Lattice Enthalpy and Free Energy Flashcards

1
Q

Define enthalpy change of formation and give example with NaCl

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

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

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Define enthalpy change of ionisation and give example with Na

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Define enthalpy change of electron affinity and give example with Cl

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Define lattice enthalpy change and give example with NaCl

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Is lattice enthalpy change likely to be positive or negative

A

Negative- bonds are formed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

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

A

2K+ (g) + CO3 2- (g) →K2CO3 (s)

3Mg 2+ (g) + 2N 3- (g) → Mg3N2 (s)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

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

A
  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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Explain what sign you would expect for enthalpy change of atomisation

A
  1. Positive- bonds holding the atoms together in their standard state are broken
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Is the first electron affinity endothermic or exothermic

A
  1. Exothermic because an electron is being added and will e attracted to the nucleus
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Are subsequent electron affinities endo or exothermic

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Describe a generic Born-Haber cycle

A
  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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Describe the calculation you can do to find △fH

A

△fH = all the other enthalpy changes added together

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Define standard enthalpy change of solution

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Define standard enthalpy change of hydration

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

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

A
  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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What would the sign of △hydH be

A

negative

18
Q

What two things that affect the △Le H

A
  1. Charge (ionic charge)

2. Size (ionic radius)

19
Q

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

A
  1. Nuclear charge
  2. Number of electron shells
  3. Number of electrons shielding
  4. Charge on the ion
20
Q

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

A
  1. Becomes more negative

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

21
Q

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

A
  1. Becomes less negative

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

22
Q

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

A

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

  1. The Cs + ion has a larger radius than the Na + and I- ion has a larger radius than the Cl- ion
  2. In CsI there will be a smaller attraction between the ions than in NaCl
23
Q

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

A

It will become more negative

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

24
Q

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

A

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

25
Q

What affects the enthalpy change of hydration

A
  1. Ionic radius

2. Ionic charge

26
Q

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

A
  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
27
Q

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

A
  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
28
Q

Define the term entropy

A

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

29
Q

Order the states in increasing entropy

A

Lowest- solid→liquid→gas→ plasma- highest

30
Q

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

A

Decrease- two gases become one liquid

31
Q

What sign are entropy values

A

Always positive- there is always disorder

32
Q

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

A

Positive values of △S - processes that increase in disorder

33
Q

Define free energy change

A

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

34
Q

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

A

Negative- the process is spontaneous and is possible

35
Q

What should you remember in Gibbs energy calculations

A
  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)
36
Q

Why might a feasible reaction not actually appear to occur

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

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

37
Q

If △H is -ve and △S is +ve what is △G

A

Always -ve so it is spontaneous

38
Q

If △H is +ve and △S is -ve what is △G

A

Always + so it is not spontaneous

39
Q

If △H is -ve and △S is -ve what is △G

A

-ve only at lower temps

40
Q

If △H is +ve and △S is +ve what is △G

A

-ve only at higher temps