rate of the reaction at a given time (ie. t = 2 s)
The rate of the reaction is ____ to the concentration of the reactants
proportional
The rate constant depends on ____ & ____.
Temperature & reaction specific properties
rate law
rate = k [A]^ x [B]^y
where x & y are integers that give the order of the reaction w/ respect to each reactant
Zero order
No dependence on the concentration ([A]0 = 1); change in concentration does not equal change in rate
First order
The rate of the reaction is directly proportional to the reactant concentration ([A]^1 = [A])
Second order
The rate of the reaction is directly proportional to the square of the reactant concentration ([A]^2 = [A]^2)
Integrated rate law
Describes the relationship between the concentrations of the reactants and time (depends on the order of the reaction)
Zero order Integrated Rate Law
[At] = -kt + [A]0
First order IR Law
ln [A]t/[A]0 = -kt
Second order IR Law
1/[A]t = kt + 1/[A]0
Slope for zero order IR
-k
Slope for first order IR
-k
Slope for second order IR
+k
Half-life
Time required for the concentration of a reactant to fall to 1/2 of its initial value
Find half-life on graph
see y-intercept and divide by 1/2 - the x-intercept (time) = half life
Zero order half life
t 1/2 = 1/k x [A]0/2
First order half life
t 1/2 = 1/k * 0.693
Second order half life
t 1/2 = 1/k x 1/[A]0
Factors that affect reaction rates
1) Concentrations
2) Temperature
3) Catalysts
Catalysts ____ the activation energy by providing a ____ energy path from reactants to products
lower; alternate, lower energy path
Arrhenius Equation
Relates the rate constant (k) and the temperature in kelvins
k = A x e^ -Ea/RT
The exponential factor of the Arrhenius equation ____ with increase in temperature
increases
The exponential factor of the Arrhenius equation ____ with an increase in activation energy
decreases
The higher the Ea, the _____ the reaction rate at a given temperature
slower
Hrxn < 0, the reaction is ___
exothermic
Hrxn > 0, the reaction is ___
endothermic
Increase in temp in the Arrhenius equation also results in ___
increase in rate constant (k)
The frequency factor in the Arrhenius equation (A) is smaller when ____
the molecules do no not collide in the right way, resulting in a slower rate
How would the rate of a reaction change if a catalyst was added (decreasing activation energy) and everything else was the same?
Reaction rate increases
How would the rate of a reaction change if a catalyst was added (decreasing activation energy) and everything else was the same?
Reaction rate increases
Elementary step
Process that occurs in a single step
In multi-step reactions, the elementary steps ___
must add up to give overall equation
Two reactant molecules participate in each step, so they are both _____
bimolecular
Each ___ on the graph is associated with the _____.
peak; activation energy
The number of valleys on the graph is equal to ______ the number of steps in the mechanism
one less than
2 valleys = ___ reaction steps
3
The barrier with the ____ activation energy is the _____.
largest; rate-determining step
The number of peaks tells you the ______.
number of transition states
The difference in the energy of the products and reactions tells us whether the reaction ____ or ___ energy
emits; absorbs
Slow step = ____
higher activation energy
Reaction mechanism must have these two conditions met:
1) elementary steps in the mechanism must sum to the overall reaction
2) rate law predicted by the mechanism must be consistent with observed rate law
Catalyst
Changes the speed of a chemical reaction without undergoing a permanent chemical change itself
Catalyzed vs. Uncatalyzed graphically
catalyzed (lower Ea)
If you multiply a reaction by a certain number (n), ____ the original Kc you multiplied by (n).
raise the original Kc to the n power
If you add two reactions together, ____ the two K together to get the new K.
multiply
When the equation for the reversible rxn is written in the opposite direction, the new Kc becomes ____.
a reciprocal (1/Kc)
K > 1 @ equilibrium, then ___
[products] > [reactants]
K is very large (10^10), then ___
reaction goes to completion (favors forward rxn)
K is very large (10^-10), then ___
reaction does not occur (no product is formed)
Kp & Kc relationship?
Kp = Kc (RT)^n
where n = products - reactant coefficients [consider stoichiometry]
