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Chemistry A2 > Thermodynamics > Flashcards

Thermodynamics Flashcards

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1
Q

Explain why methanol is easy to liquefy

A

As its hydrogen bonding is a strong enough force to hold methanol molecules together in a liquid

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2
Q

suggest why the hydration of the chloride ion is an exothermic process

A

as water is polar.

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3
Q

explain in terms of molecules why the entropy is zero when the temp is 0 kelvin

A

as at 0 k particles are stationary so there is no disorder

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4
Q

why evap of wter is spontaneous eventhough the change is endothermic (4)

A

The molecules become more disordered / random when water changes from
a liquid to a gas / evaporates

Therefore the entropy change is positive / Entropy increases

TΔS>ΔH

ΔG<0

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5
Q

State what would happen to a sample of gaseous water that was heated to a
temperature higher than that of your answer to part (c)(i).
Give a reason for your answer.
What would happen to gaseous wate

A

higher than 59 whatever then gibbs is lower so

it would decompose into hydrogen and oxygen because the delta g for this reaction would be above 0

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6
Q

use your knowledge of bonding to explain why delta h is positive for h20(g)–>h20(l)

A

positive means its endothermic so bonds broken

hydrogen bonds between water mols must be broken and energy is required to break them.

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7
Q

the freezing of water is an exothermic process, give one reason why the temp of a sample of water can stay at a constant value of 0 degrees when it freezes

A

The heat given out escapes

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8
Q

when talking about why when temp goes below 0 on the graph why is that good

A

as delta g becomes NEGATIVE

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9
Q

the line is not drawn below a temp of 240 k because its slope changes at this point, suggest what happens to the ammonia at 240k that causes the slope of the line to change

A

Ammonia liquefies and changes in shape

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10
Q

use the equation N2+O2–> 2NO to suggest why the free energy change for the reaction to form nitrogen monoxide stays approx constant at diff temps

A

there is no change in the number of moles of gases so entropy stays approx constant

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11
Q

in terms of the forces acting on particles suggest one reason why the first electron affinity of oxygen is an exothermic process

A

There is an attractive force between the nucleus of an O atom and an external electron

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12
Q

suggest one reason why a sample of magnesium aooears to be stable in air at room temp despite the reac for the formation of Mgo with a negative value for Delta g

A

forms a protective layer, this layer of Mgo will prevent oxygen attacking the Mg

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13
Q

in terms of the reactants and products and their physical states account for the sign of the entropy change that you calculated in part I

Mg(s) +1/202 (g)—> MgO(S)

A

1 mol of solid and 0.5 moles of gas reactants form 1 mol of solid products.
the system becomes more ordered.

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14
Q

at temps below the value calculated in part e, decomposition of no into its elements should be spotnaneus. However in car exhausts this decomposition reaction does not take place in the abscence of a catalyst. suggest why this spontaneous decomp does not take place

A

the activation energy may be high

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15
Q

when potassium nitrate dissolves in water, write an equation for this

suggest why the entropy change for the positive

A

its basically breaking the solid product to its constituent ions

KNO3(s)–> k+(aq) + NO3-(aq)

increase in disorder because solid to solution

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16
Q

give two reasons why methanol synthesised from carbon dioxide and hydrogen may not be a carbon neutral fuel

A

co2 may be produced during h2 manufacture or during transport.

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17
Q

suggest why step F (Electron affinity) Is an endothermic process

A

as the negative s- ion repels the added electron (energy used)

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18
Q

using a born haber cycle a value of -905kjmol-1 was determined for the lattice enthalpy of silver chloride. a value for the lattice enthalpy of silver chloride using the ionic model was -833kjmol-1.

explain what a scientist would be able to deduce from a comparison of these values

A

the model used assumes that the ions are spherical and in a lattice
the calculated value is smaller than the cycle value indicating some covalent character

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19
Q

State one change in the conditions which would both increase the rate of reaction
and decrease the value of Kp. Explain your answers

A

temp as it would increase the amount of collisions as E>Ea but
the forward direction may be exothermic therefore equilibrium may move to the left and this decreases the value of kp.

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20
Q

Ethyl ethanoate can be prepared by the reactions shown below.
Reaction 1
CH3COOH(l) + C2H5OH(l) ->CH3COOC2H5(l) + H2O(l) ΔH = –2.0 kJ mol–1
Reaction 2
CH3COCl(l) + C2H5OH(l) →

A

reaction 1 is not reversible and therefore goes to completion
disadv= vigorous reaction
ch3cocl expensive
hcl evolved is toxic

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21
Q

when using a born haber cyle to work out the first electron affinity of chlorine when given the rest of the data

A

cycle clockwise
equal it all to 0
and make sure you do your cl value times 2 for cl2

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22
Q

when you have a negative enthalpy of formation value in the born haber cycle and your asked to use it to calculate the value for temp with gibbs free energy

A

dont use the negative ignore the minus and just do that/ that bc its not going to be a minus temp

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23
Q

why does mg2+ have a higher enthalpy of hydration value compared to Na+

A

mg2+ is a smaller ion with a higher charge density, outer electrons are attracted more strongly to the nucleus, this then attracts water mols more strongly.

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24
Q

use data to calculate a value for the enthalpy change when one mol of magnesium chloride dissolves in water

A

mgcl2–> mg2+ +2cl-

and use the lattice enthalpy of formation value not enthalpy of formation because this is getting broken down

25
Q

write an equation for the process that has an enthalpy change equal to the electron affinity of chlorine

A

cl(g)+e- –> cl-(g)

26
Q

in terms of electrostatic forces suggest why the electron affinity of fluorine has the most neg value

A

there is an attraction between the nucleus and the added electron
exo energy is released when the electron is gained

27
Q

the theoretical enthalpy of lattice disassociation for silver chloride is +770jkmol-1
explain why this value is less than the value for silver fluoride

A

this means agf more endothermic, means more energy was required to break bonds

stronger attraction between Ag+ and f-
Fluoride ion is smaller than the chloride ion
so more energy required to break bonds

28
Q

suggest why the hydration of the chloride ion is an exothermic process

A

water is polar

chloride attracts the H in water

29
Q

explain the meaning of the term perfect ionic model

A 
ions are point charges
ions are perfect spheres
only electrostatic forces between ions
no covalent interaction
only ionic bonding
30
Q

suggest 2 properties of ions that influence the value of a lattice enthalpy calculated using a perfect ionic model

A

ionic radius and ionic charge

31
Q

By describing the nature of the attractive forces involved, explain why the value for the enthalpy of hydration for the chloride ion is more negative than that for the bromide ion

A

More negative hydration= greater

Smaller than bromide ions, forces of attraction between chloride ions and water is stronger, chloride ions attract the polar H of water more strongly.

32
Q

For a neg Gibbs free energy t delta s needs to be

A

Bigger than enthalpy change.

33
Q

by descibing the nature of the attractive forces why is the enthalpy of hydration more neg for the chloride ion than for the bromide ion

A

more neg= greater

smaller than the bromide ion
force of attraction between chloride ions and water is stronger, chloride ions attract the delta + on the H of water more strongly.

34
Q

why is the lattice disassociation enthalpy of magnesium chloride greater than that of calcium chloride

A

greater= more energy needed to break bonds

magnesium ion is smaller, stronger attraction between ions.

35
Q

explain why the lattice disassociation enthalpy of magnesium oxide is greater than that of magnesium chloride.

A

as the oxide ion has a greater charge so it attracts the magnesium ion more strongly.

36
Q

whenever asked anything ab attracting water molecules

A

always write water is polar

delta - on o and delta + on hydrogen

37
Q

suggest why a value for the enthalpy of sol of magnesium oxide is not found in any data books

A

magnesium oxide reacts with water to form mag hydroxide which is sparingly soluble. or mgo does not dissolve in water

38
Q

state the meaning of the theta in enthalpy change

A

standard pressure (100kpa) and a stated temp

39
Q

pure ice can look pale blue when illuminated by white light, suggest an explanation for this

A

The complementary colour to blue is absorbed

red is absorbed by the ice

40
Q

explain the meaning of the term perfect ionic model

A

ions are perfect spheres

only electrostatic attraction, assumes no covalent interactions

41
Q

whenever being asked questions like this: predict whether raising the temp will increase, decrease or have no effect on the amount of solid calcium chloride that will dissolve in a fixed mass of water,

A

remember, theres prob an equation see if the forward reac is exo or endo and then its about the equilib shifting

42
Q

use your knowledge of the equation energy change= hv to suggest what happens to the electrons in fluorite when uv light is absorbed and when visible light is given out

A

uv absorbed= electrons are excited and move up to a higher energy level
visible light given out= electrons move back down to a lower energy level

43
Q

why is the electron affinity of chlorine an exothermic change

A

net attraction between chlorine and an extra electron

44
Q

state whether you would expect the value of the theoretical enthalpy of lattice disassociation for silver chloride to be greater than equal to or less than that for silver bromide

A

greater, because chlorine ions are smaller and chloride ions attract more strongly to the silver ions

45
Q

theoretical vs experimental data

A

when born haber (experimental) and theoretical is the same the mol is 100 percent ionic. the bigger the different the more covalent characteristics as the charge cloud is distorted.

born haber is more than theoretical if covalent.

46
Q

suggest why your answer to part d is greater than the theoretical value for the enthalpy of lattice disassociation for silver chloride

A

As there is some covalent interaction, the forces in the lattice must be stronger than pure ionic attractions

47
Q

why is the second electron affinity of s an endothermic process

A

as the negative s- repels the added electron so energy is required to be put in

48
Q

explain why the enthalpy change for step d is larger than that for step c (2nd ie more than 1st ie)

A

electron is nearer to the nucleus, and electrons are now removed from a more positive species with a stronger attraction

49
Q

using a born haber cycle, a value of -905 was determined for the lattice enthalpy of silver chloride. a value for the lattice enthalpy of silver chloride in the ionic model was -833.
explain what a scientist would be able to deduce from a comparison of these values (3)

A

The model used assumes the ions are spherical and in a lattice
The calculated value is smaller than the cycle value Indicating some covalent character

50
Q

explain why the following reac occurs at room tempe even tho the reac is endothermic, talk about t delta s (3)

A

for a reac to occur gibss <0
delta s is positive and large as a gas is evolved
t delta s is larger than enthalpy change and delta g is neg

51
Q

explain why the following reac does not occur at v high temps even though the reac is exothermic (3)

A

delta s is negative
less moles of gaseous product evolved
at hight temps, t delta s is larger than enthalpy change and delta g is positive

52
Q

explain why a larger t delta s being larger than enthalpy change can result in both a negative and positive delt g

A

as temp increases, t delta s will get bigger. when delta s is positive a bigger t delta s will be made making delta g more neg

as temp increases when delta s is negative, it will make delta g more positive. this is why these are preferred at low temps

53
Q

when writing in the enthalpy changes on a born haber cycle

A

just write atomisation of magnesium, electron affinity of chlorine etc dont worry about cl2 etc. and the last arrow going down put- lattice formation enthalpy of mgcl2 for eg

54
Q

explain why the bond enthalpy of a cl-cl bond is greater than that of a br-br bond

A

bond pair is closer to the nucleus
stronger attraction between the nucleus and the bond pair

cl smaller atom, stronger attrac between nucleus and electron

55
Q

explain why the hydration enthalpy becomes less exothermic from li to k+

A

ions get bigger, charge density increases
electrostatic attraction between positive metal ion and delta neg of the oxygen decreases, so doesnt attract water as strongly

56
Q

effect of a catalyst on kp

A

none, speeds up the RATE of both the bawckwards and forwards direction EQUALLY therefore not affecting the position of equilibrium

57
Q

Explain why your answer to part (c) is different from the lattice enthalpy of
dissociation for magnesium chloride

A

magnesium is smaller than the calcium ion
so it attracts the chloride more strongly stronger ionic bonding

not talking ab hydration, talking ab lattice enthalpy so how strong the bonds are

58
Q

Suggest two properties of ions that influence the value of a lattice enthalpy
calculated using a perfect ionic model.

A

(Ionic) radius / distance between ions / size

(Ionic) charge / charge density

Chemistry A2 (13 decks)

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