HSC Chemistry Question Thread

[Fahim486]

Hey! The first question is sort of just a by-definition one; radioactivity is when elements shed electrons/helium atoms due to an instability in the nucleus. This can occur because of three reasons; One, the ratio of neutrons to protons is too high. Two, the ratio of neutrons to protons is too low. Three, there are just too many neutrons and protons. Thus, the answer is definitely D.

Question 9 is the same; the salt bridge is literally used just to complete the circuit. Remember from like year 8 science that, if you want a circuit to work, it needs to be a complete loop. Thus, the electrons might flow one way (through the external circuit), but the 'left over' electrons/ions need some way to continue to flow. Thus, we use a salt bridge soaked in a conductive substance.

For your last question, looks like you need to do some research on radioactive isotopes. The questions you need to ask yourself are;

- Is a half life of 12 days long or short? If it is long, then you don't want it in the human body (as it will continue to damage tissue for ages!). If it is short, it is likely suitable.
- What is beta decay? Is it harmful to the human body?
- What is low energy gamma radiation? Is it harmful to the human body?

As this is only a two marker, a very brief over view of the above points is likely enough. Generally, however, isotopes used for medicine have a much shorter half-life (ie. 6 hours), and will only emit very low energies of one type of radiation. So, this substance is unlikely to be suitable for medical purposes.

Thanks so much for your help!!!!

[jakesilove]

Then, look to the table. Does the fuel source provide as much energy per gram as Petrol? How about energy per litre? Will cars need to fill up with more fuel if they use a non-petrol substance? Is the substance even a liquid? How can it be stored? Will it corrode the engine? Those are the sorts of discussion points that will get you full marks.


Thanks so much
So could I say that Ethanol  and Kerosene do not provide as much energy per gram but hydrogen does. Only kerosene  releases as much and more energy  per Litre. If cars are to use these fuels, they would need to use them such as Ethanol as a petrol extender. Hydrogen is not even liquid hence will be very hard to contain and use as a fuel. When the ethanol is subjected to fractional distillation after fermentation, it will still have water in it and this will corrode the engines. Hence, Kerosene and ethanol have the most potential as alternative fuels based on their physical and chemical properties. However, Kerosene is obtained from crude oil which is a non-renewable resource, hence its supply is slowly decreasing. Hence the only alternative left is ethanol which is derived from a renewable resource (biomass) but it does have disadvantages as the engines will need modification.
I know it's a messy response but I'm just making sure my ideas are correct and then I would structure it.

Your answer is perfect! Great job!!

[bluecookie]

Hey! You generally ignore the concentration of the solution, and just look at the acid/base used.

If a strong acid and a strong base is used, then the appropriate indicator is Bromothymol Blue, which has an end point around a pH of 7

If a strong acid and a weak base is used, then the appropriate indicator is Methyl Orange, which has an end point around a pH of 4-6

If a weak acid and a strong base is used, then the appropriate indicator is Phenophthalein, which has an end point around a pH of 8-10

Thank you

[shreya_ajoshi]

Hey there,
How would you do part c) of the question. Im talking about PHB as my biopolymer.

Thanks

[jakesilove]

Hey there,
How would you do part c) of the question. Im talking about PHB as my biopolymer.

Thanks

Hey! This is just something you need to have in your notes somewhere, and will depend on the research that you have conducted. Figure out what the deficiencies in PHB are (ie. is it expensive? Time consuming to produce?). Then, all you have to do is suggest that research must be done in those areas to improve the potential of the biopolymer as a resource more widely used!

[hanaacdr]

hi
could you please go through everyday uses of soil testing on pH please
thank you

[hanaacdr]

also, in titration
is the titrant the unknown solution or the known solution?
thanks

[cookiedream]

also, in titration
is the titrant the unknown solution or the known solution?
thanks

The titrant is the solution of the known concentration.

[jakesilove]

hi
could you please go through everyday uses of soil testing on pH please
thank you

Hey! We test the pH of soil to ensure it isn't too acidic/basic. This is important, as crops and trees will only grow in soil of a certain (approximately neutral) pH. So, if we don't quickly identify acidic soil, crops will die. Additionally, plants growing in acidic soil may be acidic themselves. This will damage any animals who rely on those plants for food. Finally, soil will leach it's pH into waterways adjacent. So, we want to make sure soil is neutral else waterways become acidified, killing marine life and anything relying on the waterway

[bananna]

Hi!

Just wondering what's the difference between amphoteric and amphiprotic elements? Also, examples would be greatly appreciated !

Thank you

[jakesilove]

Hi!

Just wondering what's the difference between amphoteric and amphiprotic elements? Also, examples would be greatly appreciated !

Thank you

Hey! This is really beyond the curriculum, and confuses a lot of students. Suffice it to say that if you treat them as exactly the same, you will ALWAYS get the marks.

An amphiprotic substance is anything that can both accept and donate protons (thus, it acts as a B-L acid/base). And amphoteric substance, however, ACTS as an acid and a base without necessarily donating/accepting a proton. For instance, Aluminium trioxide will act as an acid, despite not containing any hydrogen atoms!



Again, you don't need to know this. For the purpose of the HSC, just know that these substances act as an acid and as a base.

[Gabby Mangion]

Hi. I was just going over my notes for my chem half yearly and came across the oxidation states and their rules. I was wondering if someone could please explain it to me and give an example of how to solve it in a question??

[jakesilove]

Hi. I was just going over my notes for my chem half yearly and came across the oxidation states and their rules. I was wondering if someone could please explain it to me and give an example of how to solve it in a question??

Hey Gabby! Welcome to the forums!

Oxidations states is a useful tool we can use to decide whether a substance is oxidising or reducing. There are some pretty simple rules that we need to know before we can attack any question.

1. Oxygen has an oxidation state of -2

Ever Oxygen we see will have an oxidation state of -2. So, if there are two oxygen atoms, we will say that the total state is -4!

Elements have an oxidation state of 0

This is because it is 'neutral'

The oxidation state of each element will add up to the oxidation state of the molecule

This is just sort of intuitive. Let's first look at an easy example.



Now, the initial oxidation state of Copper is 0 (as it is in elemental form). The final compound containing Copper has an overall charge of zero. However, we know that CO3 has an oxidation state of -1. Therefore, Copper must have a change of +1. So, as it has increased in oxidation state, it has oxidised.

The initial oxidation state of Silver is +1, as NO3 is -1. The final oxidation state of Silver is 0. Thus, the oxidation state has decrease, so Silver has reduced!

Let me know if you have a specific question you want us to work through, to help you better understand the theory!

[Gabby Mangion]

Hey Gabby! Welcome to the forums!

Oxidations states is a useful tool we can use to decide whether a substance is oxidising or reducing. There are some pretty simple rules that we need to know before we can attack any question.

1. Oxygen has an oxidation state of -2

Ever Oxygen we see will have an oxidation state of -2. So, if there are two oxygen atoms, we will say that the total state is -4!

Elements have an oxidation state of 0

This is because it is 'neutral'

The oxidation state of each element will add up to the oxidation state of the molecule

This is just sort of intuitive. Let's first look at an easy example.



Now, the initial oxidation state of Copper is 0 (as it is in elemental form). The final compound containing Copper has an overall charge of zero. However, we know that CO3 has an oxidation state of -1. Therefore, Copper must have a change of +1. So, as it has increased in oxidation state, it has oxidised.

The initial oxidation state of Silver is +1, as NO3 is -1. The final oxidation state of Silver is 0. Thus, the oxidation state has decrease, so Silver has reduced!

Let me know if you have a specific question you want us to work through, to help you better understand the theory!

Thank you!! I know this is probably a really simple question so please bear with me.

There is a question in a past paper I'm doing and it says (It's a multiple choice):
Zn + 2MnO2 -> ZnO + MnO3
In this state the change in oxidation state of Manganese is:

A) From O to +2
B) From +4 to +3
C) From +4 to +6
D) no change

I understand it with Zinc but sadly not Manganese.

Thanks

[jakesilove]

Thank you!! I know this is probably a really simple question so please bear with me.

There is a question in a past paper I'm doing and it says (It's a multiple choice):
Zn + 2MnO2 -> ZnO + MnO3
In this state the change in oxidation state of Manganese is:

A) From O to +2
B) From +4 to +3
C) From +4 to +6
D) no change

I understand it with Zinc but sadly not Manganese.

Thanks

No worries! So, the initial state is



Which has a net charge of zero (or they would have specified a net charge). As oxygen has a charge of -2, and the charges add up to form the net charge, we can say that



Where x is the charge on Manganese. So, the initial oxidation state of Manganese is going to be 4 (solving for x)

Now, the final state is



Again, we create a formula, noting that the net charge is zero



So, the oxidation number has INCREASED to 6. Thus, the answer is C.

If you figured out the correct answer for Zinc, you can use logic to figure out what's going on with Manganese. You would have found that zinc REDUCES (ie oxidation number decreases). Therefore, the Manganese must OXIDISE). This limits it to A and C. Finally, note that the initial condition of Manganese is not in an elemental form. Thus, it cannot have an initial oxidation state of 0. So, the answer can only be C