Paper 3

Ideal Gas Equation

pV = nRT

p = Pascals

V = m³

R = 8.314

T = K (+273C)

Polar/Non polar molecules

Dipoles across covalent bonds are caused by significant differences in electronegativity values of bonded atoms (>0.4)

C-H bonds classed as non-polar

Why aren’t symmetrical molecules polar?

Symmetrical molecules are generally non polar. Polar bonds are arranged in a way that the individual bond dipoles cancel each other out, leaving no overall dipole on the molecule

Why are non-symmetrical molecules polar?

Individual bond dipoles do not cancel out.

What causes asymmetry?

Asymmetry caused by presence of different terminal atoms, or presence of lone pairs around central atom makes molecules non symmetrical

Electron structure

Orbitals can only hold 2 electrons

s orbitals = spherical shape

p orbitals = dumbbell shape

s subshells can hold 2 electrons

p subshells can hold 6 electrons

d subshells = 10 electrons

f subshells = 14 electrons

Draw an enthalpy cycle, or otherwise, to show how Δ_rH may be determined using average bond enthalpies.

Enthalpy changes

• △H = IN - OUT

• E_IN = Energy needed to break the bonds between atoms in the reactants

• E_OUT = Energy released when bonds between atoms formed in the products

Calorimetry questions

q = mc△T

The m is the mass of liquid the thermometer is in

Don’t forget to multiply △H values by the number of moles in the equation

△H values must have a sign ±

Why are the experimental calorimetry results different to the data book values?

• heat loss to surroundings

• non standard conditions

• incomplete combustion (alcohols)

• specific heat capacity of apparatus ignored

How to calculate E_cell?

E_pos - E_neg

Draw a diagram to measure E^o value.

Draw with SHE

Salt bridge allows a flow of ions

Fuel cells

reactants supplied constantly

Storage cells (batteries)

reactants stored in the cell

K_c or K_p > 1

Equilibrium position is to the right/products side

K_c or K_p >> 1

Equilibrium position is REALLY to the right/products side

K_c or K_p < 1

Equilibrium position is to the left/reactants side

K_c or K_p << 1

Equilibrium position is REALLY to the left/reactants side

Partial pressures should add up to

total pressure

Mole fractions should add up to

1

The value of K_c and K_p only alters by changes in what?

Temperature

Transition element definition

Forms at least 1 ion with an incomplete d subshell

What 2 examples are not transition elements?

Sc and Zn

Sc only forms Sc³⁺ (which is 3d⁰)

Zn only forms Zn²⁺ (which is 3d¹⁰)

What’s unusual with Cu and Cr?

Cu and Cr are unusual with their 4s¹ configuration rather than the normal 4s²

Hydroxide ppt

• All aqueous TM ions produce hydroxide ppt with a small amount of OH^-

  • Only Cr(OH)₃ dissolves in excess to form [Cr(OH)₆]^3-

• All aqueous TM ions produce hydroxide ppt with small amount of NH₃

  • Only Cu(OH)₂ and Cr(OH)₂ dissolve in excess to form [Cu(NH₃)₄(H₂O)₂]²⁺ and [Cr(NH₃)₆]³⁺

Isomerism in complexes

3x bidentate ligand complexes show optical isomerism

2x monodentate / 2x bidentate ligands show trans isomerism, and do not show optical isomerism

Test for NH₄⁺

• Add NaOH (aq) and warm gently

  Test NH3 gas with damp red litmus paper.

• Damp red litmus paper turns blue

• NH₄⁺ + OH^- → NH₃ + H₂O

Test for Fe²⁺

• Add NaOH (aq) until in excess

• Pale green ppt, insoluble in excess NaOH

• Fe²⁺_(aq) + 2OH^- _(aq) → Fe(OH)_{2 (aq)}

Test for Fe³⁺

• Add NaOH (aq) until in excess

• Orange/brown ppt, insoluble in excess NaOH

• Fe³⁺_(aq) + 3OH^- _(aq) → Fe(OH)_{3 (aq)}

Test for Cu²⁺

• Add NaOH (aq) until in excess

• Pale blue ppt, insoluble in excess NaOH

• Cu²⁺_(aq) + 2OH^- _(aq) → Cu(OH)_{2 (aq)}

Test for Mn²⁺

• Add NaOH (aq) until in excess

• Pale brown ppt, insoluble in excess NaOH

• Mn²⁺_(aq) + 2OH^- _(aq) → Mn(OH)_{2 (aq)}

Test for Cr³⁺

• Add NaOH (aq) until in excess

• Grey-green ppt, soluble in excess NaOH, forming green solution

• Cr³⁺_(aq) + 3OH^- _(aq) → Cr(OH)_{3 (aq)}

  Cr(OH)_{3 (aq)} + 3OH^- → [Cr(OH)₃]³⁺

Test for Carbonate

• Add dilute HNO_{3 (aq)} and test gas by bubbling through limewater

• Bubbles/effervescence

  Limewater turns cloudy

• CO₃^2-_(aq) + 2H⁺_(aq) → CO₂ + H₂O

Test for Sulfate

• Add Ba²⁺ (like Ba(NO₃)₂)

• White ppt

• Ba²⁺ + SO₄^2- → BaSO₄

Test for Halides

• Add AgNO₃ followed by NH₃

• White ppt of AgCl, soluble in dilute NH₃

  Cream ppt of AgBr, soluble in conc NH₃

  Yellow ppt of AgI, insoluble in conc NH₃

• Ag⁺ + Cl^- → AgCl

  Ag⁺ + Br^- → AgBr

  Ag⁺ + I^- → AgI

Correct order for anion tests

1. Carbonate

2. Sulfate

3. Halide

Why are alkanes unreactive?

High bond enthalpies of C-C and C-H bonds

C-C and C-H bonds are non polar

Why are alkanes bad at making a single organic substance?

• Multiple termination steps

• Further substitution

• Substitution can be anywhere on the C chain

Oxidation of secondary alcohols

Only makes a ketone

Oxidation of tertiary alcohols

Doesn’t oxidise

Amines

• Can be primary, secondary, or tertiary

• Acts as bases (can accept H+ using lone pair on N → Ammonium salts

Primary aliphatic amines made from haloalkanes with excess ammonia dissolved in ethanol

Haloalkane → Primary aliphatic amines

Excess ammonia dissolved in ethanol

Excess is used to avoid further substitution to secondary and tertiary amines

Nitrobenzene → Aromatic Amines

Sn and conc HCl

6 moles of [H] per NO₂ group reduced

Nitriles → Amines

H₂ / Ni catalyst

(CN → CH₂NH₂, so 2H₂ is needed)

Primary, secondary, tertiary amides

How is the amide group hydrolysed?

With hot aqueous acid and alkali

a-amino acids

Carboxylic acid + metal

Carboxylic acid + metal oxides, metal hydroxides

Carboxylic acid + carbonates

Carboxylic acid + metal → Carboxylate + Hydrogen

Carboxylic acid + metal oxides, metal hydroxides → Carboxylate salt + water

Carboxylic acid + carbonates → Carboxylate salt + water + carbon dioxide

Carboxylate salts

(RCOO^-)_nMⁿ⁺

(RCOO^-)₂M²⁺

NH₂ → Ammonium salts

React with acids

There are 2 ways to form Nitriles. What are they?

1. From haloalkanes - Nucleophilic Substitution

   Haloalkane → Nitriles

2. From aldehydes & ketones - Nucleophilic Addition

   Carbonyl → Hydroxynitrile

Carbonyl → Hydroxynitrile

• NaCN

• Dilute H₂SO₄ (or H⁺)

• RTP

• Nucleophilic addition

Haloalkane → Nitriles

• NaCN dissolved in ethanol

• Heat under reflux

• Nucleophilic Substitution

• This reaction increases the chain length. In this reaction, 1-bromopropane forms butanenitrile

Nitrile → Carboxylic acid

• H⁺ / H₂O (could be HCl_(aq))

• Heat under reflux

Optical Isomerism

Same structural formula, different spatial arrangement of atoms/groups

Non superimposable mirror images of each other (enantiomers)

Possible if a molecule has a chiral centre

For each chiral centre → Pair of optical isomers (Total number of isomers= 2^{no. of chiral centres})

Draw the optical isomer of this molecule with 2 chiral centres

Carbon-Carbon bond formation

• Haloalkane → Amine

  Need to generate the nitrile (-CN) functional group by reaction with ethanolic CN- then react nitrile with H₂/Ni to generate the amine

• Haloalkane → Carboxylic acid

  Need to generate the nitrile (-CN) functional group by reaction wit ethanolic CN- then react nitrile with HCl to generate the carboxylic acid

• C-C bonds also formed when carbonyls → hydroxynitriles

• C-C bonds formed in alkylation and acylation reactions with benzene rings

Synthesis of an Organic Solid

1. Dissolve the impure solid crystals in a minimum amount of hot solvent

2. Leave to cool and allow crystals to reform

3. Filter the crystals under reduced pressure

4. Wash with a little cold solvent

5. Allow purified crystals to dry

Buchner Flask Diagram

How to determine purity of an organic solid?

• Measure melting point and comparing to known data values

• Run TLC and measure Rf value to compare to known data values

• Run TLC and compare to chromatogram of a pure sample

• Run NMR/IR/Mass spectrum, to compare spectral database of pure compound

Distillation Apparatus Diagram

Heat under reflux Apparatus Diagram

Tests for Organic Functional Groups

Thin Layer Chromatography

Separation by relative adsorption - components in the mixture bind to the solid stationary phase by differing amounts (strongly adsorbed not moved as far as weakly adsorbed)

How do you find out the substances in TLC?

Calculate Rf value and compare Rf value with known data values

Gas Chromatography

• Separation by relative adsorption if stationary phase is solid

• Separation by relative solubility if stationary phase is liquid

• Retention times (time from injection to detection)

How do you know what you got in gas chromatography?

Compare retention times to known data values

What does the area under peaks in gas chromatography represent?

• Relative amounts of components in sample.

• The relative proportions are measured by dividing area under peak by the total area under all the peaks.

If you want to know the actual concentrations of the substance in gas chromatography, what would you do?

Measure the area under peak for known concentrations

Then plot a calibration curve.

Concentration is found by comparing the area under a sample peak to calibration curve

How to reduce carbonyls?

• NaBH₄

• Catalytic Hydrogenation (H₂/Ni catalyst)

Carboxylic acid → Acyl chloride

SOCl₂

• Acyl chloride

• SO₂

• HCl

Acyl Chloride → Carboxylic acid

Water

Acyl chloride → Ester

Alcohol

How would the the enthalpy changes of hydration of F^- and Cl^- differ?

∆_hyd of F^- would be more exothermic than ∆_hyd of Cl^- because F^- has a smaller size than Cl^-

What is the equation and how does entropy change in the standard enthalpy change of atomisation of iodine?

I_{2 (s)} → I_{2 (g)}

The entropy increases because there is a state change from solid to gas, and gas has more disorder

The lattice enthalpy of sodium oxide is more exothermic than that of potassium oxide. Explain why.

The ionic radius of Na⁺ is smaller.

Na⁺ has a stronger attraction to O^2-

Why is it difficult to predict whether the enthalpy change of solution becomes more exothermic or less exothermic down the group from MgF₂ to MgI₂

• Halide ion gets larger down the group

• Lattice enthalpy is less exothermic down group

• Hydration enthalpy less exothermic down group

• Difficult to predict whether lattice enthalpy or hydration enthalpy has a bigger effect

Does hydration enthalpy increase or decrease down group?

Decrease

Enthalpy change of hydration

The enthalpy change when one mole of gaseous ions is dissolved in water to give one mole of aqueous ions

Enthalpy change of solution

The enthalpy change when 1 mole of a substance dissolves

Standard Enthalpy Change of Reaction

The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions

Standard Enthalpy Change of Formation

The enthalpy change when one mole of a compound is formed from its elements under standard conditions.

Standard Enthalpy Change of Combustion

The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions

Lattice Enthalpy

The energy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions.

Enthalpy change of Atomisation

The enthalpy change when one mole of gaseous atoms are formed from the element in its standard state.

First electron affinity

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.

Average bond enthalpy

The breaking of one mole of bonds in gaseous molecules

Weak Acid Equations

Strong Bases equations

Buffer Equation

[H⁺] = Ka x ([HA]/[A^-])

“Strong acid – Strong base” pH curve

1. pH = 1

2. pH = 13

3. pH = 7

4. Volume at neutralisation = 25cm³

5. Steep between pH 4-10

“Strong acid - Weak base” pH curve

1. pH = 1

2. pH = 9

3. pH = Less than 7

4. Volume at neutralisation = 25cm³

5. Steep between pH 4-7

“Weak acid – Strong base” pH curve

1. pH = 3

2. pH = 13

3. pH = More than 7

4. Half neutralisation volume x2

5. Steep between pH 7-10

“Weak acid - Weak base” pH curve

1. pH = 3

2. pH = 9

3. No equivalence point

4. No volume at which equivalence occurs

5. No steep part of curve

Why is CO toxic to humans?

O₂ co-ordinately bonds to Fe²⁺

When required, O₂ is replaced by H₂O or CO₂

CO forms stronger bonds than O₂ so prevents O₂ from attaching to haemoglobin. With CO, its stability constant is greater than with complex in O₂

Proton Exchange

Since D₂O has an even number of nucleons (²H) its nucleus does not possess spin property, so do not give a signal in ¹HNMR

Definition of E/Z isomer

There is restricted rotation around the C=C double bond. There are two different groups/atoms attached both ends of the double bond.

Definition of Cis/Trans Isomers

A type of E/Z isomerism in which two of the substituent groups are the same.

What’s a zwitterion?

A zwitterion is a molecule that has both a positive and a negative charge - but the overall charge is neutral.