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A - Chemistry > Unit 1: Section 6 - Thermodynamics CDS * > Flashcards

Unit 1: Section 6 - Thermodynamics CDS * Flashcards

enthalpy definitions lattice enthalpy and born-haber cycles enthalpies of solution entropy free energy change

1
Q

what is enthalpy change?

A

the heat energy transferred in a reaction at constant pressure

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2
Q

what are standard conditions?

A

298 K

100 kPa

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3
Q

what is the enthalpy change for exothermic and endothermic reactions?

A

exothermic - negative ΔH, heat is given out

endothermic - positive ΔH, heat is taken in

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4
Q

what is the enthalpy change of formation?

A

the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

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5
Q

what is bond dissociation enthalpy?

A

the enthalpy change when all the bonds of the same type in 1 mole of gaseous molecule are broken

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6
Q

what is the enthalpy change of atomisation?

A

the enthalpy change when 1 mole of monatomic gaseous atoms is formed from an element/compound in its standard state

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7
Q

what is the first ionisation energy?

A

the enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms

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8
Q

what is the second ionisation energy?

A

the enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions

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9
Q

what is the first electron affinity?

A

the enthalpy change when 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms

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10
Q

what is the second electron affinity?

A

the enthalpy change when 1 mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions

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11
Q

what is the enthalpy change of hydration?

A

the enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions

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12
Q

what is the enthalpy change of solution?

A

the enthalpy change when 1 mole of an ionic compound dissolves into its aqueous ions

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13
Q

what is the lattice enthalpy of formation?

A

the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions

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14
Q

what is lattice enthalpy of dissociation?

A

the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions

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15
Q

what does Hess’s law state?

A

the total enthalpy change of a reaction is always the same, no matter which route is taken

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16
Q

what is a born-haber cycle?

A

start with the enthalpy of formation (elements in standard states –> ionic lattice)
then use enthalpy of atomisation and ionisation ( turn into gas, and turn diatomic molecules into monatomic e.g. Cl2 –> 2Cl then remove electrons)
use electron affinity (electron from ionisation added to other atom)
then lattice enthalpy of formation (turns gaseous ions into ionic lattice)

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17
Q

how to work out theoretical lattice enthalpy?

A

doing some calculations on the purely ionic model of a lattice.

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18
Q

what is the purely ionic model of a lattice?

A

it assumes that all ions are spherical, and have their charge evenly distributed around them

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19
Q

what is experimental lattice enthalpy?

A

finding lattice enthalpy from experiments, the value is often different to theoretical because most ionic compounds have some covalent character

20
Q

what is covalent character?

A

the positive and negative ions in a lattice aren’t usually exactly spherical.
positive ions polarise neighbouring negative ions to different extents, the more polarisation, the more covalent the bonding

21
Q

what does it mean if a ionic compound has covalent character?

A

the bonding in the lattice is stronger than expected, because covalent bonds are stronger than ionic.
experimental enthalpy will be a lot higher than theoretical

22
Q

what happens when a solid ionic lattice dissolves?

A

bonds between ions break to give free ions - endothermic
bonds between ions and water are made - exothermic
enthalpy change of solution is the overall effect on the enthalpy of these

23
Q

why can water molecules bond to ions?

A

oxygen is more electronegative than hydrogen, so it draws electrons towards itself, creating a dipole.
positive hydrogen can form bonds with negative ions
negative oxygen form bonds with positive ions

24
Q

when do substances dissolve?

A

substances generally only dissolve if the energy released is roughly the same, or greater than the energy taken in.
so soluble substances tend to have exothermic enthalpies in solution

25
Q

how can you use Hess’s law to work out enthalpy of solution?

A

solution - converts lattice to aqueous ions
hydration - converts gaseous ions to aqueous ions
lattice dissociation - converts gaseous ions into the lattice
a triangle can be constructed
ΔSOL = ΔHYD(+ve ion) + ΔHYD(-ve ion) - ΔLEFORM

26
Q

how to tell if a substance is insoluble in water?

A

if the enthalpy of solution is positive and very big, so very endothermic, the substance is insoluble

27
Q

what is entropy? (S)

A

a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles
the more disordered the particles, the higher the entropy

28
Q

what factors affect entropy?

A

physical state affects entropy

more particles means more entropy

29
Q

why does physical state affect entropy?

A

solid particles vibrate about a fixed point - not much disorder - low entropy
gas particles move fast wherever they want - most disorder - highest entropy

30
Q

why do more particles mean more entropy?

A

the more particles, the more ways their energy can be arranged

31
Q

what makes a reaction feasible?

A

substances tend to disorder because it makes them more energetically stable, so they move to increase their entropy, this means an endothermic reaction can happen without energy

32
Q

how to calculate entropy change?

A

entropy of products - entropy of reactants

33
Q

what is the standard entropy of a substance?

A

the entropy of 1 mole of that substance under standard conditions

34
Q

what does it mean if total entropy of a reaction increases?

A

it is feasible for the reaction to occur, but it is not guaranteed

35
Q

what is free energy change?

A

a measure used to predict whether a reaction is feasible

36
Q

what is ΔG when a reaction is feasible?

A

when ΔG ≤ 0

then a reaction is feasible

37
Q

what is the equation for Gibbs free energy change?

A 
ΔG = ΔH - TΔS
ΔH = enthalpy change (kJ/mol)
ΔS = entropy change (J/mol/K)/1000 --> (kJ/mol/K)
T = temperature in kelvins
must all have same units so ΔS / 1000
38
Q

when is a reaction always feasible?

A

when the reaction is exothermic and has a positive entropy change

39
Q

when is a reaction never feasible?

A

when the reaction is endothermic and has a negative entropy change

40
Q

how to calculate the temperature that a reaction becomes feasible?

A

when ΔG is 0 a reaction becomes feasible
so when
T = ΔH/ΔS
the reaction becomes feasible

41
Q

what do the direction of the arrows show in the Born Haber cycle?

A

the direction of energy, if its pointing up its endothermic

down means exothermic

42
Q

what happens if there is a 2- ion in a born Haber cycle?

A

the 1st electron affinity will go below the equation of the gaseous ions then back up in the second electron affinity
1st is exothermic
2nd is endothermic - repulsion between electron and negative ion means 2nd Ea requires energy to add the electron

43
Q

how to know which ionic bond is stronger?

A

the bigger the ions, the weaker the ionic bonds
the higher the charge, the stronger the bond
charge takes priority over size

44
Q

what does the graph of entropy by temperature look like?

A

as temperature increases, entropy increases
when a change of state happens, entropy increases dramatically (vertical line), this change is bigger going from liquid to gas then solid to liquid

45
Q

when does a change of state become feasible?

A

ΔG = 0

this is when a substance melts or boils

46
Q

How are diatomic molecules treated differently in born-Haber cycles?

A

The atomisation energy is needed twice e.g. to turn Br2 into 2 Br
They will also need 2x ionisation energy or 2x electron affinity

A - Chemistry (26 decks)

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