Block 3 - Kinetics Flashcards
Chemical kinetics
The study of the rate of chemical reactions and the factors that affect them
Reaction rates define…
The change in concentration of a reactant per unit time
Measured in mol L-1 s-1
Thus the reaction rate is:
- the number of times a reaction happens per second
- how fast the reactant decreases
- how fast the product increases
Instantaneous rate
The rate at a particular point in time, mathematically given by d[A] / dt
Average rate
Given by the change in concentration over time, mathematically given by Δ[A] / Δt
Initial rate
The instantaneous rate at t = 0 when only reactants are present –> know amount/conc of reactants at beginning
Factors affecting rate of reaction
Nature of reaction - some reactions are inherently fast compared to others
Temperature - most reactions occur faster as the temp is raised
Action of light - some reactions happen rapidly in presence of light and may depend on exact wavelength
State - the greater the SA, the faster the reaction
Concentration - the more of something we have, the faster it may react
Catalysts - substances that increase reaction rate (often via diff reaction pathway), but do not get used up. often react in an early step, then are regenerated again in a later step
The instantaneous rate of many reactions is related to the concentrations of reactants by the…
Rate law
Rate law
If rate law is known, can calculate rate of reaction for any given conc of reagents
To determine rate law, must know how much the rate depends on reactant [A], i.e. the order of the reaction with respect to A
For a reaction A –> products, rate? If…
Rate doesn’t change when conc of A changes –> rate is proportional to [A]^0 –> zero order in A –> rate = k
Rate doubles when conc of A doubles –> rate is proportional to [A]^1 (linear) –> first order in A –> rate = k[A]
Rate is 4x when conc of A doubles –> rate is proportional to [A]^2 –> second order in A –> rate = k[A]^2
Prediction of order from equation
You CAN’T predict the order from the balanced overall equation
Rate constant
The nature of the reaction and the temperature
k tells us about the inherent nature of reaction
Units of k depend on order of reaction:
Zero order: mol L-1 s-1
First order: s-1
Second order: L mol-1 s-1
Third order: L^2 mol-2 s-1
Method of determining rate law
Carry out an experiment to measure concentrations at various times after the reaction starts, then determine the slope of an appropriate graph
To do this, we need to know what form of graph to plot, which involves integrating the instantaneous rate expressions
What is [R] = [R]0 - kt used for
Zero order reaction
What is ln[R] = ln[R]0 - kt used for
First order reaction Rearranged: [R] = [R]0 e^(-kt) --> indicates an exponential decrease in conc Where: [R] = conc of reactant [R]0 = initial conc of reactant k = rate t = time (s)
Graph - first, second, or third order?
First order: If ln[R] = ln[R]0 - kt, then a plot of ln[R] against time will be a straight line whose slope is -k
Second order: not a straight line
Second order reactions occur for reactions where…
There is a single reactant that is second order (rate = k[A]^2)
OR
There are two reactants that are both first order (rate = k[A][B])
Second order reactions: 2 reactants - problem and solution
Problem: concentrations might not initially be the same, or might change at diff rates
Solution: make the conc of one of the two compounds very large, then changes are negligible –> becomes like a first order problem; pseudo-first order
rate = k[A][B] –> rate = k’ [A]
What is the equation 1/[R] = 1/[R]0 + kt used for
Second order reactions with a single reactant
Half-life
The time (usually in s) for conc of reactant to fall to half of its initial value Denoted by symbol t1/2 Can be related to rate constant of reaction, but the relationship depends on order of reaction
t1/2 = [R]0 / 2k
Order?
Zero order
t1/2 = ln2 / k
Order
First order
Equation doesn’t feature conc –> half-life of a first order reaction is not dependent on conc
t1/2 = 1 / [R]0k
Order
Second order
Reaction profile
A graph showing the reaction pathway/co-ordinates vs energy
Activation energy
The energy required to activate the transition state and proceed with the reaction
The difference between energies at the transition state and molecules initial resting state
Always positive
A collision is effective if…
- The molecule has enough energy to surpass the activation energy
- The molecules are correctly aligned
Boltzmann distribution
Distribution of the range of diff molecule energies at a given temp
Temp proportional to average energy, but only if you use the absolute temp scale
Higher temp –> higher energy –> more molecules reach required energy to pass Ea barrier
Absolute temp scale
0 celcius = 273.15K
Arrhenius equation
k = Ae^ (-Ea/RT)
Used to calculate activation energy for a reaction
Links effect of temperature to Ea
ln(k) = ln(A) - Ea/RT
k = rate constant A = about orientation Ea = activation energy (J mol-1) R = gas constant (8.314 J K-1 mol-1) T = temp (K; kelvin)
Elementary reaction
Single-step reaction
Rate law is given by the stoichiometry of reactants
Most reactions aren’t elementary reactions so can’t determine rate law
We can predict the order of reactions if…
The elementary steps are known because the overall reaction rate depends on the rate of the slowest step
Slowest step
AKA rate determining step; if can determine slowest step, can determine overall rate law
If can speed up slowest step, can speed up overall rate of reaction
Reaction profile: Reactive intermediates
Represent points of minimum energy
Reaction profile: Transition states
Represent points of maximum energy
Types of elementary reactions
Unimolecular = one
Bimolecular = two; must collide in certain orientation
Termolecular = three; extremely rare
Never more than three
Acid-catalysed
Where rate of reaction is increased in presence of H3O+ ions
Homogeneous vs heterogeneous - catalysts
Catalysts that work in the same phase as reaction are homogeneous, and usually appear in the rate law
Catalysts that work in a diff phase as reaction are heterogeneous e.g. H2/pt
Catalysts - elementary steps
Catalysts change the elementary steps in a reaction
Product distribution depends on…
Relative activation energies of the competing reactions
Catalysts - outcome of reactions
Since catalysts affect Ea, they can also alter the outcome of reactions
A catalyst can provide a pathway for one of the reactions but not the other, changing the products observed
Catalysed reaction, either___ or ____
Either the activation energy of the new pathway is lower OR the molecules are aligned by the catalyst
Most catalytic processes do both
Adding a catalyst affects…
MAY affect value of A
MAY affect value of Ea
ALWAYS affects value of k
Decrease in Ea = increase in k
K(catalysed) vs K(uncatalysed)
K(catalysed) > K(uncatalysed)
Intermediate vs catalyst
Intermediate: formed as a product of an elementary step and used up as a reactant in a following elementary step
Catalyst: added to reaction before it begins - always appears on LHS first, then reformed on RHS
Key principles of reaction mechanisms and rate laws
Mechanisms must be made up of elementary steps
The slow step is the rate determining step
The rate law for the reaction can be written directly from the slow step
Intermediates can’t appear in the final rate law
