Thermodynamics definitions and mini q's

Standard Enthalpy of formation

The standard enthalpy of formation Δ_fH^Θ is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states

What is the enthalpy of formation of an element and why?

• Zero

• This is because forming the element from itself requires no chemical reaction or energy change

Standard Enthalpy of Atomisation

The standard enthalpy of atomisation (of an element) Δ_atH^Θ is the enthalpy change when 1 mole of gaseous atoms are formed from an element in its standard state (under standard conditions)

Why is atomisation always endothermic?

Because energy is always required to separate the atoms (solid to gas)

Is Δ_atH^Θ 1 mol of atoms being formed or 1 mol of element being atomised

It is 1 mole of the atoms being formed

Firsrt ionisation energy

The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous +1 ions

Equation for first ionisation energy

M(g) → M+(g) + e^-

Second ionisation energy

The energy required to remove one mole of electrons from one mole of gaseous +1 ions to form one mole of gaseous +2 ions

Equation for second ionisation energy

M⁺(g) → M²⁺(g) + e^-

First electron affinity

First electron affinity (ΔH_EA), is the standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, each with a single negative charge

Equation for first electron affinity

X(g) + e^-→ X^- (g)

Second electron affinity

Second electron affinity (ΔH_EA), is the standard enthalpy change when a mole of electrons is added to a mole of gaseous ions with a single negative charge to form ions each with two negative charges

Equation for second electron affinity

X^-(g) + e^- → X^2-(g)

Bond dissociation Enthalpy

Enthalpy change when breaking one mole of bonds to form gaseous atoms.

Give an example equation for bond dissocation enthalpy

Cl2(g) → 2Cl(g)

Mean bond enthalpy

Mean bond enthalpy is the average energy required to break one mole of a specified covalent bond in a gaseous molecule, averaged over a range of different compounds

Lattice formation enthalpy

The Lattice formation enthalpy (ΔHL) is the standard enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions

What kind of number is lattice formation enthalpy and why?

• A large, negative number

• This is because the reaction is very exothermic

Lattice dissociation enthalpy

Enthalpy change when one mole of solid ionic compound is broken down into its gaseous ions