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Chemistry Year 13 > Thermodynamics > Flashcards

Thermodynamics Flashcards

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1
Q

What is the strength of bonding in a lattice given by?

A

Its lattice enthalpy

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2
Q

What are the two ways to define lattice enthalpy?

A

The enthalpy of lattice formation and the enthalpy of lattice dissociation

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3
Q

What is the enthalpy of lattice formation?

A

The enthalpy change when one mole of a crystalline compound is formed from gaseous ions scattered an infinite distance apart. The enthalpy change for an ionic compound MC consisting of ions with single charges is represented as:
M+(g) + X-(g) -> MX (s)
It is an exothermic process

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4
Q

What is the enthalpy of lattice dissociation?

A

The enthalpy change when one mole of lattice is broken up to produce gaseous ions an infinite distance apart. For an ionic compound MX consisting of ions with single charges this is represented as:
MX (s) -> M+(g) + X-(g)
It is an endothermic process

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5
Q

What does the value of a lattice enthalpy depend on?

A
  • the charges on the ions
  • the size of the ions
  • the type of lattice formed (the pattern in which they pack together)
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6
Q

What is a larger exothermic enthalpy of lattice formation favoured by?

A

A greater charge on the ions, smaller ions and a closer packing in the lattice

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7
Q

Draw a simple enthalpy diagram showing the lattice enthalpy of sodium chloride

A

Check snap camera roll

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8
Q

Why are lattice enthalpies of interest?

A
  • to test our ideas of ionic bonding. By doing some mathematics theoretical values for lattice enthalpies can be worked out
  • to increase ohr understanding of why some compounds do not exist
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9
Q

How can we calculate lattice enthalpies?

A

From other experimental data using a special form of Hess’s law called the Born-Haber cycle

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10
Q

What data do you need to calculate the lattice enthalpy using a Born-Haber cycle?

A
  • enthalpy of formation
  • ionisation energy
  • enthalpy of atomisation
  • bond enthalpy
  • electron affinity
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11
Q

Why would the value of lattice enthalpy of formation be higher?

A
  • if the ions are smaller (they would approach eachother more closely so there would be a greater force of attraction between oppositely charged ions)
  • if the ions have a greater charge (the higher the charge the greater the force of attraction between oppositely charged ions)
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12
Q

What is the standard enthalpy of atomisation?

A

The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state

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13
Q

Is atomisation exothermic or endothermic and what is the symbol?

A

Endothermic because chemical bonds are broken. The symbol delta atH° (circle with a line through) is used

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14
Q

What is the standard enthalpy of atomisation for sodium?

A

Na (s) -> Na (g) delta at H° = +107 KJ mol -1

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15
Q

What is the enthalpy of atomisation relared to?

A

The forces of attraction between atoms, e.g. metallic bonds in metals or covalent bonds in non metals

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16
Q

What will mean that an element has a greater enthalpy of atomisation?

A
  • a greater charge

* a smaller size

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17
Q

What is the trend with the enthalpies of atomisation for the halogens?

A
  • they are all quite similar because they all have single covalent bonds that are broken
  • the values decrease as the halogen atoms get larger, since the forces holding the atoms together are weaker
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18
Q

Why are the enthalpies of atomisation so high for oxygen and nitrogen?

A

Because oxygen involves breaking a double bond and nitrogen a triple bond

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19
Q

What is the bond enthalpy?

A

The enthalpy change when one mole of covalent bonds in a gaseous element is broken. It is endothermic

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20
Q

For covalently bonded substances that are gases under standard conditions ehat is the enthalpy of atomisation?

A

Half the bond dissociation enthalpy

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21
Q

Why are the enthalpies of atomisation of bromine and iodine more than half their bond dissociation enthalpies?

A

Because they are not gases in their standard states so in addition to breaking bonds there are changes of state

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22
Q

What is the first electron affinity? Eea

A

The enthalpy change when one mole of electrons is gained by one mole of gaseous atoms to produce singly charged ions

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23
Q

What is the first electron affinity for a chlorine atom?

A

Cl (g) + e- -> Cl- (g) Eea= -349 KJ mol -1

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24
Q

Where are electron affinities exothermic?

A

For atoms on the right hand side of the periodic table

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25
Q

Why can electron affinities be exothermic or endothermic?

A
  • if one electron is added to an atom energy may be released (exothermic)
  • if more than one electron is added (to complete the outer shell the second and third electron affinities are endothermic. This is because a negatively charged ion repels the electron and this repulsive force must be overcome. Furthermore if you consider an ion and an atom with the same electronic configuration since the ion has a lower nuclear charge than the atom the attraction of an electron by a nucleus of the ion is less than that of the nucleus of the atom
26
Q

How do you work out a born-haber cycle?

A 
The same way you work out a hess cycle but this time you write the equations for:
• enthalpy of sublimation
• ionisation energy
• enthalpy of atomisation
• bond enthalpy
• electron affinity

And then cancel them out and swap ect. to get the equation you want and then add up all the values

27
Q

What do born haber cycles enable chemists to calculate?

A
  • the enthalpy of lattice formation of an ionic compound from experimental data
  • the enthalpy of formation of an ionic compound from a theoretical value together with experimental data
  • any missing value if the others are known
28
Q

How do you work out values other than enthalpy of formation for a born-haber cycle

A

You use the same process but use the equation of the substance you are trying to work out

29
Q

How can theoretical values for lattice enthalpies be calculated?

A

From Coulomb’s law. Complete electron transfer is assumed to happen when ionic bonds form. The radii of the sperical ions that form are estimated. Then coulombs law is used to calculate the electrostatic forces of attraction between the oppositely charged ions as a function of the charges on the ions and the distance between them

30
Q

What provides evidence that the model of spherical ions packed together in a lattice and held together by electrostatic forces of attraction us a good one?

A

Agreement between values calculated using born-haber cycles and theoretical values

31
Q

Why are the many examples where the differences between the theoretical and experimental values are significant?

A

Because the bonding is not purely ionic. The bonds have some covalent character because the electron clouds surrounding the negative ions can be polarised by the positive ion.

32
Q

What is the degree of covalent character and polarisation in an ion increased by?

A

Having larger charges in either of the negative or the positive ions, by having smaller positive ions or by having larger negative ions

33
Q

Why is a water molecule polar?

A

Because of the electronegativity difference between oxygen and hydrogen

34
Q

What happens in the process of dissolving?

A
  • the polar ends of the water molecules are attracted to the ions in the ionic lattice of a solid.
  • attractive forces between the water molecules the ions in the lattice compete with the attractive forces between the ions in the lattice
  • ions are surrounded and carried off into solution helped further by the kinetic energy of the moving water molecules
  • the ions become hydrated. Cations have a positive charge and are attracted to the negative end of the polar water molecules. Anions have a negative charge and are attracted to the positive end of water molecules.
  • the formation of bonds between water molecules and the cations and anions releases energy. It is an exothermic process
  • water molecules surrounding the cations and anions hydrogen-bond to further water molecules, releasing more energy.
  • when ions move through the solution the attached water molecules move with them, the outer water molecules are held less strongly
  • you can sometimes get average values for the extent of hydration (the number of water molecules attached to a particular ion)
35
Q

What is the enthalpy of solution?

A

The enthalpy change when one mole of an ionic compound dissolves in sufficient water to produce a solution of infinite dilution

It can be exothermic or endothermic

36
Q

How can the enthalpy of solution be determined?

A

In the labatory using calorimetry, making sure there is enough water present so that adding any more does not cause a further enthalpy change

37
Q

What is the enthalpy of hydration?

A

The enthalpy change when one mole of gaseous ions dissolves in sufficient water to produce a solution of infinite dilution

When the gaseous ions that came from an ionic compound dissolve in water to form hydrated ions

It is an exothermic process as bonds are being formed

38
Q

What would mean that the enthalpy of hydration is larger?

A

The smaller and more highly charged an ion is

39
Q

What happens for ions in the same period in regard to the enthalpy of hydration?

A

The enthalpy of hydration increases significantly along a period as the ionic charge increases and the ionic size decreases

40
Q

What happens for ions in a group with regard to enthalpy of hydration?

A

It decreases down the group but not very dramatically as the charge stays the same and the ion size increases and the ionic charge is a more significant factor than ionic size for producing larger hydration enthalpies

41
Q

What do the immediate hydration spheres of some metal ions, especially transition metal ions have?

A

A characteristic coordination number and geometry (the water molecules bonded directly to the metal ions have a characteristic geometry)

42
Q

How would you write an equation for enthalpy of hydration?

A

Mx+ + aq -> Mx+(aq)

43
Q

How do you work out the enthalpy of solution using hess cycles?

A

Write the equations for the enthalpy of hydration and the lattice enthalpy

44
Q

What is entropy?

A

The degree of disorder in a system. As the disorder increases so does the entropy value

45
Q

Where are particles more ordered?

A

The particles of a substance in a solid are more ordered than in liquids and the particles of a substance in a liquid are more ordered than in gases

46
Q

What happens in regard to order if a crystal melts?

A

There is a dramatic change from an ordered state in the crystal to a disordered state in the liquid. There is an increase in entropy

47
Q

What happens in regard to entropy when a crystalline solid dissolves in water?

A

Entropy always increases (the ions can move around more freely and are not as ordered)

48
Q

What happens in regard to entropy with any reaction that produces a gas from non gaseous reactants?

A

The entropy increases

49
Q

How does entropy change in regard to temperature?

A

It always decreases as the temperature of the system decreases

50
Q

What would you use to calculate standard entropy changes deltaS°(with a line through)

A

You can calculate absolute standard entropy values S° and use these to calculate standard entropy changes

51
Q

What is the equation for the entropy change?

A

DeltaS° = q/T

Where q= energy transferred by heating/ J and
T= temperature/ K

52
Q

What are the units for standard entropy values?

A

J K-1 mol -1

53
Q

What is the entropy change during a chemical reaction?

A

The difference between the total entropy values of the products and reactants

54
Q

How do you calculate the entropy change from standard entropy values?

A

Sum of S° of products - sum of S° of reactants

55
Q

How can substances condense if entropy values must always increase?

A

Although there is a decreases in the entropy value of the system (the products and reactants) if you take into account the surroundings the entropy increases as energy is transferred to the surroundings.

56
Q

How do you work out the total entropy change?

A

DeltaS° total = deltaS° system + deltaS° surroundings

57
Q

What does feasibility mean in chemistry?

A

To mean whether or not a physical or chemical change could take place spontaneously

58
Q

The feasability depends on the combination of enthalpy and entropy changes. What equation is this given by?

A

DeltaG = deltaH - TdeltaS

  • deltaG = gibbs free energy change/ KJ mol -1
  • delta H = enthalpy change/ KJ mol -1
  • delta S = entropy change/ kJ K-1 mol -1
  • T = temperature/ K
59
Q

What does the value of deltaG° need to be in order for change to be feasible and spontaneous?

A

Negative

60
Q

How can you determine the temperature at which change becomes feasible?

A

By using gibbs free energy equation and making deltaG° = 0 and rearranging the equation for temperature

61
Q

How do you determine the value of deltaS and deltaH from a graph of deltaG versus T?

A

It is a straight line graph with an intercept on the g axis. The gradient of the line gives you deltaS° and the i intercept of the y axis will give you the value of deltaH°

Chemistry Year 13 (16 decks)

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