Chemistry A level 16/03/26

What is the enthalpy change of formation?

What is the enthalpy change of combustion?

Standard enthalpy change of formation of a compound is the energy transferred when 1 mole of the compound is formed from its elements under standard conditions (298 K and 100 kpa) and states.

Enthalpy change of combustion is the enthalpy change when one mole of a substance is burned completely in oxygen in standards conditions and states.

Enthalpy change of atomisation

Bond dissociation enthalpy

Enthalpy change where one mole of a gaseous atoms is formed from its element in standard states.

Enthalpy change when one mole of a covalent bond is broken into two gaseous atoms/free radicals

First electron affinity

Second electron affinity

Enthalpy change when one mole of gaseous atoms gains one mole of electrons forming one mole of gaseous 1- ions. (exothermic- electrostatic attraction with nucleus)

Enthalpy change when one mole of gaseous 1- ions gains one mole of electrons forming one mole of gaseous 2- ions. (endothermic- repulsion between negative ion and electron)

First ionisation enthalpy

Second ionisation enthalpy

Enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge

Enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produces one mole of gaseous 2+ ions

Enthalpy of lattice formation

Enthalpy of lattice dissociation

Enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form. (Na+ (g) + Cl− (g) → NaCl (s))

Enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in gaseous form

Enthalpy of hydration

Enthalpy of solution

Enthalpy change when one mole of gaseous ions become aqueous ions (exothermic)

Enthalpy change when one mole of an ionic solid dissolves in water to form an infinitely dilute solution.

What are the trends for lattice enthalpy?

What are the assumptions for lattice enthalpy

Trends: larger ion=larger enthalpy, greater charge= larger enthalpy

Assumptions: perfect ionic model, 100% ionic and spherical

Perfect ionic= no difference between theoretical and experimental (born haber) values.

What is the formula for entropy?

Entropy change?

ΔS = ΣS products - ΣS reactants (J K^-1 mol^-1)

Change of state, change of moles

Gibbs free energy

∆G = ∆H − T∆S

Negative= reaction is feasible as a spontaneous reaction

Rate equation

The Arrhenius equation

r = k[A]^m[B]^n, mol dm^3 s^-1

k = Ae^-EA/RT, A= Arrhenius constant, R= gas constant

Can be mapped out as ln k = ln A – EA/r x 1/T, with +x and -y, 1/T against ln k

Kp equation

How to work out partial pressure

Kp = pp(products)^m/pp(reactants)^n, only temp has effect on Kp (not pressure or catalyst)

mole fraction = moles of gas/ total moles of all gases

partial pressure = mole fraction x total pressure of all gases

How to work out emf

Cathode vs anode

EMF = rhs - lhs

Cathode is the negative electrode where reduction takes place

Anode is the positive electrode where oxidation takes place

Hydrogen fuel cell

Requires continuous supply of oxygen and hydrogen

Only has water as by-product

Danger = hydrogen flammable

How are carbonyls reduced?

Nucleophilic addition: Uses reducing agent (NaBH4- source of nucleophilic hydride ions that attack the positive C=O carbon), aqueous ethanol

Catalytic hydrogenation: Nickel catalyst, H2, high pressure

Carbonyl addition with hydrogen cyanide

Nucleophilic addition with potassium cyanide (forms :CN- nucleophile) and sulfuric acid (supplies extra hydrogen to form alcohol group from C=O)

Forms racemate: planar carbonyl group has equal chance of being attacked from both sides

Carboxylic acid properties

Small chains up to C4 are soluble, forming hydrogen bonds with the water molecules, larger chains have hydrophobic tails that get in the way of the hydrogen bonds

Larger chain = weaker acid (electron density pushed towards COO- of carboxylate making it less stable, and therefore less likely for the acid to dissociate into)

Electronegative chlorine withdraws electron density from COO- making it more stable.

What is esterification?

Carboxylic acid + alcohol ←→ ester + water

Requires strong acid catalyst, heat under reflux, low yield, reversible

What can esters be used for?

Flavourings and perfumes- sweet smelling, non-toxic, unreactive with water

Use as plasticizer for polymers

Use as a solvent (for polar organic substances)

What is ester hydrolysis?

With acid: ester ←→ carboxylic acid + alcohol (dilute HCl, heat under reflux)

With base ester → sodium carboxylate salt + alcohol (dilute NaOH, heat under reflux)

Acid vs base hydrolysis of esters

Acid hydrolysis is reversible, doesn’t give good yield.

Base hydrolysis is not reversible, carboxylate salt is resistant to attack by weak nucleophiles, therefore reaction goes to completion.

What is the reaction of Acyl chloride/acid anhydride with water?

Acyl chloride + water → carboxylic acid

RCOCl + H2O → RCO2H + HCl

Acid anhydride + water → carboxylic acid

(RCO)2O + H2O → 2RCO2H

Nucleophilic addition elimination

What is the reaction of Acyl chloride/acid anhydride with alcohol?

Acyl chloride + alcohol → ester

RCOCl (l) + CH3CH2OH → RCO2CH2CH3 + HCl (g)

Acid anhydride + alcohol → ester

(RCO)2O + CH3CH2OH → RCO2CH2CH3 +RCO2H

Nucleophilic addition elimination

What is the reaction of Acyl chloride/acid anhydride with ammonia?

What is the reaction of Acyl chloride/acid anhydride with primary amines?

Acyl chloride + ammonia → primary amide

Acid anhydride + ammonia → primary amide

Acyl chloride + primary amine → secondary amide

Acid anhydride + primary amine → secondary amide

Nucleophilic addition elimination

Why are acid anhydrides used instead of acyl chlorides?

Safer and less violent reaction

Doesn’t produce dangerous corrosive HCl fumes as a byproduct

Cheaper to produce and use

More stable and easier to store (less hydrolyzed by moisture)

What is a transition metal?

What is a ligand?

A metal with its atoms or ions containing incomplete d subshells

A molecule that can donate an electron pair, forming a coordinate bond

What is the chelate effect?

The substitution of monodentate ligand with a bidentate or a multidentate ligand leads to a more stable complex

Increase of entropy - more moles of products

Enthalpy change close to zero- same number of dative coordinate bonds, same energy required to break them

Therefore, Gibbs free energy is negative, the complex is stable

TM: What shapes do the complexes form?

Depends on coordinate number:

6 = octahedral, 4= tetrahedral (larger ligands), square planar, 2= linear