Chapter 5-Thermochem

Question 1

Enthalpy

Answer 1

The amt of energy possessed by a chemical substance

→ stored as chemical potential energy in chemical bonds

Question 2

Enthalpy change only apply to reactions at constant___?

Answer 2

PRESSURE

Question 3

Thermal Standard Conditions

Answer 3

1) Temp of 25°C (298K)
2) Pressure of 100kPa (1atm)
3) Conc of 1moldm^-1

Question 4

Temperature

Answer 4

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

Question 5

Heat

Answer 5

Measure of the total energy in a substance

→ DOES depend on the amt of substance present

Question 6

Heat capacity

Answer 6

The amt of energy required to increase the temp. of an object by 1°C

Question 7

Why is the use of Hess’s Law important?

Answer 7

Cuz it enables the determination of the enthaply change for compounds which can’t be formed by the direct combination of the elements

Question 8

Standard Enthalpy change of Formation

Answer 8

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)

Question 9

Standard Enthalpy change of Combustion

Answer 9

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)

Question 10

ELEMENTS CANNOT BE FORMED therefore?

Answer 10

ΔHf⦵ = 0!

Question 11

Enthalpy of combustion can help determine what?

Answer 11

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!

Question 12

Bond Enthalpies

Answer 12

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!

Question 13

Average Bond Enthalpy

Answer 13

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)

Question 14

Standard Enthalpy of Atomisation

Answer 14

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

Question 15

BE can only be used when?

Answer 15

Substances are in their GASeous state!

Question 16

Born-Haber Cycle

Answer 16

The energy cycles for the formation of ionic compounds.

Question 17

Born-Haber Cycle shape?

Answer 17

→ → → 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

Question 18

Electron Affinity

Answer 18

The enthalpy change when 1 mole of gaseous atoms gain electrons to form a mole of negatively charged gaseous ions (anions)

Question 19

Ionisation energy

Answer 19

The enthalpy change for 1 mole of a gaseous element to lose e-‘s to form a mole of positively charged gaseous ions (cations)

Question 20

Lattice enthalpy

Answer 20

The energy required to convert 1 mole of the solid compound into gaseous ions
→ HIGHLY ENDOTHERMIC

Question 21

The Magnitude of the lattice enthalpy depends on the nature of the ions involved (2)

Answer 21

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

Question 22

Enthalpy change of solution

Answer 22

The enthalpy change when 1 mole of the substance (solute) is dissolved in water to form a dilute aq. solution

Question 23

Enthalpy change of hydration

Answer 23

The enthalpy change (always exothermic) when 1 mole of the gaseous ions is added to water to form a dilute solution