Reaction Kinetics 2 Flashcards
Initial rates method of finding order:-
For A + B -> products.
Set up 2 or more experiments where everything is kept same except conc of A. Measure initial rates then relate change in [A] to change in rate.
Use conc vs time graph for initial rates from tangents.
Relating rates to order:-
For example, when A initial rate at 0.5moldm^-3 =1.0moldm^-3 s^-1 and at 1.0 is 4.0, difference = x4 = 2^2 do second order.
Zero order initial rates method:-
Rate doesn’t change.
First order initial rates:-
Rate multiplied by same factor as conc.
Second order initial rates:-
Increasd by factor squared.
Clock reaction method:-
Find approx value for initial rate by measuring time for product to form, sometimes by measuring colour change.
Use moles/time on curve.
Analysing clock reactions:-
Times for colour change (t) are measured for range of diff initial reactant concs.
1/t calced.
1/t plotted vs conc, used to identify order in normal way.
Rate determinung step:-
Slowest step in a multi step reaction, number of particles involved are equal to order of reaction for that substance.
Proposing reaction mechanism:-
Propose slow step as first, those in rate equation as initial reactants. Step needs to balance and doesn’t matter as long as reactants are right in end. Call other steps fast.
T in Arrhenius equation:-
Temperature in K.
Arrhenius equation (in formula booklet but useful to know.):-
K= A e^ -(Ea/RT)
R in Arrhenius equation:-
Gas constant
Ea in Arrhenius equation:-
Activation energy
A in Arrhenius equation:-
Pre-exponential factor- relates concs of reactants to freq of collisions. Units same as K.
Y intercept on graph.
Using Arrhenius equation to determine Ea:-
Ln both sides, so Ln k =
- Ea/RT +ln A. Ln k vs 1/t graph = linear with grad
- Ea/R and y intercept ln A. Rearrange grad to get activation energy. Convert to kJmol^-1.
