Reaction Kinetics Flashcards
Defn of rate of rctn
units
eqn
change in conc of a particular prod/rct per unit time
mol dm^-3 s^-1
rate of eqn = change in conc / time taken
Defn of rate eqn
eqn
Experimentally determined mathematical exp b/w the rctn rate and the conc of rcts
Rate = k [A]^m [b]^n
Defn of order of rctn
the order of rctn wrt a rct is the power in its conc term in the rate eqn and is experimentally determined
Defn of rate const
Is the proportionality const in the exp-determined rate eqn. It is a const at a given temp
Defn of instantaneous & initial & ave rate
Instantaneous rate - is the rate at a particular time
initial rate - the instantaneous rate at time=0
Ave rate - is the change in conc of a rct or a product over that time interval
In prac, why may the initial rate ≈ ave rate provided（2）
- time interval is small enough
2. time interval starts at t=0
Interpreting rate eqn
units
formula
what order rctn
When
rate = k ——– zero-order rctn
units of k: mol dm^-3 s^-1
rate = k[A] ——- first-order rctn
units of k: s^-1
rate = k[A][B] &. =k[A]^2 ——– second-order rctn
units of k: mol^-1 dm^3 s^-1
Defn of first order rctn & eqn & graph
One in which the rctn rate is directly prop to the conc of a single rct
rate = k[A]
Defn of 2nd order rctn & eqn & graph
One in which the rctn rate is directly prop to the sq of the conc of a single rct
rate = k[A][B] & =k[A]^2
Defn of zero-order rctn & eqn & graph
One in which the rctn rate is independent of the conc of a single rct
rate = k
Defn of half life
how to find it (2)
& working on graph
The proving statement too
Time taken for the conc of a rct to decrease to half its initial value
From the graph,
1st t1/2 = time taken for […] to decrease from …. to …. = t1
2nd t1/2 = time taken for […] to decrease from …. to …. = t2
Since t1 = 12, t1/2 is const
1st order wrt [A]
On graph draw the &
in ans: Since the half-life is approximately constant (averaget1/2 = 15.5min), the reaction is first order with respect to N2O3
ALSO t1/2 = ln2/k
Pseudo-order rctns (3)
and the way of writing the k’ eqn
Presence of a large excess of a rcnt
Rcnt is also the solvent
Presence of a catalyst
eg rate = k[S2O82−][I−] = k’[I-] where k’ = k[S2O8 2-] = (0.2)(0.5)
Finding order of rctn (3)
By inspection method
By Calc
By conc-time graph
Find k
sub values into rate eqn for a certain exp (any exp will do)
Why is sth used in large excess
Ensure that [Q] stays effectively const throughout the rctns so that the effect of changing [P] on the rate of the rctn can be studied
Exp-tech
Physical methods
- gas (1)
- colour intensity (3)
Chem methods
- sampling followed by quenching then titrimetric method (3)
- ‘clock rctns’ (2)
look at notes
Factors affecting rate of rctn (4)
?
‘Clock rctns’
why must the beaker be the same
why must the total vol be const
why is initial value inversely prop to the time taken for a___ solid/colour to appear
ensure depth of rctn mixture remains the same, same amt of __ needs to be precipitated before the cross is obscured
ensures initial conc of each reactant in the rctn mixture is directly prop to its vol used
time interval is small and measured from 0
Defn of rctn mechanism
collection of elementary steps in the proper sequence showing how rcnt particles are converted into products. It is the explanation of how a rctn takes place
Defn of elementary step
distinct step in a rctn mechanism which describes a single molecular event that involves breaking or making bonds
Defn of rate-determining step
Slowest step and highest Ea (find actual defn from tut?)
What is the molecularity of an elementary step
eg unimolecular step, bimolecular step
No. of rct particles taking part in that step
Defn of an intermediate
a species that is formed in 1 step of a rctn mechanism and consumed in a subsequent step
Defn of single/multi-step rates
How to find it from stoic eqn
deducing rate eqn
Single step rctns is one that takes place in a single step and is termed an elementary rctn
Maxwell-Boltzmann distribution curve
-
Defn of catalyst &
types of catalyst (2)
- 4 eg
- 3 eg
Catalyst is a substance which increases the rate of rctn without undergoing any permanent chemical change
Homogenous Catalysis - catalyst and rcts are in same phase (state)
* Fe2+ / Fe3+ rctn b.w peroxodisulfate ions and iodide ions
- No2 catalytic oxidation of atmospheric sulfur dioxide
Mn2+ autocatalytic rctn
Heterogeneous catalysis - catalyst and rcts are in diff phase (state)
Ni catalytic reduction / hydrogenation of alkenes to form alkanes
*Fe haber process
V2O5 contact process
Defn of autocatalysis
graph
explain graph
test for autocatalyst is to..
-
Defn of Enzymes
Are proteins which catalyse chemical rctns in living systems. They are often called biological catalysts or biocatalysts.
Definition of inhibitor
is a substance which decreases the rate of a chemical rctn
Factors affecting rate of enzyme-catalysed rctn (3)
& graph(s) for one of the factors
temp
pH
conc of enzyme
Enzymes - models (2)
lock and key model
induced-fit model
5 properties of enzymes
Nature and size efficiency specificity temp sensitivity to pH
k is affected by
temp or [catalyst] / lower Ea basically
Process of catalytic reduction/hydrogenation of ethene (5)
Diffusion - diffuse towards the catalyst surface
Adsorption - rct molecules become adsorbed onto the active sites of the catalyst surface. This increase in conc of rcts at the catalytic surface and also weakens the covalent bonds in the molecules
Rctn - Adjacent molecules react to form products. This rctn has lower Ea than the uncatalysed rctn
Desorption - the reverse of adsorption. The prod molecule eventually breaks free from the catalyst surface
Diffusion - product molecules diffuse away from the surface. The vacant active sites are now available for adsorbing other rct molecules
Eg outline how the rate of each of the following rctns may be followed experimentally
general strategy?
- Identify what can be monitered over time, then identify method to use:
- change in [rct] or [prod] with time can be monitored by diff methods eg calorimetry - Perform the exp and collect data over time
- Plot a graph of data against time - (3 to 5 data pts)
- Instantaneous rate may be found by drawing a tangent to the curve at the particular time and finding its gradient g1
- rate = -g1 if data ∝ [rct] while rate = g1 if data∝ [prod]
