17 Thermodynamics

Question 1

Standard molar enthalpy of formation

Answer 1

Enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states

Question 2

Standard molar enthalpy change of combustion

Answer 2

Enthalpy change when one mole of substance is completely burnt in oxygen

Question 3

Standard enthalpy of atomisation

Answer 3

Enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in standard state under standard conditions

Question 4

First ionisation energy

Answer 4

Standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge

Question 5

Second ionisation energy

Answer 5

Loss of a mole of electrons from a mole of singly positively charge ions

Question 6

First electron affinity

Answer 6

Standard enthalpy change when one mole of gaseous ions each with a single negative charge

(Always negative as energy is given out when an electron is attracted to positively charged nucleus)

Question 7

Second electron affinity

Answer 7

Enthalpy change when one mole of electrons is added to mole of gaseous ions each with a single negative charge to from ions each with two negative charges

(Always positive negative electron has to overcome the repulsion between the electron and negative ion)

Question 8

Lattice enthalpy of formation

Answer 8

Standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions

Question 9

Enthalpy of lattice dissociation

Answer 9

Standard enthalpy change when one mole of solid ionic compounds dissociated into its gaseous ions

Question 10

Enthalpy of hydration

Answer 10

Standard enthalpy change when water molcules surround one mole of gaseous ions

Question 11

Enthalpy of solution

Answer 11

Standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart not to interact with each other

Question 12

Mean bond enthalpy

Answer 12

Enthalpy change when one mole of gaseous molecules each break covalent bonds to form two free radicals averaged over a range of compounds

Question 13

Hess’s law

Answer 13

The enthaply change accompanying a chemical change is independent of the route by which the chemical change occurs

Question 14

Trends in lattice enthalpies

Answer 14

Larger ions lead to smaller lattice enthalpies. This is because the opposite charges do not approach each other as closely when the ions are larger

Charge to size ratio increase

Question 15

Explain why the enthalpy of lattice dissociation of potassium oxide is less endothermic than that of sodium oxide

Answer 15

The potassium ion is bigger to the electrostatic forces of attraction between ions is weak

Question 16

Suggest why the electron affinity of fluorine has a negative value

Answer 16

The nucleus which is positive attracts the negative electron when an electron is gained energy is given out

Question 17

Explain why the theoretical enthalpy of lattice dissociation for silver fluoride is different form the experimental value that can be calculated using the born-harbour cycle

Answer 17

Theoretical value is calculated assuming that the bonding is perfecting ionic. Experimental value allows for the bonding to have some covalent character

Question 18

Why would the lattice dissociation for silver chloride be less than silver fluoride

Answer 18

Chloride ion is larger so the attraction between Cl- and Ag+ is weaker

Question 19

Define the term enthalpy of lattice dissociation

Answer 19

Enthalpy change when one mole of an compound is broken into its ions in the gaseous state

Question 20

Explain why the bond enthalpy of Cl-Cl bond is greater than that of a Br-Br bond

Answer 20

Bond pair is closer to the nucleus so attraction to bonding pair is stronger

Question 21

Explain the meaning of the term perfect ionic model

Answer 21

Ions can be regarded as point charges ( perfect spheres)

Question 22

Factors that increase polarisation

Answer 22

Positive ion small size and high charge

Negative ion larger size and high charge

Question 23

Spontaneous

Answer 23

Occurring as a result of a sudden impulse

Question 24

Entropy

Answer 24

A numerical measure of disorder in a chemical system

Question 25

Positive 🔺G

Answer 25

Means the reaction isn’t feasible at room tempeture

Question 26

Determining entropy change

Answer 26

At a boiling point water is equally likely to exist as liquid or vapour

🔺H = T🔺S

Question 27

Extracting metals

Answer 27

Extracting metals form their oxide ore is to heat them with carbon ( coke is cheap)

Question 28

Kinetic factors

Answer 28

Neither enthalpy change nor entropy changes tells us anything about how quickly or slowly a reaction is likely to go ( large activation energy)

Question 29

Partial pressure of A

Answer 29

Mole fraction of a ✖️ total pressure

Question 30

Mole fraction of A

Answer 30

Number of moles of A / total moles of gas

Question 31

Effect of pressure on gaseous equilibrium

Answer 31

Pressure only effects in there are a different number of moles

Increasing the pressure on a gas phase reaction will increase the rate at which equilibrium is reached because there would be more collisions

Question 32

Effect of temperature of a gaseous equilibrium

Answer 32

Increasing the temperature will also increase the rate of equilibrium is reach with a catalyst

Question 33

what does ∆H mean?

Answer 33

∆H, whilst important, is not suf cient to explain feasible change

Question 34

equation

Answer 34

∆G = ∆H – T∆S