Thermodynamics

Question 0

Define enthalpy change of combustion

Answer 0

Enthalpy change when 1 mole of a substance undergoes complete combustion in oxygen under standard conditions

Question 1

Define enthalpy change of formation

Answer 1

Enthalpy change when 1 mole of a substance is formed from its elements in their standard States under standard conditions

Question 2

Define enthalpy change of ionisation

Answer 2

The first ionisation energy is the enthalpy change when 1 mole of gaseous atoms loses 1 electron per atom to become 1 mole of gaseous 1+ ions

The second ionisation energy is the enthalpy change when 1 mole of 1+ gaseous ions loses another electron to become 1 mole of gaseous 2+ ions

Question 3

Define enthalpy change of solution

Answer 3

Enthalpy change when 1 mole of a solute is dissolved in enough solvent that the ions are separated and no further enthalpy change occurs on further dilution

Question 4

Define enthalpy change of atomisation

Answer 4

Enthalpy change when 1 mole of gaseous atoms are formed from an element or compound in its standard state under standard conditions

Question 5

Define lattice enthalpy of formation

Answer 5

Enthalpy change when 1 mole of a solid ionic compound is formed from its constituent gaseous ions

Question 6

Define electron affinity

Answer 6

The first electron affinity is the enthalpy change when 1 mole of gaseous atoms gains 1 electron per atom to become 1 mole of gaseous 1- ions

The second electron affinity is the enthalpy change when 1 mole of gaseous 1- ions gain another electron to become 1 mole of gaseous 2- ions

Question 7

Define enthalpy change of hydration

Answer 7

Enthalpy change when 1 mole of gaseous ions are fully dissolved in water to form 1 mole of aqueous ions. This always gives out energy (EXO (negative))

Question 8

Define enthalpy change

Answer 8

the heat energy transferred in a reaction at constant pressure

Question 9

Define bond dissociation enthalpy

Answer 9

Enthalpy change when all the bonds of the same type in 1 mole of gaseous molecules are broken

Question 10

Theoretical lattice enthalpies are often different from experimental values. Explain

Answer 10

You can work out a theoretical lattice enthalpy by doing some calculations based on the purely ionic model of a lattice. The purely ionic model of a lattice assumes that all the ions are spherical and have their charge evenly distributed, there is no covalent character. But the experimental lattice enthalpy from born Haber cycle is usually different. This is evidence that ionic compounds usually have some covalent character. The positive and negative ions in a lattice aren’t usually exactly spherical. Positive ions polarise neighbouring negative ions to different extents and the more polarisation the more covalent the bonding will be

Question 14

What happens when a solid ionic lattice dissolves in water

Answer 14

The bonds between the ions break (ENDO). This enthalpy change is the lattice enthalpy of dissociation. Bonds between the ions and water are made (EXO). This enthalpy change is enthalpy change of hydration. The enthalpy change of solution is the overall effect on the enthalpy of these 2 things

Question 15

What do you need to know to be able to complete an enthalpy cycle

Answer 15

Lattice dissociation enthalpy of the compound and enthalpies of hydration of the ions

Question 16

How does physical state affect entropy

Answer 16

Solid particles wobble around a fixed point, hardly any randomness so lowest entropy

Liquids have some disorder

Gas particles have most disorder, most random arrangements of particles so highest entropy

Question 17

How does dissolving affect entropy

Answer 17

Dissolving a solid increases entropy as dissolved particles can move freely as they’re no longer held in one place. Increases vibrations

Question 18

How do more particles affect entropy

Answer 18

The more particles, the more entropy as the more ways they and their energy can be arranged. Entropy increases as number of moles increases

Question 19

Entropy increase may explain spontaneous endothermic reactions. Explain

Answer 19

A spontaneous (or feasible) reaction is one that’ll happen by itself (don’t have to give it energy). Normally have to supply energy to make an endothermic reaction happen, but in some reactions the entropy increases such a lot that the reaction will happen by itself, without a supply of energy

Question 20

Reactions won’t happen unless the total entropy change is ……

Answer 20

Positive

During a reaction, there’s an entropy change between reactant and products (entropy change of the system). The entropy change of the surroundings alters too ( as energy is transferred to or from the system). The total entropy change is the sum of the entropy changes of the system and the surroundings.

Question 21

Define lattice enthalpy of dissociation

Answer 21

Enthalpy change to separate 1 mole of a substance into its gaseous ions

Question 22

Mean bond enthalpies are only approximations. Explain

Answer 22

Type of bond will vary in strength in different compounds. Only bond enthalpies in diatomic molecules will always be same. So calculations done using mean bind enthalpies will never be perfectly accurate. Will get much more exact results from experimental data obtained from specific compounds

Question 23

What is entropy

Answer 23

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 24

What things affect entropy

Answer 24

Physical state
Dissolving
More particles

Question 25

In terms of the forces acting on the particles, suggest 1 reason why the first electron affinity of oxygen is an exothermic process

Answer 25

There is a strong net attraction between the nucleus and outer electrons, takes a lot of energy to break attraction

Question 26

Explain the interactions between water molecules and fluoride ions when the fluoride ions become hydrated

Answer 26

Water is polar

The F- ions attracts water (attracted to the positive charge on the hydrogen)

Question 27

Explain why the electronegativity of fluorine is greater than that of chlorine

Answer 27

The fluorine atom is smaller, there is less shielding

The bond pair of electrons are attracted more strongly to the nucleus

Question 28

Explain why the hydration enthalpy of the fluoride ion is more negative than that of the chloride ion

Answer 28

Fluoride ions are smaller, they have a larger charge density

The positive charge attracts the positive charge on the hydrogen on water more strongly