Chapter 6 - Chemical Kinetics Flashcards
define chemical kinetics
the area of chemistry that deals with rates of reactions
what is a rate?
a change in a measurable quantity over time
define reaction rate
the change in concentration of a reactant or a product of a chemical reaction per unit time
how do chemists track the progress of a chemical reaction to calculate reaction rates?
observe the appearance of a product or disappearance of a reactant at particular time points
what are commonly used methods by chemists to track progress of a chemical reaction?
changes in gas volume, colour, mass, pH, electrical conductivity
what is the kind of method that chemists use when tracking progress of a chemical reaction?
those that do not interfere with reaction progress or rate and that are as simple as possible to carry out
what is the first step in determining the rate of a chemical reaction?
collecting data on changes in the quantity of substances involved in a chemical reaction
define average reaction rate
change in reactant or product concentration over a given time interval
what is the equation for average reaction rate
rateA = Δ[A]/Δt
what does A indicate in the average reaction rate
the product of reactant
what does the square brackets represent in average reaction rate?
indicate concentration of A in mol/L
what does the concentration of a product change with?
increases with reaction progress
what does the concentration of a reactant change with?
decreases over time
what happens if A in average reaction rate is a product
Δ[A] will be positive
what happens if A in average reaction rate is a reactant
Δ[A] will be negative
what sign does Δ[A] always have
positive
how do we ensure Δ[A] is always positive?
if working with disappearance, include negative number at the beginning of equation
what sign does the average reaction rate always have?
positive
explain how the rate of a chemical reaction is not typically constant
average reaction rate decreases as more and more reactants are converted to products and as the reaction completes, the average reaction rate nears zero
define secant
a line that intersects two points on a curve
define slope
a measure of how steep the line is relative to the horizontal axis
average rate of reaction formula for a line on a graph
rateA = Δy (concentration)/ΔX (time)
what does plotting the concentration of a reactant gives?
a falling curve with steadily decreasing negative slope
what does plotting the concentration of a product give?
a rising curve with steadily decreasing positive slope
what does plotting a product and reactant give if they are the same mol?
inverse relationship
define instantaneous reaction rate
the rate of a chemical reaction at a single point in time
what is the instantaneous reaction rate equal to?
the slope of the tangent to the curve at that particular instant in time
what does a steeper curve for reaction rates mean?
the steeper the curve, the rate of appearance/disappearance is faster
stoichiometric rate relationships, what sign does the reactants have?
negative
stoichiometric rate relationships, what sign does the products have?
positive
how to calculate rate of appearance and consumption from stoichiometric rate relationships?
use inverse of coefficient
by convention, how are reaction rates expressed?
as positive values, can be absolute values
define collision theory
the theory that chemical reactions can occur only if reactants collide with proper orientation and with enough kinetic energy to break reactant bonds and form product bonds
what does the rate of reaction depend on?
the frequency and proportion of collisions that convert reactants into products
does an increased in the frequency of collisions increase or decrease the reaction rate?
increases reaction rate
do all orientations for collisions cause reations?
no
what is the symbol for activation energy
Ea
define activation energy
the minimum energy that reactant molecules must possess for a reaction to be successful
potential energy of reactant entities in collisions
energy stored in the bonds within and among the entities of the reactants
kinetic energy of reactant entities in collisions
movement
what will sufficient kinetic energy do in collisions?
break the reactant bonds
define activated complex
an unstable arrangement of atoms containing partially formed and unformed bonds that represent the maximum potential energy point in the change
what is activated complex also called?
the transition state
potential energy of reactants and products in an exothermic reaction
reactants have higher potential energy
potential energy of reactants and products in an endothermic reaction?
products have higher potential energy
effect of temperature on reaction rates
a small change in temperature will have a large effect on reaction rate
increase in temperature on reaction rates
reaction rate increases because it increases the number of entities that will have enough kinetic energy to equal or exceed the activation energy, therefore the probability of effective collisions increases
what 3 characteristics of reactants determine the activation energy needed for a successful collision?
bond type, strength and number
reaction rates and reactant bond number relationship
reactions involving breaking of fewer bonds per reactant proceed faster than those breaking a larger number of bonds per reactant
reaction rates and bond strength
weaker bonds are broken faster than stronger bonds
reaction rates and bond types and explain
reactions between ions are faster than reactions between molecules because in molecules, covalent bonds need to be broken and new bonds formed, which slows down reaction rate
reaction rate and size/shape of molecule/ion and explain
smaller, less complex entities have a faster reaction rate than complex molecules/ions because less bonds need to be broken and its more likely they will collide in the correct orientation
concentration and reaction rate relationship
increase concentration of reactant, the probability of collisions increase and a greater number of effective collisions will occur so reaction rate will increase
surface area and concentration rate relationship, specifically with solids and explain
increasing surface area of a solid reactant increases the probability of effective collisions because only entities at the surface of the solid reactant can collide with entities of the other reactant
what does increases concentration and surface area both do?
increases total number of collisions and therefore total number of effective collisions
what do catalysts not do?
don’t increase the number of collisions between reactant entities or increase kinetic energy of the entities of the reactants
what do catalysts do?
provide an alternate pathway for the reaction, which has a lower activation energy
how do catalysts increase reaction rate?
since they lower activation energy, at any given temperature, a larger fraction of entities of the reactants will have kinetic energy equal to or greater than this new lower activation energy and since there are a greater number of collisions, the reaction rate increases
energy difference using a catalyst between products and reactants?
does not change
define rate law
the mathematical expression that allows calculation of reaction rate as a function of reactant concentration at a given temperature and pressure
why do industries determine rate laws?
to set reaction conditions that maximize output while minimizing production costs
when does the concentration of reactants and temperature inside reaction vessel change in a chemical reaction?
as soon as the reaction begins
what is reaction rate proportional to?
proportional to the product of the initial concentrations of the reactants, where each concentration is raised to some exponential value
how are exponents in a rate law equation determined and how are they not determined?
determined experimentally, cannot be determined from the balanced equation
what can the values of the exponents be in a rate law equation?
any real number
name the placeholder letters for the first, second, and third exponent value for reactants in rate law equation
m, n, p
instead of using a proportionality sign, what is the equation for rate law?
rate = k[A]^m[B]^n
define rate law equation
the relationship among rate, the rate constant, the initial concentration of the reactants, and the orders of reaction with respect to the reactants
define rate constant
determined empirically/experimentally and is unique for a single reaction at a specified temperature
what is the symbol for rate constant
k
why must temperature be kept constant in reactions determining rate law equation?
the rate constant (k) changes with temperature
define order of reaction
the exponent used to describe the relationship between the initial concentration of a particular reactant and the rate of the reaction
if the exponent for concentration of A is 1, what is the reaction with respect to A?
first order
if the exponent for concentration of A is 2, what is the reaction with respect to A?
second order
define total order of reaction
the sum of the exponents in the rate law equation
what can the total order of reaction be used for?
to predict reaction rates
what is the reaction rate proportional to for each 3 reactants
proportional to concentration of reactant A raised to an exponential value, proportional to concentration of reactant B, raised to an exponential value, and proportional to concentration of reactant C, raised to an exponential value
if the order of reaction is one, what happens to the rate when we double or triple the initial concentration of the reactant?
it doubles and triples
if the order of reaction is two, what happens to the rate when we double or triple the initial concentration of the reactant?
it quadruples and x9
if the order of reaction is zero, what happens to the rate when we double or triple the initial concentration of the reactant?
rate does not change
graph of reaction rate vs initial concentration when the reaction is zero order with respect to [A]
straight line across
graph of reaction rate vs initial concentration when the reaction is first order with respect to [A]
diagonal line going up
graph of reaction rate vs initial concentration when the exponent n is greater than 1
curved up with positive slope
graph of reaction rate vs initial concentration when the reaction is second order with respect to [A]
diagonal line going up
what is a common method for chemists to use when determining the rate law equation for a reaction
initial rates
define initial rate
the instantaneous rate determined just after the reaction begins, just after t = 0 seconds
list the 3 steps needed when using the method of initial rates to determine rate law equation
measure instantaneous rate of reaction before any significant changes in concentration to the reactants; carry out several runs using different initial concentrations, determining initial rate resulting from each run; compare results to see how initial rate depends on initial concentration
why are products not in the rate law equation?
we deal with instantaneous initial rate and at this stage, none or close to none have been formed
when looking for pairs of data in runs for initial concentration and initial rate, what do we compare?
first run to other runs
how can order of reaction be determined from the rate constant, K and why?
the units because they change with the total reaction order
units of k when total reaction order is 0
mol/L x s
units of k when total reaction order is 1
s^-1 or 1/s
units of k when total reaction order is 2
L/mol x s
units of k when total reaction order is 3
L^2/mol^2 x s
what do balanced chemical equations provide?
overall summary of a chemical reaction
how do most chemical reactions occur?
in a sequence of simpler reactions
what provides clues about the steps of a reaction mechanism?
experiments
define elementary step
a step involving a one, two, or three entity collision that cannot be explained by simpler reactions
define reaction mechanism
a series of elementary steps by which a chemical reaction occurs
what are the two types of elementary steps?
a slow step and fast step
define rate-determining step
the step in a reaction mechanism that determines the rate of the overall reaction, the slowest step in the reaction mechanism
define reaction intermediate
a species that is neither a reactant nor a product but is formed and consumed during the reaction sequence
how many reactant entities do most reactions have?
one or two
if a reaction has one reactant entity, what happens?
a single entity collides with the sides of the container and breaks apart into smaller entities
why is a reaction with 3 reactant entities rare?
collisions of 3 entities simultaneously is more rare than the collision of 2 entities simultaneously
how can the rate law equation be written and not be written from?
cannot be written from overall balanced equation but can be written from balanced equations representing elementary steps (from rate-determining equation)
what is the rate law equation if 2 reactants are the same?
rate = k[A]^2
what are the 2 requirements of a plausible reaction mechanism
summing elementary steps in the reaction mechanism must give the overall balanced equation for the reaction; the reaction mechanism must agree with the experimentally determined rate law
how do you determine the slowest step in a reaction mechanism?
the experimentally determined rate law for the overall reaction indicates that the slowest step is rate determining and it matches the rate law written for the elementary step
