Thermodynamics

Question 1

Define enthalpy of formation

Answer 1

The enthalpy of formation (ΔHfꝋ) is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions
at a standard condition temperature of 298 K and a pressure of 100 kPa

Question 2

what is energy change?

Answer 2

energy change is the sum of the bonds broken and formed, so the enthalpy change can have positive or negative values

Question 3

what type of reaction is the enthalpy of formation i.e. endo or exothermic

Answer 3

can be endothermic or exothermic (as the energy change is the sum of the bonds broken and formed, so the enthalpy change can have positive or negative values)

Question 4

What does the enthalpy of formation refer to i.e. elements, molecules

Answer 4

enthalpy of formation only refers to compounds
enthalpy of the formation of elements is zero

Question 5

define ionisation energy

Answer 5

The ionisation enthalpy (ΔHieꝋ) of an element is the amount of energy required to remove an electron from a gaseous atom of an element to form a gaseous ion under standard conditions

Question 6

what type of reaction is ionisation enthalpy

Answer 6

always endothermic

Question 7

Why is it this type of reaction is ionisation enthalpy

Answer 7

as energy is needed to overcome the attraction between an electron and the nucleus

Question 8

i.e. what is the first ionisation energy?

Answer 8

The energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions

Na (g) → Na+ (g) + e– ΔHieꝋ = +500 kJ mol-1

Question 9

Define Enthalpy change of atomisation (ΔHatꝋ)

Answer 9

The enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions

Question 10

What type of reaction is enthalpy of atomisation

Answer 10

endothermic

Question 11

Why is enthalpy of atomisation this type of reaction

Answer 11

energy is always required to break any bonds between the atoms in the element, to break the element into its gaseous atoms

Question 12

Will the value be positive or negative for energy of atomisation

Answer 12

the enthalpy change will always have a positive value as its endothermic

Question 13

Define is bond enthalpy

Answer 13

The amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation energy

Question 14

What is bond dissociation energy?

Answer 14

Bond dissociation energy (E) is usually just simplified to bond energy or bond enthalpy

Question 15

How do you write the type of bond broken in bond dissociation?

Answer 15

the type of bond broken is written in brackets after E
i.e. E (H-H) is the bond energy of a mole of single bonds between two hydrogen atoms

Question 16

How is bond enthalpy different to enthalpy of atomisation

Answer 16

atomisation = produces 1 mole of atoms
bond enthalpy = expressed per mole of bonds

Question 17

Define Electron Affinity (ΔHeaꝋ)

Answer 17

The energy change when one mole of electrons is gained, by one mole of gaseous atoms of an element to form one mole of gaseous ions under standard conditions

Question 18

Example of electron affinity

Answer 18

Cl (g)+ e– → Cl- (g) ΔHeaꝋ = -364 kJ mol-1

Question 19

What type of reaction is the first electron affinity of an atom

Answer 19

Always exothermic as energy is released when electrons are attracted to the atoms

Question 20

What type of reaction is the second electron affinity of an atom and why

Answer 20

Can be endothermic or exothermic
because a large force of repulsion must be overcome between the negatively charged ion and the second electron requiring a large input of energy

Question 21

Define lattice energy (ΔHlattꝋ)

Answer 21

enthalpy change when 1 mole of an ionic crystal is formed from the gaseous ions

Question 22

What type of reaction is lattice energy

Answer 22

exothermic, as when the ions are combined there is an extremely large release of energy
So it has a large negative value

Question 23

What does this large negative value suggest?

Answer 23

That the ionic compound is much more stable than its gaseous ions

Question 24

Why is the ionic compound more stable than its gaseous ions?

Answer 24

• Strong electrostatic forces of attraction between the oppositely charged ions in the solid lattice
• No electrostatic forces of attraction between the ions in the gas phase, the gaseous ions are less stable than the ions in the ionic lattice
• More exothermic the lattice enthalpy value is, the stronger the ionic bonds within the lattice are

Question 25

Write down the equations which represent the lattice energy of:

(i) Magnesium oxide

(ii) Lithium chloride

Answer 25

Answer 1: Mg2+ (g) + O2- (g) → MgO (s)

Answer 2: Li+ (g) + Cl- (g) → LiCl (s)

Question 26

What does the endothermic reaction mean in the Born-Haber Cycle

Answer 26

reaction goes up, energy increases

Question 27

What does the exothermic reaction mean in the Born-Haber Cycle

Answer 27

reaction goes down, energy decreases

Question 28

What does the Born-Haber cycle show

Answer 28

all the steps needed to turn atoms -> gaseous ions and gaseous ions -> the ionic lattice