Physical Chemistry 3 - Kinetics - Order of a Reaction, The Rate Constant, k and Reaction Mechanisms Flashcards
For the reaction:
A + B -> C,
what is the rate constant?
Rate = k [A][B]
What is a zero order reactant?
What is a first order reactant?
What is a second order reactant?
What is a third order reactant?
0: a reactant that does not affect rate when its concentration is doubled.
1st: a reactant that causes rate to double when its concentration is doubled
2nd: a reactant that causes rate to quadruple when its concentration is doubled
3rd: a reactant that causes rate to octuple when its concentration is doubled
If the rate of a reaction doubles once the concentration of reactant B doubles, it is said to be…
first order with respect to B - (B is first order)
A reaction is second order with respect to C if doubling C’s concentration causes the rate to ___.
quadruple
The order of a reactant is represented by a…
power (eg A^2 would be second order)
Under what circumstances would you not include a reactant in the rate equation?
Why is this?
if it is zero order.
because it doesn’t affect rate
If reactant A is second order, B is zero order and C is first order, what would the rate equation look like?
Rate = k [A]^2 [C]^1
*it’s a good idea to write ^1, but it’s not mandatory
*also notice how there’s no B!
What is the overall order of a reaction?
What about this one?
Rate = k[A]^2[B]^1
the sum of the powers to which the concentrations of the reactants are raised
3
How do you find k?
by rearranging the rate equation: k = Rate/[A][B]
What do you sub into the rate equation to find k, and from where?
k = Rate/[A][B]
the rate, the concentration of the reactants, and their order.
all of these values must be taken from the same row of the table.
The order of each reactant can be determined from the table.
How do you find the units for k?
sub in the units for the rate and each of the reactants and simplify
The reactants will be measured in ___, and the rate will typically be measured in ____ (unless otherwise specified).
mol/l, mol/l/s
Finding the units for k may look like this:
k = Rate/[A]^1[B]^2
k = molL-1s-1 / [molL-1]^1 [molL-1]^2 ** sub in
k = s-1 / mol2L-2 **cancel one [molL-1] from top and bottom
k = mol^-2L^2s^-1 ***bring ‘mol2L-2’ up to top by inverting powers
Done!
Remember that everything has an invisible ___ over it, which needs to be taken into account when expanding!
^1 (power one)
Eg [molL-1]^2 becomes mol^2L^-2
When bringing the denominator of a fraction up, you simply…
What would s^-1 / mol^2 L^-2 become?
invert the power!
mol^-2 L^2 s^-1
