Thermodynamics P1

Enthalpy of atomisation ΔH (θ) atm

The standard enthalpy change for the formation of one mole of gaseous atoms on an element in its standard state under standard conditions.

Example of enthrall of atomisation for chlorine

½ Cl2 (g) → Cl (g)

Enthalpy of ionisation ΔH (θ) IE

• the standard enthalpy change when one mole of gaseous atoms is converted into the one mole of gaseous ions.

Example of first ionisation energy of magnesium

Mg (g) → Mg+ (g) + e-

Example of second ionisation energy of magnesium

Mg+ (g) → Mg2+ (g) + e-

Ensure to be careful when calculating enthalpy of atomisation of chlorine as two molecules

• this is twice the Enthalpy of atomisation

• Cl2 (g) → 2Cl (g)

What is electron affinity?

• the standard enthalpy change when one mole of gaseous atoms is converted into one mole of gaseous ions with a single negative charge.

Is electron affinity typically exothermic or endothermic?

exothermic

Ensure to be careful when calculating electron affinity of chlorine as two molecules (as in not a single mole)

2Cl (g) + 2e- → 2Cl- (g)

Twice the Enthalpy of atomisation for ½ Cl2

Which element has a first and second electron affinity?

Oxygen

Example of first electron affinity of oxygen

O (g) + e- → O- (g)

Example of second electron affinity of oxygen

O- (g) + e- → O^(2-) (g)

Why type of process in the 1st EA and why?

• ALWAYS Negative

• Energy is given out as electron attracted to positive nucleus.

Why type of process in the 2nd EA and why?

• ALWAYS positive

• Energy taken in to overcome repulsion between electron and negatively charged ion.

Enthalpy of lattice formation ΔH (θ) L

Is the standard enthalpy change when one mole of a solid ionic compound is formed from it gaseous ions.

Example of lattice formation of NaCl

Na+ (g) + Cl- (g) → NaCl (s)

What type of process is lattice formation?

Exothermic

Enthalpy of lattice dissociation

• reverse of lattice formation

• Standard Enthalpy change when one mole of a solid ionic compound is broken down from its gaseous ions.

Example of lattice dissociation of NaCl

NaCl (s) → Na+ (g) + Cl- (g)

What steps sum up to form enthalpy of formation?

Enthalpy of atomisation (one for each element)

Electron affinity (one for each element)

Enthalpy of lattice formation

What is Enthalpy of formation?

The standard Enthalpy of formation is the enthalpy change for the formation of one mole of a compound from its elements under the standard conditions, with reactants and products in their standard states.

How many steps are in the Born Haber cycle?

Six

What are the steps of the Born Haber cycle?

Enthalpy of atomisation (one for each element)

Electron Affinity (one for each element)

Enthalpy of lattice formation

Enthalpy of formation

For the Born-Haber cycle how are endothermic amd exothermic reactions represented?

Endothermic - arrows going upward

Exothermic - arrows going downward

What does it mean if the enthalpy of lattice formation agrees with the theoretical value?

The compound is perfectly ionic.

What are the characteristics of a perfectly ionic compound?

• no covalent charachter

• No polarisation

• Ions are perfect spheres

Why does the experimental enthalpy of lattice formation disagree with the theoretical value for some compounds?

• the bond in question has some covalent charachter

• Therefore the binds/forces holding the lattice together are stronger.

What type of compounds are not perfectly ionic?

• the positive ion is small and high in charge

• The negative ion is large in size and high in charge

• Greater polarising effect distorts the electron cloud more.

What is enthalpy is hydration defined as?

Standard Enthalpy change when one mole of gaseous ions are completely dissolved by water molecules.

What is the equation for Enthalpy of solution?

In order for an ionic solid to dissolve in water, what must the Enthalpy of solution be?

Negative value

Why might a compound with a small positive Enthalpy of solution value still dissolve in water?

Entropy

What factors affect Enthalpy of hydration?

Charge and size of the ions

How do charge and size affect the Enthalpy of hydration in down a group?

The ionic charge (name charge) remains constant but the ions become larger so Enthalpy of hydration decreases

Therefore name ion which are smaller than name ion would attract the delta negative O in water more strongly and have a more exothermic enthalpy of hydration.

How do charge and size affect the enthalpy of hydration up a group Of anions?

The name ions are smaller and have a higher charge density and would attract the delta positive H in water more strongly and have a more exothermic enthalpy of hydration.

How do charge and size affect the enthalpy of hydration in elements along a period?

Name all ions have the same electronic structure but different nuclear charges and sizes.

Therefore name most positive ion attracts water molecules very strongly and so has a greater Enthalpy of hydration value than name less positive ions.