Kinetics
09 January 2018 10:03
Reactions that are thermodynamically viable and occur For a reaction to occur:
spontaneously Molecules must first collide and the rate of collision is dependent
A-LEVEL REVIEW e.g. Na(s) + H2O(l) => Na+(aq) + OH-(aq) + 1/2H2(g) on:
• Thermodynamics tells us whether or Some reactions do not proceed at any appreciable rate, • Concentration of molecules
not a reaction is energetically feasible: despite being thermodynamically favourable • The speed at which they are travelling
△G = △H - T△S e.g. diamond => graphite can take millions of years • Their size
• If △G < 0, reaction is spontaneous, but Thermodynamics: position of equilibrium between states The molecules must collide with the correct orientation:
this gives no indication to how fast the Kinetics: rate of change between states (how quickly a • This is known as the steric requirement (e.g. in the reaction
reaction occurs. system find equilibrium point) between OH- and CH3Br, the OH- would need to approach
Definitions from the opposite side of the Br, the leaving group)
Rate equation: e.g. the SN2 reaction between OH- and CH3Cl, rate = For the general scheme A + B => Products, rate ∝ [A]n[B]m
k[OH-][CH3Cl]
Concentration: number of reactant molecules in a volume written as Overall order of reaction: sum of the
[OH-], [CH3Cl], with units mol/dm3, moldm-3, molL-1, or simply M orders w.r.t. reactants; n+m
Rate constant, k: measure of how fast the reaction proceeds Molecularity: for single-step reactions
Order of reaction: how the reaction rate depends on the concentration or elementary steps,
of each reactant molecularity=overall order of reaction.
Molecularity: the number of molecules involved in the elementary step For complex reactions e.g. multiple
steps with intermediates, undefined.
For the SN2 reaction between OH- and CH3Cl (SN1 involves unimolecular):
1 molecule of OH- reacts with 1 molecule of CH3Cl in the elementary step Transition states and activation energies: The products are more
thermodynamically stable
than the reactants, but the
transition state acts to block
the progress of the reaction
Overall order of reaction: rate=k[OH-][CH3Cl], hence order is 1+1=2 The larger the activation
Molecularity: in this single-step reaction 2 molecules take part, hence the energy Ea the slower the
overall order of reaction = molecularity of 2 => bimolecular reaction proceeds i.e. fewer
• For an elementary step, the rate equation can enable us to molecules make it over the
understand the mechanism for the reaction. barrier.
TOPIC 1 - THE RATE OF REACTION, ELEMENTARY STEPS AND
MOLECULARITY, DETERMINATION OF RATE LAW, INTEGRATED RATE LAWS
The Rate of Reaction
• The rate at which the product molecules are produced and reactants are lost
• Conservation of mass: rate of reactants depleted = rate products appear
• For the reaction
with concentrations of reactants: [A], [B] product: [P]
The rate at which A and B are consumed = the rate at which P is produced
-rate of change [A] = -rate of change [B] = rate of change of [P]
Rate of change of P with time = rate of change A or B with time - 1
Problem
a. For a reaction A + B => P, sketch the [B] vs time b. Sketch the [A], [B] and [P] vs time
function given that the initial concentrations of functions if [A]0 = 2[B]0
A and B are equal ([A]0 = [B]0). Sketch the [P] vs
time function for (a) on the same graph. For the general process:
aA + bB => cC + dD (lowercase indicates stoichiometric coefficient)
Example:
1. Define the reaction rate for the combustion of propane
Slower reaction compared with [A], less B, C3H8 + 5O2 => 3CO2 + 4H2O
B runs out, not all A consumed. A excess
Practically all A and B consumed and turned into reagent, B limiting reagent.
P, [A]initial=[B]initial=[P]final
Elementary steps and molecularity
For a complex reaction (multiple steps):
• If such a rate law can be obtained at all: overall reaction order = m+n, molecularity is undefined.
• Powers m and n are to be determined experimentally, they are nothing to do with the
stoichiometry of the reaction.
• Rate laws are often more complicated for complex reactions
e.g. gas phase reaction
For an elementary reaction (single step):
• Overall reaction order = 1+1 = 2
Molecularity: 2 = bimolecular (as two molecules take part in the reaction)
• Overall reaction order = 1
Molecularity: 1 = unimolecular (as one molecule takes part in the reaction) Determination of Rate Laws
Determining Rate Laws - Initial rates method:
Physical Chemistry I Page 1
09 January 2018 10:03
Reactions that are thermodynamically viable and occur For a reaction to occur:
spontaneously Molecules must first collide and the rate of collision is dependent
A-LEVEL REVIEW e.g. Na(s) + H2O(l) => Na+(aq) + OH-(aq) + 1/2H2(g) on:
• Thermodynamics tells us whether or Some reactions do not proceed at any appreciable rate, • Concentration of molecules
not a reaction is energetically feasible: despite being thermodynamically favourable • The speed at which they are travelling
△G = △H - T△S e.g. diamond => graphite can take millions of years • Their size
• If △G < 0, reaction is spontaneous, but Thermodynamics: position of equilibrium between states The molecules must collide with the correct orientation:
this gives no indication to how fast the Kinetics: rate of change between states (how quickly a • This is known as the steric requirement (e.g. in the reaction
reaction occurs. system find equilibrium point) between OH- and CH3Br, the OH- would need to approach
Definitions from the opposite side of the Br, the leaving group)
Rate equation: e.g. the SN2 reaction between OH- and CH3Cl, rate = For the general scheme A + B => Products, rate ∝ [A]n[B]m
k[OH-][CH3Cl]
Concentration: number of reactant molecules in a volume written as Overall order of reaction: sum of the
[OH-], [CH3Cl], with units mol/dm3, moldm-3, molL-1, or simply M orders w.r.t. reactants; n+m
Rate constant, k: measure of how fast the reaction proceeds Molecularity: for single-step reactions
Order of reaction: how the reaction rate depends on the concentration or elementary steps,
of each reactant molecularity=overall order of reaction.
Molecularity: the number of molecules involved in the elementary step For complex reactions e.g. multiple
steps with intermediates, undefined.
For the SN2 reaction between OH- and CH3Cl (SN1 involves unimolecular):
1 molecule of OH- reacts with 1 molecule of CH3Cl in the elementary step Transition states and activation energies: The products are more
thermodynamically stable
than the reactants, but the
transition state acts to block
the progress of the reaction
Overall order of reaction: rate=k[OH-][CH3Cl], hence order is 1+1=2 The larger the activation
Molecularity: in this single-step reaction 2 molecules take part, hence the energy Ea the slower the
overall order of reaction = molecularity of 2 => bimolecular reaction proceeds i.e. fewer
• For an elementary step, the rate equation can enable us to molecules make it over the
understand the mechanism for the reaction. barrier.
TOPIC 1 - THE RATE OF REACTION, ELEMENTARY STEPS AND
MOLECULARITY, DETERMINATION OF RATE LAW, INTEGRATED RATE LAWS
The Rate of Reaction
• The rate at which the product molecules are produced and reactants are lost
• Conservation of mass: rate of reactants depleted = rate products appear
• For the reaction
with concentrations of reactants: [A], [B] product: [P]
The rate at which A and B are consumed = the rate at which P is produced
-rate of change [A] = -rate of change [B] = rate of change of [P]
Rate of change of P with time = rate of change A or B with time - 1
Problem
a. For a reaction A + B => P, sketch the [B] vs time b. Sketch the [A], [B] and [P] vs time
function given that the initial concentrations of functions if [A]0 = 2[B]0
A and B are equal ([A]0 = [B]0). Sketch the [P] vs
time function for (a) on the same graph. For the general process:
aA + bB => cC + dD (lowercase indicates stoichiometric coefficient)
Example:
1. Define the reaction rate for the combustion of propane
Slower reaction compared with [A], less B, C3H8 + 5O2 => 3CO2 + 4H2O
B runs out, not all A consumed. A excess
Practically all A and B consumed and turned into reagent, B limiting reagent.
P, [A]initial=[B]initial=[P]final
Elementary steps and molecularity
For a complex reaction (multiple steps):
• If such a rate law can be obtained at all: overall reaction order = m+n, molecularity is undefined.
• Powers m and n are to be determined experimentally, they are nothing to do with the
stoichiometry of the reaction.
• Rate laws are often more complicated for complex reactions
e.g. gas phase reaction
For an elementary reaction (single step):
• Overall reaction order = 1+1 = 2
Molecularity: 2 = bimolecular (as two molecules take part in the reaction)
• Overall reaction order = 1
Molecularity: 1 = unimolecular (as one molecule takes part in the reaction) Determination of Rate Laws
Determining Rate Laws - Initial rates method:
Physical Chemistry I Page 1
