Rate of reaction Flashcards
What is the rate of reaction equation?
Rate of reaction = 1/a d(A)/dt
What are the two types of rate?
Instantaneous - at a point in time
Average - over a period of time
What are the factors that affect rate of reaction?
Nature of reaction Temperature Action of light State of reactants Concentration Catalysts
What are catalysts?
A compound that increases the rate of reaction and is not used up - reformed
What does zero order mean?
The concentration of the reactant has no effect on rate of reaction
What does first order mean?
The concentration of the reactant is proportional to the rate of the reaction
What does second order mean?
The concentration of the reactant squared is proportional to the rate of reaction
What is the rate equation?
Rate = k[a][b]
What is the rate constant?
A constant in the rate equation which provides information about the nature of the reaction and the temperature
What is the unit for a first order reaction?
s -1
What is the unit for a zero order reaction?
mol L-1 s-1
What is the unit for a second order reaction?
L mol -1 s-1
What is the unit for a third order reaction?
L2 mol-2 s-1
What is the integrated rate law equation for a first order reaction?
ln([R]) = ln([R])o - kt [R] = [R]o x e^-kt
How do we check if a reaction is first-order?
A plot of ln[R] on the y axis and time on the x-axis should show a gradient of -k with a y-intercept of ln[R]o
What is a second order reaction?
two reactants each first order or a single reactant thats second order
What is a pseudo-first order reaction?
When considering a second order reaction with two first order reactants, use an excess of reactants for one reactant so that the change in the conc. of that reactant is negligible and hence the reaction can be considered as a normal first order reaction with the integrated rate law equation for first order being applied.
Rate = k[CH3Br][OH-}
=k’[CH3Br}
where [OH-] is in excess
What is the integrated rate law equation for a second order reaction and when does this apply?
Apply when you have a second order reaction with just one reactant
1/[R] = 1/[R]o +kt
What is half-life?
Time taken for amount of reactant to halve in a reaction
What is the integrated rate law equation for a zero order reaction?
[R] = [R]o - kt
How do you work out formula for half-life?
Substitute values into the integrated rate law equations so that the concentration of R is half the concentration of Ro
What is the equation for half-life regarding a first order reaction?
t 1/2 = ln(2) / k
Hence, the half-life for a first order reaction can always be worked out using the k-value and the initial concentration of the reactant has no effect on half-life
HALF LIFE IS CONSTANT
What is collision theory?
Reactions happen when molecules collide at a specific orientation with enough energy
What is 0K?
-273.15 degrees Celsius
What is the boltzman distribution?
A graph of number of molecules against the energy of the molecules - shows change in proportion of molecules with certain energies as temperature changes
What is the arrhenius equation?
k = Ae^(-Ea/RT)
What is R?
constant = 8.314 J K-1 mol-1
What is the natural log version of the arrhenius equation?
ln k = ln A - (Ea/RT)
How do you determine Ea experimentally?
Measure rate constant at various temperatures then use the follow equation:
ln (k2/k1) = -Ea/R x (1/T2 - 1/T1)
How do you determine the overall reaction rate?
From the rate determining step - the slowest step of the mechanism
How many molecules can be involved in elementary reactions?
Unimolecular - collision with a catalyst
Bimolecular - collision of two molecules
Termolecular - impossible
How do you determine the rate equation from the reaction mechanism?
Add the reactants from the rate determining step and add the number in front as the power
What is the difference between Sn1 and Sn2 nucleophilic substitution mechanisms?
The overall reaction equation is the same however due to the two step process in Sn1 the concentration of the nucleophile does not affect the rate of the reaction
How do you determine the rate equation for reaction mechanisms involving equilibrium reactions?
use the equilibrium constant
