Chemical Energetics

!!!!Trends in electron affinity of Group 16 & Group 17 elements

Define lattice energy (ΔHlatt⊖​)

The energy change when 1 mole of ionic compound is formed from it’s gaseous ions under standard conditions

e.g: Na⁺(g) + Cl^-(g) → NaCl (s)

• more -ve = stronger ionic bond = stable ionic compound

Factors affecting ΔHlatt⊖​

• Radius of ion: the smaller the radius/smaller the size of the ion, the greater the charge density

• Charge on ion: Greater charge density, greater electrostatic attraction between ions,​ more exothermic, more stable compound

Define lattice energy (ΔHlatt⊖​)

The energy change when 1 mole of ionic compound is formed from it’s gaseous ions under standard conditions

e.g: Na⁺(g) + Cl^-(g) → NaCl (s)

• more -ve = stronger ionic bond = stable ionic compound

Factors affecting ΔHlatt⊖​

• Radius of ion: the smaller the radius/smaller the size of the ion, the greater the charge density

• Charge on ion: Greater charge density, greater electrostatic attraction between ions,​ more exothermic, more stable compound

What are the standard conditions

• 298 K

• 101 kPa

Trend in 1st to 3rd electron affinity

• 1st electron affinity is exothermic (-)

• 2 - 3rd is endothermic, addition of electron is added to an already negative ion , more energy to overcome repulsive forces (+)

Trend of electron affinity down grp 7

• 1st electron affinity is exothermic(-)

• Electron affinity decreases down group due to shielding

• except Fluorine , small atom , crowded , more repulsion

Define standard enthalpy change of atomisation (ΔHat⊖)

The enthalpy change when 1 mole of gaseous atom is formed from its elements under standard conditions

e.g: 1/2Cl₂ (g) →→ Cl (g)

• Value is always POSTIVE (endothermic)

Define first electron affinity

When 1 mole of electrons is added to 1 mole of gaseous atoms to form one mole of gaseous anion under standard conditions

• Generally exothermic (negative)

Formula for the bohr haber cycle

ΔH_f^θ = ΔH₁^θ + ΔH_latt^θ

Define ΔHsol

• The enthalpy change of solution when 1 MOL of solute dissolves in sufficient water to form a solution of infinite dilution

Define ΔHhyd

• Enthalpy changes when 1 MOL of specific Gaseous ions dissolve in sufficient water to form a very dilute solution

ΔHhyd and ΔHsol formula

ΔHhyd = Δlatt + ΔHsol

Equation to find ΔH1

ΔHf = ΔH1 + ΔHlatt

• Factors affecting ΔH_hyd

• factors affecting ΔH⊖_lat

• The solubility of ionic salts depends on the value of ΔH⊖_sol: the more –ve = more soluble

• More exothermic the ΔH⊖_{sol The lower the solubility}

ΔHhyd down group 2

• Smaller ionic radii have a higher ΔHhyd

• Gets less exothermic down the group

• large decrease in the group

ΔHlatt down group 2

• Higher ΔHlatt with smaller lattices

• less exothermic down the group

• Not a large decrease down the group

Trend of ΔHsol down group

• It gets more endothermic down the group , less soluble

What is ion polymerization

Distortion of the electron cloud on an anion by a neighbouring cation

What is Polarising power

ability of a cation to attract electrons and distort and anion

Factors affecting ion polymerization

• Cation has a higher charge density: high positive charge and small size

• Anion has greater polarizability: high negative charge and large size

What takes place with the increase of polarisation

• The greater the polarisation = easier to weaken carbon- oxygen bond in the carbonate to form carbon - dioxide and metal oxide.

Explain why the solubility of group 2 sulfates decrease down the group

• ΔHlatt and ΔHhyd become less exothermic down the group

• ΔHlatt at a lesser extent

• ΔHyd becomes more endothermic

Define entropy

The number of possible arrangements of the particles and their energy given systems

What are systems

• The chemical reaction itself

Describe the surroundings

• test tube , anything dipped into test tubes , air , water used as a solvent

What has the highest entropy

gas > liquid > solid