Block 3 - Kinetics Flashcards
(44 cards)
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
