Chapter 14: Chemical Kinetics

Chapter 14: Chemical Kinetics

chemical kinetics – area of chemistry dealing with speeds/rates of reactions
rates of reactions affected by four factors
1) concentrations of reactants
2) temperature at which reaction occurs
3) presence of a catalyst
4) surface area of solid or liquid reactants and/or catalysts

14.1 Reaction Rates

reaction rate – speed of a chemical reaction

14.1.1 Rates in Terms of Concentrations

rate calculated in units of M/s
brackets around a substance indicate the concentration

instantaneous rate – rate at a particular time
instantaneous rate obtained from the straight line tangent that touches the curve at a specific point
slopes give instantaneous rates
instantaneous rate also referred to as the rate

14.1.2 Reaction Rates and Stoichiometry

for the reaction

14.2 The Dependence of Rate on Concentration

equation used only if C and D only substances formed
Rate = k[A][B]

Rate law – expression that shows that rate depends on concentrations of reactants
K = rate constant

14.2.1 Reaction Order

Rate = k[reactant 1]^m[reactant 2]ⁿ
m, n are called reaction orders
m+n, overall reaction order
reaction orders do not have to correspond with coefficients in balanced equation
values of reaction order determined experimentally
reaction order can be fractional or negative

14.2.2 Units of Rates Constants

units of rate constant depend on overall reaction order of rate law
for reaction of second order overall
units of rate = (units of rate constant)(units of concentration)²
units of rate constant = M^-1s¹

14.2.3 Using Initial Rates to Determine Rate Laws

zero order – no change in rate when concentration changed

first order – proportional changes in rate

second order – increase rate by 2² or 3³, etc…
rate constant does not depend on concentration

14.3 The Changes of Concentration with Time

rate laws can be converted into equations that give concentrations of reactants or products

14.3.1 First-Order Reactions

corresponds to y = mx + b
equations used to determine:
1) concentration of reactant remaining at any time
2) time required for given fraction of sample to react
3) time required for reactant concentration to reach a certain level

14.3.2 Half-Life

half-life of first order reaction

half-life – time required for concentration of reactant to drop by one half initial value
t_1/2 of first order independent of initial concentrations
half-life same at any given time of reaction
in first order reaction – concentrations of reactant decreases by ½ in each series of regularly spaced time intervals

14.3.3 Second-Order Reactions

rate depends on reactant concentration raised to second power or concentrations of two different reactant each raised to first power
Rate = k[A]²
half life dependent on initial concentration of reactant

14.4 Temperature and Rate

chemiluminescent reaction – reaction that produces light
rate constant must increase with increasing temperature

14.4.1 The Collision Model

collision model – molecules must collide to react
greater number of collisions the greater the reaction rate
for most reactions only small amount of collisions lead to a reaction

14.4.2 Activation Energy

Svante Arrhenius
Molecules must have a minimum amount of energy to react
Energy comes from kinetic energy of collisions
Kinetic energy used to break bonds
Activation energy, E_a – minimum energy required to initiate a chemical reaction
Activated complex or transition state – atoms at the top of the energy barrier
Rate depends on E_a
Lower E_a means faster reaction
Reactions occur with collisions and orientation of molecules

14.4.3 The Arrhenius Equation

reaction rate data:
(Arrhenius Equation)
k = rate constant, E_a= activation energy, R = gas constant (8.314 J/mol K), T = absolute temperature, A = frequency factor
A relates to frequency of collisions, favorable orientations
ln k vs 1/t graph has slope –E_a/R and y-intercept ln A
for two temperatures:
used to calculate rate constant, k₁ and T₁

14.5 Reaction Mechanisms

reaction mechanism – process by which a reaction occurs

14.5.1 Elementary Steps

elementary steps – each step in a reaction

molecularity – if only one molecule involved in step

unimolecular – if only one molecule involved in step

bimolecular – elementary step involving collision of two reactant molecules

termolecular – elementary step involving simultaneous collision of three molecules
elementary steps in multistep mechanism must always add to give chemical equation of overall process

intermediate – product formed in one step and consumed in a later step

14.5.2 Rate Laws of Elementary Steps

if reaction is known to be an elementary step then the rate law is known
rate of unimolecular step is first order (Rate = k[A])
rate of bimolecular steps is second order (Rate = k[A][B]
first order in [A] and [B]
if double [A] than number of collisions of A and B will double

14.5.3 Rate laws of multistep mechanisms

rate-determining step – slowest elementary step
determines rate law of overall reaction

14.5.4 Mechanisms with an Initial First Step

intermediates usually unstable, low and unknown concentrations
whenever a fast step precedes a slow one, solve for concentration of intermediate by assuming that equilibrium is established in fast step

14.6 Catalysis

catalyst – substance that changes speed of chemical reaction without undergoing a permanent chemical change

14.6.1 Homogeneous Catalysis

homogeneous catalyst – catalyst that is present in same phase as reacting molecule
catalysts alter E_a or A
generally catalysts lowers overall E_a for chemical reaction
catalysts provides a different mechanism for reaction

14.6.2 Heterogeneous Catalysis

exists in different phase from reactants
initial step in heterogeneous catalyst is adsorption

adsorption – binding of molecules to surface
adsorption occurs because ions/atoms at surface of solid extremely reactive

14.6.3 Enzymes

biological catalysts
large protein molecules with molecular weights 10,000 – 1 million amu

catalase – enzyme in blood and liver that decomposes hydrogen peroxide into water and oxygen

substrates – substances that undergo reaction at the active site

lock-and-key model – substrate molecules bind specifically to the active site

enzyme-substrate complex – combination of enzyme and substrate
binding between enzyme and substrate involves intermolecular forces (dipole-dipole, hydrogen bonding, and London dispersion forces)
product from reaction leaves enzyme allowing for another substrate to enter enzyme

enzyme inhibitors – molecules that bind strongly to enzymes

turnover number – number of catalyzed reactions occurring at a particular active site
large turnover numbers = low activation energies