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OCR A Level Chemistry > Chapter 22 - Enthalpy and Entropy > Flashcards

Chapter 22 - Enthalpy and Entropy Flashcards

1
Q

What is lattice enthalpy, is it exo or endo?

A

The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
e.g. K+ (g) + Cl- (g) → KCl (s)
EXOTHERMIC - ionic bond formation

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2
Q

What is the standard enthalpy of formation?

A

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states

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3
Q

What is the standard enthalpy change of atomisation? Is it exo or endo?

A

The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
e.g. Na (s) → Na(g) or 1/2Cl2 (g) → Cl (g)
ENDOTHERMIC
Because bonds are broken to form gaseous atoms

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4
Q

What is first ionisation energy? Is it exo or endo?

A

The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
ENDOTHERMIC - energy required to overcome attraction between-ve electron and +ve nucleus

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5
Q

What is first electron affinity? Is it exo or endo?

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
EXOTHERMIC - electron being added is attracted in towards nucleus

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6
Q

How can you construct a Born Haber cycle?

A

Route 1 - formation of lattice from elements
Route 2 - formation of gaseous atoms, gaseous ions and then lattice formation from gaseous ions

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7
Q

Why are second electron affinities endothermic?

A

A second electron is being gained by a negative ion, which repels the electron away. Therefore energy must be put in to force the -ve electron onto the negative ion

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8
Q

What is the standard enthalpy change of solution?

A

The enthalpy change that takes place when one mol of a solute dissolves in a solvent (WATER).

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9
Q

How can you experimentally determine the enthalpy change of solution?

A
  1. weigh out set amount of ionic solid
  2. put 25cm3 of water in polystyrene cup with lid
  3. measure starting temp of water
  4. quickly tip in all of ionic solid and stir
  5. record final temp when all of solid has dissolved and temp remains constant.
    THEN USE Q = MCΔT m = the mass of the water + the mass of the salt)
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10
Q

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

A
  • ionic lattice breaks up
  • water molecules are attracted to and surround ions
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11
Q

What 2 types of energy change are involved in the dissolving process?

A
  • ionic lattice broken up (opposite energy change of lattice enthalpy)
  • gaseous ions interact with polar water molecules to form hydrated aq ions (enthalpy change of hydration)
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12
Q

What is the enthalpy change of hydration?

A

The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
e.g. Na+ (g) + aq → Na+ (aq)

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13
Q

What affects the cycle to calculate the enthalpy change of solution?

A

Whether the enthalpy change of solution is endothermic or exothermic
ENDO: arrow up
EXO: arrow down

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14
Q

What are the general properties of ionic compounds?

A
  • high melting and boiling points
  • soluble in polar solvents
  • conduct electricity when molten or in aq solution
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15
Q

What is the effect on lattice enthalpy of ionic size/radius increasing?

A

As ionic radius increases, attraction between ions decreases. This means that lattice energy is less negative and melting point decreases

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16
Q

What is the effect on lattice enthalpy of ionic charge increasing?

A

As ionic charge increases, attraction between ions increases. Therefore lattice energy becomes more negative and melting point increases

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17
Q

What is the effect of ionic size and charge across Period 3?

A

→ (2 supporting forces) increasing charge and decreasing size gives more attraction
← (2 opposing forces) increasing charge gives more attraction but increasing size gives less attraction

18
Q

What can metal oxides like MgO be used for and why?

A

Used as a protective coating for the inside of furnaces and refractories as they are stable and have very exothermic lattice enthalpies and very high melting points

19
Q

How does increasing ionic radius affect hydration enthalpies?

A

As ionic radius increases, the attraction between ion and water molecules decreases so hydration energy becomes less negative

20
Q

How does increasing ionic charge affect hydration enthalpies?

A

As ionic charge increases, the attraction with water molecules increases and hydration energy becomes more negative

21
Q

What must happen for an ionic compound to dissolve in water, what can we use to predict this?

A

The attraction between ions in the ionic lattice must be overcome
If sum of hydration enthalpies is larger than magnitude of lattice enthalpy, the overall enthalpy change (solution) will be exo and the compound SHOULD dissolve (but doesn’t always)

22
Q

What is entropy?

A

The dispersal of energy within the chemicals making up the chemical system

23
Q

How does entropy vary with solids, liquids and gases?

A

Solids have the smallest entropies, liquids have greater entropies, and gases have the greatest entropy

24
Q

What are the units for entropy?

A

J K-1 mol-1
The greater than entropy value, the greater that energy is spread out per Kelvin per mole

25
Q

How does entropy change with temperature?

A

Particles vibrate/move faster at higher temps so the entropy is greater

26
Q

What is entropy at 0K?

A

At 0K, everything has an entropy of 0

27
Q

What are the tendencies of systems with positive/negative ΔS?

A

A system with positive ΔS has a tendency to become more disordered so energy is more spread out
A system with negative ΔS has a tendency to become less disordered so less energy is spread out

28
Q

How can you see from a chemical equation whether a reaction has a change in enthalpy?

A

Reactions producing gases result in an increase in entropy.
Reactions where there is a decrease in the number of gas molecules results in a negative enthalpy

29
Q

What is standard entropy?

A

The entropy of one mole of a substance under standard conditions.
ALWAYS POSITIVE

30
Q

How can you calculate entropy change of reaction from standard entropies?

A

Entropy of products - entropy of reactants

30
Q

How can you calculate entropy change of reaction from standard entropies?

A

ΔS° = ΣS° (products) - ΣS° (reactants)
PRODUCTS - REACTANTS

31
Q

What is feasibility?

A

Used to describe whether a reaction is able to happen and is energetically feasible

32
Q

What is free energy and what two types of energy is it made up of?

A

The overall change in energy during a chemical reaction is called free energy change
Made up of
- enthalpy change ΔH (heat transfer between chem system and surroundings)
- entropy change at temp of reaction TΔS (dispersal of energy within the chemical system itself)

33
Q

What is the Gibb’s equation?

A

ΔG = ΔH - TΔS

34
Q

What does the value of ΔG tell us?

A

Whether or not the reaction is feasible at a certain temperature.
NEGATIVE ΔG VALUE = feasible
POSITIVE ΔG VALUE = not feasible

35
Q

What are the limitations of ΔG?

A

Indicates thermodynamic feasibility but takes no account of kinetics or rate of reaction

36
Q

What does the feasibility of a reaction depend on?

A

The balance between ΔH and TΔS.
T increases the significance of ΔS when it changes

37
Q

What is the condition for feasibility?

A

ΔG < 0

38
Q

Why is the entropy of water zero at 0K?

A

The particles are in a maximum state of order

39
Q

Why is the entropy change between liquid to gas greater than that of solid to liquid?

A

There is a greater increase in disorder between liquid to gas.

OCR A Level Chemistry (29 decks)

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