Difficult Questions Thread - Chemistry

[ArtyDreams]

Hi everyone!! I have made this thread to help you all in your preparations for your VCE chemistry exam. Every week or two, I will post some difficult questions from past exams for you all to have a go solving (i.e. questions <50% of students got correct.) All these questions will be taken from exams older than 2010 to help save the newer ones for closer to exams. When you answer the questions, please put your answer in a spoiler, so you don’t spoil it for anyone else

As these are from old exams, the format of the questions will vary slightly to what you will get now. However, the content of them is still applicable I'll post the answers at the end of the week, and provide feedback to any short answers!

To kick us off, here is a set of questions from 2005.

MULTIPLE CHOICE

1. Equal masses of the two gases oxygen (O2) and sulfur dioxide (SO2) are placed in separate vessels. Both vessels have the same volume and are at the same temperature. The pressure exerted by the oxygen is 100 kPa. The pressure, in kPa, exerted by the SO2 is closest to:
A. 25
B. 50
C. 100
D. 200

2. The rechargeable nickel-cadmium cell is used to power small appliances such as portable computers. When the cell is being used, the electrode reactions are represented by the following equations.
NiO2(s) + 2H2O(l) + 2e- → Ni(OH)2(s) + 2OH-(aq)
Cd(s) + 2OH-(aq) → Cd(OH)2(s) + 2e-

Which of the following occurs during the recharging of the nickel-cadmium cell?
* I  cadmium is deposited on the negative electrode 

* II  the pH of the electrolyte increases 

* III  the direction of electron flow in the external circuit is from the anode to the cathode

A. I only
B. I and II only
C. II and III only
D. I and III only

3. An identical disc is to be gold-plated with a solution containing Au3+(aq) as the electrolyte using a current of 1.50 amps.
The ratio of the time that is needed to plate the disc with 0.150 g of gold to the time needed to plate the disc with 0.150 g of silver is closest to
A. 1 to 3 

B. 1 to 1.6 

C. 1.6 to 1 

D. 3 to 1 


SHORT ANSWER


One type of breathalyser instrument used by police for the measurement of the concentration of alcohol in a driver’s breath is a fuel cell. An acidic electrolyte is used. Ethanol is oxidised to ethanoic acid at one electrode and oxygen from the air is converted to water at the other.
The overall equation for this reaction is
 C2H5OH(aq) + O2(g) → CH3COOH(aq) + H2O(l)

a. Write the equation for the half reaction at the anode. (2 marks)

b. A motorist who has consumed alcohol blows into the fuel cell. If the breath entering the cell provides alcohol at the rate of 3.0 × 10^-5 g per second, calculate the maximum current, in amps, that the cell would produce. (3 marks)


c. The nature of the electrodes in the cell is essential to the effective operation of the breathalyser. State two important functions that the electrodes must perform. (2 marks)

[Billuminati]

Spoiler

https://drive.google.com/file/d/1mgOtAiLOodf4Y0W-GZ0mQihyguc4eXCl/view?usp=drivesdk

Geez electrochem was harder than I last remembered, had to BS question c in the short answer

[wingdings2791]

Thanks heaps for this ArtyDreams

Spoiler

MULTIPLE CHOICE
1.\(n(SO_2)=\frac{1}{2}n(O_2)\) and \(PV=nRT\)
\(\therefore P(SO_2)=\frac{1}{2}P(O_2)\\ =50kPa\)(B)

2.
I: correct (\(Cd(OH)_2\) reduced to \(Cd\) at the the cathode)
II: incorrect (no net change in \([OH^-]\) so no change to pH)
III: correct (electrons flow from anode to cathode in electrolysis)
(D)

3. \(\frac{3}{M_R(Au)}:\frac{1}{M_R(Ag)}\approx1.6:1\) (C)

SHORT ANSWER
a) \(C_2H_5OH_{(aq)}+H_2O_{(l)}\rightarrow\ CH_3COOH_{(aq)}+4H^+_{(aq)}+4e^-\)
b) \(Q=It=n(e^-)\times\ F\) and \(4n(C_2H_5OH) = n(e^-)\) when \(t=1\)
\(\therefore n(e^-)=4n(C_2H_5OH)=\frac{3.0\times\ 10^{-5}g}{M_R(C_2H_5OH)}\times\ 4 \\ \therefore Q=\frac{3.0\times\ 10^{-5}g}{46.0gmol^{-1}}\times\ 4 \times\ 96500Cmol^{-1} \\ \approx0.25A\)
c)

• Provide surface area for reactants to contact catalysts
• Unreactive and prevents spontaneous reactions (directly between reactants and between electrodes and reactants)