HSC Chemistry Question Thread

[cnimm2000]

Hey guys,
I was struggling to do this 2010 HSC Question:
HSC Question:
In which of the following reactions does he metal atom show the greatest change in oxidation state?
a)   MnO4- to Mn2+
b)   MnO2 to Mn(OH)3
c)   PbO2 to PbSO4
d)   VO2+ to VO2+
Could someone please explain it. I know the rules and stuff but i get confused on how to find the oxidation states for particular compounds such PbSO4.

[itssona]

Hey guys,
I was struggling to do this 2010 HSC Question:
HSC Question:
In which of the following reactions does he metal atom show the greatest change in oxidation state?
a)   MnO4- to Mn2+
b)   MnO2 to Mn(OH)3
c)   PbO2 to PbSO4
d)   VO2+ to VO2+
Could someone please explain it. I know the rules and stuff but i get confused on how to find the oxidation states for particular compounds such PbSO4.

sure so we would need to find the oxidation numbers and most important thing to remember is that the sum of the oxidation states of all the atoms or ions in a neutral compound is zero.

So to find PbO2 oxidation state, we first know that O has an oxidation number of -2. Since we have 2 O atoms, the total oxidation state for oxygen is 2 x -2 which is -4. To make the element sum add to zero, Pb has to have an ox number of 4.

Similarly for PbSO4. We see that the polyatomic ion SO4 is there and it has a charge of 2-. So that means Pb is 2.

Does this help?

[itssona]

hey guys I was looking at this question
excess HCL is added to 5g Copper. What is the volume of gas emitted at 0 degrees C and 100kPA.

so do I find the moles and multiply by constant? In the answer, they found the moles of copper but how is that the same as the moles for HYDROGEN?

[vcestressed]

Hello there,
so basically oxidation is loss of electrons. I got the answer A, because it has a difference of oxidation number of -5. If someone finds any mistake with my working out, please let me know. Try to find out the difference in oxidation numbers for c and d by yourself.
(Attached a picture for a and b). 

[MisterNeo]

hey guys I was looking at this question
excess HCL is added to 5g Copper. What is the volume of gas emitted at 0 degrees C and 100kPA.

so do I find the moles and multiply by constant? In the answer, they found the moles of copper but how is that the same as the moles for HYDROGEN?

The moles of copper are of a 1:1 ratio to the moles of hydrogen. The answers probably just assumed that the reader would know it’s a 1:1 ratio even though you should show that ratio step in an exam.

[itssona]

Hi so I was looking at this question : Q. Use Le Chatelier’s Principle to explain why carbonated soft drinks are often served cold.

CO2 + H2 --> H2CO3

I understand that if temperature decreases, the equation would move forwards so that products are favoured to produce more heat.
The answer said that that is correct but also said that : as a result the concentration of the H2CO3 would increase and concentrations of the CO2 and H2O would decrease. (How the freak do we know that??)

This change would also effect reaction (2)
H2CO3 + H2O --> H3O+ + HCO3 -
 as the concentration of the H2CO3 has increased. According to Le Chatelier’s principle the system will want to reduce the concentration and this can be achieved by favouring the forward reaction. This will increase the concentration of H3O+ and HCO3-. H3O+ gives the favourable acidic taste and hence a cooler temperature is desired.

I dont understand how they know about the concentration.. any input is greatly appreciated

[Lear]

Concentration is basically how much of something there is in a certain amount of space. Now, if the reaction moves forward it is obvious that the products are favoured over the reactants meaning more products are formed (which uses up reactants). Therefore there exists MORE h3o+ and HCO3 now.

Since there is more in the same amount of space, their concentrations have increased.
Likewise, we have used up more H2CO3 and there is less of H2CO3 in the space than before = decrease in concentration.

Feel free to correct me. This is just my understanding of what I’ve read on Le chatelier so far.

[itssona]

Concentration is basically how much of something there is in a certain amount of space. Now, if the reaction moves forward it is obvious that the products are favoured over the reactants meaning more products are formed (which uses up reactants). Therefore there exists MORE h3o+ and HCO3 now.

Since there is more in the same amount of space, their concentrations have increased.
Likewise, we have used up more H2CO3 and there is less of H2CO3 in the space than before = decrease in concentration.

Feel free to correct me. This is just my understanding of what I’ve read on Le chatelier so far.

ohh this makes sense!! seems right, thank you so much!!!!

[RuiAce]

Hi
I was going through the 2009 HSC chemistry  paper:
An electrochemical cell is constructed using two half cells. One half cell consists of
an inert platinum electrode and a solution of Fe2+ and Fe3+. The other half cell consists
of a lead electrode and a solution of Pb2+.
Identify an appropriate electrolyte to use in the salt bridge.
How would we know if what electrolyte is appropriate?

For any such cell, a suitable electrolyte for the salt bridge should be anything that will not be reactive. The salt bridge should only facilitate the transfer of ions, and not affect the chemistry of the cell itself.

So one that will work in every case is KNO₃, because both the K⁺ and NO₃ ions are very unreactive.

[itssona]

how do I account for the ionisation rate?
Calculate the pH of 0.05 mol L-1 citric acid if it has an ionisation rate of 50%

[RuiAce]

how do I account for the ionisation rate?
Calculate the pH of 0.05 mol L-1 citric acid if it has an ionisation rate of 50%

Recall: [something] = C_something, i.e. it's concentration.
I just used H⁺ for convenience; of course it should actually be H₃O⁺

[rozz.godly]

Hi, I was wondering if we need to know the process of *thermal (steam) cracking* and *catalytic cracking* from the production of materials topic.
Thanks

[cnimm2000]

Hi, I was wondering if we need to know the process of *thermal (steam) cracking* and *catalytic cracking* from the production of materials topic.
Thanks

Hi,
I would definitely know both as they are the two methods used to break long-chained hydrocarbons into smaller chains. i would also recommend a chemical equation as an example for both of these methods.
Here is a summary for both to help you

Catalytic Cracking (carried out in absence of air and pressures somewhat above atmospheric):
The heavy crude oil is heated in the presence of a catalyst composed of a zeolite (crystalline aluminium silicate).
Hydrogen atoms are removed from the hydrocarbons onto the catalyst’s surface. The positive hydrocarbon ion that forms undergoes further reactions leading to the formation of the final products: one alkane of shorter chain length (used for petrol) and one small alkene, until the desired product such as ethylene is created.

Thermal cracking:
2.   Thermal cracking (in the Naphtha- fraction)
It is a non-catalytic process in which a mixture of alkanes with steam passes through very hot metal tubes and at just above atmospheric pressure. Long pyrolysis coils are used to heat the hydrocarbon feedstocks and steam to about 750–900C inside the furnace
There are three main steps in the process:
a)   Initiation:
Free radicals are produced when the hydrocarbon chains are split into fragments at high temperatures. These radicals are reactive, as they have an unpaired electron.
E.g. Decane is split into two carbon chains: 2 penty radicals (C5H11•)
C10H22 --> 2C5H11•
b)   Propagation:
The free radicals decompose to produce smaller free radicals and release alkenes such as ethylene.
E.g. Each penty radical decomposes into a propyl radical (C3H7•) and ethylene
C5H11• --> C3H7• + C6H14
c)   Termination:
Hydrocarbon molecules can reform when free radicals react with other free radicals. These hydrocarbons can be recycled for further cracking
E.g. The two propyl radicals formed in the propagation process undergo termination to form hexane and this hydrocarbon can be recycled for further cracking
2C3H7• --> C6H14
For thermal cracking its best to create a flowchart to help you to remember the process

[cnimm2000]

Hi guys
For this dot point:
T.2.4 Identify factors which can affect the equilibrium in a reversible reaction
Im struggling to understand what the effect of pressure is
i know that an increase in pressure will favour the side of the fewer moles but i dont understand why?
Also what happens if there isa decrease in temperature
Really appreciate the help

[kiwiberry]

Hi guys
For this dot point:
T.2.4 Identify factors which can affect the equilibrium in a reversible reaction
Im struggling to understand what the effect of pressure is
i know that an increase in pressure will favour the side of the fewer moles but i dont understand why?
Also what happens if there isa decrease in temperature
Really appreciate the help

Hey!! Pressure is caused by gas molecules hitting the side of the container - so the more molecules in the container, the higher the pressure will be. So, when there is an increase in pressure, equilibrium will shift to favour the side that reduces the pressure (by Le Chatelier). This will be the side that produces fewer moles, as this will reduce the number of molecules in the reaction vessel, thus reducing pressure.

When there is a decrease in temperature, equilibrium will shift to favour the side which increases the temperature (by Le Chatelier) - this will be the exothermic reaction. If the equation has an exothermic forward reaction, it will shift right; if it has an endothermic forward reaction, it will shift left.

Let me know if any of that doesn't make sense!!