Chem kinetics pt 1 L13 Flashcards
Two imp things of a chem reaction
*How far it goes (thermodynamics)
*How fast it goes (kinetics)
TWO ARE NOT RELATED
What are the two parts to studying kinetics?
Experimental and Theoretical
Experimental
measure reaction rates i.e measure speed and study the factors that influence them i.e what speeds/slows down a reaction
Theoretical
understand rates in molecular terms i.e what makes a reaction inherently fast/slow and use the exp data to probe reaction mechanisms
What is a reaction mechanism?
intermediate steps that a reaction takes to reach completion
What do we ask in a rate of reaction?
How does concentration change over time
To quantify a curved rate of reaction:
Find the gradient of the reactant over time, this is called a rate
What does a -ve rate mean?
We cannot have a negative rate so this means that the conc. is decreasing (the -ve sign in front of a term of reactant)
For general reaction:
aA + bB –> cC + dD
rate = -1/a (dA)/dt = 1/c (dC)/dt
Factors that affect the rate of reaction
*Chem nature
*Physical states of the reactant + products
* Conc.
*temp
NOT INVOLVED IN STOIC - no net change
*Catalyst
*inhibitor
How does conc. of reactants affect the rate?
*Conc. of reaction increases then speed increases
*Conc. of reaction increases but no affect on speed eg u drink too much beer but ur body only processes a certain amount in time
Rate law, order and constant
Rate law: rate = k [A]ˆx [B]ˆy
What does rate law say?
If you change the conc.,this is how it will change the rate
Can i ignore stoic for rate law
YAS
How to determine rate law?
Through experiments as we cannot find x and y without it
What is x and y
x - order with respect to A
y = order with respect to B
x + y = total order of reaction
Usually integers from 0 - 2
what do 1st order reactions do?
Behave the same
How to get 1st order reactions?
*1st order in A where reaction has only one reactant
*1st order in A where reaction has 2+ reactants but all other reactants are 0
Pseudo 1st order reaction
Increasing the conc. of B in a reaction so that its conc. is relatively constant i.e it is not being used up much so we can write rate exp in terms of one reactant
k’ = k[B]
Rate = k’[A]
Purpose of a pseudo 1st order reaction
Easier to find the rate by working with only 1 reactant (forcing the other reactant to not change)
Rate of order changes with conc.
- 0 order - change in conc. has no affect on rate change
- 1st order - rate doubles
*2nd order - rate increases by 4 = k[A]ˆ2
rate increases by 2 = k[A][B]
Rate constant k
*Independent of conc.
*increases with temp
*Gives constant of proportionality
*units depend on overall x+ y
units when order = 0
mol-1s-1
units when order = 1
s-1
units when order = 2
L mol-1s-1
units when order = 3
Lˆ2 mol-2 s-1
Relationship between rate and time
Inverse relationship
How does the rate constant change
If the order changes
What is the isolation method?
Varying the conc. of one reactant and holding the others constant
