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Level 149

Rates & Mechanisms of Chemical Reactions


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chemical kinetics
study of how fast chemical reactions occur
reaction rate
speed at which chemical reactions occur
concentrations of reactants
Four (Controllable) Factors that Affect Rate
Effect of Concentration on Rate
MOLECULES MUST COLLIDE TO REACT!!
MOLECULES MUST MIX TO REACT!!
Effect of Physical State on Rate
Effect of Temperature on Rate
MOLECULES MUST COLLIDE WITH ENOUGH ENERGY!!
Rate of [A]
= -?[A] / ?t ?reactant
Instantaneous Rate
rate at a particular instant during the reaction
Initial Rate
instantaneous rate at the moment the reactants are mixed @ t=0
aA + bB ? cC = dD
Rate = -(1/a)(?[A]/?t) = -(1/b)(?[B]/?t) = (1/c)(?[C]/?t) = (1/d)(?[D]/?t)
rate law
an equation that relates reaction rate and concentration at a certain temperature
Rate Constant (k)
specific for a given reaction at a given temperature and does NOT change as the reaction proceeds
Reaction Orders
-exponents (m & n)
How to Determine Initial Rate
from plot of concentration vs time
3 Methods to Determine Concentration
1)Spectrometric Methods: measure concentration of component that absorbs/emits characteristic wavelengths of light
Order of Reaction (Overall)
sum of exponents (ind. rxn orders) in rate law
First Order Rxn
rate doubles when [A] doubles
Second Order Rxn
rate quadruples when [A] doubles
Zero Order Rxn
rate does not change when [A] doubles
Determining Reaction Orders Experimentally
-run a series of experiments in which one reactant concentration changes while the other is kept constant, and measure the effect on the initial rate in each case
mol/L·s
Units of K for Overall Reaction Order 0
1/s
First order unit
L/mol·s
Units of K for Overall Reaction Order 2
L²/mol²·s
Units of K for Overall Reaction Order 3
Integrated Rate Laws
different form of the rate laws
ln([A]0/[A]t)=kt ?(rate=k[A])
Integrated Rate Law for First Order Rxn
(1/[A]t)-(1/[A]0)=kt ?where rate: k[A]²
Integrated Rate Law for 2nd Order Rxn (1 Reactant)
Integrated Rate Law for Zero Order Rxn
[A]t - [A]0 = -kt ?rate: k[A]° = k
Fraction of Decomposition
([A]0 - [A]t)/[A]0
Half-Life(t?1/2)
of a reaction is the time it takes for the reactant concentration to reach half its initial value
Half Life of First-Order Rxns
t1/2 = ln2/k = 0.693k
t1/2= 1/(k[A]0) ?rate=k[A]²
Half Life of Second Order Rxns
t1/2= [A]0/2k
Half Life of Zero Order Rxns
collision theory
for a set of reactions to occur, molecules must collide
Arrhenius Equation
An equation Expressing the temperature dependence of a reaction rate constant. K=Ae^Ea/RT
Activation Energy (Ea)
an energy threshold that the colliding molecules must EXCEED to react
Effect of Temp on Collision Energy
causes higher collision frequency AND energy
Calculating Activation Energy
if we know rate constants and temps at 2 ts
Effective Collisions
the collisions that actually lead to product because the atoms that become bonded in the product make contact
Frequency Factor
a in the arrhenius equation
Transition State Theory
focuses on high-energy species that exists at the moment of an effective collision when reactants are becoming products
transition state
In each individual step of a reaction, the state of highest energy between reactants and products. The transition state is a relative maximum (high point) on the reaction-energy diagram.
Reaction Energy Diagram
plots how potential energy changes as reaction proceeds form reactants to products (rxn process)
Reaction Mechanism
a SEQUENCE of single reaction steps that sum to the overall equation
Elementary Reactions(/Steps)
the individual steps that make up a reaction mechanism
Molecularity
characterizes elementary steps
Unimolecular Reaction
-one that involves the 'decomposition or rearrangement' of a SINGLE particle
Bimolecular Reaction
one in which TWO particles react
Termolecular Reaction
EXTREMELY rare because the probability of three particles colliding SIMULTANEOUSLY with enough energy and effective orientation is EXTREMELY small
Unimolecular
A reactions involving a single species. A unimolecular reaction occurs when a molecule rearranges itself to produce one or more products.
bimolecular
2A?product (elementary step)
termolecular
2A + B?product
Rate-Determining Step
limits how fast the overall reaction proceeds(the slowest step)
Reaction Intermediate
a substance formed and used up throughout the steps of a reaction mechanism
A Valid Mechanism must meet 3 Criteria:
1)elementary steps must add up to overall balanced equation
Mechanisms w/ Slow Initial Step
step 1 is rate determining state, same as overall rate law
Mechanisms w/ Fast Initial Step
1)write rate laws for fast step(fwd&rev) and slow step
catalyst
a substance that increase reaction rate w/o being permanently used up or changed. it lowers the activation energy needed for the reaction so more molecules have the necessary energy for a reaction
homogeneous catalyst
a catalyst that is in the same phase as all the reactants and products
heterogeneous catalyst
a catalyst that is in a different phase
Hydrogenation
A reaction involving the addition of Hydrogen.
enzymes
biological catalyst
Active Site
-small part of enzyme's surface (like a hollow carved into a mountainside)
Substrates
reactant molecules
Lock and Key Model
model of enzyme action
Induced-Fit Model
more current model of enzyme action
Enzyme-Substrate Complex (ES)
intermediate formed from enzyme catalysis
kinetic-molecular theory
The model that accounts for macroscopic gas behavior at the level of the particles (atoms or molecules) is known as the...?
postulate
a thing suggested or assumed as true as the basis for reasoning
assumed to be zero
Postulate 1 of the kinetic-molecular theory, states that PARTICLE VOLUME is...?
Postulate 2 of the kinetic-molecular theory, states that PARTICLE MOTION is...?
constant, random, and straight-line until they bump into the container wall or each other
elastic (total kinetic energy "Ek" is constant)
Postulate 3 of the kinetic-molecular theory, states that PARTICLE COLLISIONS are...?
average kinetic energy
At any given temperature, all gases have the same...?
gas particles and walls
The kinetic-molecular theory states that pressure arises from countless collisions between...?
because the particles more frequently collide with the container
Why does an increase in the number of particles increase the internal pressure of a container...?
because collisions of the particles with the walls of the container becomes more frequent
Why does an increase in pressure exerted on a gas (Pext) increase the pressure of a gas (Pgas) in a container...?
free space between their particles
The fact that liquids and solids cannot be compressed implies there is little, if any...?
because more particles are colliding with the walls per second
Why does an increase in the amount of a gas (in moles) increase the pressure of the gas (Pgas)...?
Why does an increase in temperature (T) increase the pressure ( P ) of a gas...?
because the average kinetic energy of the particles increases which causes them to collide more with the walls
the pressure exerted by the atmosphere (Patm)
At equilibrium, for an amount of gas (n), the pressure of the gas (Pgas) is equal to...?
the pressure of the gas (Pgas) equals the pressure exerted by the atmosphere (Patm)
Whether increasing temperature (T), or increasing the amount of gas (n), the volume of the gas (V) will increase until...?
because larger molecules move more slowly. (KE= 1/2 m v^2)
How do larger gas molecules, such as oxygen (O2), have the same kinetic energy, and thus pressure, as smaller gas molecules such as hydrogen (H2)...?
Average Kinetic Energy of particles
is directly proportional to the absolute temperature.
Why does the mercury in a thermometer rise when placed over a flame...?
because the kinetic energy of the high energy gas molecules is being transferred to the lower energy particles of mercury
reaction speed
The change in the concentration of reactants (or products) as a function of time is known as...?
the nature of the reactants
Under any given set of conditions, each reaction has its own reaction rate, determined by...?
What four factors can we control that affect the rate of a reaction...?
reactant concentration, physical state of reactants, temperature of reaction, & the use of a catalyst
collisions between reactant molecules
Concentration affects rate of reaction by influencing the frequency of...?
the interface between the phases
When the reactants are in different phases, contact occurs only at...?
reactants can mix
Physical state affects rate of reaction by determining how well...?
Why does steel wool burn more readily than a steel nail...?
because it has greater surface area and therefore greater rate of collision with oxygen gas
Why do you start a campfire with twigs, and not logs...?
because of the greater surface area of twigs are able to react more readily with oxygen gas
frequency & energy of collisions between reactant molecules
Temperature increases the rate of reaction by influencing what two things...?
more frequently
At a higher temperature, collisions occur...?
energy of the collisions between reactant molecules
Temperature affects reaction rate by influencing the frequency and, more importantly, the...?
increase
as the size of halogens increase, the melting and boiling points
negative
at the beginning of a redox reaction, the charge of the anode is
Square brackets "[ ]" indicate...?
a concentration in moles per liter (mol * L^-1)
Positive
Anode (Electrolytic Cell)
a reaction proceeds
In most cases, reaction rate varies as...?
equal
If the moles of two reactants are equal, then the rates of reactant concentration for each reactant (-?[A1]/?t & -?[A2]/?t), is presumably...?
two points of time
The AVERAGE RATE is the rate of change in reactants (or products) over...?
instantaneous reaction rate
In general we use the term reaction rate to mean...?
which reaction component is being referenced
The expression for a reaction rate and its numerical value depend on...?
concentration
a mixture that has a large amount of solute in the solvent; a strong solution
concentrations and temperature
The rate law, or rate equation, expresses the rate as a function of...?
reactant concentration ([A] & [B])
Reaction orders (m & n) define how the rate is affected by...?
reactant orders (m & n)
The coefficients of reactants in a balanced equation (a & b) are not related in any way to...?
Experiment
+/- 1 phenylethylamine is resolved using tartaric acid L (+) tartaric acid
each reactant
A reaction has an individual order with respect to...?
sum of the individual orders
A reaction has an overall order, which is the...?
the rate doubles
If a reactant is to the FIRST ORDER, then when the concentration of that react ([A]) doubles, then...?
the rate quadruples
If a reactant is to the SECOND ORDER, then when the concentration of that reactant ([A]) doubles, then...?
the rate is unaffected
If a reactant is to the ZERO ORDER, then when the concentration of that reactant ([A]) doubles, then...?
experimental data
Reaction orders CANNOT be deduced from the balanced equation but MUST be determined from...?
the square root of the concentration of the reactant
If a reactant and equation are half order (R=k[A]^1/2), then the rate is...?
A negative reaction order means that the rate...?
decreases when the concentration of that component increases
is kept constant
To find the reaction order values (m & n), we run a series of experiments in which one reactant concentration changes while the other...?
one reactant at a time to see its effect on the rate
With an accurate method for obtaining initial rates, reactions orders are determined experimentally by varying the concentration of...?
the rate constant (k)
By substituting the known rate, concentrations, and reaction orders into the rate law, we solve for...?
calculus
We integrate over time to obtain the integrated rate law of reactions, using methods of...?
reaction orders and rate constants graphically
Rearrangements of the integrated rate laws, that give equations in the form of a straight line, allow us to determine...?
first order (with respect to that reactant)
If you obtain a straight line when you plot the natural logarithm of a reactant (ln[A]) vs. time (t), the reaction is...?
second order (with respect to that reactant)
If you obtain a straight line when you plot 1 divided by the concentration of the reactant (1/[A]) vs. time (t), the reaction is...?
zero order (with respect to that reactant)
If you obtain a straight line when you plot the concentration of a reactant ([A]) vs. time (t), the reaction is...?
half of its initial value
The half-life (t1/2) of a reaction is the time it takes for the reactant concentration to reach...?
temperature (T)
A half-life (t1/2) is a characteristic of a reaction at a given...?
order of the reaction
The mathematical expression for the half-life (t1/2) depends on the overall...?
reactant concentration ([A])
For a first-order reaction, the time it takes to reach one-half the starting concentration is a constant (ln(2)/k) and, thus, independent of...?
inversely (1/k[A]0)
How is the initial reactant concentration of a second-order reaction related to its half life (t1/2)...?
directly ([A]0/2k)
How is the initial reactant concentration of a third-order reaction related to its half life (t1/2)...?
react
According to the collision theory, reactant particles must collide to...?
multiplying the number of reactant molecules
According to the collision theory, the number of possible collisions in a reaction is found by...?
increases the rate constant (k)
As Arrhenius equation (k = A*e^(-Ea/RT)) shows, a rise in temperature (T) increases the rate because it...?
change to products
If reactant molecules collide with a certain minimum energy, they reach an activated state, from which they can...?
exothermic or endothermic
The relative activation energy (Ea) values for the forward and reverse reactions depend on whether the overall reaction is...?
equal to or greater than activation energy (Ea)
The equation f=e ^ -Ea/RT shows at a given temperature the fraction (f) of collisions with energy...?
smaller orientation probability factor ( p ) values
More complex molecular structures, have what type of orientation probability factor ( p ) values...?
transition state (or activated complex)
The activation energy of a reaction is used to reach the...?
highest potential energy
The transition state (or activated complex) is a very unstable species that exists only at the instant of...?
reactants to products (reaction progress)
A reaction energy diagram plots how potential energy changes as the reaction proceeds from...?
revert to reactants
Transition states either continue toward products, or...?
elementary reactions
The individual steps that make up a reaction mechanism are called...?
reaction orders in the rate law
Only for elementary steps, can we use the equation coefficients as the...?
how fast the overall reaction occurs
The rate-determining (slowest) step in a mechanism determines...?
rates
All the elementary steps in a mechanism have their own...?
how fast the overall reaction proceeds
The rate-determining step in a mechanism, is much slower then the rest, and limits...?
the overall reaction
The rate law for the rate-determining step is the rate law for...?
the rate-determining step
Reactants do not appear in the rate law (reaction order = 0) when they take part in the mechanism AFTER...?
overall balanced equation
Elementary steps must add up to the...?
the rate constant (k) of the overall rate law
The rate constant (k) of the rate-determining step is equal to...?
the rate law of the backward reaction
For a reversible reaction, the rate law of the forward reaction is equal to...?
transition states
Each step in a mechanism has its own peak on a reaction energy diagram, representing...?
unstable and reactive
Intermediate species are higher in energy than reactants or products, because they are...?
activation energy required (Ea)
The slow (rate-determining) step of a mechanism has a higher 'what' than the fast step...?
lowering the activation energy (Ea)
In general, catalysts increase the rate of a reaction by...?
yield (with or without a catalyst)
A catalyst speeds up the forward and reverse reactions. Thus, a reaction has the same...?
faster
A catalyst causes the product to form...?
mechanism for the reaction
A catalyst causes a lower total activation energy by providing a DIFFERENT...?
forward activation energy (Ea) of the uncatalyzed pathway
The total of the activation energies for both steps of the catalyzed pathway (Ea a1'fwd' + Ea a2'fwd') is less than the...?
regenerated
The catalyst is not consumed, but rather used and then...?
reactants & products
Chemists classify catalysts based on whether or not they act in the same phase as the...?
homogenous catalysis
What type of catalysis exist in solution with the reaction mixture, so it must be a gas, liquid, or soluble solid...?
a heterogenous catalyst
What type of catalyst speeds up a reaction in a different phase...?
gaseous or liquid reactants
Most often heterogeneous catalysis involve solids interacting with...?
the catalyst's surface
When using a heterogeneous catalyst with enormous surface area, the rate-determining step occurs on...?
increase the rate further
Once a reactant covers a heterogenous catalyst surface, adding more reactant cannot...?
metal catalyst
the hydrogenation of carbon-carbon double bonds takes place on a solid...?
the breakage of hydrogen molecules (H2)
In using a metal catalyst, what rate-determinig step is sped up in the hydrogenation of carbon-carbon double bonds...?
amino acid side chains involved in catalyzing the reaction
The active site of an enzyme, is a region whose shape results from the...?
Enzymes act as both...?
homogenous & heterogenous catalysis
immobilized to wait for another reactant nearby
Enzymes act as heterogeneous catalysis by providing a surface on which one reactant is temporarily...?
the active site groups with the aqueous substrates
What part of an enzyme acts as a homogenous catalyst...?
groups at the active site
Enzymes are highly specific due to the particular...?
the lock-and-key model (outdated)
What earlier model of enzyme action proposed that a specifically shaped substrate would fit into a specifically shaped active site...?
the induced-fit model
According to 'what' more recent, and accepted, model of enzyme activity do enzymes change shape when the substrate lands at the active site...?
rate of reaction (not the [S])
The concentration of enzyme-substrate complexes ([ES]) determines the...?
bond cleavage
Many hydrolases have acidic groups that provide hydrogen ions (H+) to speed...?
lysozyme
What hydrolase, found in tears, protects the eye from bacterial infection by hydrolyzing their polysaccharide cell walls...?
chymotrypsin
What hydrolase, found in the small intestine, hydrolyzes proteins during digestion...?
stabilizing the reaction's transition state
No matter what their mode of action, all enzymes catalyze by...?
breakdown of stratospheric ozone (O3)
Chlorine atoms from chlorofluorocarbon (CFC) molecules act as catalysis and speed the...?