Chapter 5-Thermochem Flashcards
Enthalpy
The amt of energy possessed by a chemical substance
→ stored as chemical potential energy in chemical bonds
Enthalpy change only apply to reactions at constant___?
PRESSURE
Thermal Standard Conditions
1) Temp of 25°C (298K)
2) Pressure of 100kPa (1atm)
3) Conc of 1moldm^-1
Temperature
The measure of av. kinetic energy of the particles present in the substance (it’s HOTNESS) = independent of amt of substance present
→ measure in °C or K
Heat
Measure of the total energy in a substance
→ DOES depend on the amt of substance present
Heat capacity
The amt of energy required to increase the temp. of an object by 1°C
Why is the use of Hess’s Law important?
Cuz it enables the determination of the enthaply change for compounds which can’t be formed by the direct combination of the elements
Standard Enthalpy change of Formation
The amt of energy evolved or absorbed in the formation of 1 mole of the compound, in its standard state, from its constituent elements, in their standard state.
ΔH⦵ = Sum of ΔfH⦵ (product) - Sum of ΔfH⦵ (reactants)
Standard Enthalpy change of Combustion
The energy change when 1 mole of the compound undergoes complete combustion in excess oxygen under standard conditions.
+ ALWAYS EXOthermic
ΔH⦵ = Sum of ΔcH⦵ (reactants) - Sum of ΔcH⦵ (product)
ELEMENTS CANNOT BE FORMED therefore?
ΔHf⦵ = 0!
Enthalpy of combustion can help determine what?
The enthalpy change of any reaction in which BOTH the reactants and products can be readily converted to their oxides
→ EVEN if reaction can’t occur in practice!
Bond Enthalpies
The amt of energy (in kJ) required to break 1 mole of a particular covalent bond in the GASeous state into GASeous atoms (under standard thermodynamics)
→ The measure of strength of the bond!
Average Bond Enthalpy
The energy needed to break one mole of a bond in a gaseous molecule AVERAGED over similar compounds
ΔH = Sum of BE (bonds broken) - Sum of BE (bonds made)
Standard Enthalpy of Atomisation
The energy change required to convert 1 mole of a substance in its standard state into 1 mole of gaseous atoms.
→ Numerically = to 1/2 of its BE
BE can only be used when?
Substances are in their GASeous state!
Born-Haber Cycle
The energy cycles for the formation of ionic compounds.
Born-Haber Cycle shape?
→ → → Gaseous anions and cations ← ←
↑ionisation energy ↑
Gaseous anions (-) and metal atoms ↑
↑ electron affinity ↑
Elements as gaseous atoms ↑
↑ E of atomisation ↑
↑ Enthalpy of formation ↑
Elements in their standard state → Solid compound
Electron Affinity
The enthalpy change when 1 mole of gaseous atoms gain electrons to form a mole of negatively charged gaseous ions (anions)
Ionisation energy
The enthalpy change for 1 mole of a gaseous element to lose e-‘s to form a mole of positively charged gaseous ions (cations)
Lattice enthalpy
The energy required to convert 1 mole of the solid compound into gaseous ions
→ HIGHLY ENDOTHERMIC
The Magnitude of the lattice enthalpy depends on the nature of the ions involved (2)
1) The greater the charge on the ions, the greater the electrostatic attractions and hence the greater the lattice enthalpy
2) Ionic radius
→ The smaller the ions = the closer together they are = the stronger the forces of attraction betw/ oppositely charged ions = greater lattice enthalpy
Enthalpy change of solution
The enthalpy change when 1 mole of the substance (solute) is dissolved in water to form a dilute aq. solution
Enthalpy change of hydration
The enthalpy change (always exothermic) when 1 mole of the gaseous ions is added to water to form a dilute solution
