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Chemistry A-Level COPY > Enthalpy and Entropy (Chapter 22) > Flashcards

Enthalpy and Entropy (Chapter 22) Flashcards

1
Q

What is the definition of lattice enthalpy?

A

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

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2
Q

Give an example of a lattice enthalpy equation

A

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

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3
Q

Is lattice enthalpy exothermic or endothermic?

A

Always exothermic, therefore enthalpy change is always negative

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4
Q

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

A

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

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5
Q

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

A

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

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6
Q

Why is 1st E.A exothermic?

A

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

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7
Q

Why is 2nd E.A endothermic?

A
  • 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
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8
Q

What is the definition of the enthalpy of atomisation?

A

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

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9
Q

Why is the enthalpy of atomisation endothermic?

A

Because bonds are broken to form gaseous atoms

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10
Q

What is the definition of the enthalpy change of solution?

A

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

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11
Q

What is the definition of the enthalpy change of hydration?

A

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

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12
Q

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

A
  • 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
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13
Q

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

A
  • The ionic lattice breaks up

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

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14
Q

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

A
  • 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)
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15
Q

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

A
  • It depends on the relative sizes of the lattice enthalpy and the enthalpy of hydration
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16
Q

What are the two factors that affect lattice enthalpy?

A

Ionic size and ionic charge (charge density)

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17
Q

How does ionic size affect lattice enthalpy?

A
  • Ionic size increases
  • Charge density decreases
  • Attraction between ions decreases
  • Lattice enthalpy becomes less negative (less exothermic)
  • Melting point decreases
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18
Q

How does ionic charge affect lattice enthalpy?

A
  • Ionic charge increases
  • Charge density increases
  • Attraction between ions increases
  • Lattice enthalpy becomes more negative
  • Melting point increases
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19
Q

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

A

Because there are more protons attracting the same number of electrons

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20
Q

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

A
  • Increasing charge gives more attraction

- Increasing size gives more attraction

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21
Q

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

A
  • Increasing charge gives more attraction

- Increasing size gives less attraction

22
Q

What is a use of stable metal oxides?

A

Protective coating for the inside of furnaces

23
Q

How does ionic size affect hydration enthalpy?

A
  • Ionic radius increases
  • Attraction between ion and water molecules decreases
  • Hydration enthalpy becomes less negative (bonds between ions and water molecules are weaker)
24
Q

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

A

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

25
Q

When should a compound dissolve?

A

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

26
Q

What is the symbol for enthalpy of solution?

A

deltaHsol

27
Q

What is the symbol for enthalpy of hydration?

A

deltaHhyd

28
Q

What is the symbol for entropy?

A

S

29
Q

What is entropy?

A
  • 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
30
Q

What happens in natural thermodynamic processes?

A

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

31
Q

What is the ‘universe’ in relation to entropy?

A

The system and the surroundings

32
Q

What are the units for entropy?

A

J/mol/K

33
Q

How can you calculate deltaS?

A

deltaS products - deltaS reactants

34
Q

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

A
  • Melting and boiling increase the randomness of particles

- Energy is spread out more (there is more disorder) and deltaS is positive

35
Q

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

A
  • Production of gas increases the disorder of particles as energy is spread out more
36
Q

Are standard entropies positive or negative?

A

Always positive, because at 0K, entropy is 0

37
Q

When does a reaction happen?

A
  • When the products have an overall lower energy than the reactants
  • When there is a decrease in free energy
  • When deltaG < 0
38
Q

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

A

Feasible or spontaneous

39
Q

What is the free energy change (deltaG) ?

A

The overall change in energy during a chemical reaction

40
Q

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

A

deltaG = deltaH - TdeltaS

41
Q

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

A

Make deltaG equal to 0

42
Q

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

A

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

43
Q

What is lattice enthalpy a measure of?

A

The strength of ionic bonding in a giant ionic lattice

44
Q

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

A

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

45
Q

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

A

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

46
Q

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

A 
Sol = -latt + hyd
(hyd = sol + latt)
47
Q

How does ionic charge affect hydration enthalpy?

A
  • Ionic charge increases
  • Attraction with water molecules increases
  • Hydration enthalpy becomes more negative
48
Q

How do you calculate entropy change?

A

The entropy of products - entropy of reactants

49
Q

If a reaction occurs what must deltaG be?

A

Negative

50
Q

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

A

That there is a positive entropy change

Chemistry A-Level COPY (27 decks)

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