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chem > enegetics ii > Flashcards

enegetics ii Flashcards

1
Q

What are giant ionic lattices and what holds the positive and negative ions together?

A

Giant ionic lattices are regular structures formed by ionic compounds, where positive and negative ions are held together by electrostatic attractions.

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2
Q

What is lattice energy and how is it defined?

A

Lattice energy (DLEH) is the energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions.

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3
Q

What does the standard lattice energy measure?

A

The standard lattice energy measures the strength of ionic bonds. A more negative lattice energy indicates stronger bonding.

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4
Q

How does the charge on ions affect the energy released when an ionic lattice forms?

A

The higher the charge on the ions, the more energy is released when an ionic lattice forms due to stronger electrostatic forces between the ions.

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5
Q

What is the relationship between lattice energy and the charge on ions?

A

Higher charges on ions result in more exothermic lattice energies, leading to more negative values.

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6
Q

How does the size of ions affect lattice energy?

A

Smaller ionic radii result in more exothermic (more negative) lattice energies. Smaller ions have higher charge densities and can sit closer together in the lattice, increasing the strength of attractions between ions.

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7
Q

What does Hess’s law state regarding the total enthalpy change of a reaction?

A

Hess’s law states that the total enthalpy change of a reaction is always the same, regardless of the route taken. This principle is known as the conservation of energy.

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8
Q

How can lattice energy be calculated indirectly?

A

Lattice energy cannot be directly calculated. Instead, a Born-Haber cycle is used to determine the enthalpy change if an alternative, indirect route is taken.

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9
Q

What is the first step in constructing a Born-Haber cycle?

A

Start with the enthalpy of formation at the bottom right, represented with an arrow going downwards.

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10
Q

What comes next in a Born-Haber cycle after the enthalpy of formation?

A

Place the enthalpies of atomization and ionization above the enthalpy of formation, indicated with arrows going upwards.

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11
Q

Where is the electron affinity positioned in a Born-Haber cycle?

A

The electron affinity is placed on the top right, represented with an arrow going downwards.

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12
Q

Where does the lattice energy go in a Born-Haber cycle?

A

Lattice energy is positioned below the electron affinity, indicated with arrows going downwards.

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13
Q

What are the two ways to determine a lattice energy?

A

1) The experimental way, using experimental enthalpy values in a Born-Haber cycle.
2) The theoretical way, which involves calculations based on the purely ionic model of a lattice.

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14
Q

What is involved in calculating the theoretical lattice energy?

A

To calculate the theoretical lattice energy, you assume that all ions are spherical with evenly distributed charges in a purely ionic lattice. Then, you determine the strength of attraction between ions based on their charges and distance, yielding the energy change when the ions form the lattice.

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15
Q

What does the close match between experimental and theoretical lattice energy values indicate about the structure of sodium halides?

A

The close match suggests that the structure of the lattice for these compounds is quite close to being purely ionic.

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16
Q

What does the difference between experimental and theoretical lattice energy values indicate about the bonding in magnesium halides

A

The difference suggests that the bonding in magnesium halides has more covalent character and is more polarized

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17
Q

Why do magnesium halides have more covalent character in their ionic bonds compared to sodium halides?

A

Magnesium halides have a higher charge density on the cation (Mg2+), which allows it to polarize the bond and exhibit some covalent character.

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18
Q

What is the relationship between the charge density of the cation and the match between experimental and theoretical lattice energy values?

A

The greater the charge density of the cation, the bigger the difference between experimental and theoretical lattice energy values.

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19
Q

What is polarisation in ionic compounds?

A

Polarisation in ionic compounds occurs when the positive charge on the cation attracts electrons towards it from the anion.

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20
Q

How does the size and charge of ions affect polarisation in ionic compounds?

A

Small cations with a high charge and large anions with a high charge are more polarising and polarisable, respectively.

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21
Q

What happens when a compound contains a highly polarising cation and a easily polarisable anion?

A

If a compound contains a cation with a high polarising ability and an anion which is easily polarised, some of the anion’s electron charge cloud will be dragged towards the positive cation.

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22
Q

What occurs when a compound is polarised enough in terms of bonding?

A

If the compound is polarised enough, a partially covalent bond is formed.

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23
Q

What happens to an ionic bond as it becomes more polarised?

A

As an ionic bond becomes more polarised, it gains more covalent character.

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24
Q

How does increased covalent character in an ionic bond affect the properties of compounds?

A

compounds with more covalent character in their ionic bonds exhibit different properties compared to those with purely ionic bonds.

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25
Q

What is electronegativity?

A

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond.

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26
Q

What scale is typically used to measure the electronegativity of an atom?

A

The Pauling Scale is usually used to measure electronegativity.

27
Q

How does the difference in electronegativity affect the polarity of a bond?

A

The greater the difference in electronegativity, the greater the shift in electron density, and the more polar the bond becomes.

28
Q

When is a bond considered polar based on electronegativity values?

A

Bonds are polar if the difference in Pauling electronegativity values is more than about 0.4.

29
Q

Predict whether a C–Cl bond will be polar, given that the Pauling electronegativity values of carbon and chlorine are C = 2.5 and Cl = 3.0.

A

The difference between the electronegativities of chlorine and carbon is: 3.0 – 2.5 = 0.5. So the bond will be polar. The chlorine atom will have a slight negative charge and the carbon atom will have a slight positive charge.

30
Q

What happens when a solid ionic lattice dissolves in water?

A

1) When a solid ionic lattice dissolves in water, two things happen:
2) The bonds between the ions break — this is endothermic. The enthalpy change is the opposite of the lattice enthalpy.
3) Bonds between the ions and the water are made — this is exothermic. The enthalpy change here is called the enthalpy change of hydration.

31
Q

What is the enthalpy change of hydration, ΔhydrH?

A

ΔhydrH is the enthalpy change that occurs when 1 mole of gaseous ions dissolves in water.

32
Q

What is the enthalpy change of solution, ΔsolH?

A

ΔsolH is the enthalpy change that occurs when 1 mole of solute dissolves in water.

33
Q

What is the relationship between the enthalpy change of solution and solubility?

A

Soluble substances typically have exothermic enthalpies of solution, meaning they release energy when dissolving. This energy release aids dissolution, as substances generally only dissolve if the energy released is roughly the same or greater than the energy taken in.

34
Q

What property makes ions with higher charges better at attracting water molecules?

A

Higher charge leads to stronger electrostatic attraction with water molecules, resulting in a more exothermic enthalpy of hydration

35
Q

How does ion size affect its ability to attract water molecules?

A

Smaller ions with higher charge density attract water molecules better, resulting in a more exothermic enthalpy of hydration compared to larger ions.

36
Q

What is entropy?

A

Entropy is a measure of the disorder of a system.

37
Q

What does entropy measure?

A

Entropy tells you the number of ways particles can be arranged and the number of ways energy can be shared among them.

38
Q

How does particle disorder affect entropy?

A

The more disordered the particles, the higher the entropy.

39
Q

What does a large, positive entropy value indicate?

A

A large, positive entropy value signifies a high level of disorder in the system.

40
Q

Describe the behaviour of solid particles and what it means for their entropy

A

Solid particles wobble about a fixed point with little randomness, leading to the lowest entropy.

41
Q

Describe the behaviour of gas particles and what it means for the particles entropy

A

Gas particles move freely and randomly, resulting in the highest entropy.

42
Q

How does dissolving a solid affect its entropy?

A

Dissolving a solid increases its entropy because dissolved particles can move freely without being held in one place.

43
Q

How does dissolving a solid affect its entropy?

A

Dissolving a solid increases its entropy because dissolved particles can move freely without being held in one place.

44
Q

How does dissolving a solid affect its entropy?

A

Dissolving a solid increases its entropy because dissolved particles can move freely without being held in one place.

45
Q

Why does entropy increase in a reaction like N2O4(g) → 2NO2(g)?

A

Entropy increases because the number of moles increases, leading to more ways the particles and their energy can be arranged.

46
Q

What happens to the energetic stability of substances as disorder increases?

A

Substances become more energetically stable when there’s more disorder, leading particles to move to increase their entropy.

47
Q

Why are some reactions feasible even with an endothermic enthalpy change?

A

Some reactions occur spontaneously because particles move to increase entropy, allowing them to happen without the addition of energy despite an endothermic enthalpy change.

48
Q

During a reaction, what represents the entropy change of the system?

A

ΔSsystem = Sproducts - Sreactants

49
Q

What does a positive entropy change indicate about the feasibility of a reaction?

A

Likely feasible.

50
Q

Why does the entropy of the surroundings change during a reaction?

A

Energy transfer to or from the system.

51
Q

What does the total entropy change represent in a reaction?

A

ΔStotal = ΔSsystem + ΔSsurroundings

52
Q

What formula is used to calculate the change of entropy of the surroundings?

A

ΔSsurroundings = -(ΔH / T)

53
Q

What three factors determine the tendency of a process to occur?

A

Entropy (ΔS), Enthalpy (ΔH), and Temperature (T).

54
Q

What does the free energy change (ΔG) indicate about the feasibility of a reaction?

A

A more negative ΔG value indicates a more feasible reaction.

55
Q

What is the formula for calculating the Gibbs free energy change (ΔG)?

A

ΔG = ΔH - TΔSsystem

56
Q

How can you determine the temperature at which a reaction becomes feasible using the Gibbs free energy equation?

A

By rearranging the equation to solve for temperature when ΔG equals zero.

57
Q

How can you predict the feasibility of a reaction using the Gibbs free energy equation?

A

When ΔH is negative and ΔS is positive, ΔG will be negative, indicating feasibility. When ΔH is positive and ΔS is negative, ΔG will be positive, indicating non-feasibility.

58
Q

What does an equilibrium constant measure in a reversible reaction?

A

The ratio of product to reactant concentrations at equilibrium.

59
Q

What type of reactions, in terms of ΔG, have equilibrium constants greater than 1?

A

Reactions with negative ΔG, indicating feasibility.

60
Q

What type of reactions, in terms of ΔG, have equilibrium constants smaller than 1?

A

Reactions with positive ΔG, indicating non-feasibility.

61
Q

What equation represents the relationship between Gibbs free energy (ΔG) and equilibrium constant (K)?

A

ΔG = -RT ln K

62
Q

What aspect of a reaction does the value of ΔG not provide information about?

A

Reaction rate.

63
Q

Why might a reaction with a negative ΔG still not occur?

A

High activation energy or slow kinetics.

chem (16 decks)

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