Chapter 12 - Kinetics Flashcards
What are the factors which alter rate of a reaction? Explain how they affect rate.
- Concentration - Influences # of overall collisions
- Temperature - Influences KE of each collision and whether able to overcome activation energy or not + more frequent collisions
- Surface area - Breaking up particles into smaller pieces = more collisions
- Adding a catalyst - Lowers activation energy of slow step, speeding up rate of reaction
- Having fewer bonds/strength of bond (nature of reactants) - Reactants require less energy to break bonds, and are more readily able to react
What is the rate constant (k) dependent on?
temperature
What is a catalyst?
A substance which is added to a reaction, speeding up the slowest step, but is not consumed by the OVERALL reaction
Homogeneous vs. heterogeneous catalysts
Homogeneous - Same phase/state of matter as reactants
Heterogeneous - Different phase/state of matter as reactants (is most often solid)
What is instantaneous rate? How do you determine it?
1.The rate of a point extrapolated at a certain point in time (line that doesn’t match the curve but does match the individual point on the graph)
2. Determined by tangent line being estimated, with any two points on that line used to calculate slope
When is the rate equation multiplied by -1?
When the rate is of reactants (cannot have a negative rate)
From what information is the integer n derived?
Experimental data
Are products ever considered in rates? Why?
No, as that would mean the amount of products has accumulated enough to cause reverse reactions, overcomplicating calculations. An example of negligible reverse reaction is the release of oxygen gas in a solution decomposition. The oxygen will not return to the solution substantially.
What rate law is the “default?”
Differential rate law (depends on concentration)
What is the integrated rate law?
The rate law expressing concentration/time
What is the purpose of the rate law?
Determines the slowest step of a reaction, and thus the speeds of and steps which a reaction takes to occur
Formula for determining order:
(rate2/rate1) = ([x2]/[x1])^n
How do you find [A] of trial (any number) when [B] is (any number) M? (calculations between MULTIPLE reactants)
- Subtract initial [B] value in data table w/new [B] value
- Multiply that number by the coefficient of A
- Subtract initial [A] by value determined in step 2
Adsorption vs. absorption
Equation for rate law (works for initial rate too)
Rate = k [A]^n
Rate = k [A]^n [B]^m
Rate = k [A]^n [B]^m [C]^p
etc.
Half-life equation for a 1st order reaction
t(1/2) = 0.693/k
How do you determine which order a reaction is based on graphs?
The one with the constant slope/straight line
1st order integrated equation
Does an increase in temperature correlate to an increase in total product?
Even though it increases the rate, the actual amount of material to react remains the same.
In what type of reaction is a reactant reacted with itself?
Dimerization
In which direction are forward reactions and reverse reactions read?
Forward: left to right
Reverse: right to left
Activation energies are always _____(lower/higher)
higher, as the activation energy is the point with the most energy required in the reaction
Define an activated complex
A “transition state” grouping of reactants during an elementary step (very unstable)
Which is the “rate-defining” step? The step with the most activation?
The “slow” step
Catalyst vs. intermediate (ex. intermediate = reactant, then product)
Catalyst - Reactant first, then product
Intermediate - Product first, then reactant
Define intermediate/transition state
Temporary chemical formed and consumed during a reaction (think the species at the top of the curve w/the highest potential energy)
Define Reaction Mechanism
The sequence of elementary steps of a chemical reaction
How do we figure out the CORRECT reaction mechanisms?
Research and experimentation of scientists in rate studies. You specifically are not able to do this.
When is order determined by coefficients?
In elementary steps
3 requirements for a reaction:
- Reactants colliding frequently
- Reactants having equal or greater energy than the activation energy to break bonds
- Reactants must collide in the correct orientation for the molecule to form
Why do multiple step reactions exist, instead of all reactions occurring in 1 step?
Because it is incredibly unlikely for a reaction between multiple molecules to occur in one step (ex. 5 m’cules colliding at the exact orientation at the exact same time with enough energy to break the bonds)
Units of K for zero, 1st, 2nd, and 3rd reactions
M^1-n x s^-1
units dependent on OVERALL order of reaction
Are termolecular reactions fast or slow?
Slow
When writing the rate law of a mechanism, do you use the reactant, product, or both concentrations? On which step is the rate law based?
- Reactant
- Slow/rate-determining step
Intermediates ____(can sometimes/cannot) exist in the rate law of a mechanism
cannot
What replaces an intermediate in a rate law?
The reactants that created the intermediate
What types of plots are for zero, 1st, and 2nd order reactions?
0: Time vs. [A]
1: Time vs. ln[A]
2: Time vs 1/[A]
For which reaction order is slope positive?
2nd
What can you change to change the rate constant k?
temperature, changing activation energy, and using a catalyst (more energy to overcome activation energy and lowering activation energy respectively)
What are the 2 requirements for a successful collision?
- Adequate energy
- Correct collision geometry (angle of collision)
Does temperature increase collision frequency?
yes
What are the TWO ways that a catalyst decreases activation energy?
- Lower energy intermediate
- Alternate pathway for rxn w/lower energy
Units of rate
Mol/L x s (or other unit of time)
Rate law _______(does/does not) effect consumption of other reactants when asked
Does not, instead use balanced equation to determine ratio
What is chemical kinetics?
The study of reaction “movement” or rate (reactants being used up and products being created)
What are relative rates?
Rate laws that are equal to each other, and can be of either reactant or product. Relative rates are found in the form of (change in [X])/(change in time)
Rate laws have coefficients that are relative to 1 mole
These coefficients are negative in the case of reactants and positive for products
(ex. 2NO2 —-> 2NO + O2, change in [O2]/change in time = 1/2[NO]/delta t = -1/2[NO2]/delta t)
The units of concentration in rate are always:
M (mol/liter)
What is initial rate?
The rate of reaction at t=0
What is the average rate?
The rate given the slope of the overall rxn over a certain period of time
When are differential rate laws used? When are integrated rate laws used?
Differential: When the question revolves around concentration vs. rate
Integrated: When the question revolves around concentration vs. time
so cool https://www.youtube.com/watch?v=Efloac5qC8M
When finding k, always find time and concentration in units of…
moles and seconds
When potential energy is high, kinetic energy is ______(high/low)
low
Endothermic reactions must run at a ______ temperature
high
Integrated equations can only be used if the reactant order matches the:
reactant order
How do you know if a half life is zero order? first or second order?
zero: decreasing half life
First: constant half life
second: increasing half life
Define spontaneous
A reaction requiring no external energy (regardless of speed)
