Topic 8: Energetics I

Question 1

Standard enthalpy change of formation

Answer 1

Enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions

Question 2

What are standard conditions

Answer 2

298K and 100kPa

Question 3

First ionisation energy

Answer 3

The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous unipositive cations

Question 4

Second ionisation energy

Answer 4

The energy required to remove one mole of electrons from one mole of gaseous unipositive cations to form one mole of gaseous 2+ cations

Question 5

Lattice enthalpy of formation

Answer 5

The energy released when one mole of an ionic compound is formed from its ions in the gaseous state.

Question 6

First electron affinity

Answer 6

The energy released when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous uninegative anions.

Question 7

Second electron affinity

Answer 7

The energy required for one mole of gaseous uninegative anions to gain one mole of electrons to from one mole of gaseous 2- anions.

Question 8

Standard enthalpy change of atomisation of an element

Answer 8

The enthalpy change when one mole of gaseous atoms is formed from its element in standard state under standard conditions

Question 9

Enthalpy change of atomisation of a compound

Answer 9

Enthalpy change when one mole of gaseous atoms are formed from a species in its standard state
i.e. 1/2 H_2 (g) -> H(g)

Question 10

What is Hess’s Law

Answer 10

The total enthalpy change of a reaction is always the same, regardless of the reaction route taken.

Question 11

What is the assumption which makes theoretical lattice enthalpies possible?

Answer 11

Ionic compounds contain no covalent character - they are purely ionic

Question 12

What does it mean if the experimental and theoretical lattice enthalpies are very close?

Answer 12

The ionic compound contains very little covalent character (almost none)

Question 13

What makes a cation polarising?

Answer 13

Small ionic radius and high charge - high charge density

Question 14

What makes an anion easily polarised?

Answer 14

High charge and large ionic radius

Question 15

What is the formula for predicting ΔHr using bond enthalpies?

Answer 15

ΔHr = ΣBE(reactants) - ΣBE(products)

Question 16

Bond dissociation enthalpy

Answer 16

The energy required to break one mole of bonds are broken in the gaseous state to form gaseous atoms

Question 17

What is the relationship between enthalpy of atomisation and bond enthalpy for diatomic gases?

Answer 17

Bond enthalpy = enthalpy of atomisation
For diatomic molecules, the enthalpy of atomization is equal to the enthalpy of bond dissociation

Question 18

What does a smaller bond enthalpy mean?

Answer 18

Weaker bond

Question 19

What is the definition of standard bond enthalpy values

Answer 19

Average of the dissocation energies of a specific bond in different molecules

Question 20

Why are the ΔHr values a bit off when you use average bond enthalpies?

Answer 20

The strength of the bond also depends on the environment which means it is not actually the bond enthalpy of the specific bond involved in the reaction.

Question 21

What is enthalpy change

Answer 21

The heat energy change measured at constant pressure

Question 22

What is the standard enthalpy of combustion

Answer 22

The enthalpy change when one mole of a substance in its standard state reacts completely with oxygen under standard conditions

Question 23

What is enthalpy of neutralisation

Answer 23

The enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water.

Question 24

What is average bond enthalpy

Answer 24

The amount of energy required to break one mole of bonds of a particular type between two atoms in gaseous state, averaged over different molecules.