Chapter 12 - Chemical Kinetics
Integrated law second order formula
1/[A] = kt+ 1/[A0]
The reaction rate for the change in [No2] will be negative; but since it is customary to work with positive reaction rates we deinfe the rate of this partiuclar reaction as
- delta[NO2]/ deltat
For a first order reaction slope is
-k
For a zero order reaction slope is
-k
What is an intermediate species?
A species that is neither a reactant or product; it is consumed and produced during reaction sequence. (appears on reactants for one equation and products for antoher equation)
In order to determine the order for two or more reactants what must you do? ex: NH4+(AQ) + NO2-(aq) -> N2(g) + 2H2O(l) given three experiments with initial concentrations of NH4+, three initial concenrtaitons of NH2-(NO2-), and three initial rates(mole/L *s)
1) Find the order of each reactant seperatley. 2) Do this by finding two experiments that has a change in the initial concentration, and a change in the initial rate. The reactant that changes is the order you are finding.
What is important about the integrated rate law equation?
1) If initial concentration of A and rate constant K are known, the conentration of A at any time can be calculated. 2) The equation is in y=mx+b form. The reaction is first order if when t is plotted against ln[A], the line is straight. 3) The integrated rate law can be expressed in terms of a ratio kt= ln ([A]0/[A])
Writing rate laws and integrated rate laws normally only work for the type
aA reactions; only 1 reactant is involved.
What is the overall reaction order?
adding all the exponents of the reactant concentrations.
The order of a reactant can be an zero or a fraction/
any integer.
If 2N2O5(soln) -> 4NO2(soln.) + 02(g) were a first order reaction, and N2O5 in a flask was doubled, the rate ofproduction of NO2 and O2 would
double.
Define kinetic energy
energy that a body posseses by virtue of being in motion.
The proportionality constant(k) and order of reactant(n) must both be determined by
experiment.
The value of exponent n must be determined by
experiment; it cannot be writing from chem eq.
Integrated Rat Law
expresses how concentrations depend on time.
Sponatneous does not mean
fast
In the reaction: 2NO2(g) -> 2NO(g) + O2(g) in an empty container, the change in concentration initally dpends on the forward reaction. After a period of time, enough products accumulate so delta[NO2] depends on the difference in the rates of the...
forward and reverse reactions
What is the most common method for determining the integrated rate law?
given time and concentrations measured at various values of t, you would see which integrated rate law fits the data; whichever integrated rate law would give you straight line is the order. The value of k can also be determined from the slope.
The time required for a reactant to reach half its original concentration is called the
half life of a reactant and is designated by the symbol t1/2
Don't forget the negative sign when calculating the
instanenous rate.
For a second order reaction slope is
k
For an integrated rate law, the slope represents
k.
Rate= -delta[A]/ deltat =
k[A]
For a zero order reaction, rate = k because rate=
k[A]^0=k*1= k
The area of chemistry that concerns reaction rates is called
kinetics
To understand a reaction, we must know its
mechanism.
One method for determining the form of the rate law for a reaction is the
method of initial rates.
Define molecularity
number of species that must collide to produce the reaction.
Spontaneity refers to the inherent tendency for the process to
occur.
We will always define rate as a
positive quantity.
For a second order reaction, each successive half life is double the
preceeding one.
For a second order rate law; k = k[A]^2 doubling the concentration of A ____ the reaction rate. Tripling the concentration of A increases the rate by ____ times.
quadruples; nine
The first step in understanding how a given chemical reaction occurs is to determine the form of the
rate law.
An expression which shows how the rate depends on the concentrations of reactants is called a
rate law. Rate= k[NO2]^n k is the proportionality constant n is order of reactant
General equation for differential rate law:
rate= k[A]
What is the general form of the rate law when given the equation: BrO3-(aq) +5Br-(aq) + 6H+(aq) -> 3Br2(l) + 3H2O(l)
rate= k[BrO3-]^n [Br-]^m [H+]^p
Knowing that the reactant N2O5 is first order, and the rate= -delta[N2O5]/deltat, what is the differential rate law form?
rate= k[N2O5]
The initial rate of a reaction is the instaneous rate(slope) determined just after the
reaction begins. (just after t=0)
What is the term for understanding the steps of a reaction
reaction mechanism
To be useful, reactions occur at a
reasonable rate.
The second half life is much longer than the first half life in a
second order reaction. (This is a characteristic pattern of second order)
time is in the unit
seconds.
any rate expression involving a reactant will include a negative
sign. (since its concentration always decrease)
The idea is to determine the instantaneous rate before the intial concentrations of reactants have changed
signifigantly.
one of the main purpose in kinetics is to learn as much as possible about
steps involved in a reaction.
What is the half life equation for a first order reaction?
t1/2 = 0.693/k
What is the difference between a first order and second order reaction?
t1/2 depends on both k and [A0] for a second order reaction. For a first order reaction, t1/2 depends only on k. A constant time is required to reduce the conentration of the reactant by half in first order reactoion.
Half life(second order) formula
t1/2= 1/k[A0]
The half life of a zero order reaction
t1/2= [A]0/ 2k
Half life for zero order
t1/2= [A]0/2k
In order to discuss the rate of the reaction in terms of the product, you must take into account
the coefficients of the balanced chemical reaciton.
The value of the rate at a particular time is called
the instantaneous rate
In order to calculate how much time is required for a reaction to be 75 % complete, what equation must you use?
the integrated rate law as a ratio. kt= ln([Ao]/[A]) !!(pay attention to flipped numerator and denominator) You must then look at [A]/[Ao] *100 % = 25% since 75% was consumed and 25% is left. [A]/[Ao] is 0.25 and flipping the num and denom would be 4.0. The equation would be ln(4.0) = (3.47*10-2)t since you already calculated k from the half life for first order equation.
What is the most common method for experimentally determining the differential/rate law?
the method of initial rates. (given initial reactant concentrations and initial rates)
What do I mean by determining the form of the rate law?
the power to which each reactant concentrationis raised to.
what does k1, and k2 over the arrows represent?
the rate constants for each individual reaction.
A rate law that expresses how the rate depends on concentrations is called the differential rate law, but is often simple called
the rate law
In order to avoid the above complication(difference in rates of forward and reverse reactions is complicated), we must view the reaction at a point in which
the reactant is just mixed and before the products form.
A balanced equation does not tell us
the reaction mechanism( the steps of how reactants become products)
If we choose a reaction in which the reverse reaction is neglected,
the reaction rate will only depend on the reactants concentration.
rate laws reveals
the steps by which a reaction occurs.
Given the differential rate time(dependent on concentrations) we want to be able to write it as a function of
time
Rate(reaction rate) is not constant and decreases with
time.
Rate is constant; it does not change as concentration increases for zer order reaction; true or false?
true
The rate constant, k can be calculated by plugging in any experiment set
values
BE CAREFUL: When given various initial rates and initial concentrations for various reactants, you can only determine the order for a reactant if..
you choose two sets of values in which everything BUT THAT reactants initial concentration is constant.
2NO2(g)-> 2NO + O2(g) can tell us that
No is produced at the same rate NO2 is consumed. No is the same shape as the curve for NO2 except that the curve is inverted. The slope will be the negative slope for NO2. O2 is produced at half the rate of No2. The slope of NO is twice the slope of 02.
12.2
Rate Laws: An Introduction
What is the rate law of 2N2O5(soln) -> 4No2(soln. ) + O2(g)?
Rate= -delta[N2O5]/deltat= k[N2O5]
12.1
Reaction Rates
12. 6
Reaction mechanisms
How do you determine rate laws for complex reactions(involving more than one reactant)?
Step 1: Determine which reactant has the smallest concentration. The smallest concentration means it will be the most likely affected. Step 2: Write k' = k[A]n [B]m for the reactants with large initial given concentrations. Large cocnentrations will be considered constant. Step 3: Write full rate law with k'. The order is of the reactant with the least amount of concentration.
12.4
The Integrated Rate Law
In summary, what do we always assume for rate laws written for reactions?
The rate law is only written for reactant concentrations and only proceeds in the forward direction.
What does A represent in the reaction rate of a chemical reaction?
The reactant OR product being considered. The square brackets indicate concnetration in mol/L.
Why do the concentrations of products not appear in the rate law?
The reaction rate is being studied under conditions where the reverse does not contribute to rate.
The rate constant, k has what units?
The units can be determined by plugging in every value for the rate law(k[A]^n) and only looking at the resulting units.
Given: Rate= ksubprime[A]^n how is the value of n determined? how is the value of ksubprime deterimned? How is the value of n, m, and p determined?
The value of n is determined by plotting [A] vs t, ln[A] vs t, or 1/[A] vs t. Whichever is straight, is the order. The value of ksubprime is the slope for the respective plot. n, m, and p can be determined by the value of k' at several concentrations of B and C.
What is the main goal of kinetics?
To understand the the steps by which a reaction takes place.
What is characteristic of a zero order reaction experimentally?
Zero order reactions typically require a metal surface or an enzyme to react. ex: N2O5 molecules on a hot platinum surface will not vary in rate because the molecules are only influenced by what happens on the pad.
Reaction rate=
[(concentration of A at time t2- concnetration of A at time t1)]/ [(t2-t1)] =delta[A]/deltat
Integrated rate law zero order
[A] = -kt + [A0]
The integrated rate law for a zero over expression
[A]= -kt + [A0]
What does delta represent?
a change in a given quantity. NOTE that the change can be positive(increase) or negative(decrease); leading to a positive or negative reaction rate by definition.
Pseudo first order rate law
a rate law that was obtained by simplifying a more complicated one( use k' to represent the reactants that have a large initial conentration, and the reactants must have a zero subscript to indicate)
Most chemical reactions occur by a series of steps called
a reaction mechanism
What is an elementary step?
a reaction whose rate law can be written from its molecularity.
Chemical reactions are reversible: 2No2(g) -> 2NO(g) +O2(g)
2NO(g) + O2(g) -> 2NO2(g) As NO and O2 accumulate, they can react to reform 2No2.
For first order integrated rate law, what is the unit of k?
6.93 * 10^-3 / s
How do you determine if a reaction is first order or second order given concentration and time data values?
By plotting a graph. ln[A] plotted against t should be straight. This is single order 1/[A] vs t should be straight if it is second order.
12.3
Determining the form of the Rate Law
When attempting to calculate the rate constant k, provided the integrated law and knowledge that it is first order, what must you do?
In the equation, -k represents m(the slope). Thus you must do y2-y1/x2-x1. Here y has to represent ln[A] and x has to represent time. In the end, you will find the slope m but remember k is negative. So given m, the rate constant is -(m).
What is the integrated rate law?
Expresses the concentration of the reactant as a function of time. ln[A]= -kt + ln[A]0
How do you calculate the slope for a single order or second order reaction?
Given concenrations, calculate the change in concentration for the respective ln[A] or 1/[A]. NOTE: ALWAYS USE THE FIRST AND FINAL POINT.
Reaction rate of a chemical reaction is defined as
change in concentration of a reactant or product per unit of time.
The order cannot be determined by
coefficients in the balanced chemical equation
We can choose which rate law to use by determining which data is the easiest to
collect
The order of a reactant concentration can only be determined by:
comparing the two reactant concentrations and the two rates. For zero order: The rate is constant even when reactant concentrations change. For first order: rate of reaction is directly proportional to reactant concentration. For second order: doubling reactant A will quadruple the rate.
The value of the rate at a particular time(the instantaneous rate) can be obtained by
computing the slope of a line tangent to the curve at that point.
For a first order reaction, the half life does not depend on
concentration.
