Thermodynamics

Question 1

What is enthalpy change?

Answer 1

It is the heat energy transferred in a reaction at constant pressure.

Question 2

What is enthalpy of formation?

Answer 2

It is the enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.

Question 3

What is the enthalpy change of atomisation of an element?

Answer 3

It is the enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.

Question 4

What is the first ionisation energy?

Answer 4

It is the enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms.

Question 5

What is the first electron affinity?

Answer 5

It is the enthalpy change when 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms.

Question 6

What is the enthalpy change of hydration?

Answer 6

It is the enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions.

Question 7

What is the bond dissociation enthalpy?

Answer 7

It is the enthalpy change when all the bonds of the same type in 1 mole of gaseous molecules are broken.

Question 8

What is the enthalpy change of atomisation of a compound?

Answer 8

It is the enthalpy change when 1 mole of a compound in its standard state is converted into gaseous atoms.

Question 9

What is the second ionisation energy?

Answer 9

It is the enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions.

Question 10

What is the second electron affinity?

Answer 10

It is the enthalpy change when 1 mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions.

Question 11

What is the enthalpy of solution?

Answer 11

It is the enthalpy change when 1 mole of solute is dissolved in enough solvent that no further enthalpy change occurs on further dilution.

Question 12

What is lattice enthalpy?

Answer 12

It is a measure of ionic bond strength

Question 13

What is lattice enthalpy of formation?

Answer 13

It is the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions.

Question 14

What is the lattice enthalpy of dissociation?

Answer 14

It is the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions.

Question 15

What is the enthalpy of combustion?

Answer 15

It is the enthalpy change when 1 mole of a substance is completely burnt in oxygen with all the reactants and products in their standard states under standard conditions.

Question 16

What is the enthalpy of neutralisation?

Answer 16

It is the enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions.

Question 17

What is the enthalpy of vaporisation?

Answer 17

It is the enthalpy change when 1 mole of a liquid is turned into a gas.

Question 18

What is the enthalpy of fusion?

Answer 18

It is the enthalpy change when 1 mole of a solid is turned into a liquid.

Question 19

What is Hess’ Law?

Answer 19

The enthalpy change for a reaction is independent of the route taken

Question 20

Which factors affect lattice enthalpies?

Answer 20

The size of the ions

The charges on the ions

Question 21

How does size affect the lattice enthalpy?

Answer 21

The larger the ions, the less negative the enthalpies of lattice formation. The charges are further apart, therefore, there is a weaker force of attraction between them.

Question 22

How do charges on the ions affect the lattice enthalpy?

Answer 22

The larger the charge on the ion, the greater the attraction between the ions, therefore, the stronger the lattice enthalpy.

Question 23

What are the assumptions of theoretical lattice enthalpies?

Answer 23

The assumptions are of a perfect ionic model

• the ions are 100% ionic
• the ions are spherical
• the attractions are purely electrostatic

Question 24

Why are the theoretical and experimental values different?

Answer 24

This is evidence that the ionic compounds have some covalent character. The positive ions polarise the neighbouring negative ions.

Question 25

What type of reaction is the lattice enthalpy of dissociation and why?

Answer 25

The bonds between the ions break to give free ions

Question 26

What type of reaction is the enthalpy change of hydration and why?

Answer 26

Exothermic | Bonds between the ions and the water are made

Question 27

Why can water molecules bond to the ions?

Answer 27

Oxygen is more electronegative than hydrogen, and therefore draws electrons towards itself, creating a dipole. The dipole means that the positively charged hydrogen atoms can form bonds with negative ions and the negatively charged oxygen can form bonds with positive ions.

Question 28

When do substances generally dissolve?

Answer 28

If the energy released is roughly the same or greater than the energy taken in.

Question 29

What is entropy?

Answer 29

It is a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles.

Question 30

What is the effect of change in physical state on the entropy?

Answer 30

Solids have the lowest entropy because the particles vibrate about fixed points. Gases have the highest entropy because the particles move around rapidly and randomly.

Question 31

What is the effect of having more particles on the entropy?

Answer 31

The more particles there are, the more ways there are for them and their energy to be arranged

Question 32

How does temperature affect entropy?

Answer 32

The higher the temperature, the faster the particles vibrate/move and so the greater the entropy.

Question 33

Why is the entropy increase from liquid to gas greater than the increase from solids to liquids?

Answer 33

There is a large amount of disorder in gases compared to solids and liquids.

Question 34

How do you calculate entropy(S) change?

Answer 34

change in S = S(products) - S(reactants)

Question 35

What is free energy change(G)?

Answer 35

It is a measure used to predict whether a reaction is feasible/spontaneous.

Question 36

What is the condition for a reaction to be feasible?

Answer 36

change in G must be negative or zero

Question 37

Why might a reaction not happen or be so slow even if it is feasible?

Answer 37

The reaction might have a high activation energy

Question 38

How can change in G be calculated?

Answer 38

change in G(J/mol) = enthalpy change(J/mol) - temperature(K)*entropy change(J/K/mol)

Question 39

What is G when the reaction is exothermic and has a positive entropy change?

Answer 39

change in G is always negative | The reactions are feasible at any temperature

Question 40

What is G when the reaction is endothermic and has a negative entropy change?

Answer 40

change in G is always positive | The reactions are not feasible at any temperature

Question 41

How do you calculate the temperature at which the reaction is just feasible?

Answer 41

T= enthalpy change/entropy change

Question 42

What is the standard entropy of a substance?

Answer 42

It is the entropy of 1 mole of that substance under standard conditions.