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Chemistry > Thermodynamics > Flashcards

Thermodynamics Flashcards

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1
Q

The standard enthalpy of formation

A

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

Na(s) + 1/2 Cl2(g) -> NaCl

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2
Q

First ionisation enthalpy

A

The standard enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to give one mole of gaseous ions each with a single positive charge

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3
Q

The standard enthalpy of atomisation

A

The enthalpy change when one mole of gaseous atoms are formed from an element in its standard state

1/2 Br2 (l) -> Br(g)

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4
Q

Mean bond enthalpy

A

The enthalpy change when one mole of gaseous molecules each break a covalent bond to form two free radicals averaged over a range of compounds

Br2 (l) -> 2Br (g)

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5
Q

First electron affinity

A

The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single negative charge under standard conditions

O(g) + e- -> o- (g)

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6
Q

Lattice formation enthalpy

A

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

Mg2+ (g) + 2Br- (g) -> MgBr2 (s)

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7
Q

Lattice dissociation enthalpy

A

The standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions
MgBr2 (s) -> Mg2+ (g) + 2Br- (g)

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8
Q

Standard enthalpy of hydration

A

The standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions

Cl-(g) + aq -> Cl- (aq)

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9
Q

Standard enthalpy of solution

A

The standard enthalpy change when one mole of solute dissolves to form its aqueous ions

NaCl(s) + aq -> Na+ (aq) + Cl-

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10
Q

What can effect the strength of an ionic bond

A

The size of the ions- smaller ions have a strange attraction

The charge of the ions- higher charged ions have a stronger attraction

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11
Q

What do stronger ions lattices have

A

More exothermic lattice formation enthalpy
More endothermic lattice dissociation enthalpy

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12
Q

Compare different techniques for measuring ionic bond strength

A

One method is theoretical also known as the perfect ionic model it assumes that ions are perfect spheres. The nature of the bonding is purely ionic

One method is experimental also known as the born hater cycle it assumes that ions are polarisable. The nature of the bonding is ionic plus some covalent character

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13
Q

what are the two key factors that determine how exothermic a lattice enthalpy will be

A

charge on ions -the greater the charge on an ion has the greater it’s attraction to the oppositely charged ion
size of ions (ionic radius) -the smaller the ion the greater the attraction to an oppositely charged ion

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14
Q

CRAM

A

Charge on the ions
Radius of the ions
Attraction between ions
More exothermic/endothermic

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15
Q

which compound will have more exothermic lattice formation enthalpy MgO or Na2O

A

Mg2+ has a greater charge than Na +
Mg 2+ is smaller than Na +
Mg 2+ is more strongly attracted to O2-

MgO has the more exothermic lattice formation enthalpy

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16
Q

describe covalent character using Zn 2+ and Se 2-

A

the Zn2+ ion is quite small with quite a strong +ve charge
the Se 2- ion is large with a diffuse electron cloud
the 2 with ionically bond because of electrostatic attraction

the Zn 2+ ion is strongly polarising
the electron cloud about the Se 2- ion becomes distorted
some the electron density is shared
this is covalent character

17
Q

what characteristics will compounds with covalent character have

A

a +ve ion which is small and highly charged (very polarising)

a -ve ion which is very large and highly charged (very polarisable)

18
Q

the theoretical lattice enthalpy model

A

perfect ionic model
ions are point charges
and the bonding is perfectly ionic

19
Q

the experimental model for lattice enthalpy

A

born haber
ions are polarisable
covalent character

20
Q

what’s the ^H LF calculated by perfect ionic model like in comparison to ^H LF calculated by experimental model
when there is NO COVALENT CHARACTER

A

equal

21
Q

what’s the ^H LF calculated by perfect ionic model like in comparison to ^H LF calculated by experimental model
when there IS COVALENT CHARACTER

A

less exothermic as the born haber model allows for covalent character and predicts bonding

22
Q

what happens in terms of bonds when you dissolve an ionic compound

A

1.break the ionic bonds
2. form bonds between the water molecules and the ions

23
Q

What two factors affect how exothermic a lattice enthalpy will be

A

Charge of ions
Size of ions

24
Q

How does the charge of ions affect the how exothermic a lattice enthalpy will be

A

The greater the charge on an ion has the greater it’s attraction to me oppositely charge ion

25
Q

How does the size of the ions affect how exothermic a lattice enthalpy will be

A

The smaller the ion the greater the attraction to an oppositely charge ion

26
Q

What is meant by covalent character in Zn and Se

A

Zn2+ is quite small and has a strong positive
Se2- is large with a diffuse electron cloud
The 2 will ionically bond because of electrostatic attraction

The Zn2+ ion is strongly polarising
The electron cloud about the se2- ion becomes distorted
Some of electron density is shared
This covalent character

27
Q

How can ypu predict which compounds will have covalent character

A

A +ve ion which is small and highly charged (very polarising)
A -ve ion which is very large and highly charged (very polarisable)

28
Q

Describe the theoretical model

A

Perfect ionic model
Ions are point charges
Perfectly ionic

29
Q

Describe the experimental model

A

Born haber
Ions are polarisable
Covalent character

30
Q

What is the ^H Lf for perfect ionic model like compared to the the ^H Lf when there is covalent character

A

The ^H Lf for the perfect ionic model is less exothermic then the one for the experimental model
As the experimental allows for covalent character so predicts stronger bonding

31
Q

What happens in terms of bonds when an ionic compound dissolves

A

Break the ionic bonds
Form bonds between the water molecules and the ions

32
Q

What is entropy

A

Disorder
^S

33
Q

Solid -> liquid
What is the entropy change

A

Positive
As the particles become more disordered

34
Q

Gibbs free energy equation

A

^G=^H - T^S

35
Q

When is a reaction feasible

A

When ^G=<0

36
Q

In the Gibbs free energy equation what is ^S measured in

A

KJ K-1mol-1

37
Q

Why may a reaction still not occur even if ^G<0

A

Kinetic factors
A reaction may have a very high activation energy such that a few particles have sufficient energy to react or it may occur at a very slow rate

38
Q

Describe how Gibbs free energy equation relates to Y=mx+C

A

^G=y
-^ST=mx
^H=c

39
Q

If the gradient is +ve what is ^S

A

Negative

Chemistry (24 decks)

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