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Chemistry > Unit 1: Section 6 - Thermodynamics > Flashcards

Unit 1: Section 6 - Thermodynamics Flashcards

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1
Q

What is enthalpy change?

A

The heat energy transferred in a reaction at constant pressure

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2
Q

What is enthalpy change of formation?

A

ΔfH - the enthalpy change when 1 mole of a substance is formed from its constituent elements with all substances in their standard states - exothermic for most substances
2Na(s) + 1/2O2(g) -> Na2O(s)

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3
Q

What is enthalpy of combustion?

A

ΔcH - When 1 mole of a substance undergoes complete combustion in oxygen with all substances in their standard states - exothermic
H2 (g) + 1/2O2(g) -> H2O(l)

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4
Q

What is enthalpy of neutralisation?

A

ΔneutH - when 1 mole of water is formed in a reaction betweeen an acid and alkali under standard conditions - exothermic
1/2H2SO4(aq) + NaOH(aq) -> 1/2Na2SO4(aq) + H2O(l)

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5
Q

What is ionisation enthalpy?

A

ΔieH
1st ionisation enthalpy - enthalpy change whe each atom in 1 mole of gaseous atoms loses 1 electron to form 1 mole of gaseous 1+ ions - endothermic
Mg(g) -> Mg+(g) +e-
2nd ionisation enthalpy - enthalpy change when each ion in 1 mole of gasous 1+ ions loses 1 electron to form 1 mole of gaseous 2+ ions - endothermic
Mg+(g) -> Mg2+ +(g) +e-

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6
Q

What is electron affinity?

A

ΔeaH
1st electron affinity - enthalpy change when each atom in 1 mole of gaseous atoms gains 1 electron to form 1 mole of gaseous 1- ions - exothermic for many non-metals
O(g) + e- -> O-(g)
2nd electron affinity - enthalpy change when each ion in 1 mole of gasous 1- ions gains 1 electron to form 1 mole of gasoues 2- ions - endothermic
O-(g) + e- -> O2-(g)

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7
Q

What is enthalpy of atomisation?

A

ΔatH - when 1 mole of gaseous atoms is produced from an element in its standard state - endothermic
1/2I2(s) -> I(g)

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8
Q

What is Hydration enthalpy?

A

ΔhydH
When 1 mole of gaseous atoms dissolve in water (become hydrated) - exothermic
Mg2+(g) + aq -> Mg2+(aq)

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9
Q

What is enthalpy of solution?

A

ΔsolH
When 1 mole of an ionic solid disolves in an amount of water large enough so that the dissolved ions are well separated and don’t interact with each other - varies
MgCl2(s) + aq -> Mg2+(aq) + 2Cl-(aq)

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10
Q

What is bond dissociation enthalpy?

A

ΔdisH
When 1 mole of covalent bonds is broken in the gaseous state - endothermic
I2(g) -> 2I (g)

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11
Q

What is lattice enthalpy of formation?

A

ΔLEFH
When 1 mole of a solid ionic compound is formed from its consistuent ions in the gas phase - exothermic
Mg2+(g) + 2Cl-(g) -> MgCl2(s)

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12
Q

What is lattice enthalpy of dissociation?

A

When 1 mole of a solid ionic compound is broken up into its constituent ions in the gas phase - endothermic
MgCl2(s) -> Mg2+(g) + 2Cl-(g)

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13
Q

What is enthalpy of vaporisation?

A

ΔvapH
Whe 1 mole of a liquid is turned into a gas - endothermic
H2O(l) -> H2O(g)

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14
Q

What is enthalpy of fusion?

A

ΔfusH
When 1 mole of a solid is turned into a liquid - endothermic
Mg(s) -> Mg(l)

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15
Q

What are Born-Haber cycles used for?

A

Used to calculate lattice enthalpies

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16
Q

What is the perfect ionic model of a lattice?

A

Assumes that all the ions are spherical, and have their charge evenly distributed around them

17
Q

Why is the perfect ionic model often not accurate?

A
  • Ions are not perfectly spherical
  • Polarisation often occurs when small positive ions or large negative ions are involved
  • Some lattices are not regular and the crystal structure can differ
18
Q

What does covalent character show?

A

When you find the lattice enthalpy experimentally, the value that you often get is not the theoretical lattice enthalpy, this is evidence that most ionic compounds have some covalent character - the positive and negative ions in a lattice aren’t exactly spherical, positive ions polarise neighbouring negative ions to different extents, and the more polarisation there is, the more covalent the bonding will be

19
Q

What characteristics does covalent character affect?

A
  1. Solubility - makes the compound less soluble
  2. General conductivity
20
Q

What 2 things happen when a solid ionic lattice dissolves?

A
  1. The bonds between the ions break to give free ions - endothermic
  2. Bonds between the ions and the water are made - exothermic
    The enthalpy change of solution is the overall effect on the enthalpy of these 2 things
21
Q

When do substances dissolve?

A

Generally only dissolve if the energy released is roughly the same, or greater than the energy taken in , so soluble substances tend to have exothermic enthalpies of solution

22
Q

What is entropy?

A

A measure of the number of ways that particles can be arranged and the number of ways the energy can be shared out between the particles in a system
The more disordered the particles are, the higher the entropy is - a large, positive value of entropy shows a high level of disorder

23
Q

What affects entropy?

A

Physical state - solids have the lowest entropy as they just vibrate about a fixed position but gases have the highest entropy as they have the most disordered arrangements, so they have the highest entropy
More particles - more ways they can be arranged and the more energy there is

24
Q

What makes a substance more stable?

A
  1. Substances tend towards disorder - they’re more energtically stable when there’s more disorder
  2. This is why some reactions are feasible even when the enthalpy change is endothermic
25
Q

How can you calculate the entropy change for a reaction?

A

ΔS = Sproducts - Sreactants

26
Q

What is the standard entropy of a substance?

A

The entropy of 1 mole of that substance under standard conditions

27
Q

What is free energy change?

A

ΔG, a measure used to predict whether a reaction is feasible
If ΔG is negative or equal to 0 then the reaction might happen by itslef

28
Q

How do you calculate free energy?

A

ΔG = ΔH - TΔS
Make sure the units are all the same

29
Q

How does temperature affect the feasibility of reactions?

30
Q

How to calculate the temperature at which a reaction becomes feasible?

A
  1. When ΔG=0 a reaction is just feasible
  2. Rearrange the equation to T=ΔH/ΔS
  3. In an exam question write down the og equation and the replacement of ΔG with 0 to gain all marks
31
Q

What affects lattice enthalpy?

A

Size of the ion - the bigger the ion, the lower the magnitude of lattice enthalpy
Charge - higher the magnitude of charge, the bigger the magnitude of lattice enthalpy

32
Q

When is ΔH and ΔS favourable?

A

ΔH - as negative as possible
ΔS - as positive as possible

33
Q

How can you increases the lattice enthalpy of a compound? Why does this increase it?

A
  • Smaller ions - the charge centres will be closer together
  • Increased charge - there will be a greater electrostatic force of attraction between the oppositely charged ions
  • Increasing the charge on the anion has a much smaller effect than increasign the charge on the cation, since increasing anion charge also has the effect of increasing ionic size
34
Q

How can Born-Haber cycles be used to see if compounds could theoretically exist?

A
  • Use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable
  • Was used to predict the existence of the first noble gas containing compound
35
Q

What does sponatneous and feasible mean?

A

If a reaction is spontatneous and feasible, it will take place of its own accord; does not take account of rate of reaction

36
Q

What are the limitations of using G as an indicator of whether a reaction will occur?

A
  • Gibbs free energy only indicates if a reaction is feasible
  • It does not take into account the rate of reaction (the kinetics of the reaction)
  • In reality, many reactions that are feasible at a certain temperature have a rate of reaction that is so slow that effectively no reaction is occuring

Chemistry (16 decks)

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