Enthalpy

Question 1

Standard Enthalpy of Formation

Answer 1

Enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements.

= sum of all Enthalpy changes

Usually EXOTHERMIC

Question 2

Enthalpy Change of Atomisation

Answer 2

Enthalpy change that takes place when 1 mole of gaseous atoms forms from the element.

Always ENDOTHERMIC

K(s) –> K(g)

Question 3

First Ionisation Energy

Answer 3

Enthalpy change accompanying removal of 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

ENDOTHERMIC - electron overcomes nuclear attraction

K(g) –> K+(g) + e-

Question 4

Second Ionisation Energy

Answer 4

Enthalpy change that accompanies the removal of 1 electron from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions.

ENDOTHERMIC

Ca+(g) –> Ca2+(g) + e-

Question 5

First Electron Affinity

Answer 5

Enthalpy change that accompanies the addition of 1 electron to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions.

EXOTHERMIC - electron attracted in by nucleus/nuclear attraction

Cl(g) + e- –> Cl-(g)

Question 6

Second Electron Affinity

Answer 6

Enthalpy change that accompanies the addition of 1 electron to each ion in 1 mole of gaseous 1- ions to form 1 mole of gaseous 2- ions.

ENDOTHERMIC - more energy needed so more energy is released

O-(g) + e- –> O2-(g)

Question 7

How does ionic size affect lattice Enthalpy?

Answer 7

As ionic size increases, the attraction decreases and the lattice Enthalpy is less negative/EXOTHERMIC.

Large ions = attract weakly
Small ions = attract strongly

Question 8

How does ionic charge affect lattice Enthalpy?

Answer 8

As ionic charge increases, ionic size decreases due to increased attraction so lattice Enthalpy is more negative/EXOTHERMIC

Question 9

Hess’ Law

Answer 9

States that if a reaction can take place by more than 1 route and the initial and final condition are the same, the total Enthalpy change is the same for each route.

Question 10

Standard Conditions

Answer 10

298K

100kPa

Question 11

Why can lattice Enthalpy not be measured directly?

Answer 11

Cannot form 1 mole of ionic lattice from gaseous ions

Question 12

Standard Enthalpy Change of Solution

Answer 12

Enthalpy change that takes place when 1 mole of a compound completely dissolved in water.

EXOTHERMIC = CaCl2 in water

or ENDOTHERMIC = Ammonium nitrate in water (NH4NO3)

KCl(s) + (Aq) –> K+(Aq) + Cl-(Aq)

Question 13

What happens when an ionic solid dissolves?

Answer 13

Ionic lattice breaks down into gaseous ions

Ions are hydrated

Question 14

Standard Enthalpy Change of Hydration

Answer 14

Enthalpy change that takes place when 1 mole of isolated gaseous ions is dissolved in water, forming 1 mole of aqueous ions.

EXOTHERMIC

K+(g) + (Aq) –> K+(Aq)

Question 15

How does ionic size affect Enthalpy change of hydration?

Answer 15

As ionic size decreases, hydration becomes more negative bc small ions attract water more strongly.

Question 16

How does ionic charge affect Enthalpy of hydration?

Answer 16

As ionic charge increases, ions attract water more strongly and hydration becomes more negative.

EXCEPTION!!
Sodium, Magnesium and aluminium ions decrease in size BUT INCREASE in charge so hydration becomes more negative.

Question 17

Lattice Enthalpy

Answer 17

Enthalpy change that accompanies the formation of 1 mole of an ionic compound from its gaseous ions.

= sum of all enthalpies + positive Enthalpy of formation

Shows strength of ionic bonds/electrostatic forces of attraction between oppositely charged ions in lattice