FSN Chemistry Unit 4 Exam - Answers (By Collin)

[Ahmad]

Collin's answers. Ain't he a gun?

SECTION A - Multiple-choice questions

Question 1
CrO3 => +6
Cu2S => +1 (The sulfide ion is -2)
MnCl2 => +2
K2Cr2O7 => +6
The answer is B.

Question 2
The reaction is a fusion reaction involving species smaller than iron-56, so: I. "The reaction is endothermic" is incorrect.
Energy is released, and according to E=mc^2, mass is lost to compensate for the release in energy, so: II. "The mass of the carbon-12 nucleus is greater than the combined masses of the reactants" is incorrect.
The answer is D.

Question 3
Let a be the proportion of rhenium-187:
185(1-a) + 187a = 186.2
=> 2a = 1.2
=> a = 0.6 = 60%
The answer is C.

Question 4
SO3(g) + H2O(l) --> H2SO4(aq)
The salt will be Na2SO4, while the oxide ion and protons have fun neutralising each other.
The answer is D.

Question 5
Ligands must have a spare electron pair, or be negatively charged, to be attracted to the central metal cation:
Set A contains H+: incorrect
Set B appears correct
Set C contains Na+: incorrect
Set D contains CH4: incorrect (no free electron pair, all are participating in C-H bonds)
The answer is B.

Question 6
Transition metal compounds tend to exhibit colour: barium, aluminium, potassium and sodium are not transition metals, but manganese (Mn) is.
The answer is D.

Question 7
Enzymes are class of proteins that are known for their catalytic ability in biological reactions. Statements I and II are correct. III is incorrect, a catalyst does not affect the equilibrium constant at all.
The answer is A.

Question 8
Antioxidants sacrifically oxidise in reaction with oxidants in defence of foods that would otherwise become rancid. In order to sacrificially oxidise, it must be a good reductant. (answer B or D)
A key feature of antioxidants is the number of OH groups they possess, so antioxidants are also soluble in water.
The answer is D.

Question 9
The compound is not a protein, because there is no nitrogen. This means it is a carbohydrate or fat, which consist of hydrogen only:
=> Hydrogen: 100 - 76.2 - 11.3 = 12.5%
n(C) : n(H) : n(O) = 76.2/12 : 12.5/1 : 11.3/16
= 6.35 : 12.5 : 0.706
= 9 : 18 : 1
It is most likely a fat of the molecular formula: C18H36O2. Carbohydrates have a much higher oxygen content.
The answer is A.

Question 10
In process I, NH3 accepts a proton, it acts as a base.
In process II, NH4+ is oxidised from an oxidation state of -3 to +5.
The answer is D.

Question 11
(+) Half cell I | Half cell II (-) => B2+ is oxidising while A2+ is reducing
(+) Half cell II | Half cell III (-) => C is oxidising while B3+ is reducing
While B3+ is a stronger oxidant than C+, A2+ is a stronger oxidant than B3+ (reduces more readily). Therefore A2+ would be a stronger oxidant than C+ too.
The answer is A.

Question 12
The positive section is the anode, where the oxidation of Cl- occurs. X is chlorine.
The negative section is the cathode, where the reduction of H2O occurs. Y is hydrogen (see electrochemical series).
The answer is C.

Question 13
The porous diaphgram allows the movement of ions (Na+ particularly) so that NaOH can be formed.
The answer is B.

Question 14
The highly concentrated salt solution exceeds the standard conditions of 1M, thus creating non-standard conditions that cause the order of the chlorine and water reaction to swap, allowing chlorine to be produced in preference to oxygen gas.
The answer is B.

Question 15
While generating electricity, M acts as the anode. Therefore, a reduction is occurring in the nickel half-cell, with NiO(OH) being reduced, thus acting as an oxidant.
The answer is A.

Question 16
While being recharged, the positive terminal is the anode where oxidation occurs. In the forward reaction, NiO(OH) acts as the oxidant and is reduced to Ni(OH)2. Therefore in the reverse reaction, Ni(OH)2 must be oxidised instead.
The answer is C.

Question 17
The answer must be C or D, as they are the only options that show oxidation reactions at the anode.
The reductant is methanol, as it becomes oxidised at the anode (as suggested by options C or D, and by commonsense that oxygen is an oxidant).
The answer is C.

Question 18
The calibration factor = energy input / change in temperature. Energy input is Q*V, while the change in temperature is ∆T1. (Typically, we use E = VIt, but since Q = It, we can say E = V*Q)
The answer is D.

Question 19
∆H = - (CF * ∆T2) * 2/n
The factor (2/n) was to adjust the energy output (CF * ∆T2) into per mol. There are 2 moles of butane per mole of reaction, and n moles of butane was combusted, so this is a compensation factor.
VCAA forgot the negative sign!
The answer is B.

Question 20
For a fixed amount of electrical output, we would require the highest amount of chemical energy in coal and oxygen, as this is the first step in the process, and a lot of the energy is lost as heat.
The key is identifying which steps are the earliest in the power production process. The generation of steam occurs next, and there will be more energy in the steam then there will be from the turbine spinning, because not all of the thermal energy of steam will be transferred to the turbine. (I < III < II)
The answer is C.

Since this is the last MC question, I'll digress: this question takes a while to digest. Here's an alternative, more risky route for those who couldn't comprehend it. From an earlier post:

Heh, answer is C without even reading the question. Options A and C indicate II is certainly the last one. Options B and C indicate I is certainly the first one. C is the intersection. (edit: you can even reinforce this further by using this reasoning on the middle one, it's certainly III by options C and D, C is the ultimate intersection!)

An illustrative example (this is a fitting final MC, because it is confusing):
Suppose there is a 1000 units of chemical energy. 400 units of this becomes thermal energy, while 600 units is lost as heat. Then, 300 units of the remaining 400 units is converted into the mechanical energy of the turbine. 100 units must be lost from heat. We are producing a fixed amount of electricity (300 units), where the required units of input vary (the earlier steps require more input to compensate for the certain loss of energy from heat).

END OF SECTION A

SECTION B - Short answer questions

Question 1
i. Mg
ii. U
iii. Al
iv. B
v. O
vi. Sn

Question 2
a)
Any two of these three features:
- Elements with similar chemical properties were placed in the same "group."
- There were spaces where Mendeleev predicted "missing elements."
- The table was ordered by increasing atomic mass.

b)
i. Mass spectrometer
ii. Protons: 113, Electrons: 113, Neutrons: 171
iii. Group: III, Period 7
iv. Tl
v. Fr is in group I, so it has a weaker nuclear core charge than Uut, which means the electrons in Uut are more tightly bound to the nucleus, hence the atomic radius of Uut is smaller than Fr.
vi. Al is in period 3 and since it has the same nuclear core charge as Uut, it has a stronger attraction on the fewer number of electrons it has compared to the electrons on Uut, hence electrons on Uut are easier to ionise.
vii.

Code: [Select]

283         4     279
   Uut --->  He +    Rg
113         2     111

Question 3
a)
i. [He]2s22p2
ii. [Ar]3d64s2
iii. [Ar]3d6
(These should be superscript instead)

b)
Group: VI, Period 5

Question 4
a)
i. C6H12O6
ii. C12H22O11 + 12O2 ---> 12CO2 + 11H2O
iii. ΔH = -2816*2 = -5632 kJ/mol. 1 mole of maltose can hydrolyse into 2 moles of glucose.

b)
i. C15H29COOH => 30 hydrogen atoms
ii.

Code: [Select]

 H
  |
H-C-O-H
  |
H-C-O-H
  |
H-C-O-H
  |
  H

c)
i. CH3OH + C17H35COOH ---> H2O + C17H35COOCH3
ii. C17H35[COO]CH3 (in square brackets)

d)
Lecithin has a non-polar half, and polar half, which allows lecithin to interact with both water and oil, and can create oil in water suspensions, and vice versa.

Question 5
a)
i.

Code: [Select]

   CH3
    |
H2N-C-COOH
    |
    H
ii.

Code: [Select]

  H
   |
HO-C=O
   |
 H-C-H
   |
   N-H
   |
 O=C
   |
 H-C-CH3
   |
   NH2

b)
i. Circle any one of the many:

Code: [Select]

  |
   N-H
   |
 O=C
   |
ii. Z2 (-CH2CH2CH2CH2NH2)
iii. Z1 (-CH2SH)
iv. Enzymes are sensitive to pH and temperature. Under different pH conditions, it's shape can change and it can denature.
v. Urea:

Code: [Select]

H   H
 \ /
  N
  |
  C=O
  |
  N
 / \
H   H

Question 6
a)
i. E = (100-18.5)[550*4.18 + 150*0.900] = 198*10^3 J = 198 kJ
ii. n(C2H5OH) = 198/1364 = 0.145 mol
=> m(C2H5OH) = 0.145 * 46.0 = 6.69 g
iii. mass = (1/0.35) * 6.69 = 19.1 g

b)
n(C2H5OH) = 10.0 / 46.0 = 0.217 mol => E = 0.217 * 1364 = 296 kJ
n(C4H10) = 6.00 / 58.0 = 0.103 mol
=> ΔH = -296*2 / 0.103 = -5730 kJ/mol
(2 moles of butane per mole of reaction)

Question 7
a)
i. Platinum
ii. temperature: 25?C, pH = 0

b) Cadmium
An increase in pH indicates H+ has been consumed. This means the forward reaction involves the reduction of H+(aq) into H2(g). Thus, the stronger reductant out of H2(g) and Cd(s) is cadmium.

c)
i. n(X2+) = n(initial) - n(final) = 0.100*(1.00 - 0.725) = 0.0275 mol
ii. Q = 2654 => n(e-) = 2654/96500 = 0.0275 mol
=> n(X2+) = n(e-) = 0.0275 mol
=> n(X2+):n(e-) = 1:1
iii. X3+
iv. X2+ ---> X3+ + e-

Question 8
a)
cathode: Cu2+(aq) + 2e- ---> Cu(s)

b)
Q = 1.62*581 = 941 C
n(Cu) = 0.306/63.6 = 4.81*10^-3 mol

c)
n(e-) = 2*n(Cu) = 9.62*10^-3 mol
=> F = Q/n(e-) = 941 / (9.62*10^-3) = 97800 C/mol

d)
Any of the following:
- Only a fraction of the extra copper was weighed (due to flaking).
- Inefficient cell and/or resistance in wires causing a lower n(e-) than expected.

END OF EXAM

[Khangfu]

20 was wrong. Its a trick question. It basically ask for the efficiency, thus coal will be least efficient because so much of its energy is lost ( input/output) . the turbine will be highest( Keep in mind this was conversion to eletrical energy, not from thermal---> mechanical)

Edit: wait shit. Did i circle the wrong option O.o ignore me delete this post XD Let em check exam again :/

Edit: no wait its definitely D

[esiarp]

No you're wrong it's C

It asks for "The amount of energy in each of these forms" "From Lowest to Highest" Given that no extra energy is given into the system.

It does not "basically ask for the efficiency"

Obviously Coal has the most as it fuels the energy in the other forms and then some more energy from it is lost as excess heat, then logically the amount of steam energy >mechanical as steam "fuels" the mechanical... duh It is C

[Ahmad]

Updated. Section B is out. Well done Collin!

[Collin Li]

20 was wrong. Its a trick question. It basically ask for the efficiency, thus coal will be least efficient because so much of its energy is lost ( input/output) . the turbine will be highest( Keep in mind this was conversion to eletrical energy, not from thermal---> mechanical)

Edit: wait shit. Did i circle the wrong option O.o ignore me delete this post XD Let em check exam again :/

Edit: no wait its definitely D

Yeah, I disagree with your interpretation. The question is hard to understand, but it does mean something clear.

[ninwa]

Thank you so much for that coblin sigh, I'm shattered, I worked way too hard for this exam to lose marks because I made idiotic mistakes :cry:

Oh well, at least now I can make a realistic prediction of how I went and so won't be disappointed on December 17

[Khangfu]

Actually it does ask for efficiency. It says the amount fi each, not how much each one contributes.

Let me elaborate. The energy converted from turbine to electricity is highest. Nearly all of it is converted. Hence most of the energy ni turbine is converted. Coal on the otherhand most fo energy in it is lost. Hence not amount of energy in coal is converted to electricity.

Look at it this way. Nearly ALL of the energy spinning turbine is converted to electricity. Hence it has the greatest amount of its energy converted. if that makes any sense. The coal most of its energy is lost as heat. NOT ALL ENERGY IN COAL GOES TO MAKE ELECTRICITY!

[bilgia]

gosh i made heaps of little errors in section B....lost like 10marks there..guess that leaves me hopefully with a mid A...
with a B+ midyr exam score, A+ sacs and A here what ss is possible

[Khangfu]

OK i got a clear explanation to 20. It asks for how much of each step converts to electricity. Hence coal is least. NOT ALL OF COAL GOT TURNED INTO ELECTRICITY. However most of the energy from spinning turbine did . There we go =D

Edit: Omg my english so bad. Manual wipe after vce XD

[Collin Li]

OK i got a clear explanation to 20. It asks for how much of each step converts to electricity. Hence coal is least. NOT ALL OF COAL GOT TURNED INTO ELECTRICITY. However most of the energy from spinning turbine did . There we go =D

Edit: Omg my english so bad. Manual wipe after vce XD

The amount of energy in each of these forms that take part in the generation of a fixed quantity of electricity is, from lowest to highest

Let's say we wanted to create 300 units of electricity: this is the fixed quantity we require. We need much more coal for that, than we do for steam energy, than we do for mechanical energy.

It doesn't ask how much of a "fixed amount of energy" in each of these forms makes a portion of a final electricity output, instead it says we are to produce a fixed quantity of electricity (i.e.: 300 units), so we must find the order: highest to lowest of energy units in terms of other energy forms that will result in this output.

[Ahmad]

I have read the question a few times (found the wording a bit tricky) but came to the same conclusion as Collin. I also agree with all his answers posted above.

[ninwa]

Yeah, I agree with coblin, and so does my chem teacher.

"The amount of energy in each of these forms that take part in the generation of a fixed quantity of electricity"

By "take part" I'm assuming they mean being involved in the energy conversions which eventually end up as electrical energy. Almost all of the energy obtained from burning the coal would be converted to thermal energy in steam. A greater amount of this thermal energy is converted to mechanical energy, than the amount of mechanical energy converted to electrical energy.

If that makes sense.

If only I saw it this clearly in the exam it seems so simple now that I'm not under stress and pressure

[Khangfu]

Amount of energy in coal----> many transformation---> electricity. Least amount fo energy from coal
Amount of energy from steam---> mechanical ---> electrical not alot fo energy from steam to electrical
Mechanical --> electrical.

 The question asked for the amount of energy in each form converted to electrical. Not how much energy was in the coal initially, in each form... so the chemical energy, a lot of it will be lost and yeah. Really annoying. I know what your saying, that coal has most energy, however most of this energy is lost as waste heat. Not all is converted to electricity. They key is to identify how much of each energy was converted to electricity so basically efficiency.

[AkirA]

Question 20 is wrong - it's C, not D. I interpreted it a different way.

[Collin Li]

Amount of energy in coal----> many transformation---> electricity. Least amount fo energy from coal
Amount of energy from steam---> mechanical ---> electrical not alot fo energy from steam to electrical
Mechanical --> electrical.

 The question asked for the amount of energy in each form converted to electrical. Not how much energy was in the coal initially, in each form... so the chemical energy, a lot of it will be lost and yeah. Really annoying. I know what your saying, that coal has most energy, however most of this energy is lost as waste heat. Not all is converted to electricity. They key is to identify how much of each energy was converted to electricity so basically efficiency.

There's no way your interpretation makes sense, since the question says:
"The amount of energy in each of these forms that take part in the generation of a fixed quantity of electricity is, from lowest to highest."

For a fixed quantity of electricity, the amount of energy is 100% from the coal, steam and the turbine, it's just that the coal, steam and turbine also produced other forms of energy.

We actually need to find out, for a fixed quantity of electricity, how much input do we need of each form of energy so that we will get that same fixed quantity of electricity. This then answers the question: which forms of energy require the most "energy input" in order to yield the fixed quantity of electricity we are supposed to produce.