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Chemistry ALEVEL - Ocr A > 22 - Enthalpy And Entropy > Flashcards

22 - Enthalpy And Entropy Flashcards

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1
Q

What does lattice enthalpy measure

A

Ionic bond strength

  • energy given out when gaseous ions combine to make a solid lattice
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2
Q

Standard lattice enthalpy definition

A

The enthalpy change when 1 mole of an ionic lattice is formed from its gaseous ions under standard conditions

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3
Q

What are standard conditions

A

298k
25c

100kpa

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4
Q

The more negative the lattice enthalpy the…

A

Stronger the bonding

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5
Q

Standard enthalpy change of formation definition

A

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

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6
Q

enthalpy change of formation equation

A

The normal reaction equation

Elements in standard state - > solid compound

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7
Q

Standard enthalpy change of atomisation definition

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

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8
Q

enthalpy change of atomisation

Is it endothermic or exothermic

A

Endothermic - because bonds are broken to form gaseous atoms

Elements in standard state -> gaseous atoms

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9
Q

First ionisation energy

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+ atoms

Gaseous atom -> gaseous ion

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10
Q

First ionisation

Endo or exo

A

Endo

Energy required to overcome attraction between a negative electron and the positive nucleus

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11
Q

First electron affinity definition

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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12
Q

First electron affinity

Endo or exo

A

Exo

- the electron added is attracted in towards the nucleus

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13
Q

Why is the second successive electron affinity endothermic

A

A second electron is gained by a negative ion, energy is need to force the electron onto the negative ion, due to repulsion

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14
Q

Enthalpy change of solution definition

A

The enthalpy change that takes place when one mole of a solute dissolves in a solvent under standard conditions

Ionic compound + aq -> aqueous ions

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15
Q

What is the experimental determination of the enthalpy change of solution

Procédure

A

1) weigh out sample of KCl
2) pour 25cm3 of distilled water into a plastic cup in a beaker
3) measure initial temp
4) tip all of KCl in cup - stir till dissolved, temp is constant
5) record temp to nearest 0.5c

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16
Q

What is the experimental determination of the enthalpy change of solution

Calculations

A 
1) use q=mct 
C= 4.18
Mass of water - (1gcm-3 x volume used)
Mass of solution - (water + solid)
- energy = kJ

2)find moles of solid compound

3) calculate enthalpy change in solution
- 0.05 mol of KCl gains 278kj energy from solution - 1 mol would gain

Energy / mol

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17
Q

What is the dissolving process steps

A

1) the bonds between the ions break to give gaseous ions - endo = Lattice enthalpy

2) the separate gaseous ions interact with polar water to form hydrated aqueous ions
= enthalpy change of hydration

18
Q

Enthalpy change of hydration definition

A

Enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions

19
Q

Enthalpy change of hydration

Équation

A

Can be exo or endo - depending on relative size of LE

Gaseous ions + aq -> aqueous ions

20
Q

List 2 factors affecting Lattice enthalpy

A

Ionic size

Ionic charge

21
Q

Affect of ionic charge on LE

A
  • ionic radius increases
  • lower charge density
  • ions further apart in lattice
  • attraction between ions decrease
  • lattice enthalpy less negative
  • melting point decreases
22
Q

Effect of ionic charge on LE

A
  • ionic charge increases
  • more energy released when ionic lattice forms
  • attraction between ions increases
  • LE becomes more negative
  • mp increases
23
Q

List factors affecting hydration

A

Ionic size

Ionic charge

24
Q

Ionic size affect on hydration enthalpy

A
  • ionic radius increases
  • lower charge density
  • attraction between ion and water molecules decreases
  • hydration enthalpy is less negative
25
Q

Ionic charge on hydration enthalpy

A
  • ionic charge increases
  • attraction with water molecules increases - stronger electro station attraction
  • more energy released when the bonds are made
  • hydration enthalpy more negative
26
Q

Entropy definition

A

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

27
Q

The higher the entropy (positive) …

A

The more disordered a system

28
Q

What is entropy the measure of

A

The number of ways that particles can be arranged

The number of ways that the energy can be shared out between the particles

29
Q

How physical state affects entropy

A

Solid -> liquid -> gas

Gases:
Randomness increases, most disorder, have the greatest entropy

30
Q

Why more particles means more entropy

A

The more particles
The more ways their energy can be arranged
Entropy increases because the number of moles increases
Energy is spread out more

31
Q

Why are some reactions feasible

A

Substances like disorder, they’re more stable

Particles move to increase their entropy

32
Q

What does thermodynamically stable mean

A

When a substance reaches its maximum entropy state

Won’t react further without energy input

33
Q

How to calculate entropy change

A

Change in S =

S of products - S reactants

34
Q

What does a positive entropy mean

A

The reaction is likely to be feasible

React without the addition of energy

35
Q

Why does negative free energy not guarantee a reaction

A

Free energy change doesn’t tell you about rate of reaction

It might have a high activation energy or happen very slowly

36
Q

What is free energy

A

The overall change in energy during a chemical reaction

Tells you if a reaction is feasible or not

37
Q

What 3 factors determine whether a process will take place

A

Température - kelvin

Entropy - j K-1mol-1= convert by dividing by a thousand

Enthalpy - kjmol-1

38
Q

Free energy change equation

A

G = H - tS

39
Q

What two types of energy make up free energy change

A

The enthalpy change

  • heat transfer between the chemical system and the surroundings

Entropy change at temperature of reaction

  • dispersal of energy within chemical system
40
Q

Limitations of predictions made for feasibility

A
  • high activation energy

- slow rate of reaction

Chemistry ALEVEL - Ocr A (8 decks)

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