Topic 18: Kinetics Y2 Flashcards
Rate of reaction at particular instant
rate = change in conc. / time
The order of a reaction (with respect to a species)
The power to which the conc. of that species is raised in the rate expression.
The overall rate of a reaction
The sum of all the orders of reaction in the rate expression.
Units for rate
mol dm−³ s−¹
How do you determine rate from conc-time graph?
Draw a tangents at different point these determine the rates at certain points of the reaction.
Determining order of a reaction from rate-conc. graphs
zero order- horizontal straight line
1st order - straight line through the origin
2nd order - wont be a straight line, plot rate against [X]^2 if straight line, order 2
Using initial rate to determine order with respect to each species.
As initial conc of species is *2 if order 2 rate will *2^2. If initial conc of species is *3 rate will *3^3 if order 2.
General rule for increase in temp and rate of reaction.
As temp increases by 10K, rate of reaction doubles.
What is the effect of increasing the temperature on k (the rate constant)?
Increasing the temperature increases the rate constant.
As you increase the temperature more particles have energy greater than or equal to the activation energy.
So increased frequency of successful collisions.
The Arrhenius Equation
k =Ae^(-Ea/RT)
A in the Arrhenius equation
A pre-exponential factor, the frequency of correctly oriented collisions.
Ea
Activation Energy is the minimum amount of energy particles require in order for a successful collision to occur causing the reaction to happen.
R
The gas constant (usually 8.31)
e^(-Ea/RT)
The fraction of collisions with enough energy to react.
Unit for T in Arrhenius equation
K
Unit for -Ea in Arrhenius equation
Jmol^-1
Unit for R in Arrhenius equation
JK^-1mol^-1
Log form of Arrhenius equation
lnk=ln(Ae^(−Ea/RT))
= (−Ea/R)(1/T)+lnA
y=mx+c
How to use the log form of the Arrhenius equation
Drawing a graph of 1/T and lnk, the gradient will be (-Ea/R) and can be used to determine the activation energy.
Rate Determining Step
The slowest step in the reaction so it determines the overall rate.
Anything _____ the rate determining step wont affect the rate.
After, these will be quick in comparison and not affect the rate or be in the rate equation, steps before will potentially be.
Determining the rate determining step
Either the slowest, the has the reactants in the rate expression, the number of those reactants add to their respective order.
a+b=c
b+c=d
d+c=e
rate equation = k[a][b]^2
step 2 is the rate determining step.
The iodine clock reaction step 1
H202(aq)+2I-(aq)+2H+(aq) —-> I2(2)+2H20(l) rate determining step (hydrogen peroxide) (potassium iodide)(sulfuric acid)
The iodine clock reaction step 2
I2(aq) +2S203^-2(aq)——> S406^-2(aq)+2I-(aq) (sodium thiosulfate) (tetrathionate) Excess iodine will react with the added starch and
How does the iodine clock reaction work?
Step 1 then Step 2. Excess iodine will react with the added starch and turn it from colourless and clear to blue black. Time it with varying conc. of H202.
