17 - Thermodynamics

Question 1

Standard molar enthalpy of formation

Answer 1

Enthalpy change when one mole of substance is formed from its constituent elements under standard conditions, all reactants and products in standard states.

Question 2

Standard molar enthalpy of combustion

Answer 2

Enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions, all reactants and products in standard states.

Question 3

What are the standard conditions?

Answer 3

100kPa (1 atmosphere)
298K

Question 4

Standard enthalpy of atomisation

Answer 4

Enthalpy change due to the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.

Question 5

1st electron affinity

Answer 5

Standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, each with a single negative charge.

Question 6

2nd electron affinity

Answer 6

Enthalpy change when a mole of electrons is added to a mole of gaseous ions each with a single negative charge to form ions each with two negative charges.

Question 7

Lattice enthalpy of formation

Answer 7

The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.

Question 8

Enthalpy of hydration

Answer 8

Standard enthalpy change when water molecules surround one mole of gaseous ions.

Question 9

First ionisation energy

Answer 9

Standard enthalpy change when one mole of gaseous atoms is converted into one mole of gaseous ions each with a single positive charge.

Question 10

Second ionisation energy

Answer 10

The standard enthalpy change when one mole of electrons are lost from a mole of singly positive charged ions, forming one of mole of two positive charge.

Question 11

Enthalpy of solution

Give equation.

Answer 11

Standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules/ions are far enough apart not to interact with each other.

∆H(sol) = ∆H(lattice diss) + Σ∆H(hydration)

Question 12

Lattice enthalpy of dissociation

Answer 12

Standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions.

Question 13

Mean bond enthalpy

Answer 13

Standard enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form free radicals, averages over a range of compounds.

Question 14

What do lattice enthalpies depend on?

Answer 14

• Size of ions (larger = weaker attraction as further apart = less negative)
• Charges on ions (bigger = greater attraction = stronger lattice enthalpy of dissociation = more negative)

Question 15

Why are Born Haber calculated values (experimental) larger than theoretical values?

Answer 15

Covalent character in bond

Question 16

What do theoretical lattice enthalpies assume?

Answer 16

Perfect ionic model, with 100% spherical ions and purely electrostatic attraction

Question 17

Define a spontaneous process.

Words not equation

Answer 17

A process which will proceed on its own without any external influence.

Question 18

In terms of entropy and order, what do things tend too?

Answer 18

Higher entropy
More disordered

Question 19

What is entropy a measure of?

Answer 19

The disorder of something, or the number of ways it can be arranged.

Question 20

Which state has the highest entropy?

Answer 20

Gas, as they are the most disordered.

Question 21

When does a reaction involve an increase in entropy?

Answer 21

Solid/liquid to gas
Increase in number of molecules

Question 22

Give an equation for ∆S?

Answer 22

∆S = ΣS(products) - ΣS(reactants)

Question 23

When does a molecule/atom have 0 entropy?

Answer 23

At 0K

Question 24

What is the unit of entropy?

Answer 24

J K-1 mol-1

Question 25

Give Gibbs free energy equation.

What units are these in?

Answer 25

∆G = ∆H - T∆S

kJ mol-1, kJ mol-1, K, J K-1 mol-1

Question 26

According to Gibbs, when is a reaction feasible?

Answer 26

∆G < 0

Question 27

How can feasibility points be determined if given enthalpy and entropy changes for a reaction?

Answer 27

∆G = 0
Solve for T.

Question 28

What can be known about an insoluble substance’s DeltaHSolution? Why?

Answer 28

Lattice enthalpy (to break up molecule) > hydration enthalpy (to coat molecule in water)
Not energetically favourable.
Endothermic DeltaHSolution