Enthalpy and Entropy (Chapter 22)

Question 1

What is the definition of lattice enthalpy?

Answer 1

The enthalpy change that accompanies the formation of one mole of an ionic compound (lattice) from its gaseous ions, under standard conditions

Question 2

Give an example of a lattice enthalpy equation

Answer 2

Na+(g) + Cl-(g) = NaCl(s)

Question 3

Is lattice enthalpy exothermic or endothermic?

Answer 3

Always exothermic, therefore enthalpy change is always negative

Question 4

What is the definition of first electron affinity (E.A)?

Answer 4

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 5

What is the definition of second electron affinity (E.A)?

Answer 5

The enthalpy change when one electron is added to each 1- ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions

Question 6

Why is 1st E.A exothermic?

Answer 6

Because the electron being added is attracted in towards the nucleus (doesn’t require energy, does it on its own) + mexo

Question 7

Why is 2nd E.A endothermic?

Answer 7

• Because a second electron is being gained by a negative ion, which repels the electron away
• Therefore, energy must be put in to force the negatively-charged electron onto the negative ion

Question 8

What is the definition of the enthalpy of atomisation?

Answer 8

The enthalpy change when one mole of gaseous atoms is formed from its element

Question 9

Why is the enthalpy of atomisation endothermic?

Answer 9

Because bonds are broken to form gaseous atoms

Question 10

What is the definition of the enthalpy change of solution?

Answer 10

The enthalpy change when one mole of a solute is completely dissolved in a solvent

Question 11

What is the definition of the enthalpy change of hydration?

Answer 11

The enthalpy change that occurs when one mole of isolated gaseous ions is dissolved in water to form one mole of aqueous ions

Question 12

What happens to the ions when an ionic compound is dissolved in water?

Answer 12

• The ions from the ionic lattice are surrounded with water molecules as aqueous ions
• The d- oxygen atom is attracted to the positive ion
• The d+ hydrogen atoms are attracted to the negative ion

Question 13

What are the two processes that take place when a solid ionic compound dissolves in water?

Answer 13

• The ionic lattice breaks up

- Water molecules are attracted to, and surround, the ions

Question 14

What are the two types of energy change involved when a solid ionic compound dissolves in water?

Answer 14

• The ionic lattice is broken up forming separate gaseous ions (opposite of lattice enthalpy)
• The separate gaseous ions interact with polar water molecules to form hydrated aqueous ions (enthalpy change of hydration)

Question 15

Why can the enthalpy change of solution be exothermic or endothermic?

Answer 15

• It depends on the relative sizes of the lattice enthalpy and the enthalpy of hydration

Question 16

What are the two factors that affect lattice enthalpy?

Answer 16

Ionic size and ionic charge (charge density)

Question 17

How does ionic size affect lattice enthalpy?

Answer 17

• Ionic size increases
• Charge density decreases
• Attraction between ions decreases
• Lattice enthalpy becomes less negative (less exothermic)
• Melting point decreases

Question 18

How does ionic charge affect lattice enthalpy?

Answer 18

• Ionic charge increases
• Charge density increases
• Attraction between ions increases
• Lattice enthalpy becomes more negative
• Melting point increases

Question 19

Why does the ionic size of cations across a period decrease?

Answer 19

Because there are more protons attracting the same number of electrons

Question 20

What is the effect of ionic charge and size for cations in period 3?

Answer 20

• Increasing charge gives more attraction

- Increasing size gives more attraction

Question 21

What is the effect of ionic charge and size for anions in period 3?

Answer 21

• Increasing charge gives more attraction

- Increasing size gives less attraction

Question 22

What is a use of stable metal oxides?

Answer 22

Protective coating for the inside of furnaces

Question 23

How does ionic size affect hydration enthalpy?

Answer 23

• Ionic radius increases
• Attraction between ion and water molecules decreases
• Hydration enthalpy becomes less negative (bonds between ions and water molecules are weaker)

Question 24

What must happen when an ionic compound is dissolved in water?

Answer 24

The attraction between the ions in the ionic lattice must be overcome

Question 25

When should a compound dissolve?

Answer 25

When the sum of the hydration enthalpies is larger than the magnitude of the lattice enthalpy, the overall enthalpy change (of solution) will be exothermic

Question 26

What is the symbol for enthalpy of solution?

Answer 26

deltaHsol

Question 27

What is the symbol for enthalpy of hydration?

Answer 27

deltaHhyd

Question 28

What is the symbol for entropy?

Answer 28

S

Question 29

What is entropy?

Answer 29

- A measure of the dispersal of energy in a system which is greater, the more disordered a system - A measure of the 'disorder' of a system

Question 30

What happens in natural thermodynamic processes?

Answer 30

The sum of the entropies of the interacting thermodynamic systems increases i.e. the entropy of the universe must always increasee

Question 31

What is the 'universe' in relation to entropy?

Answer 31

The system and the surroundings

Question 32

What are the units for entropy?

Answer 32

J/mol/K

Question 33

How can you calculate deltaS?

Answer 33

deltaS products - deltaS reactants

Question 34

Why does entropy increase as substances go from solid to liquid to gas?

Answer 34

- Melting and boiling increase the randomness of particles | - Energy is spread out more (there is more disorder) and deltaS is positive

Question 35

Why does the production of gas particles (or more moles of gas particles) increase entropy?

Answer 35

- Production of gas increases the disorder of particles as energy is spread out more

Question 36

Are standard entropies positive or negative?

Answer 36

Always positive, because at 0K, entropy is 0

Question 37

When does a reaction happen?

Answer 37

- When the products have an overall lower energy than the reactants - When there is a decrease in free energy - When deltaG < 0

Question 38

What term is used to describe whether a reaction is able to happen?

Answer 38

Feasible or spontaneous

Question 39

What is the free energy change (deltaG) ?

Answer 39

The overall change in energy during a chemical reaction

Question 40

What is the relationship between deltaG, deltaH, deltaS and T?

Answer 40

deltaG = deltaH - TdeltaS

Question 41

How do you calculate the minimum temperature that a reaction will take place?

Answer 41

Make deltaG equal to 0

Question 42

Why don't some reactions with a negative deltaG take place?

Answer 42

Because the reaction is too slow due to its very high activation energy

Question 43

What is lattice enthalpy a measure of?

Answer 43

The strength of ionic bonding in a giant ionic lattice

Question 44

How do you work out any missing values in a Born-Haber cycle?

Answer 44

Go from the start of the arrow to the end of the arrow the long way round

Question 45

What do you need to remember when doing Born-Haber cycle calculations?

Answer 45

Negative values when going down the page, positive when going up

Question 46

What is the relationship between enthalpy change of solution, hydration and lattice enthalpy?

Answer 46

``` Sol = -latt + hyd (hyd = sol + latt) ```

Question 47

How does ionic charge affect hydration enthalpy?

Answer 47

- Ionic charge increases - Attraction with water molecules increases - Hydration enthalpy becomes more negative

Question 48

How do you calculate entropy change?

Answer 48

The entropy of products - entropy of reactants

Question 49

If a reaction occurs what must deltaG be?

Answer 49

Negative

Question 50

If bubbles are produced in an experiment, what does this indicate about the entropy change?

Answer 50

That there is a positive entropy change