Thermodynamics Flashcards
Explain why methanol is easy to liquefy
As its hydrogen bonding is a strong enough force to hold methanol molecules together in a liquid
suggest why the hydration of the chloride ion is an exothermic process
as water is polar.
explain in terms of molecules why the entropy is zero when the temp is 0 kelvin
as at 0 k particles are stationary so there is no disorder
why evap of wter is spontaneous eventhough the change is endothermic (4)
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
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
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
use your knowledge of bonding to explain why delta h is positive for h20(g)–>h20(l)
positive means its endothermic so bonds broken
hydrogen bonds between water mols must be broken and energy is required to break them.
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
The heat given out escapes
when talking about why when temp goes below 0 on the graph why is that good
as delta g becomes NEGATIVE
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
Ammonia liquefies and changes in shape
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
there is no change in the number of moles of gases so entropy stays approx constant
in terms of the forces acting on particles suggest one reason why the first electron affinity of oxygen is an exothermic process
There is an attractive force between the nucleus of an O atom and an external electron
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
forms a protective layer, this layer of Mgo will prevent oxygen attacking the Mg
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)
1 mol of solid and 0.5 moles of gas reactants form 1 mol of solid products.
the system becomes more ordered.
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
the activation energy may be high
when potassium nitrate dissolves in water, write an equation for this
suggest why the entropy change for the positive
its basically breaking the solid product to its constituent ions
KNO3(s)–> k+(aq) + NO3-(aq)
increase in disorder because solid to solution
give two reasons why methanol synthesised from carbon dioxide and hydrogen may not be a carbon neutral fuel
co2 may be produced during h2 manufacture or during transport.
suggest why step F (Electron affinity) Is an endothermic process
as the negative s- ion repels the added electron (energy used)
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
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
State one change in the conditions which would both increase the rate of reaction
and decrease the value of Kp. Explain your answers
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.
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) →
reaction 1 is not reversible and therefore goes to completion
disadv= vigorous reaction
ch3cocl expensive
hcl evolved is toxic
when using a born haber cyle to work out the first electron affinity of chlorine when given the rest of the data
cycle clockwise
equal it all to 0
and make sure you do your cl value times 2 for cl2
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
dont use the negative ignore the minus and just do that/ that bc its not going to be a minus temp
why does mg2+ have a higher enthalpy of hydration value compared to Na+
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.
use data to calculate a value for the enthalpy change when one mol of magnesium chloride dissolves in water
mgcl2–> mg2+ +2cl-
and use the lattice enthalpy of formation value not enthalpy of formation because this is getting broken down
write an equation for the process that has an enthalpy change equal to the electron affinity of chlorine
cl(g)+e- –> cl-(g)
in terms of electrostatic forces suggest why the electron affinity of fluorine has the most neg value
there is an attraction between the nucleus and the added electron
exo energy is released when the electron is gained
the theoretical enthalpy of lattice disassociation for silver chloride is +770jkmol-1
explain why this value is less than the value for silver fluoride
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
suggest why the hydration of the chloride ion is an exothermic process
water is polar
chloride attracts the H in water
explain the meaning of the term perfect ionic model
ions are point charges ions are perfect spheres only electrostatic forces between ions no covalent interaction only ionic bonding
suggest 2 properties of ions that influence the value of a lattice enthalpy calculated using a perfect ionic model
ionic radius and ionic charge
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
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.
For a neg Gibbs free energy t delta s needs to be
Bigger than enthalpy change.
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
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.
why is the lattice disassociation enthalpy of magnesium chloride greater than that of calcium chloride
greater= more energy needed to break bonds
magnesium ion is smaller, stronger attraction between ions.
explain why the lattice disassociation enthalpy of magnesium oxide is greater than that of magnesium chloride.
as the oxide ion has a greater charge so it attracts the magnesium ion more strongly.
whenever asked anything ab attracting water molecules
always write water is polar
delta - on o and delta + on hydrogen
suggest why a value for the enthalpy of sol of magnesium oxide is not found in any data books
magnesium oxide reacts with water to form mag hydroxide which is sparingly soluble. or mgo does not dissolve in water
state the meaning of the theta in enthalpy change
standard pressure (100kpa) and a stated temp
pure ice can look pale blue when illuminated by white light, suggest an explanation for this
The complementary colour to blue is absorbed
red is absorbed by the ice
explain the meaning of the term perfect ionic model
ions are perfect spheres
only electrostatic attraction, assumes no covalent interactions
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,
remember, theres prob an equation see if the forward reac is exo or endo and then its about the equilib shifting
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
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
why is the electron affinity of chlorine an exothermic change
net attraction between chlorine and an extra electron
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
greater, because chlorine ions are smaller and chloride ions attract more strongly to the silver ions
theoretical vs experimental data
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.
suggest why your answer to part d is greater than the theoretical value for the enthalpy of lattice disassociation for silver chloride
As there is some covalent interaction, the forces in the lattice must be stronger than pure ionic attractions
why is the second electron affinity of s an endothermic process
as the negative s- repels the added electron so energy is required to be put in
explain why the enthalpy change for step d is larger than that for step c (2nd ie more than 1st ie)
electron is nearer to the nucleus, and electrons are now removed from a more positive species with a stronger attraction
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)
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
explain why the following reac occurs at room tempe even tho the reac is endothermic, talk about t delta s (3)
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
explain why the following reac does not occur at v high temps even though the reac is exothermic (3)
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
explain why a larger t delta s being larger than enthalpy change can result in both a negative and positive delt g
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
when writing in the enthalpy changes on a born haber cycle
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
explain why the bond enthalpy of a cl-cl bond is greater than that of a br-br bond
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
explain why the hydration enthalpy becomes less exothermic from li to k+
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
effect of a catalyst on kp
none, speeds up the RATE of both the bawckwards and forwards direction EQUALLY therefore not affecting the position of equilibrium
Explain why your answer to part (c) is different from the lattice enthalpy of
dissociation for magnesium chloride
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
Suggest two properties of ions that influence the value of a lattice enthalpy
calculated using a perfect ionic model.
(Ionic) radius / distance between ions / size
(Ionic) charge / charge density
