Topic 16.1: Rate expression and reaction mechanism Flashcards
How can the rate law between two reactants (A / B) be determined?
Experimentally
Not from stoichiometric equations
Mathematical relationship of reaction rate with reactant concentration
Rate = k [A]^x [B]^y
Rate constant (k)
Particular for a reaction at a specific temperature
Overall order (a+b)
Sum of the magnitude orders
Method to determine the rate law
The initial rate is worked out using a fixed amount of B and changing the concentration of A.
Zero order reaction
a) Description
b) Rate expression
c) Units of rate constant
a) The rate does not depend on concentration
b) Rate = k
c) mol dm-3 s-
First order reaction
a) Description
b) Rate expression
c) Units of rate constant
a) The rate is directly proportional to concentration.
b) Rate = k[A]
c) s-1
Second order reaction
a) Description
b) Rate expression
c) Units of rate constant
a) The rate is proportional to the square of the concentration.
b) Rate = k[A]^2
c) mol-1 dm3 s-
Third order reaction
a) Description
b) Rate expression
c) Units of rate constant
a) The rate is proportional to the cube of the concentration.
b) Rate = k[A]^3
c) mol-2 dm6 s-
Graphs (Rate vs Concentration)
a) Zero order
b) First order
c) Second order
a) Constant rate
b) Linear rate
c) Square rate
Graphs (Concentration vs Time)
a) Zero order
b) First order
c) Second order
a) Negative linear
b) Negative curve with constant half-life
c) Negative curve with no constant half-life
Half-life definition
Time required for the concentration of a reactant to reach half its initial value.
Half life for a first order rate law
It does not depend on the initial concentration of any reactant, so it remains constant throughout the reaction.
Reaction mechanism definition
Sequence of steps by which a chemical reaction occurs
Elementary reactions definition
Reactions that occur in a single step
Special feature of the rate law of an elementary reaction
Its rate law is based directly on its molecularity
Molecularity definition
Number of molecules that participate as reactants in an elementary reaction
Why do most chemical reactions occur by mechanisms that involve two or more elementary reactions?
Since the chances of a successful collision between three or more particles are extremely small compared to the one between two.
What is the overall result of the chemical equations for the elementary reactions in a multistep mechanism?
Chemical equation of the overall process
Intermediates definition
Substances that are consumed in one elementary reaction and consumed in the next
The rate law of a multistep mechanism based on the ones of the elementary reactions
The overall rate of reaction depends upon the rate of the slowest step, also known as rate determining step.
Rate law for mechanisms with a slow initial step
a) The rate of the overall reaction depends on the one of step 1.
b) The rate law of the overall reaction equals the one of step 1
Rate law for mechanisms with a fast initial step.
We can solve for the concentration of an intermediate by assuming that an equilibrium is established in the fast step.
Rate Determining Step (RDS)
The step with the highest Ea, which acts as a limit on the rate of reaction (slow)
How do catalysts speed up a chemical reaction?
By changing a reaction mechanism that has a lower Ea than its original one.
Rules for writing mechanisms (4)
a) The mechanism must agree with the overall stoichiometric equation.
b) A maximum of two particles can react in any step.
c) All species in the rate equation must appear in the mechanism in or before the RDS.
d) The power of a particular reactant’s concentration in the rate equation indicates the number of times it appears in the mechanism up to and including RDS.
