Enthalpy and Entropy Flashcards

1
Q

Define lattice enthalpy

A

Lattice enthalpy is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

e.g Na+ (g) + Cl- (g) = NaCl (s)
gaseous ions solid ionic lattice

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2
Q

What kind of energy change will lattice enthalpy always be?

A

lattice enthalpy is Exothermic and hence Enthalpy change is negative because bond forming is exothermic.

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3
Q

What is the Born-Haber cycle?

A

A way of indirectly calculating lattice enthalpy from other known energy changes because it cant be calculated directly.

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4
Q

What is the lattice formation and what energy change is it?

A

lattice formation is Changing gaseous ions into the solid ionic lattice and its Exothermic.

gaseous ions to ionic lattice

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5
Q

Define standard enthalpy change 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.

Na (s) + 1/2 Cl2 (g) = NaCl (s)
reactants to one mole of a product

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6
Q

Define standard enthalpy change of atomisation

A

The standard enthalpy change of atomisation is the enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.

Na (s) = Na (g)
1/2 Cl2 (g) = Cl (g)

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7
Q

Define 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+ ions.

Na(g) = Na+ (g) + e-

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8
Q

Why is ionisation energy always endothermic?

A

ionisation energy is endothermic because Energy is required to overcome the attraction between a negative electron and a positive nucleus.

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9
Q

Define first electron affinity

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.

Cl (g) + e- = Cl- (g)

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10
Q

Why is first elctron affinity always exothermic?

A

Electron being added is attracted in towards the nucleus.

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11
Q

what do group 2 compounds require in there born haber cycle?

A

Born-haber cycles for group two compounds require both first and second ionisation energies.

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12
Q

what do oxides and sulfides require in born haber cycles?

A

born-haber cycles for oxides and sulfides require first and second electron affinities/

O+ e- = O-
O- + e- = O 2-

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13
Q

why is the second electron affinity endothermic?

A

the second electron affinity is negative because

1) an electron is being gained to a negatively charged atom which repels s there the same charge
2) Energy is needed to force the negatively charged electron onto the ion

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14
Q

Why are successive electron affinities endothermic?

A

The negative ion repels the additional electron, so energy is required to force the electron onto it.

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15
Q

Define standard enthalpy change of solution

A

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

Na+Cl- + aq= Na+ (aq) + Cl- (aq)
solute to dissolved

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16
Q

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

A

when a solid ionic compound dissolves in water
the Ionic lattice breaks up
Water molecules are attracted to and surround the ions.

17
Q

What are the energy changes involved in these processes of a solid ionic compound dissolving in water?

A

When a solid ionic compound dissolves in water, the Ionic lattice is broken up forming separate gaseous ions.
gaseous ions interact with polar water molecules forming hydrated aqueous ions.
This is Called enthalpy change of hydration.

18
Q

Define enthalpy change of hydration

A

The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.

Na+ + aq = Na + (aq)

19
Q

Describe the effect of ionic size on lattice enthalpy and melting point

A

as Ionic radius increases
Attraction between ions decreases
Lattice enthalpy becomes less negative ( less exothermic)
Melting point decreases as less energy is needed to overcome the decreased attraction

20
Q

Describe the effect of ionic charge on lattice enthalpy and melting point

A

as Ionic charge increases
Attraction between ions increases
Lattice enthalpy becomes more negative (more exothermic)
Melting point increases as more energy is needed to overcome increased attraction

21
Q

Describe the effect of ionic size on hydration enthalpy

A

as Ionic size increases
Attraction between ion and water molecules decreases
Hydration energy less negative

22
Q

Describe the effect of ionic charge on hydration enthalpy

A

Ionic charge increases
Attraction with water molecules increases
Hydration energy becomes more negative

23
Q

Why is hydration enthalpy an inaccurate method of predicting solubility?

A

Many compounds with endothermic enthalpy changes of solution are soluble.

24
Q

Define entropy

A

entropy is a measure of the dispersal of energy within the chemicals in a chemical system.

25
Q

What are the units of standard entropy?

A

J/K/mol

26
Q

How does entropy relate to physical states?

A

In order of size of entropy:
Gases > liquids > solids
solid has the least entropy and gases the most

27
Q

Define standard entropy

A

The entropy of one mole of a substance, under standard conditions : pressure of 100kpa and temp of 298 k
(Always positive)

28
Q

How is an entropy change calculated for a reaction?

A

entropy Change= products entropy sum- reactants entropy sum

29
Q

What is feasibility?

A

Whether a reaction is able to happen and is energetically feasible (or spontaneous). if products have less energy than reactants its ‘energetically feasible’

30
Q

What is the equation for free energy change?

A

ΔG = ΔH - TΔS

31
Q

What is the requirement for the free energy change such that a reaction is feasible?

A

ΔG < 0

there must be A decrease in free energy for it to happen.

32
Q

Even though a reaction may have a negative value for ΔG, why might the reaction not take place at room temperature?

A

High activation energy resulting in a very slow rate.

33
Q

what are zwitterions?

A

zwitterions have both a negative and positive charge but are neutral overall

34
Q

On a ΔG and T/K graph, how do you find the ΔS and ΔH

A

ΔS = Gradient
ΔH = Y intercept