Lattice Enthalpy and Free Energy

Question 1

Define enthalpy change of formation and give example with NaCl

Answer 1

Enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions
△fH(planet symbol
Na (s) + 1/2 Cl2 → NaCl (s)

Question 2

Define enthalpy change of atomisation and give example with Na and Cl

Answer 2

The enthalpy change that takes place when one mole of gaseous atoms are formed form its elements in their standard state
△aH
Na (s) → Na(g)
1/2 Cl2 (g) →Cl (g)

Question 3

Define enthalpy change of ionisation and give example with Na

Answer 3

Enthalpy change when 1 electron is removed form each atom of one mole of gaseous atoms
△ieH (planet symbol)
Na (g) →Na+ (g) + e-

Question 4

Define enthalpy change of electron affinity and give example with Cl

Answer 4

Enthalpy change when one electron is added to each atom of one mole of gaseous atoms
△eaH (planet symbol)
Cl (g) + e- →Cl-(g)

Question 5

Define lattice enthalpy change and give example with NaCl

Answer 5

Enthalpy change when one mole of a compound in its standard state is formed from its ions in the gaseous state, under standard conditions.
△leH (planet symbol)
Na+ (g) + Cl- (g) → NaCl (s)

Question 6

Is lattice enthalpy change likely to be positive or negative

Answer 6

Negative- bonds are formed

Question 7

Give the equations for changes represented by lattice enthalpy for potassium carbonate and magnesium nitride

Answer 7

2K+ (g) + CO3 2- (g) →K2CO3 (s)

3Mg 2+ (g) + 2N 3- (g) → Mg3N2 (s)

Question 8

Explain the sign of ionisation energy and what happens to the numerical value as more electrons are removed

Answer 8

1. Positive- electrons are pulled away from attraction to the nucleus
2. The numerical value increases as more electrons are removed, the remaining electrons are pulled closer to the nucleus, increasing the attraction, so more energy is needed to remove further electrons

Question 9

Explain what sign you would expect for enthalpy change of atomisation

Answer 9

1. Positive- bonds holding the atoms together in their standard state are broken

Question 10

Is the first electron affinity endothermic or exothermic

Answer 10

1. Exothermic because an electron is being added and will e attracted to the nucleus

Question 11

Are subsequent electron affinities endo or exothermic

Answer 11

Endothermic because negatively charged electron is being added to a negatively charged ion - necessary to overcome repulsion

Question 12

Describe a generic Born-Haber cycle

Answer 12

1. Bottom layer is the compound
2. Layer above on left s elements in their standard state
3. They are then atomised to form the gaseous atoms
4. Then the ionisation energy to form a positive ion
5. Then electron affinity to form negative ion
6. Then lattice enthalpy pointing down to the bottom layer to form the compound

Question 13

Describe the calculation you can do to find △fH

Answer 13

△fH = all the other enthalpy changes added together

Question 14

Define standard enthalpy change of solution

Answer 14

The enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions.

Question 15

Define standard enthalpy change of hydration

Answer 15

The enthalpy change that takes place when one mole of gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions

Question 16

Describe the generic look of a standard enthalpy change of solution Hess cycle

Answer 16

1. Bottom is the solution of aqueous ions
2. Up to the let is the ionic solid- arrow down to bottom is △Hsol
3. Above that is the gaseous ions on the top row- arrow down to ionic solid is △leH
4. Down to the right is the △H hyd of one of the ions turning it to aqueous
5. An arrow from that points to the bottom row and is the △H hyd of the other ion

Question 17

What would the sign of △hydH be

Answer 17

negative

Question 18

What two things that affect the △Le H

Answer 18

1. Charge (ionic charge)

2. Size (ionic radius)

Question 19

What four things need to be considered when we think about the radius of an ion

Answer 19

1. Nuclear charge
2. Number of electron shells
3. Number of electrons shielding
4. Charge on the ion

Question 20

State and explain how the lattice enthalpy will change as the radius of the cation decreases

Answer 20

1. Becomes more negative

2. Charge will be distributed over a smaller volume so attraction for negative ion will increase

Question 21

Sate and explain how lattice enthalpy will change as the radius of the anion increases

Answer 21

1. Becomes less negative

2. The charge will be distributed over a larger volume so attraction for the positive ion will decrease

Question 22

Explain the difference you would expect between the lattice enthalpy of NaCl and CsI

Answer 22

1 Lattice enthalpy for CsI will be less negative than that of NaCl

2. The Cs + ion has a larger radius than the Na + and I- ion has a larger radius than the Cl- ion
3. In CsI there will be a smaller attraction between the ions than in NaCl

Question 23

What is likely to be the effect on lattice enthalpy of Na2O if Na+ is replaced with Ca2+

Answer 23

It will become more negative

2. Ca2+ ion has a higher charge than Na+ so will be more strongly attracted to O2- ion

Question 24

What might be the expected relationship between melting point and lattice enthalpy of an ionic compound

Answer 24

The more exothermic the lattice enthalpy is the higher the expected melting point- because the ionic bonding is stronger

Question 25

What affects the enthalpy change of hydration

Answer 25

1. Ionic radius

2. Ionic charge

Question 26

Out of K+ and Cs + which would have the least negative enthalpy change of hydration

Answer 26

1. Cs+ because it has the biggest ionic radius which means it’s charge is distributed over a larger volume.
2. Meaning bonds with water molecules are weaker

Question 27

Out of Na+ and Mg2+ which would have the most negative enthalpy change of hydration

Answer 27

1. Going form Na+ to Mg2+ the ionic charge gets larger and ionic radius gets smaller
2. This means the charge becomes much more concentrated
3. Bonds with water are much stronger, hence the enthalpy of hydration becomes more negative

Question 28

Define the term entropy

Answer 28

Used to describe the dispersal of energy and disorder within the chemicals making up a chemical system
Symbol- S
Unit- J K-1 mol-1

Question 29

Order the states in increasing entropy

Answer 29

Lowest- solid→liquid→gas→ plasma- highest

Question 30

What would the entropy change be if hydrogen and oxygen reacted to make water

Answer 30

Decrease- two gases become one liquid

Question 31

What sign are entropy values

Answer 31

Always positive- there is always disorder

Question 32

What type of processes tend to be favoured those with a positive △S or negative

Answer 32

Positive values of △S - processes that increase in disorder

Question 33

Define free energy change

Answer 33

The balance between enthalpy, entropy and temperature for a process given by:
△G = △H-T△S

Question 34

What value does Gibbs energy need to have for a reaction to be able to take place

Answer 34

Negative- the process is spontaneous and is possible

Question 35

What should you remember in Gibbs energy calculations

Answer 35

1. Temp must be in Kelvin
2. △G is in kJmol-1
3. △H is in kJmol-1
4. △S is in kJK-1 mol-1 ( need to divide this by 1000 usually to get this unit)

Question 36

Why might a feasible reaction not actually appear to occur

Answer 36

1. It is too slow- activation energy is very large

2. So rate of reaction might be too slow to be perceived

Question 37

If △H is -ve and △S is +ve what is △G

Answer 37

Always -ve so it is spontaneous

Question 38

If △H is +ve and △S is -ve what is △G

Answer 38

Always + so it is not spontaneous

Question 39

If △H is -ve and △S is -ve what is △G

Answer 39

-ve only at lower temps

Question 40

If △H is +ve and △S is +ve what is △G

Answer 40

-ve only at higher temps