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A Level Chemistry > Thermodynamics > Flashcards

Thermodynamics Flashcards

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1
Q

Define mean bond enthalpy

Is this endothermic, exothermic, or does it depend?

A

Enthalpy change for the breaking of 1 mole of covalent bonds, averaged over a range of compounds

Endothermic

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2
Q

Define Bond Dissociation

A

Enthalpy change to break the bond in one mole of gaseous molecules to form gaseous atoms

This is endothermic

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3
Q

Define standard molar enthalpy of atomisation (🔺H at)

A

Enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions

This is endothermic

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4
Q

Define first electron affinity (EA)/(🔺H ea)

A

The enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous negative ions

This is exothermic

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5
Q

Define second electron affinity (🔺H ea-)

A

The enthalpy change when a mole of electrons is added to a mole of gaseous negative ions, to form ions each with a doubly negative charge

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6
Q

Why is first electron affinity exothermic but second is endothermic

A

1st - attraction is created between electron and the nucleus

2nd- energy is required to overcome repulsion between negative electron and negative ion

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7
Q

Define lattice formation enthalpy

A

The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions

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8
Q

Define lattice dissociation energy

A

Enthalpy change to separate one mole of an ionic substance into its gaseous ions

Always endothermic (positive value)

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9
Q

Define enthalpy of hydration

A

Enthalpy change when one mole of gaseous ions become aqueous ions

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10
Q

Why is enthalpy of Hydration exothermic

A

Attraction is created between the ion and water

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11
Q

Define enthalpy of solution

A

When one mole of solid dissolves in water to form aqueous ions

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12
Q

Breaking attractions is ______

Making attractions is ______

A

Endothermic (🔺H >0)

Exothermic (🔺H<0)

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13
Q

What are the 4 stages in forming an ionic lattice?

A

Stage 1: atomisation

2: ionisation
3: electron affinity
4: lattice formation

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14
Q

For lattice formation, what does enthalpy of formation equal?

A

All enthalpy changes added together

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15
Q

What is lattice enthalpy

A

The energy released when gaseous ions combine to form an ionic lattice (formation is exothermic) or the energy required to break apart the lattice to form gaseous ions (dissociation is endothermic)

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16
Q

What are the 2 main things that affect lattice enthalpy?

Why?

A

Charge of the ion
Size of the ion

Higher the charge the smaller its radius, meaning the ions will be closer together in the lattice, increasing the attractions’ strength

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17
Q

If the attraction is stronger is more or less energy released on the formation of the lattice

A

More energy

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18
Q

Why do lattice dissociation enthalpies decrease as the size of ions increase?

A

Larger ions have a weaker electrostatic attraction between oppositely charged ions

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19
Q

What is the perfect ionic model?

Why may this value be different to the experimental value?

A

It assumes all ions are perfect spheres with purely ionic attraction

Covalent Character

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20
Q

What does covalent character do?

What does this mean when comparing theoretical and experimental values?
Why?

A

Puts extra energy into the ionic bond, making the bond stronger

Experimental value will be greater

Covalent character makes the bond stronger so more energy is released/ required

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21
Q

Define enthalpy of solution

A

Enthalpy change when one mole of a solid dissolves in water to form aqueous ions

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22
Q

What must a solvent be for ionic solids to dissolve in it

A

Polar eg water

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23
Q

If NaCl was dissolved in water, what would happen?

A

The Na+ would be attracted to the delta-negative Oxygen of the water and the Cl- would be attracted to the delta-positive hydrogen

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24
Q

What does 🔺H sol =

A

🔺H (L. D.) + 🔺H(hyd)

25
Q

What is entropy

A

A measure of disorder in a system

26
Q

What does an increase in disorder mean?

A

The reaction is ‘feasible’ or ‘spontaneous’

27
Q

NAme the states of matter from most ordered to most disordered

A

Solid
Liquid
Gas

28
Q

What affects entropy?

A

Number of moles/ particles

State

29
Q

If a solid becomes liquid, or liquid becomes gas what does this mean for the entropy?

A

Entropy increases so 🔺S is positive

30
Q

If the number of moles decreases, what does this mean for the entropy

A

Disorder decreases so 🔺S is negative

31
Q

If a question asks about entropy what must you mention

A

Disorder

32
Q

The more positive the 🔺H…

A

The more disordered a system is and the more feasible the reaction

33
Q

Equation for entropy change

A

🔺S = S(products) - S (reactants)

[SliPeR]

34
Q

What units are used to measure 🔺S

A

J/K/mol

35
Q

Why is entropy 0 at 0K?

A

There is no movement at absolute zero so there is no disorder

36
Q

When is the greatest 🔺S as temperature increases?

A

The boiling point

Substance is becoming a gas so there is a large increase in disorder

(use question’s terminology)

37
Q

What is the equation for Gibbs Free Energy?

What are the units for this?

A

🔺G=🔺H -T🔺S

kJ/mol

38
Q

When is a reaction feasible?

A

🔺G<0

So if the value is negative or 0 the reaction IS feasible. The more negative the more feasible

39
Q 
If the question asks when a reaction:
becomes feasible
Is no longer feasible 
At equilibrium 
 What do you do
A

Set 🔺G=0

40
Q

What is the 🔺G value at equilibrium?

A

0

41
Q

Why does 🔺G=0 at equilibrium?

A

Forwards reaction is equally feasible as the backward reaction

42
Q

Relate y=mx+c to

🔺G=🔺H-T🔺S

A 
y= 🔺G
m= -🔺S
x= T
c= 🔺H
43
Q

If 🔺S is positive and 🔺H is positive, when is the reaction feasible?

A

At high temperatures, when T🔺S>🔺H

44
Q

When 🔺S is negative and 🔺H is positive, when is the reaction feasible

A

Never

45
Q

When 🔺S is positive and 🔺H is negative when is the reaction feasible?

A

Always

46
Q

If 🔺S is negative and 🔺 is negative when is the reaction feasible

A

At low temperatures, when T🔺S>🔺H

47
Q

Define bond dissociation enthalpy

Give example

A

Enthalpy change to break the bond in one mole of gaseous molecules to form gaseous atoms

Eg Cl2 —> 2Cl

48
Q

Define standard molar enthalpy of atomisation

A

Enthalpy change for he formation of one mole of gaseous atoms from an element in its standard state under standard conditions

1/2 Cl2 (g) —> Cl(g)

49
Q

Difference between bond dissociation and atomisation

A

BD: breaking 1 mole of bonds

Atom: forming one gaseous mole from standard state

50
Q

Define first ionisation enthalpy

A

Minimum amount of energy required to remove one mole of e- from one mole of gaseous atoms

51
Q

Why is second ionisation energy more endothermic

A

More difficult to remove an e- from a more positive ion

52
Q

First electron affinity

A

🔺H when a mole of gaseous atoms is converted to gaseous negative ions

53
Q

Second e- affinity

A

🔺H when a mole of electrons is added to a mole of gaseous negative ions to form ions each with a doubly negative charge

54
Q

Why is 2nd electron affinity endothermic

A

Energy is required to overcome repulsion between negative e- and negative ion

55
Q

Lattice formation enthalpy

A

🔺H when one mole of a solid ionic compound is formed from its gaseous ions

56
Q

Lattice dissociation enthalpy

A

🔺H to separate one mole of an ionic substance into its gaseous ions

57
Q

🔺H hydration

A

Enthalpy change when one mole of gaseous IONS become aqueous ions

58
Q

🔺H solution definition

A

🔺H when one mole of solid dissolves in water to form aqueous ions

59
Q

Define enthalpy change

A

The heat energy change at constant pressure

A Level Chemistry (11 decks)

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