thermodynamics

Question 1

Hess’s Law

Answer 1

The enthalpy change for a chemical reaction is the same, regardless of the route taken from reactants to products.

Question 2

The Standard Enthalpy of Combustion, ∆Hɵc

Answer 2

The enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions, all reactants and products in their standard states.
C2H6(g) + 3½O2(g) à 2CO2(g) + 3H2O(l)

Question 3

The Standard Enthalpy of Formation, ∆Hɵf

Answer 3

The enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states.

Question 4

The Standard Enthalpy of Atomisation,

Answer 4

The enthalpy change when one of gaseous atoms is formed from an element in its standard state.

Question 5

Mean Bond Enthalpy ΔHBEΘ

Answer 5

.
e.g. CH4(g)—> H(g) + CH3(g)
enthalpy change needed to break the 1 mol of gasseous molecules with covalent bond into gaseous atoms, averaged over different molecules
(These values are positive because energy is required to break a bond. The definition only applies when the. substances start and end in the gaseous state.)

Question 6

First Ionisation Enthalpy, 1st∆Hɵi

Answer 6

The standard enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to give one mole of gaseous ions each with a single positive charge.

Question 7

First Electron Affinity, 1

Answer 7

The enthalpy change when one mole of electrons added to gaseous atoms then converted into a mole of gaseous ions, each with a single negative charge under standard conditions.

X(g) + e- X-(g)

Question 8

Second Ionisation Enthalpy, 2nd∆Hɵi

Answer 8

The standard enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to give one mole of gaseous ions each with a 2+ charge.

Question 9

Second Electron Affinity

Answer 9

The enthalpy change when one mole of electrons is added to a mole of gaseous ions each with a single negative charge, to form a mole of ions each with a two negative charge.

Question 10

Lattice Formation Enthalpy

Answer 10

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

Question 11

Standard Enthalpy of Hydration

Answer 11

The standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions.

Question 12

Lattice Dissociation Enthalpy

Answer 12

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

Question 13

Standard Enthalpy of Solution
and how to work out enthapy of sol

Answer 13

enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form a dilute solution. (The symbol (aq) is used to show that the solid is dissolved in sufficient water.)

NaCl (s).–>Na+(aq) + Cl-(aq)
Enthapy of sol=enthapy of Lat diss + enthapy of hydrration (of each )

Question 14

mean bond enthalpies and the formula:

Answer 14

∆H = ∑Bonds Broken – ∑Bonds Formed