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Chemistry A Level 1 > thermodynamics > Flashcards

thermodynamics Flashcards

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1
Q

what is Hess’ Law

A

enthalpy change for a reaction is independent of the route taken

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2
Q

define standard enthalpy of formation

A

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

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3
Q

what is the standard enthalpy of an element

A

0

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4
Q

define standard enthalpy of combustion

A

enthalpy change when one mole of substance is completely burnt in excess oxygen

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5
Q

define standard enthalpy of atomisation

A

enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions

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6
Q

define first ionisation energy

A

enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions

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6
Q

define first ionisation energy

A

enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions

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7
Q

define second ionisation energy

A

enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

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8
Q

define first electron affinity

A

enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions

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9
Q

define second electron affinity

A

enthalpy change when on emole of gaseous 1- ions gains one mole of electrons to form one mole of gaseous 2- ions

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10
Q

define lattice enthalpy of formation

A

enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions

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11
Q

define lattice enthalpy of dissociation

A

enthalpy change when one mole of solid ionic lattice is dissociated into its gaseous ions

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12
Q

define enthalpy of hydration

A

enthalpy change when one mole of gaseous ions become hydrated/dissolved in water to infinite dilution (water molecules totally surround the ion)

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13
Q

define enthalpy of solution

A

enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution

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14
Q

define mean bond dissociation enthalpy

A

enthalpy change when one mole of covalent bonds if broken with all species in the gaseous state

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15
Q

what is a born-haber cycle

A

thermochemical cycle showing all the enthalpy changes involved in the formation of an ionic compound. start with elements in their standard states (enthalpy of 0)

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16
Q

what factors affect the lattice enthalpy of an ionic compound

A

size of ions, charge on the ions

17
Q

how can you increase the lattice enthalpy of a compound

A

smaller ions since the charge centres will be closer together

increased charge, since there will be a greater electrostatic force of attraction between the oppositely charged ions

increasing the charge on the anion has a much smaller effect than increasing the charge on the cation, since increasing anion charge also has the effect of increasing ionic size

18
Q

how can born-haber cycles be used to see if compounds could theoretically exist

A

use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable

was used to predict the existence of the first noble gas containing compound

19
Q

what actually happens when a solid is dissolved in terms of interactions of the ions with water molecules

A

break lattice –> gaseous ions; dissolve each gaseous ion in water

aqueous ions are surrounded by water molecules (which have a permanent dipole due to polar O-H bond)

20
Q

what is the perfect ionic model

A

assumes that ions are perfectly spherical and that there is an even charge distribution (100% polar bonds)

act as point charges

21
Q

why is the perfect ionic model often not accurate

A

ions are not perfectly spherical
polarisation often occurs when small positive ions or large negative ions are involved, so the ionic bond gains covalent character
some lattices are not regular and the crystal structure can differ

22
Q

which kinds of bonds will be the most ionic

A

between large positive ions and small negative ions

23
Q

define the terms spontaneous and feasible

A

if a reaction is spontaneous and feasible, it will take place of its own accord; does not take account of rate of reaction

24
Q

is a reaction with a +ve or -ve enthalpy more likley to be spontaneous

A

-ve since exothermic

25
Q

define entropy

A

randomness/disorder of a system
high value entropy = more disordered

26
Q

what units is entropy

A

JK-1mol-1

27
Q

what is the second law of thermodynamics

A

entropy of an isolated system always increases as it is overhwlemingly more likely for molecules to be disordered than ordered

28
Q

is a reaction with positive or negative entropy change more likley to be spontaneous

A

+ve
reactions always try and increase the amount of disorder

29
Q

how would you calculate entropy change for a reaction

A

entropy change = sum of product entropy - sum of reactant entropy

30
Q

gibbs free energy equation

A

change in gibbs free energy = enthalpy change - (temperature x entropy change)

31
Q

what does the value for gibbs free energy

A

if G<0, reaction is feasible
if G=0, reaction is JUST feasible
if G>0, reaction is not feasible

32
Q

what is the significance of the temperature at which G=0

A

temperature at which reaction becomes feasible in Kelvin

33
Q

what are the limitations of using G as an indicator of whether a reaction will occur

A

gibbs free energy only indicates if a reaction is feasibke
it does not take into account the rate of reaction (kinetics of the reaction). in reality, many reactions that are feasible at a certain temperature have a rate of reaction that is so slow that effectively no reaction is occurring

34
Q

if the reaction is exothermic and entropy increases, what is the value of G and what does this mean

A

G is always negative, so reaction is always feasible - product favoured

35
Q

if the reaction is endothermic and entropy decreases, what is the value of G and what does this mean

A

G always positive, so reaction is never feasible , reactant favoured

36
Q

if reaction is exothermic and entropy decreases, what is the value of G

A

temperature dependent

37
Q

if the reaction is endothermic and entropy increases, what is the value of G and what does this mean

A

temperature dependent

38
Q

why is entropy zero at 0K

A

no disorder - molecules/atoms are not moving or vibrating and cannot be arranged in any other way
maximum possible state of order

39
Q

what are the two key things to look out for to decide if entropy increases/decreases/stays relatively the constant

A

number of moles - more moles made - increase in entropy - going from solid - liquid/gas or liquid - gas

40
Q

how is it possible for the temperature of a substance undergoing an endothermic reaction to stay constant

A

the heat that is given out escapes to the surroundings

Chemistry A Level 1 (19 decks)

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