+

ChemistryUnit 4

+ Reaction RatesIt’s the change in the amount of reactants or products per unit time

1st Order:• Rate = k[X]1

• Constant Half Lives2nd Order:• Rate = k[Y]2

• Increasing Half Lives0 Order:• Rate = k[Z]0

Continuous Rate Method

Initial Rate Method

1o Halogenoalkane + Hydroxide = SN2

3o Halogenoalkane + Hydroxide = SN1

If it’s in the Rate Equation then it’s the Rate Determining Step

+ Nucleophilic SubstitutionSN2

SN1

Iodine + Propanone

Slow Fast

Fast

Fast Fast

Slow

+ Arrhenius Equation

ln(k) = X + Constant-Ea 1 R T

R = Gas Constant (8.314)T = Temperature (Kelvin)

1/temp (k)

ln( 1/time)x

y

Gradient = XY

+ Heterogeneous Catalysts• In a different state to the reactants

• Large surface area as they’re usually powder or a mesh

• Easily separated from products & excess reactants

• Can be poisoned:• Adsorbs too strongly to surface of catalyst and doesn’t

allow other reactants to adsorb to the catalyst

• e.g. – Nickel in Hydrogenation of Vegetable Oil – Platinum in catalytic converters in cars

How do they work?

- Reactant adsorbed onto surface of catalysts at the active site- Interaction between reactant & catalyst - Reaction occurs from the interaction- Products are desorbed – breaks off catalyst

Adsorb – forms a temporary bond when something sticks to a surface

Homogeneous Catalyst is when catalyst is in the same state to reactants

+ EntropyEntropy change of a reaction is measure of order or disorder

The order within is a substance is how the quanta of energy are arranged

Reaction will occur if overall entropy is increasing, from order to disorder

If entropy is +ve then reaction will tend to occur

Ordered

Disordered

But doesn’t exist

More disorder = more +ve SΘ

Solid OrderedLiquid Disordered Gas Very Disordered

More Complex/Moles More Disordered

+

+ Stable or Inert

Stable if there is no tendency for the reaction to ‘go’

If system is +ve but surrounding are –ve we can ‘force’ the reaction by increasing the temp

C (Diamond) C (Graphite)

+ve entropy change

But Ea is too high – Kinetically Inert

= - ve

+ Solubility

Endo

Endo

Exo

Standard enthalpy change of Solution:• Enthalpy change when one mole of a compound

dissolves to form solution containing 1moldm-3 under standard conditions.

Standard enthalpy change of Hydration:• Enthalpy change when one mole of gaseous ions

is hydrated under standard conditions to form a solution in which the concentration of ions is 1moldm-3.

Factors affecting solubility:• Ionic Charge > Ionic Radii

Lattice Energy & Hydration Energy increase as Charge increases

Group 2 Compounds less soluble as solution energy is more endoGroup 1 Compounds more soluble as solution energy is less endo

+ Equilibria (Kc)

Dynamic Equilibria:- The forwards reaction and back reaction are at the same

rate so there’s no overall change in yield of products or reactants in a closed system

+ Kp

+ Effects on EquilibriaAdding a catalyst:• Equilibrium constants not affected• Position of equilibria not affected• Speeds up forward & backward reaction at same rate

Change in concentration:• Equilibrium constants not affected• Adding reactant shifts equilibria right• Adding product shifts equilibria left

Change in pressure:• Equilibria shifts to side with fewest molecules• Equilibrium constants not affected

Increase in temperature:• Endothermic = +ve shifts right, more product produced• Exothermic = -ve shifts left, more reactant produced• Kc & Kp:

• Increases if endothermic• Decreases if exothermic

• Total entropy change = RlnK (R = Gas Constant)

KTotal

EntropyProgression of Reaction

>10-10 >-191 Doesn’t Go

>10-5 >-96 Reversible pushed to Left

1 0 Equilibrium

<105 <+96 Reversible pushed to Right

<1010 <+191 Goes to Completion

+ Uses in Industry

They alter conditions to produce maximum yield

Requiring least amount of energy

Often looking for new more environmentally friendly catalysts

e.g. Ethene + H2O Ethanol Sped up by using catalyst (Silica soaked in H3PO4)

Remove product as it’s being formed

+

Arrhenius:

- Acids are H+ producers- Bases are OH- producers in H2O- Only used in aqueous solutions

Bronsted-Lowry:

- Acids are proton donors- Bases are proton acceptors

Acid Theories

A base has a lone pair of electrons which can form a dative covalent bond with a H+

Lewis:

- Acid is an electron pair acceptor- Bases is an electron pair donor

HA + H2O H3O+ + A-

Conjugate Pairs

H2O & HA are Acids & Bases as they give & accept H+

H+ = H3O+

HA is the acid & A- is it’s conjugate baseH2O is the base & H3O+ is it’s conjugate acid

Amphoteric Substances:• It acts a base or an acid H3O+ H2O OH-

AcidBase

+ pH Strong Acids or Strong BasespH = - log 10 [ H+ ] Kw @298k = 1x10

-14

[ H+ ] = 10-pH

+ pH Strong Acid & Strong Base

Excess [ H+ ]

Excess [ OH- ]

+ pH Weak AcidspKa = -log10( Ka )

Larger pKa = Weaker Acid

+ pH BuffersSubstances that resist change to pH when small amounts of acid/alkali are added

Assumptions:• [CH3COO-] = [H+]• [CH3COO-] = [CH3COONa]• [CH3COOH] same at

equilibrium & at start

[CH3COO-] from Salt

Acidic Buffers = Weak Acid + it’s Salt

+ Isomerism/Optical IsomersOnly occurs when chiral carbon present:

- Carbon with 4 different groups attached

Mirror

Enantiomers/Optical Isomers

Racemic Mixture:Contains equal amounts of each enantiomer

+ Carbonyls

Ethanal PropanoneCarbonyl Group

+ Reactions of Carbonyls

Reaction with Dichromate:- Aldehydes can be oxidised

- Orange Green- Ketones can’t be oxidised

3RCHO + Cr2O7- + 8H+ 3RCOOH + 2Cr3+ + 4H2O

Reaction with Tollens:- Aldehydes +ve Silver Mirror Forms

Reaction with Benedicts:- Aldehydes + ve Blue (Cu2+) Red Precipitate (Cu+)

Reaction with Brady’s:(2,4 DNP or 2,4 DiNitroPhenylhydrazine)- Carbonyls +ve Orange Precipitate

Reaction with Iodine:- Methyl group adjacent to C=O +ve Pale Yellow Precipitate, Antiseptic smell

Melting point used to identify Carbonyl compound

Dissolved in Methanol & conc H2SO4

AgNO3 dissolved in NH3(aq)

Dissolved in NaOH

dil H2SO4

LiAlH4 (in Dry Ether) to go from Carb Acid to 1o or Aldehyde

Presence of Alkali

Triiodomethane

+ HCN Reactions

Propanone + HCN

Ethanal + HCN

2 methyl 2 hydroxypropanenitrile

2 hydroxypropanenitrile

Nucleophilic Addition

In a lab HCN made by reacting KCN(s) + H2SO4

+ Carboxylic Acids

H-Bonding in Pure Ethanoic Acid (Dimer Shape)

Sodium Ethanoate

Identifying Carboxylic Acids:Add Sodium Carbonate – effervescence if +ve

Weak acid – Partially Dissociate

Very soluble in H2OLonger chain, less soluble

Formed from:- Oxidising a 1°- Hydrolysis of Nitrile

(Reflux with dil HCl and distil off)

Reaction with PCl5:CH3COOH + PCl5 POCl3 + HCl + CH3COCl

Ethanoyl Chloride

Add NH3 white smoke = +ve

Acid + Alcohol Ester + H2O

+ Esters

Acid Hydrolysis:- Reflux with dil HCl or H2SO4

Transesterification

Base Hydrolysis:- Reflux with dil Alkali (e.g. NaOH)

Dicarboxylic Acid + Diol Alcohol Polyester

+ Acyl ChloridesEthanoyl Chloride

Reaction with H2O:

CH3COCl(l) + H2O(l) CH3COOH(l) + HCl(g)

Reaction with Alcohol:

CH3COCl(l) + CH3CH2OH(l) CH3COOCH2CH3(l) + HCl(g) Ethyl Ethanoate

Reaction with Ammonia:

CH3COCl(l) + NH3(aq) CH3CONH2(aq) + HCl(g)

Ethanamide Reaction with Ethylamine:

CH3COCl(l) + C2H5NH2(aq) CH3CONHCH3CH2(aq) + HCl(g)

N-Ethyl-Ethanamide

N-“substituted”-Amide

-amide

+ Soap & Triglycerides

Fats solid at RTP

Triglycerides have lower melting point due to less regular shape

Hydrogenation:- Nickel catalyst @ 150°C- Unsaturated Saturated FA- Solidifies fats

+ UV & Microwave RadiationUV can initiate reactions:• In the form of electromagnetic radiation• Wavelength between Visible and X-ray (400nm-10nm)• e.g. Cl-Cl = Clo + Clo homolytic fission

Free-radical Substitution:

• Initiation = Breaks homolytically (sufficient energy in sunlight)• Propagation = Cl* + CH4 = HCl + CH3* = CH3* + Cl2 = ClCH3 + Cl*• Termination = Cl* + Cl* = Cl2 = CH3* + Cl* = CH3Cl* = CH3* + CH3* = C2H6

Microwaves are used to heat things• Longer than IR also used for communications (1mm-1m)• Most molecules in food are polar (water, fats, sugars)• Microwaves are passed through, they create an electric field• Any polar molecules align with the electric field• Whilst rotating they collide with other molecules generating heat energy

Exo

+ Mass Spectroscopy

Vapourisation:Sample put into vacuum – analysed as a gas

Ionisation:High energy e-s knock off other e-s (+vely charged sample)Electron gun used

Acceleration:-vely charged plate pulls sample up the tube

Deflection:Magnetic field introduced – lighter atoms = deflect moreAll same charge, so only mass varies

Detection:Atoms hit charged plate – small charge is created

Uses: Drugs Testing & Carbon dating

Particle Charge Mass

Proton +1 1

Neutron 0 1

Electron -1 1/1840

Parent Ion Peak = Mr

Only Ions show up

+ NMR – Nuclear Magnetic Resonance• Any molecules with odd number of nucleons (protons & neutrons) has

nuclear spin• This causes a weak magnetic field• NMR looks at how this weak field reacts when you put it in a much

larger external field• When field is applied the protons align themselves with or against the

field• When aligned protons are at a lower energy level than opposing

protons, and they can absorb radio waves of the right frequency, they flip to a higher energy level

• The opposing protons can absorb the radio waves and flip to a lower energy level

• There tends to be more aligned protons, so there’s an absorption of energy overall. NMR measures this absorption

• Proton environments can affect the amount of absorption

Chemical Shift:

• Every energy peak is relative to the peak of tetramethylsilane at ‘0’• Height of peak is no of protons

2 Proton Environment

s

2 Proton Environment

s

Chemical Shift

+ NMRHigh Resolution NMR:

• Each peak is broken down into smaller peaks, this is due to neighbouring magnetic field interacting with each other (spin-spin coupling)

2 Peaks so 2 Proton

EnvironmentsPeak at

9.5ppm due to R-CHO

(aldehyde)

Peak at 2.5ppm due to

R-COCH3 (carbonyl)

DoubletQuartet

Tetramethylsilane

Uses:• To ensure pharmaceutical products are pure• Studies internal structures of the body

+ IR – Infrared Spectroscopy• A beam of IR radiation goes through the sample• Bonds absorb the IR energy, increasing the vibrational energy• Different bonds absorb different IR wavelengths• Bonds in different places of a molecule also absorb different

wavelengths

Uses:• Able to detect when one functional group has

been changed to another in a reaction• The degree of polymerisation that has occurred• Detects weaknesses in polymer is reacted with

O2

+ Chromatography• Mobile Phase – where the molecules can move• Stationary Phase – where the molecules can’t move

Gas Chromatography:• Stationary phase is a viscous liquid (e.g. oil) which coats a coiled tube• Mobile phase is N2(g) as it’s unreactive• Sample is injected into tube as gas• Each compound adsorbs to the stationary phase differently• So each compound takes a different amount of time to be recorded

(retention time)• Recorder produces a graph – area/height shows amount of each

compound

HPLC – High Performance Liquid Chromatography:• Stationary phase is small particles of a solid in a tube• Liquid mobile phase is usually a polar mixture (e.g. methanol & H2O)• Liquid is forced through tube under high-pressure• Sample added to liquid phase and forced through tube as a solution• Mass spectrometer is used to analyse each compound as it’s collected• Mixture separates as it adsorbs to the solid differently• UV shone through liquid stream at end• UV absorbed by mixture as it comes through. Graph is produced• Can be used when sample is heat sensitive or high boiling point

Uses:• Checks chemical

equipment for impurities

Uses:• Routinely check

purity of products in a continuous process