1.8 - Thermodynamics

Question 1

What does Hesses law state?

Answer 1

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

Question 2

Define standard enthalpy of formation

Answer 2

The enthalpy change when one molecule of a compound is formed from its constituent elements in standard conditions with all products and reactants in their standard states

Question 3

What is the standard enthalpy of an element?

Answer 3

Zero

Question 4

Define standard enthalpy of combustion

Answer 4

The enthalpy change when one mole of a substance is completely burned in excess oxygen

Question 5

Define standard enthalpy of atomisation

Answer 5

Enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions

Question 6

Define first ionisation energy

Answer 6

Enthalpy change when one mole of electrons is removed from one mole of gaseous, atoms to form one more gaseous 1+ ions

Question 7

Define second ionisation energy

Answer 7

Enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions from one mole gaseous 2+ ions

Question 8

Define first electron affinity

Answer 8

Enthalpy change when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous 1- ions

Question 9

Define second electron affinity

Answer 9

Enthalpy change when one mole of gaseous 1- ions gains one mole of electrons to form one mole of gaseous 2- ions

Question 10

Define lattice enthalpy of formation

Answer 10

Enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions

Question 11

Define lattice enthalpy of dissociation

Answer 11

Enthalpy change when one mole of solid ionic lattice is dissociated into its gaseous ions

Question 12

Define Enthalpy of hydration

Answer 12

Enthalpy change when one mole of gaseous ions becomes hydrated/dissolved in water to infinite dilution

Question 13

Define enthalpy of solution

Answer 13

Enthalpy change when one mole of solute dissolves, completely in a solvent to infinite dilution

Question 14

Define mean bond dissociation enthalpy

Answer 14

Enthalpy change when one mole of a certain type of covalent bond is broken with all species in the gaseous state

Question 15

What is a born-haber cycle?

Answer 15

Thermochemical cycle, showing all the enthalpy changes involved in the formation of an ionic compound. start with elements in there standard states

Question 16

What factors affect the lattice enthalpy of an ionic compound?

Answer 16

Size of the ions
Charge on the ions

Question 17

How can you increase the lattice enthalpy of a compound? Why does it increase?

Answer 17

Smaller ions - send is the charge centres will be closer together

Increased charge - since there will be a greater electrostatic force of attraction between the oppositely charged ions

Question 18

How can born-haber cycles used to see if compounds could theoretically exist?

Answer 18

Use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable. Was used to predict the existence of the first noble gas containing compound.

Question 19

What actually happens when a solid is dissolved in terms of interactions of the ions with water molecules?

Answer 19

Break lattice - gaseous ions; dissolve each gaseous ion in water. The aqueous ions are surrounded by water molecules ( which have a permanent dipole due to polar O-H bond)

Question 20

What is the perfect ionic model?

Answer 20

Assumes that ions are perfect Lisa, very cool and that there is an even charge distribution. Act as point charges.

Question 21

Why is the perfect ionic model often not accurate?

Answer 21

Ions are not perfectly spherical. polarisation often occurs when small positive ions are large negative ions are involved so the ionic bonds gain covalent character. Some lattices are not regular and the crystal structure can do for

Question 22

What kind of bonds will be the most Ionic?

Answer 22

Between large positive ions and small negative ions

Question 23

Define the term, spontaneous and feasible

Answer 23

If a reaction is spontaneous and feasible, it will take place of its own accord, does not take account of rate of reaction

Question 24

Is a reaction with a positive or negative enthalpy change more likely to be spontaneous?

Answer 24

Negative – exothermic

Question 25

Define entropy

Answer 25

Randomness/disorder of a system
Higher value for entropy = more disordered

Question 26

What units is entropy measured in?

Answer 26

JK^-1mol^-1

Question 27

What is the second law of thermodynamics?

Answer 27

Entropy of an isolated system always increases as it is overwhelmingly, more likely for molecule to be disordered than ordered

Question 28

Is a reaction with a positive or negative entropy change more likely to be spontaneous

Answer 28

Positive – reactions always try and increase the amount of disorder

Question 29

How would you calculate the entropy change for a reaction?

Answer 29

Entropy change = sum of products entropy- sum of reactants entropy

Question 30

Define Gibbs free energy using an equation

Answer 30

G = H - TS

Question 31

What does the value for Gibbs free energy for a reaction show?

Answer 31

if G < 0 reactions feasible
if G = 0 reaction is just feasible
if G > reaction is not feasible

Question 32

What is the significance of the temperature which G = 0?

Answer 32

This is the temperature in kelvin, at which the reaction becomes feasible

Question 33

What are the limitations of using G as an indicator of whether a reaction will occur?

Answer 33

Gibbs free energy only indicates if a reaction is feasible. It does not take into account the rate of reaction. In reality, many reactions that are feasible at certain temperature have a rate of reaction that is so slow that effectively no reaction is occurring

Question 34

Why is entropy at 0 K ?

Answer 34

No disorder

Question 35

How is it possible for the temperature of a substance, undergoing an exothermic reaction to stay constant?

Answer 35

The heat that is given out, escapes to the surroundings