Thermodynamics

Question 1

Define mean bond enthalpy

Is this endothermic, exothermic, or does it depend?

Answer 1

Enthalpy change for the breaking of 1 mole of covalent bonds, averaged over a range of compounds

Endothermic

Question 2

Define Bond Dissociation

Answer 2

Enthalpy change to break the bond in one mole of gaseous molecules to form gaseous atoms

This is endothermic

Question 3

Define standard molar enthalpy of atomisation (🔺H at)

Answer 3

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

This is endothermic

Question 4

Define first electron affinity (EA)/(🔺H ea)

Answer 4

The enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous negative ions

This is exothermic

Question 5

Define second electron affinity (🔺H ea-)

Answer 5

The enthalpy change when a mole of electrons is added to a mole of gaseous negative ions, to form ions each with a doubly negative charge

Question 6

Why is first electron affinity exothermic but second is endothermic

Answer 6

1st - attraction is created between electron and the nucleus

2nd- energy is required to overcome repulsion between negative electron and negative ion

Question 7

Define lattice formation enthalpy

Answer 7

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

Question 8

Define lattice dissociation energy

Answer 8

Enthalpy change to separate one mole of an ionic substance into its gaseous ions

Always endothermic (positive value)

Question 9

Define enthalpy of hydration

Answer 9

Enthalpy change when one mole of gaseous ions become aqueous ions

Question 10

Why is enthalpy of Hydration exothermic

Answer 10

Attraction is created between the ion and water

Question 11

Define enthalpy of solution

Answer 11

When one mole of solid dissolves in water to form aqueous ions

Question 12

Breaking attractions is ______

Making attractions is ______

Answer 12

Endothermic (🔺H >0)

Exothermic (🔺H<0)

Question 13

What are the 4 stages in forming an ionic lattice?

Answer 13

Stage 1: atomisation

2: ionisation
3: electron affinity
4: lattice formation

Question 14

For lattice formation, what does enthalpy of formation equal?

Answer 14

All enthalpy changes added together

Question 15

What is lattice enthalpy

Answer 15

The energy released when gaseous ions combine to form an ionic lattice (formation is exothermic) or the energy required to break apart the lattice to form gaseous ions (dissociation is endothermic)

Question 16

What are the 2 main things that affect lattice enthalpy?

Why?

Answer 16

Charge of the ion
Size of the ion

Higher the charge the smaller its radius, meaning the ions will be closer together in the lattice, increasing the attractions’ strength

Question 17

If the attraction is stronger is more or less energy released on the formation of the lattice

Answer 17

More energy

Question 18

Why do lattice dissociation enthalpies decrease as the size of ions increase?

Answer 18

Larger ions have a weaker electrostatic attraction between oppositely charged ions

Question 19

What is the perfect ionic model?

Why may this value be different to the experimental value?

Answer 19

It assumes all ions are perfect spheres with purely ionic attraction

Covalent Character

Question 20

What does covalent character do?

What does this mean when comparing theoretical and experimental values?
Why?

Answer 20

Puts extra energy into the ionic bond, making the bond stronger

Experimental value will be greater

Covalent character makes the bond stronger so more energy is released/ required

Question 21

Define enthalpy of solution

Answer 21

Enthalpy change when one mole of a solid dissolves in water to form aqueous ions

Question 22

What must a solvent be for ionic solids to dissolve in it

Answer 22

Polar eg water

Question 23

If NaCl was dissolved in water, what would happen?

Answer 23

The Na+ would be attracted to the delta-negative Oxygen of the water and the Cl- would be attracted to the delta-positive hydrogen

Question 24

What does 🔺H sol =

Answer 24

🔺H (L. D.) + 🔺H(hyd)

Question 25

What is entropy

Answer 25

A measure of disorder in a system

Question 26

What does an increase in disorder mean?

Answer 26

The reaction is ‘feasible’ or ‘spontaneous’

Question 27

NAme the states of matter from most ordered to most disordered

Answer 27

Solid Liquid Gas

Question 28

What affects entropy?

Answer 28

Number of moles/ particles | State

Question 29

If a solid becomes liquid, or liquid becomes gas what does this mean for the entropy?

Answer 29

Entropy increases so 🔺S is positive

Question 30

If the number of moles decreases, what does this mean for the entropy

Answer 30

Disorder decreases so 🔺S is negative

Question 31

If a question asks about entropy what must you mention

Answer 31

Disorder

Question 32

The more positive the 🔺H...

Answer 32

The more disordered a system is and the more feasible the reaction

Question 33

Equation for entropy change

Answer 33

🔺S = S(products) - S (reactants) [SliPeR]

Question 34

What units are used to measure 🔺S

Answer 34

J/K/mol

Question 35

Why is entropy 0 at 0K?

Answer 35

There is no movement at absolute zero so there is no disorder

Question 36

When is the greatest 🔺S as temperature increases?

Answer 36

The boiling point Substance is becoming a gas so there is a large increase in disorder (use question’s terminology)

Question 37

What is the equation for Gibbs Free Energy? What are the units for this?

Answer 37

🔺G=🔺H -T🔺S kJ/mol

Question 38

When is a reaction feasible?

Answer 38

🔺G<0 So if the value is negative or 0 the reaction IS feasible. The more negative the more feasible

Question 39

``` If the question asks when a reaction: becomes feasible Is no longer feasible At equilibrium What do you do ```

Answer 39

Set 🔺G=0

Question 40

What is the 🔺G value at equilibrium?

Answer 40

0

Question 41

Why does 🔺G=0 at equilibrium?

Answer 41

Forwards reaction is equally feasible as the backward reaction

Question 42

Relate y=mx+c to | 🔺G=🔺H-T🔺S

Answer 42

``` y= 🔺G m= -🔺S x= T c= 🔺H ```

Question 43

If 🔺S is positive and 🔺H is positive, when is the reaction feasible?

Answer 43

At high temperatures, when T🔺S>🔺H

Question 44

When 🔺S is negative and 🔺H is positive, when is the reaction feasible

Answer 44

Never

Question 45

When 🔺S is positive and 🔺H is negative when is the reaction feasible?

Answer 45

Always

Question 46

If 🔺S is negative and 🔺 is negative when is the reaction feasible

Answer 46

At low temperatures, when T🔺S>🔺H

Question 47

Define bond dissociation enthalpy | Give example

Answer 47

Enthalpy change to break the bond in one mole of gaseous molecules to form gaseous atoms Eg Cl2 —> 2Cl

Question 48

Define standard molar enthalpy of atomisation

Answer 48

Enthalpy change for he formation of one mole of gaseous atoms from an element in its standard state under standard conditions 1/2 Cl2 (g) —> Cl(g)

Question 49

Difference between bond dissociation and atomisation

Answer 49

BD: breaking 1 mole of bonds Atom: forming one gaseous mole from standard state

Question 50

Define first ionisation enthalpy

Answer 50

Minimum amount of energy required to remove one mole of e- from one mole of gaseous atoms

Question 51

Why is second ionisation energy more endothermic

Answer 51

More difficult to remove an e- from a more positive ion

Question 52

First electron affinity

Answer 52

🔺H when a mole of gaseous atoms is converted to gaseous negative ions

Question 53

Second e- affinity

Answer 53

🔺H when a mole of electrons is added to a mole of gaseous negative ions to form ions each with a doubly negative charge

Question 54

Why is 2nd electron affinity endothermic

Answer 54

Energy is required to overcome repulsion between negative e- and negative ion

Question 55

Lattice formation enthalpy

Answer 55

🔺H when one mole of a solid ionic compound is formed from its gaseous ions

Question 56

Lattice dissociation enthalpy

Answer 56

🔺H to separate one mole of an ionic substance into its gaseous ions

Question 57

🔺H hydration

Answer 57

Enthalpy change when one mole of gaseous IONS become aqueous ions

Question 58

🔺H solution definition

Answer 58

🔺H when one mole of solid dissolves in water to form aqueous ions

Question 59

Define enthalpy change

Answer 59

The heat energy change at constant pressure