Unit 5: Thermo (Packets 1 & 2)

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Exo or Endo: Bond formation

Exothermic

Exo or Endo: Bond breakage

Endothermic

Bond Energy

Amount of energy required to break a covalent bond

To solve this equation:

1: Balance stoichiometric equation.

2: Draw Lewis Dot Structure for all reactants and products.

3: Substitute correct bonds into equation and apply mole ratio, e.g.

• ΔH = 4(C-H) + 2(O=O) - 2(C=O) - 4(O-H)

4: Substitute correct numerical values into equation.

• ΔH = 4(393) + 2(433) - 2(433) - 4(464)

• Bond energies found in DP data booklet

5: Solve the equation.

• ΔH = -890 kJ

Hess’ Law

The enthalpy change for a reaction is independent of the pathway between the initial and final states.

Standard Enthalpy Change of Combustion

Amount of heat released from 1 mol of a substance in the presence of oxygen.

• C₃H₈ + 5O₂ —> 3CO₂ + 4H₂O

• ΔH: Need to use ΔH_formation to find ΔH_combustion

Standard Enthalpy Change of Formation

Amount of heat needed to make (released) from the formation of 1 mol of a compound from its elements in standard state.

• Does not apply to diatomic and metals (they aren’t compounds)

• 0 for elements at standard state

• Standard state = state the element exists as at 25 ºC (298.15 K) and 1 atm (101 kPa)

• CANNOT USE COMBUSTION VALUES FOR FINDING ΔH OF A COMBUSTION REACTION

ΔH_formation vs ΔH_BE

• ΔH_formation: Uses standard states and not gas volumes —> accurate

• ΔH_BE: Averages of bond energies in gaseous phase and are used as a guide —> approximate

• ALWAYS A QUESTION ON IB EXAM

Born-Haber Cycle

BHC is an application of Hess’ Law used to show energy changes in the formation of an ionic compound.

Lattice Enthalpy

Energy required to make gaseous ions from 1 mol of solid ionic compound.

• Endothermic

• Experimental values at 298K are found in section 18 of data booklet

• ΔH^º_lat > 0

Enthalpy of atomization / sublimation

Standard enthalpy change that occurs on the formation of 1 mol of separate gaseous atoms of an element in its standard state.

• M_(s) —> M_(g) ΔH^º_atom > 0 Sublimation

• ½ X_{2 (g)} —> X_(g) ΔH^º_atom > 0 BE

Ionization Energy (ΔH^º_IE)

Standard enthalpy change that occurs on the removal of 1 mol of e^- from 1 mol of atoms or positively charged ions in the gaseous phase.

• For metal ions with multiple ve^- the 1^st, 2^nd and 3^rd IEs are defined.

• M_(g) —> M⁺_(g) + e^- ΔH^º_IE > 0

• M⁺_{(g) —>} M²⁺_(g) + e^- ΔH^º_IE > 0

Electron Affinity (ΔH^º_EA)

The standard enthalpy change on the addition of 1 mol of e^- to 1 mol of atoms in the gaseous phase.

• X_(g) + e^- —> X^-_(g) ΔH^º_EA < 0

Lattice Enthalpy Steps

1: ΔH_formation (↓ -ΔH (heat released))

2: ΔH_{atomisation / sublimation} (↑ +ΔH)

3: IE₁ (↑ +ΔH)

4: ½ ΔH_BE (↑ +ΔH)

5: EA (↓ -ΔH)

ΔH_lattice = ΔH_atom + IE_cation + ½ ΔH_BE + EA_anion - ΔH_f