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Edexcel A-levels - Chemistry Notes

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Condensed Notes for Chemistry Unit 4 - Inorganic:

Zero Order First Order Second Order

↑Half-life decreases ↑Half-life is constant ↑Half-life increases

SN1 - 2 step, first order, formation of carbo cation (tertiary or secondary)

SN2 - 1 step, second order (primary or secondary)

Arrhenius:

ln k = -Ea/RT + a constant

Graph of ln k against 1/T : gradient = -Ea/R

Entropy:

total entropy = entropy of system + entropy of surroundings

entropy of system = total entropy of products - total entropy of reactants

entropy of surroundings = - ΔH/T

ΔS(total) = R ln K

Equilibria:

Kc and Kp --> solids and liquids are omitted from equation

Kw --> H2O not written!

Kpartition --> concentration of substance in medium 1 / concentration of substance in medium 2

At equilibrium, ΔS is zero and ΔG is zero

At equilibrium, concentration of reactant(s) & product(s) don't change

Stronger the acid, larger its Ka

Gases --> mole fraction of A = no. of moles of A / total no. of moles (e.g. of A & B)

partial pressure of A = mole fraction of A * total pressure of system

only temperature can change value of Kp/Kc

Buffer solution = weak acid/base and its salt

pH = -log[H+] pH + pOH = 14

Kw = [OH-]*[H+]

Diluting a strong acid by a factor of 10 raises its pH by one unit

HIn ⇌ H+ + In-

Notes for Chemistry Unit 5:

CHAPTER 1 - REDOX

Thermodynamic stability is determined by: ΔH (Enthalpy change), ΔS (Change in entropy) and E^ocell (Standard electrode potential of the cell)

Kinetic stability is determined by: Activation energy (Ea)

Standard conditions are 298K, 1 atm pressure, 1 moldm-3

If E^ocell > +0.6V , then the reaction goes to completion

If E^ocell < -0.6V, then the reaction doesn't occur

Note that E^ocell is the probability of the reaction taking place. Even if E^ocell is > +0.6V, reaction may not take place if the Ea is very high and/or due to non-standard conditions.

E^ocell is not affected by the quantity of substances in the mixture.

E^ocell ∝ ΔS
E^ocell ∝ ln K

Important Definitions to know

Oxidation number: the charge the element would have if the electrons in each bond of the molecule or ion belonged to the more electronegative element.

Standard electrode potential: the electrode potential of a half-cell when measured with a standard electrode potential as the other half cell.

Standard hydrogen electrode: an electrode with a standard electrode potential of zero under standard conditions.

Titrations

1. Potassium manganate(VII) and Fe(II) ions

(Potassium Manganate(VII) is a strong oxidizing agent)

* Self indicating titration

* End point = All Fe(II) → Fe(III). Colourless → Pink

Redox reaction:

2.Sodium thiosulfate and aqueous Iodine

*indicator: starch

* End point = All I2 → I-. Blue black → colourless

Redox reaction:

This titration can be used to find concentration of oxidising agent (e.g. Cu(II) ions) that oxidises I- to I2

Reaction:

* 1:1 ratio between Cu(II) ions and thiosulfate ions

Uncertainties and inaccuracies:

Uncertainty - caused by limits of apparatus precision

Repeating measurement will increase reliability but not accuracy

Larger the measurement, lower the percentage error

Assumptions can introduce systematic errors and inaccuracy.

Fuel cells and breathalysers:

Remember: In fuel cells, anode is negative and cathode is positive but oxidation still occurs at anode and reduction occurs at cathode.

1. Hydrogen fuel cell → uses: spacecrafts, electric cars

sources of H2 → water or brine by electrolysis, natural gas

2. Alcohol Fuel cell → uses: mobile phones and laptops

source → fermentation of sugars, hydration of alkenes obtained from crude oil

3. Breathalysers

→ for conviction, IR Analysis and a blood test is required

* old fashioned → Dichromate(VI) breathalyser

* fuel cell:

* MOST RELIABLE:

IR Breathalyser - alcohol content will show as IR spectrum peak at 2950cm^-1 (C-H Absorption)

CHAPTER 2 - TRANSITION METALS

→ Go through the table of transition metals and their 1st , 2nd, and 3rd Ionisation energies (textbook)

Features of Transition Metals

variable oxidation states
form coloured ions in solution
form complex ions (monodentate and bidentate)
can act as catalysts

Complexes

Copper

* Copper(I) compounds disproportionate in aqueous solution
* CuCl(s) + Cl-(aq) → [CuCl2)-(aq) <-- stabilized form
*

Chromium

Note: All species are strong oxidising agents especially in acidic conditions

Colours

Why there is no colour:

d-subshell is full
so d-d transition cannot take place

Why there is colour:

3d orbitals/3d sub shell split by attached ligands
electrons are promoted from lower to higher energy d orbitals/levels
absorbing energy/photons of a certain frequency
reflected/transmitted light is in visible region, hence colour is seen.

Ligand Exchange Reactions

EDTA

* Polydentate

* Many lone pairs

* hence very stable due to increase in entropy

Disproportionation

Steps to find out if disproportionation will occur or not:

(same element, two equations for diff species reductions) Reverse equation with least positive E^ocell and reverse sign of its E^ocell
add/combine equations
If E^ocell is positive then disproportionation will occur

Preparation of chromium(III) ethanoate

Catalysts

reactants are ADSORBED temporarily to surface (do not confuse with absorb)
after completion of reaction, products diffuse away
catalysts are poisoned when materials permanently bind to surface

Catalytic convertors:

* fine coating of platinum catalyst

advantages: air pollution is reduced, CO and NOx compounds are removed

disadvantages: fuel economy is reduced, catalyst can be poisoned by leaded fuel.

Why development of new catalyst is needed:

energy usage
pollution
atom economy
use of waste materials from other processes

Note: Learn the colour & formula of ions/compounds in NaOH, excess NaOH, NH3 and excess NH3 for the following species: Cr(III), Mn(II), Fe(II), Fe(III), Ni(II), Cu(II) and Zn(II)

Note: Zinc and Cr are amphoteric

Uses of transition metals:

* catalysts

* chemotherapy (cis-platin and gold)

* photochromic glasses (silver and copper halides - redox reaction)

CHAPTER 3 - ORGANIC CHEMISTRY: ARENES

Benzene:

Evidence for ring model:

Thermochemical evidence - ΔH(formation) is +49 kJmol^-1 instead of +252 kJmol^-1 which suggests that benzene is more stable (energetically)
X-ray diffraction - shows that all bond lengths are same in nature and length is between the length of a single bond and that of a double bond.
Infrared - spectrum of benzene is simpler than that of cyclohexane, hence benzene must have more symmetry

Reactions of Benzene:

Reactions of Phenol:

CHAPTER 4 - ORGANIC CHEMISTRY: NITROGEN COMPOUNDS

Amine: R - NH2 Amide: C(=O) - NH

Primary amine: only one carbon atom attached to the nitrogen atom
- Fishy smell
- miscible (hydrogen bonds formed with water) --> Forms alkaline solution

phenyl amine + HNO3 --> phenylammonium nitrate (

)

* nucleophilic substitution:
phenyl amine + acyl chloride --> amide + HCl

Note: Learn formation/steps of paracetamol production

Reduction of aromatic nitro-compounds to form aryl amines

1. Sn(s) + 2HCl(aq) --> Sn2+ (aq) + 2Cl- (aq) + H2(g)
2. Sn2+ (aq) --> Sn4+ (aq) + 2e- & C6H5NO2(l) + 6H+(aq) + 6e- --> C6H5NH2(l) + 2H2O(l)
3. NaOH
4. steam distillation
5. salting out
6. solvent extraction
7. KOH
8. distillation

Reactions with nitrous acid

X-NH2(aq) + HNO2(aq) --> X-OH(aq) + N2 + H2O(l)
phenyl amine(aq) + HNO2(aq) --> phenol(aq) + N2(aq) +H2O(l)
phenyl amine(aq) + HNO2(aq) + HCl(aq) --> benzenediazonium chloride(aq) + 2H2O

Benzenediazonium chloride:

Dyes
cold NaOH
benzenediazonium chloride + phenol ------------------> 4 hydroxyphenol azobenzne + HCl

Hydroxyphenol azobenzene:

Note: Learn condensation polymerization equations of Nylon 6,6 & Kevlar & Terylene. Learn addition polymerization equations of poly(propenamide) and poly(ethenol)
For properties of polyamides: page 220 and 223 of textbook

Amino Acids

zwitter ion formation (in neutral solution):