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A Level Chemistry > Enthalpy and entropy > Flashcards

Enthalpy and entropy Flashcards

1
Q

What is the definition of lattice enthalpy

A

Enthalpy change that accompanies the formation of 1 mole of an ionic compound from its gaseous ions under standard conditions
Always exothermic, measures the strength of ionic bonding in a giant ionic lattice

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2
Q

Are ionic solid compounds stable?

A

Tend to be very stable

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3
Q

How does stability arise from ionic bonds

A

Strong electrostatic attraction between oppositely charged ions in a lattice

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4
Q

Born-Haber cycles

A

Like a Hess cycle where the lattice enthalpy cannot be measured directly

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5
Q

Standard enthalpy change of formation

A

Enthalpy change that occurs when 1 mole of a compound is formed from its elements under standard conditions
The compound will always be an ionic compound in its solid lattice

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6
Q

Standard enthalpy change of atomization

A

Enthalpy change that takes place for the formation of 1 mole gaseous atoms from the element in its standard state under standard conditions
Always endothermic, bonds broken to form gaseous atoms

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7
Q

Electron affinity

A

Opposite of ionization energy

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8
Q

1st electron affinity

A

Enthalpy change that takes place when 1 electron is added to each atom in a mole of gaseous atoms to form 1 mole of gaseous 1- ions
Always exothermic

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9
Q

Determination of lattice enthalpies

A

Start from standard states
Formation of gaseous atoms
Formation of gaseous ions
Form lattice

From standard states
Formation of lattice from elements

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10
Q

Considerations to make when determining lattice enthalpies

A

If an ion has a charge greater than 1, multiply IE and EA

If there is a diatomic atom, multiply everything by a multiple of 2

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11
Q

Successive electron affinities

A

1st EA, exothermic

2nd EA, endothermic

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12
Q

Why is the 2nd successive electron affinity endo?

A

2nd electrons is being gained by a -ve ion

Energy must be put in to force the -ve electron onto -ve ion

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13
Q

Definition of enthalpy change of solution

A

Enthalpy change that takes place when 1 mole of solute dissolves in a solvent to form aqueous ions
Can be exo or endo

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14
Q

How do ions act in solution?

A

Partial charges in water are attracted to +ve -ve ions
Lattice dissolved into separate ions
Ions are now surrounded by water to form aqueous ions

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15
Q

Determination of enthalpy change in solution

A

Calculate q=mc∆t/1000
m=mass of water and solute as its the enthalpy change of the solution we want

E/Moles=kj

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16
Q

What mass is used in enthalpy solution calculations?

A

Water+solute

as its the solution that changes temperature

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17
Q

What processes occur during dissolving?

A

Ionic lattice breaks up

Water molecules are attracted to and surround ions

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18
Q

What energy changes are involved in dissolving

A

Ionic lattice broken up, forming separate gaseous ions,

Separate gaseous ions interact with polar water molecules too from hydrated aqueous ions, enthalpy change of hydration

19
Q

Energy cycle in hydration

A

From ionic lattice
Formation of gaseous ions (opposite of lattice enthalpy)
Hydrate ions from gaseous ions to form aqueous ions

20
Q

Considerations to make when forming enthalpy cycles

A

If 2 of the same ion is used, multiply by 2
Ensure all species are balanced
Consider arrow direction

21
Q

Properties of ions compounds

A

Tend to have high melting and boiling points
Soluble in polar solvents
Conduct electricity when molten or in aqueous solution

22
Q

Factors affecting lattice enthalpy

A

Ionic size

Ionic charge

23
Q

Effect of ionic size on lattice enthalpy

A

As ionic radius increases
Attraction between ions decrease
Lattice enthalpy less -ve
Melting point decreases

24
Q

Effect of ionic charge on lattice enthalpy

A

As ionic charge increases
Attraction between ions increases
Lattice energy becomes more -ve
Melting point increases

25
Q

Combination of both effects from alkali metals to the right

A

Increasing charge
Decreasing size
Both result in more attraction

26
Q

Combination of both effects from noble gases to the left

A

Increasing charge
Increasing size
Opposing effects

27
Q

How to predict melting points

A

Magnitudes of lattice enthalpy give a good indication of melting points
Other factor such as the packing of ions may need to be considered

28
Q

Factors affecting hydration

A

Similar to lattice enthalpies

Ionic size
Ionic charge

29
Q

Effect of ionic size on hydration enthalpy

A

Ionic radius increases
Attraction between ion and water molecules decreases
Hydration energy less -ve

30
Q

Effect of ionic charge on hydration enthalpy

A

Ionic charge increases
Attraction between ion and water molecules increases
Hydration energy more -ve

31
Q

Predicting solubility

A

To dissolve a compound, attraction between ions in ionic lattice must be overcome
Requires a quantity of energy equal to lattice enthalpy
Water molecules are attracted to +ve and -ve ions, surrounding them and releasing energy equal to hydration enthalpy
If sum of hydration enthalpies is larger than the magnitude of the lattice enthalpy, enthalpy change of solution will be exothermic, should dissolve
However, some endothermic enthalpy changes are soluble, also depends on temperature and entropy

32
Q

What is entropy?

A

Dispersal of energy within the chemicals making up the chemical system
JK-1 mol-1

33
Q

What has the greatest enthalpy between the states of matter?

A

In increasing amounts of entropy
Solid
Liquid
Gas

34
Q

What affects the amount of entropy?

A

Melting and boiling points increase randomness of particle movement, energy more spread out
+S

35
Q

Entropy at and above 0K

A

At 0K, there would be no energy, all substances have an entropy value of 0
Above 0K, energy becomes more dispersed among particles, +ve entropy

36
Q

Predicting entropy changes

A

If physical states change, entropy changes

If no of gas moles changes, entropy changes

37
Q

Standard entropy

A

Entropy of 1 mole of substance under standard conditions
JK-1mol-1
Always +ve

38
Q

Calculating entropy changes

A

Entropy change=Entropy of products-entropy of reactants

39
Q

Feasibility of reactions

A

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

40
Q

Free energy

A

Overall change during a chemical reaction is the free energy change ∆G made up of 2 energies

Enthalpy change ∆H, heat transfer between chemical systems and surroundings
Entropy change at temperature of reaction T∆S, dispersal of energy within chemical system itself

41
Q

Gibbs equation

A

∆G=∆H-T∆S
Temperature in K
∆S must match kJ in ∆H, divide
∆S by 1000

42
Q

Conditions for feasibility

A

∆G must be less than 0

43
Q

Working out minimum temperature for feasibility

A

T=∆H/∆S

Convert K to C

44
Q

Limitation of predictions made for feasibility

A

Does not account for activation energies or rate of reaction

A Level Chemistry (12 decks)

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