15: Enthalpy changes

Question 1

If heat energy (enthalpy) is evolved is the reaction exothermic or endothermic

Answer 1

Exothermic

Question 2

If heat energy (enthalpy) is absorbed is the reaction exothermic or endothermic

Answer 2

Endothermic

Question 3

What is enthalpy change of atomisation of an element

Answer 3

The enthalpy change when 1 mol of gaseous atoms of an element are formed from the element in its standard state

Question 4

What is the enthalpy change of atomisation related to for a metal

Answer 4

The strength of metallic bonding in the solid element

Question 5

What is enthalpy change of atomisation related to for atoms that already exist as gaseous molecules

Answer 5

The bond enthalpy

But for diatomic molecules like Cl₂, ΔHatomisation will be half the bond enthalpy as only 1 mol of Cl atoms need to be produced

Question 6

What is the first electron affinity of an element

Answer 6

The enthalpy change when one electron is added to each atom in a mol of gaseous atoms of an element

X(g) + e¯ → X¯(g)

Question 7

Why is the first electron affinity negative for an element that commonly forms negative ions

Answer 7

The electron that is added is attracted to the nucleus more strongly than it is repelled by the other electrons

Question 8

Why are all second electron affinities positive

Answer 8

Energy must be put in to overcome the repulsion between the negatively charged X¯ ion and the e¯ that is added

Question 9

What is the lattice enthalpy of an ionic compound

Answer 9

The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions

Question 10

What is lattice enthalpy a measure of (in terms of bonding)

Answer 10

The strength of ionic bonding in a compound

Question 11

Why are all lattice enthalpies exothermic

Answer 11

When ions come together to ionic bonds, heat energy is always released

Question 12

If a compound has a stronger ionic bonding is its lattice enthalpy more or less exothermic

Answer 12

More exothermic

Question 13

What factors effect lattice enthalpy

Answer 13

The size of the ions

The charge of the ions

Question 14

How does the size of the ions affect lattice enthalpy

Answer 14

The smaller the ionic radii of the ions, the greater their charge density, hence the stronger the attraction between them and the more exothermic the lattice enthalpy

Question 15

How does the charge on the ions affect lattice enthalpy

Answer 15

The greater the charge on the ions, the greater their charge density, hence the stronger the attraction between them, and the more exothermic the lattice enthalpy

Question 16

How does the strength of ionic attraction change going down a group

Answer 16

Strength decreases

The charge on the ion stays the same but its radius increases. This decreases the charge density, so attraction to oppositely charged ions becomes weaker and lattice enthalpies become less exothermic.

Question 17

How does the strength of ionic attraction change going across a period

Answer 17

Strength increases

The charge on the ion increases and the radius of the ion decreases.
Greater attraction to other ions, so lattice enthalpies become more exothermic

Question 18

How do melting points relate to lattice enthalpies

Answer 18

The more exothermic the lattice enthalpy, typically the higher the melting point.
This is because both depend on how much energy is needed to disrupt the ionic lattice

Question 19

What would the steps be for the Born-Haber cycle for ΔfH of NaCl starting from Na(s) +½Cl₂(g)

Answer 19

1) ΔatomisationH of Na
2) 1st IE of Na
3) ΔatomisationH of Cl
4) 1st electron affinity of Cl
5) Lattice enthalpy of NaCl

Question 20

What is enthalpy change of solution (ΔsolH)

Answer 20

The enthalpy change when 1 mol of a compound dissolves in water

Question 21

What is enthalpy change of hydration (ΔhydH)

Answer 21

The enthalpy change when 1 mole of gaseous ions dissolve to form one mole of hydrated ions

Question 22

What 2 steps can you separate the dissolving of an ionic solid into

Answer 22

1) The lattice breaks up, overcoming the attraction between the oppositely charged ions
2) The separate ions are hydrated, interacting with water molecules through ion-dipole forces

Question 23

Factors that affect the enthalpy change of hydration of an ion

Answer 23

The greater the ionic charge, the stronger the attraction between the ion and the water molecules, so the more exothermic the enthalpy change of hydration

The greater the ionic radius, the weaker the attraction between the ion and the water molecules, so the less exothermic the enthalpy change of hydration