Kinetics Flashcards
What are chemical kinetics?
how fast a chemical reaction takes place/the rate of a chemical reaction
What are the qualities of burning gasoline?
Exothermic
Exergonic
fast
What are the qualities of Diamond → graphite?
Exergonic
Spontaneous
Very slow
Do the thermodynamics have anything to do with how fast a reaction goes?
no
What is the collision theory?
A+B→C
What are the 3 criteria for a reaction to occur (collision theory)
collision: Molecules must collide/interact with each other
energy: Molecules must reach a minimum threshold of energy for reaction
orientation: Molecules have to collide with the right orientation/location
What is activation energy/reaction barrier?
the amount of energy needed to have a reaction occur
Is it easier to get a reaction to happen if the activation energy is lower or higher?
lower
Activation energy controls…
the rate of a reaction
Is activation energy inversely or directly proportional to rate?
inversely
What is the rate when activation energy is large?
smaller
What is the rate when activation energy is small?
faster
What is temperature?
the measure of average kinetic energy
What happens to molecules at higher temperatures?
more molecules have enough energy to overcome the activation energy
What happens to molecules at lower temperatures?
fewer molecules have enough energy to overcome the activation energy
What is the rate at higher temperatures?
faster
What is the rate at lower tempertatures?
slower
What happens to the rate of a reaction when temperature increases by 10 degrees Celsius?
rate of reaction is doubled
What does the activation energy determine?
how fast a reaction goes
What are the units of the rate of reaction?
speed=miles/hour=𝚫distance/𝚫time
Chemical reactions=𝚫moles/𝚫time=𝚫M/𝚫t
Big M=molarity=moles/L of solution
Biers law: absorbance=epsilon x b x c
A=𝜺bc
What is absorbance directly proportional to?
concentration
How do you measure reaction rate?
A→B
How many B molecules appear over time
How many A molecules disappear over time
Can measure either reactants disappearing or products appearing over time
Why are the beginning of reactions faster?
because of higher concentration (more collisions)
Why are the end of reactions slower?
because concentration is lower (less collisions)
What happens to reaction rate as a reaction proceeds?
reaction rate decreases as reactants are “used up”
Fewer collisions therefore slower rate
What is a tangent line?
hits curve at one point (slope=instantaneous rate)
What is a Tangent line that hits at the very beginning?
initial rate=slope (instantaneous rate)
What is the average rate?
slope=rate=𝚫M/𝚫t
What is the initial rate?
rate at the very first moment of a rate
Instantaneous rate
Fastest rate during the course of a reaction
Calculated by finding the slope of the line tangent to the very first point
You know the concentration of reactants
You know the temperature
Temperature affects rate
You know the volume
You know the moles of reactants
What is instantaneous rate and what is it usually found by?
specific moment found by a tangent line
What is the rate expression A→B?
Rate: - 𝚫[A]/𝚫t = +𝚫[B]/𝚫t
[A]=concentration of A in molarity
For every one A that disappears, one B appears
Disappearing at the same rate B is appearing
What is the rate expression A→2B?
Rate: -𝚫[A]/𝚫t = +½ (𝚫[B]/𝚫t)
As A disappears, B is appearing twice as quickly as A is going away
What is the rate expression 2A+B→3C?
Rate: -½(𝚫[A]/𝚫t) = -1/1(𝚫[B]/𝚫t) = +⅓(𝚫[C]/𝚫t)
What is the generic rate of aA+bB→cC+dD?
-1/a(𝚫[A]/𝚫t) = -a/b(𝚫[B]/𝚫t) = +1/c(𝚫[C)/𝚫t) = +1/d(𝚫[D]/𝚫t)
What are the factors that affect the rate?
state of the reactions (rate constant)
concentration of reactants (rate law)
temperature (rate constant)
catalysts
How does the state of the reactions affect the rate?
Liquids, solutions, gases go much faster than solids because they can freely mix
Solids are slow
How does concentration of reactants affect the rate?
Larger concentrations have faster rates
Smaller concentrations have slower rates
To make a reaction slower, take more out
To make a reaction faster, put more in
How does temperature affect the rate?
As temperatures go up, reaction rates go up
As temperature decreases, rate decreases
What are catalysts and how do they affect the rate?
Compounds added to a reaction that increase the reaction rate
What is the rate constant, k?
Proportionality constant that appears in the rate law (relates rate and concentration) and the Arrhenius equation (relates rate to activation energy)
Is k directly or inversely proportional to rate?
directly
Often used to indicate how fast or slow a reaction is
Large values of k=faster rates
Smaller values of k=slower rates
Is k constant at constant temperature? Does k change when temperatures changes?
yes, yes
Is temperature inversely or directly proportional to k?
directly
Do the units of k change? What are the units usually?
yes, s-1, m-1s-1, m-2s-1
What is rate law?
Experimentally determined equation that shows how the reactant concentration affects the rate of a reaction
Includes information regarding the reaction order for each reactant (exponents)
Includes the rate constant
Includes the initial rate
What is the rate law equation?
Rate = k[A]x[B]y
R is Initial rate (instantaneous rate)
X and y = reaction order
What is reaction order?
Experimentally determined exponents. Shows how the concentration affects the rate
Number of molecules involved in the slowest step of a reaction
What is the reaction order when the rate depends on one molecule?
the rate is directly dependent on the concentration of that molecule, you would have a reaction order of 1: rate 𝛂[A]1
If the concentration of a reaction order of 1 doubles, what happens to the rate?
it doubles
What is the reaction order when the rate depends on two molecules?
the rate is proportional to the concentration of that molecule squared: rate 𝛂[A]2
What happens to the rate if the concentration of a reaction order of 2 doubles?
it quadruples
Can reaction order be deduced from the coefficients of a reaction?
no
Are there both individual reaction orders for each reactant and an overall reaction order for the reaction?
yes
How is the individual reaction order for each reactant found?
found by adding together all of the individual reaction orders
What is the overall order of A+B→C?
1st order in A, first order in B
If one molecule of A reacts with a molecule of B, it makes C and is second order overall
1+1=2nd order overall
What are the characteristics of 1st order reactions?
Rate=k[A]1
exponent=1
A→B
Rate is directly proportional to concentration
If the concentration of A doubles, the rate doubles
If the concentration of A triples, the rate triples
What are the characteristics of 2nd order reactions?
exponent=2
rate=k[A]^2
A + A → C
Rate is proportional to concentration^2
If the concentration of A doubles, the rate quadruples
If the concentration of A triples, the rate goes up by a factor of 9
rate=k[A]^1[B]^1
A + B → C
What are zero-order reactions?
occasionally seen for individual reaction orders
oxponent=0
rate=k[A]0
[A]0=1
rate=k
What are ½ order reactants?
rate is dependent on the square root of the concentration
rate=k[A]^1/2
What are negative reaction orders?
rate slows when concentration increases, usually due to the presence of a product in the overall reaction
rate=k[a]^-1
A and B are in equilibrium
What is the method of initial rates?
Method for experimentally determining the individual reaction orders for each reactant
What does the method of initial rates compare?
how the rate changes between experiments as reactant concentrations are changed
What is the most common method of initial rates?
change concentration by factor of 2 and observe how the reaction rate changes
What is the minimum number of experiments on the method of initial rates?
The minimum number of experiments is n+1, where n=number of reactants
For two reactants, the minimum number of experiments is three
Why is the reaction order with respect to a given reaction not always the same as the stoichiometric coefficient in a chemical equation?
Reactions happen in multiple steps, and rate law only figures out the reaction orders for one step (you can’t see all of the individual steps in rate law)
How does collision theory match with the experimental rate law?
A+B→C
rate=k[A]x[B]y
[ ] are concentrations
K is temperature, activation energy, probability of collision, orientation
Why don’t the coefficients of a reaction match the reaction orders?
Reactions don’t happen straightforward, so you don’t see all of the individual reactions
A+B→C (rate=k1[A]1 fast) matches coefficients
May occur in two steps, but it may happen such that A decomposes right away
A→D
D+B→C (intermediate) forms during the reaction and gets used up during the reaction (rate=k2[D]1[B]1 slow)
K values are not the same, can be slower or faster
Ds cancel each other out
Final reaction is A+B→C
To get C you need A and B
The rate of reaction does not depend on either one of them
Each step has its own rate law
What is a mechanism?
steps a reaction goes through on the path from reactants to products
What are elementary steps?
Most reactions occur in small, incremental steps
Usually involve 1 or 2 molecules, rarely 3, almost never 4
Does each elementary step have its own rate law?
yes
Do reactant coefficients match the reaction order for elementary steps?
yes
What is the rate determining step?
the slowest step in any reaction or process
When you measure rate or determine a rate law you are only measuring the rate-determining step
What is the intermediate step?
reactant that gets used up and is therefore not in the overall reaction
What is molecularity?
how many molecules are involved in each elementary step
What does unimolecular, bimolecular, and termolecular mean?
1 molecule, 2 molecules, 3 molecules
What do integrated rate laws allow us to do?
to answer questions regarding how much of a reactant has been used after a given amount of time
Do reaction concentrations and rates change constantly throughout a reaction?
yes
After the initial start, do concentrations increase or decrease, do rates increase or decrease?
concentrations decrease, rates decrease
Do rate laws take into account how the concentrations and rates change over time? What do we have to do?
no, we have the integrate the rate laws over time to do this
Do reactions with different overall reaction orders have different rate laws and different integrated rate laws?
yes
What is the integrated rate law equation?
[A]sub t - [A]sub 0 = -kt
What is [A]sub t?
concentration in molarity at any given time
What is [A] sub 0?
Initial concentration of A in molarity (t=0)
How is rate written?
change of concentration over the change of time
𝚫[A]/𝚫t = k
What is a half-life?
the amount of time it takes for the concentration of a reactant to reach half of its original value
t sub 1/2
[A]sub½ = ½ [A]sub0
What is a half-life first order example?
ln([A]subt1/2/[A]sub0) = -ktsub1/2
ln( ½ [A]sub0 / [A]sub0) = -ktsub1/2
ln1/2 = -ktsub1/2 (Asub0 canceled out)
-.693=-ktsub1/2
tsub1/2 = .693/k
Is tsub1/2 independent or dependent on concentration in first order?
independent
What is a half-life 2nd order example?
[A]sub½ = ½ [A]sub0
1/[A]subt1/2 - 1/[A]sub0 = ktsub1/2
1/½[A]sub0 - 1/[A]sub0 = ktsub1/2
2/[A]sub0 - 1/[A]sub0 = ktsub1/2
1/[A]sub0 = ktsub1/2
tsub1/2 = 1/k[A]sub0
Is tsub1/2 independent or dependent on concentration in a 2nd order reaction?
dependent
As [A]sub0 decreases, tsub1/2 increases
