Chemistry Topic 2 Flashcards
From the simulation we can observe that
- More atoms (Higher concentration leads to more collisions, and faster rates)
-Higher temperature (I.e more KE) gives faster rates
Chemical Kinetics :
Is the study of the rates of chemical reactions and the factors that affect them
What are the two important aspects of chemical kinetics ?
- The rate of reaction (I.e . How fast it occurs) and how it depends on various factors
- Reaction Mechanism that describes the sequence of molecular events that lead from reactants to products
What are the factors affecting reaction rate?
- The nature of the reactants - ionic vs covalent; size ; bond strength etc
- Physical form (and surface area ) - solid vs liquid vs gas , - homogeneous vs heterogeneous
- Reactant concentrations - higher concentration -> more collisions -> faster reaction
4.Temperature -> more collisions , more energy -> faster reaction - Catalyst - increases rate of reaction
What is the simple case of reaction mechanism that describes the sequence of molecular events that lead from reactants to products
A + B —-> C
A = (1) no reaction (A ) (B) separate
+ AB(transition state)
B. =(2) reaction (C)
What is the reaction mechanism simple example
A simple model: grasshoppers in a box
-jump randomly
- jump height a (alpha) temperature
-portioning (barrier) separates two sides of the box
A simple model :
N right + N left = N total (total number is constant)
Delta. N right = -Delta N left (total number is constant)
(change in number on right is balanced by the change on the left)
Reaction rates , some complications arise from?
Change in concentration of reactants (or products ) per unit time
= delta [concentration] / delta time
(Always positive since assume forward rxn )
And units will be (mol/L) / s = mol/L * s = mol L^-1 s^-1 = M s^-1 (or min^-1 , hr^-1, etc.)
Some complications with this definition arise from:
-the avg rate vs the instantaneous rate
- stoichiometry
- rate of reaction = +rate of change of products, - rate of change of reactants
What is rate of appearance and disappearance of HI
1/2 H2 (g) + 1/2 I 2(g) -> HI (g)
Rate of Apperance of HI : delta[HI]/delta time
Rate of disappearance of H2 (or I2):
- delta [H2]/ delta time = - delta [I2]/ delta time
- THESE REPRESENT AVERAGE RATES
The rate of reaction often changes as the reaction progresses. This makes it important to distinguish between an average rate and instantaneous rate
1/2H2(g) + 1/2 I2(g) -> HI (g)
The instantaneous rate :
- corresponds to a specific time t
-is defined as the slope of the tangent to the concentration vs time curve
-it is the derivative of the curve at time t
Instantaneous rate (for HI) :
= d[HI]/ dt
Initial rate: rate at t=0
Rate Laws:
relate the rate of a reaction to the concentrations of species involved
For a rxn in general form: aA + bB + cC -> pP
The general form of the rate law would be: Rate = k [A]^x [B]^y [C]^z
K = rate constant for the reaction (units depend on overall rate)
X = order of the reaction with respect to A
Y = order of the reaction with respect to B
Z = order of the reaction with respect to C
X + y + z. = the overall order of the reaction
- x, y, z are often integers positive or negative, but can also be fractions
- x,y,z (the orders) are in general unrelated to the stoichiometric coefficients (a, b, c)
- if a species has no effect on the rate the order for that species is 0
-the Value of k contains all other dependence’s (e.g T, P , etc) - products and catalysts can sometimes appear in rate laws
- rate laws must be determined experimentally
How would we deduce a rate law if it wasn’t known?
1 method: use initial rates (rates at the beginning of the reaction, when [A] and [B] are known
Rate = k [A]0 [B]0^2
- if double [A]0 but keep [B]0 constant -> rate doubles
- if double [B]0 but keep [A]0 constant -> rate quadruples
(1). Solve by inspection - use logic such as that above to deduce rate law
(2). Solve mathematically ( pair of conditions where all the concentrations except [A] are constant
Time Deependence of Concentration (Integrated Rate Laws) =
What is the equation that expresses concentration as a function of time called?
A rate law allows us to predict the rate of a reaction given concentrations
- its called integrated rate law
A -> products
Rate = -d[A]/dt
(1)Zero-order rate= k[A]^0 = k
(2) First-order rate = k[A]
(3) Second-order rate=k [A]^2
Comparing curves of. 0th, 1st, 2nd order kinetics
- the [A] goes to 0 for a 0th order reaction, BUT it never reaches. 0 for 1st and 2nd order reactions
- the half-life of a reaction t1/2 is the time it takes for half of the starting material to be consumed
- 0th order: t1/2 decreases
-1st order: t1/2 is constant
-2nd order: t1/2 increases
One way to characterize the rate of a reaction is by its __________ , ____.
The larger the half - life , the ______ the reaction
Half-life , t1/2
Slower ,
As the temperature increases, the reaction rate _______.
The ________ Equation describes how the rate constant changes with temperature
- Rate (rate constant) also _________ when: Ea ______ and A ________
- RATE increases _________ with Temperature
- The quantities Ea And A are characteristic of a particular reaction
Increases also ,
Arrhenius Equation
Ea decreases, A increases .
Rate increases exponentially
Two methods are used top determine A and Ea experimentally
- Two-point method (when u have k values at 2 temperatures)
There’s a formula for this one topic 2 slide 32 - Graphical Method; uses measured rate constants for a series of temperatures
A plot of Ink vs 1/T should give a straight line with slope = -Ea / R
Intercept = In A
More atoms (higher concentration ) leads to more _______ and _____ rates
Higher temperatures (more KE) gives _____ rates
- Collisions and faster rates
-Faster rates
Collision theory (Transition state theory)
The model assumes that reactant molecules to form an intermediate known as the ________ _______ (or __________ ___________)
Transition state = unstable _______ or ________ of atoms
A model used to describe observed concentration and temperature dependence of a reaction rates
It assumes 3 conditions must be met for a reaction to occur:
1)Reacting molecules must collide
2) The colliding molecules must have sufficient energy
3)The colliding molecules must have the right orientation
-The model assumes that reactant molecules to form an intermediate known as the transition state (or activated complex)
Transition state = unstable arrangement or grouping of atoms
The probability of a reaction then depends on:
As we have seen before _________ shifts the distribution of kinetic energies of the molecules toward higher energy (similar behaviour can be seen in distribution of potential energy)
As temperature increases the number of molecules with the minimum energy Ea, required for achieving a reaction _________ very rapidly (essentially exponentially)
- The number of collisions (we have seen previously that this depends on concentration and temperature)
- Geometric factors ( e.g the orientation of the molecules
- The energy of collisions (depends on temperature
Temperature
Grows
The reaction proceeds only when there is sufficient ______ energy to cross the _______ energy barrier
Kinetic energy to cRoss the potential energy barrier
A reaction coordinate diagram is often used for?
Ea= activation energy = always ______, the larger Ea, the ______ the reaction
delta H= enthalpy (energy) change for the reaction,= can be positive (__________) or negative (_____________
To describe the changes in energy during a reaction
Positive , the larger Ea the slower the reaction
, endothermic or negative exothermic
Reaction Mechanism:
A reaction mechanism is extremely important to understanding a chemical
reaction. It describes the specific steps by which a reaction occurs.
These steps, known as elementary steps, describe the actual reactions
that occur at the molecular level (e.g. the rearrangement of bonds).
In reaction mechanisms: we can describe A + B + C —> ABC
We can write as :
(A)A + B -> AB - two _________ reaction steps
(B) AB + C ->ABC - two ________ reaction steps
(C) A+B + C -> ABC - one ________ reaction step
1) Reaction mechanisms are often inferred from the ________ _______ law
2)For any mechanism, it is essential that the __________ steps add together to give the ______ reaction
3) The rate law cannot generally be determined from the overall reaction, BUT the rate law for any _________ steps can be determined for this reaction
(4) For any elementary step, its rate is determined by its reaction ________.
(5) True termolecular reactions are usually ____ and ______ occur
(since collisions of 3 molecules are rare).
(6) For most reactions there is usually a rate limiting step, that is an
elementary step that is much ______ than all other steps.
A) bimolecular
B) bimolecular
C)termolecular
1) overall rate law
2)elementary steps, overall reaction
3) elementary
4) stoichiometry
5)slow, rarely
6)slower
