w2 Flashcards
Arrhenius Equation relates to
energy, collision frequency, temperature, orientation
Arrhenius Equation
k = rate constant = Ae ^-Ea/RT
Frequency factor (units L mol -1 s-1)
- combines collision frequency and probability of correct orientation
- specific to each reaction
- temp dependent
fraction of molecules with minimum energy for reaction
always less than 1, changes significantly with temp
Effect of catalysts on RR
provides a mechanistic pathway with a lower activation energy, can be regenerated, very effective, specific, accelerates reactions.
Homogeneous catalysis: interconversion of but-2-ene without catalysts. Ea: 264 kJ mol-1
- large activation energy
- because C=C must be broken
- low frequency
- requires high temps
mechanism of iodine-catalysed isomerization of cis-but-2-ene (learn the circle)
- converted to trans-but-2-ene with iodine
- emphasizes the catalytic cycle
- rate = [cis-C4H8][I2]^1/2
heterogeneous catalysis: ethylene hydrogenation
- H2 molecules collide with platinum surface and form Pt-H bones, weakening H-H bones and leaves H bound to surface
- C2C4 also attaches to surface through bones from each C atom to a Pt by breaking the C=C bond in each molecule
- Rxn with adjacent H atoms first leads to formation of C2H5 and then makes ethene C2H6
- New C-H bonds are formed as Pt-C bonds are weakened and C2H6 molecules leaves the surface
- In their place, other C2H4
and H2 molecules can bond to the surface and undergo reaction.
Rxn mechanisms
- sequence of bond-making
and bond-breaking steps
that occurs during the
conversion of reactants to
products. - can’t be proved but shown through experiments
sequence of steps: rxn of Br2 (g) and NO (g)
Nitric oxide is a free radical, i.e., it has an unpaired electron. It is sometimes
denoted by a dot in its formula, i.e., ·NO. (Ozone depletion). - unlikely to have a singe step reaction
Br2 molecules and NO molecules combine to produce molecules of an
intermediate species, Br2NO.
Br2NO then reacts with another NO molecule to form two molecules of the
reaction product.
Elementary steps
Each step.
The equation for an elementary step describes particular collisions:
1. which particles collide,
2. which bonds are broken or formed,
3. and which particles are formed.
Molecularity of an elementary step
the number of molecules, ions, or atoms that collide and undergo change.
– A unimolecular step involves reaction of only one particle,
– a bimolecular step involves two particles,
– termolecularstep involves three particles (atoms, molecules or ions).
Rate equations for elementary steps
Determined directly from stoichiometric equations
1. A -> product, unimolecular, Rate = k[A]
2. A + B -> product, bimolecular, Rate = k[A][B]
3. A+A -> product, bimolecular, rate = k[A]^2
4. 2A + B ->product, termolecular, rate = k[A]^2[B]
not expected to have the same value
What controls the rate at which products are produced
the slowest elementary step (rate limiting step
Nucleophilic substitution rxns
involves the substitution of one electron-rich nucleophile (such as a chloride or a hydroxide ion) in molecules of a substance (referred to as the substrate) for an atom, group of atoms, or an ion, called the leaving group.
A good nucleophile…
needs to be able to donate an electron pair to an electron deficient (electrophilic) site on the substrate. The leaving group must accept an electron pair.
S(n)2 Mechanism
As the two particles react, the C-Br bond weakens simultaneously with the gradual formation of a C-OH bond.
In the maximum-energy transition state, the C-H, C-CH3 and C-CH2CH3 bonds are planar. As the rearrangement proceeds, the C-OH bond formation is completed. The product CH3CH9CH(OH)CH3 molecule
has stereo-chemistry inverted at the C atom. The Brion is released with the electron pair from the former C-Br bond.
Based on:
* experimental observation of
second-order kinetics
* stereochemical evidence: the
stereochemistry of chiral
substrates is inverted in the
product
These reactions proceed by a single
bimolecular step during which the
nucleophile interacts with the
substrate on the side opposite the
leaving group.
Substitution nucleophilic (unimolecular) S(n)1
There is one species involved in the rate-determining step. rate is first order with respect to the conc of the substrate (CH3)3CBr (SN1), but independent of the conc of the OH nucleophile. (OHis not in the rate equation) A rate-determining, bond-breaking elementary step is
consistent with first-order kinetics.
- Infrequent (and rate determining)
dissociation of Br ions creates a
carbocation. - Carbocations react quickly with
nucleophilic water molecules. - This produces protonated alcohol.
- Loss of a proton from the protonated
alcohol ion, gives molecules of the
alcohol product.
Racemate formation in SN1 reaction mechanism from a
chiral substrate.
As the carbocations are planar, there is a 50% probability nucleophiles will
collide with them on either side of the plane of the ion. If chiral substrate: product is racemic.
The SN1 Mechanism of Nucleophilic Substitution Reactions
The SN1 mechanism is based on the experimental observation of firstorder kinetics and the stereochemical evidence that a racemic mixture of
the product is formed from a chiral substrate.
There is one reacting species in the rate limiting step.
These reactions proceed by infrequent unimolecular dissociation of the
substrate molecules to form planar carbocations, with which the
nucleophiles can react on either side with 50% probability.
Enzymes
polypeptides that catalyse
chemical reactions.
They have well defined amino acid
sequences and structures - in biology
structure is almost synonymous with
function.
There are four levels of structure: primary, secondary, tertiary and quaternary.
increase the reaction rate by a
factor of between 10^7 and 10^14
CO2 Equilibria in Water
Carbon dioxide dissolves in water to a small extent to produce carbonic acid,
which ionizes to give H+
(aq) and HCO3-(aq) ions
Carbonic anhydrase catalyst for rxn 1 and 2.
In blood, as hemoglobin loses O2, it picks up H3O+ (aq) ions produced by ionization of H2CO3 (reaction 3). HCO3 - (aq) ions are sent to lungs.
When hemoglobin takes on O2 in the lungs it releases H3O+. H3O+ (aq) and HCO3 (aq) re-form H2CO3 (aq), from
which CO2 (g) is liberated and exhaled.
Enzyme changes shape
Hexokinase catalyzes the initial step of glucose metabolism:
The transfer of a phosphate group from adenosine tri-phosphate (ATP) to
glucose, forming glucose 6-phosphate and adenosine diphosphate (ADP).
The hexokinase structure has an active site that firmly accommodates
a 1 glucose and ATP molecule pair in a reacting orientation.
Acids and Bases background
- common in nature
- pH of lakes, rivers, oceans, and rains are controlled by dissolved acids and bases
- lots of bodily functions depend on acids and bases (stomach, blood)
Arrhenius definition of acids and bases
– an acid is a substance that contains hydrogen & can release a hydrogen
ion (H+) upon dissociation with water
– a base is compound that produces hydroxide ions (OH-) in water
- limits acids and bases to substances that can dissolve in water.
- majority of acid-base systems & reactions are aqueous, non-aqueous acids and bases exists.
Acids and bases
– React to form a salt plus water
acid + base → salt +water
e.g. HCl +NaOH → NaCl + H2O
Hydronium Ion
Aqueous ionic solutions are even more complex:
– Ions are hydrated
– The H+ ion (also called a proton) is a special case - it is always associated with a water molecule
- H+ cannot exist in water on its own – always with water – H3O+
Relative strengths of weak acids and bases
- almost all acids and bases are WEAK (almost 100% dissociation)
Strong Acid: [H3O+] = [Acid]
Weak Acid: [H3O+] «_space;[Acid]
- described by the equilibrium constant for the
dissociation reaction
How do we predict the direction of acidbase reactions – which side is favoured?
acid + base conjugate base + conjugate acid
depends on relative “strengths” of the acid & bases
- a strong acid will be an excellent proton donor
– it can effectively ‘react’ completely, donating all available protons
(E.g. HCl completely dissociates to H+ and Cl-)
– conjugate base of this acid will be a weak base (e.g. Cl-)
- a weak acid is one in which not all of the acidic protons have “reacted” with a base – (eg. acetic acid in aqueous solution does not dissociate into ions appreciably – mostly
(99%) in molecular form) - a strong base will readily accept a proton
– NaOH dissociates completely.
– conjugate acid of this base will be a weak acid - a weak base has a low propensity to accept a proton
– E.g. the nitrate ion (conjugate base of a strong acid)
General rule: The stronger the acid or base - the weaker its respective conjugate species will be
Ionisation Constants of Weak Acids and Bases
Equilibrium constants for ionization of weak acids and weak bases in
aqueous solution are called ionization constants, Ka
and Kb
.
The larger the ionization constant, the stronger the weak acid or base.
EXAMPLE:
- HA(aq) + H2O(ℓ) ⇌ H3O+ (aq) + A- (aq)
Q = [H3O][A-]/[HA] = Ka
- B(aq) + H2O(ℓ) ⇌ BH+(aq) + OH-(aq)
Q=[BH+][OH-]/[B] = Kb
Relationship btw Ka of an Acid and Kb of its conjugate base
The stronger a weak acid is (the larger Ka is), the weaker its conjugate base (the smaller Kb is).
* For any conjugate acid-base pair
Ka × Kb = Kw
pKa + pKb = pKw (=14.00 at 25°C)
Lewis Model of Acids and Bases
include substances with no transferable protons
E.g. iron(III) chloride
- Based on the “sharing” of electron pairs in orbitals
- a molecule or ion that donates a pair of electrons to another atom to form a covalent bond.
- A Lewis acid accepts a pair of electrons from another atom in the formation of a covalent bond.
- The product of bond formation between a Lewis acid and a Lewis base is called an adduct.
Visualization of Reactive Sites of
Organic Acids and Bases
Electrostatic potential map models
– Red regions indicates where the negative charge of electrons
exceeds the positive charge on the nuclei (basic sites of
molecules)
– Blue regions indicates where there is net positive charge
(acidic sites of molecules)
Salt Solutions
Can be acidic, basic or neutral
Cations: conjugate acids of strong bases are weak acids with no effect on solution pH (G1 and G2 on periodic table)
Anions: conjugate bases of strong acids are weak bases that have no effect on solution pH (Cl- and NO3-)
The pH of solutions of other salts depends on whether cations
and anions are acidic or basic.
Metal Cation ARE Lewis Acids
have a ‘free’ orbital that can accept
an electron pair
Water - lewis base: donates unbonded electron pair to other atoms and molecules
All metal cations interact with water to form hydrated cations.
Co-ordinate covalent bonds form between the metal and a lone pair of eon the O atom of H2O.
The result is a complex ion or co-ordination complex.
Metal acts as an acid, produces…
H+
The charge on the metal attracts electrons (actually electron density)
This weakens the O-H bonds
One of the weak O-H bonds is converted into a lone pair
A proton is produced – the [H3O+] increases
Thus the hydrated ion can function as a Brønsted acid
This explains why many transition metal solutions are acidic
The aqueous solutions of many metal cations are…
Acidic.
A pH measurement shows that a
solution of copper (II) sulfate is acidic.
Among the common cations, Al3+, Fe3+ and Cr3+ form the most acidic solutions.
Salts of Al3+ and Fe3+ are used by
gardeners to make soils more acidic.
Measures of acidity of a solution. Acidity refers to…
- pH of solution
- amount of base needed to neutralize the dissolved acid (neutralizing capacity)
Rxns in aqueous solution between an acid and the conjugate base of any weaker acid are product favoured and “go to completion.”
– A base can react with all of the removable protons of a weak acid (and not just the hydronium ions present) at equilibrium
pH from Ka
STRATEGY:
Designate that the concentration of the weak acid decreases by X mol L-1 as equilibrium is reached.
The stoichiometry of the reaction equation allows us to determine equilibrium concentrations.
- Write balanced equation
- Tabulate known info
- Substitute Values at equilibrium
Dependence of Percentage Ionization on Magnitude of Ka
For weak acid solutions of a given concentration and temperature, the smaller Ka is, the lesser the percentage
of the weak acid that ionizes.
Dependence of Percentage Ionization on Solution Concentration
At a given temperature, the more dilute the solution of a weak acid
is, the greater the percentage of it that ionizes.
Solutions of Polyprotic Acids or Their Bases
More than one proton can be removed from molecules of polyprotic acids in acid-base reactions.
H3PO4(aq) + H2O(ℓ) ⇌ H2PO4-(aq) + H3O+(aq)
Ka1 = 7.5 × 10-3
H2PO4- (aq) + H2O(ℓ) ⇌ HPO4 2- (aq) + H3O+ (aq)
Ka2 = 6.2 × 10-8
HPO4 2- (aq) + H2O(ℓ) ⇌ PO43- (aq) + H3O+ (aq)
Ka3 = 3.6 × 10-13
- In solutions of weak polyprotic acids with large differences in the successive Ka values, the pH depends primarily on the hydronium ions generated in the first ionization step.
- A similar effect is observed for basic solutions.
Aqueous Solutions of Weak Bases
For weak bases in aqueous solution, there is a corresponding set of relationships to those that pertain
to solutions of weak acids.
