HSC Chemistry Question Thread

[Natasha.97]

heey to calculate the potential E, do we subtract the cathode metal from the anode always?

Hi!

I'll use the previous galvanic cell as an example (Zinc and Copper).

The reaction at the Cu cathode is Cu²⁺ + 2e^- --> Cu, with a cell potential of +0.34V. For the Zn anode, reverse the standard reduction equation and cell potential on the data sheet: Zn --> Zn²⁺ + 2e^-, with a cell potential of +0.76V (was -0.76V). Then add the two cell potentials to get 1.1V.

If we calculated it using your method, it would result in the same potential, but I find that it is more beneficial to go through the "longer" method of thinking about what is happening at each electrode and why we reverse the reduction equation for the anode (in case a really tricky question comes up in the HSC).


Hope this helps

[itssona]

does alpha decay occur when the nucleus is too large? what about beta decay - unstable nucleus or too many neutrons?
thank you

[RuiAce]

does alpha decay occur when the nucleus is too large? what about beta decay - unstable nucleus or too many neutrons?
thank you

Yeah. I don't believe this is "literally" what goes on but you can think of things this way for some understanding.

When your nucleus is just too large, you just want to get rid of a bit of everything. Because if you only get rid of one thing, you're gonna do weird things to the neutron:proton ratio. So in alpha decay, you just eject a whole helium nucleus (2 protons, 2 neutrons) to make life easier.

But when your n:p ratio is what's too large, you would want to have less neutrons and more protons to balance things out. In beta decay, an electron is ejected from the nucleus. That electron is pretty much being ejected from a neutron, which decays into a proton. So what have you really done? Turned a neutron to a proton, which makes the n:p ratio smaller.

In both cases, the nucleus could be said to be unstable. But the instability is caused by different factors.

[shellmekler]

Hi,

I need to know the properties, growth conditions and recent developments which led to making PHBV - please any help would be much appreciated.
Also, the formula for PHB !

Thanks in advance,
Shelley

[itssona]

Hi,

I need to know the properties, growth conditions and recent developments which led to making PHBV - please any help would be much appreciated.
Also, the formula for PHB !

Thanks in advance,
Shelley

Biopol (PHBV) is actually a co-polymer of two other biopolymers, polyhydroxybutyrate and polyhydroxyvalerate.

Its development began after the development of PHB (since PHB makes PHBV)
During 1923-1927, Maurice Lemoigne researched about using bacteria to produce polyesters, and discovered PHB. But it was forgotten since need for this biopolymer at the time was unneccesary. In 1957, PHB was rediscovered in Britain and USA. The mass production of PHB began after the 1990's because of the growing need of a environmentally friendly plastic alternative.

It is produced by the bacteria Alcaligenes Eutrophus. When this bacteria is placed in conditions with an excess of carbon (glucose)  and the deficiency of another nutrient (usually nitrogen) . Under these conditions the organism is no longer able to increase its population but instead produces PHBV as a storage reservoir of the carbon. PHBV is produced in cytoplasmic granules (30-80% dry weight). The biopolymer is then purified by dissolving it in a chlorinated hydrocarbon (such as trichloromethane), centrifuging to remove solid waste and precipitating the final product solution and drying it to form powder.

Biopol has similar properties to common synthetic plastics; thermoplastic, elastomeric, insoluble to water, strong and resistant to chemical attack. It also has the added advantage of being biodegradable. This allows it to be used as disposable nappies, wrapping film and plastic bags.

Its applications within the medical industry have been comparatively successful, as the nontoxic and biodegradable nature of PHB removes the need for follow-up surgeries to remove medical sutures, which will now decompose over time.

However, in general, such success is constrained by its low usage. However continued research is required to overcome the economic disadvantages and improve the properties of Biopol, extending its potential for usage. Certainly, its impact upon the environment will be significant.


What is the relationship between dissolved oxygen and biochemical oxygen demand?

Mod edit: Merged posts

[Natasha.97]

What is the relationship between dissolved oxygen and biochemical oxygen demand?

Hi!

Let's start off with some definitions:
- Dissolved Oxygen (DO): Pretty self explanatory (amount of oxygen dissolved in water)
- Biochemical Oxygen Demand (BOD): The amount of oxygen consumed by microorganisms which break down organic waste

These indicators of water quality are inversely related. Take the following two scenarios for example:

(1) If there is a large amount of organic waste, microorganisms will consume more oxygen to break it down. This means that the BOD is high. At the same time, as the microorganisms are depleting the supply of oxygen in the water, the DO level decreases.

(2) If there is a low level of organic waste, microorganisms will not consume as much oxygen (BOD is low). Thus, the level of DO increases.

Hope this helps

[itssona]

Hi!

Let's start off with some definitions:
- Dissolved Oxygen (DO): Pretty self explanatory (amount of oxygen dissolved in water)
- Biochemical Oxygen Demand (BOD): The amount of oxygen consumed by microorganisms which break down organic waste

These indicators of water quality are inversely related. Take the following two scenarios for example:

(1) If there is a large amount of organic waste, microorganisms will consume more oxygen to break it down. This means that the BOD is high. At the same time, as the microorganisms are depleting the supply of oxygen in the water, the DO level decreases.

(2) If there is a low level of organic waste, microorganisms will not consume as much oxygen (BOD is low). Thus, the level of DO increases.

Hope this helps

that was a really nice explanation, thank you so much jess :3

[cnimm2000]

Hey guys
I wanted to know if i answered the dot point correctly:
T.1.2 Identify that ethylene, because of the high reactivity of its double bond, is readily transformed into many useful products

Alkenes are unsaturated thereby giving the possibility of further atoms or molecules joining the existing hydrocarbon chain. Alkenes e.g. CH2=CH2 (ethylene) are more reactive molecules than alkanes due to the high electron density of the double bond thus attracting electronegative species such as halogens to readily react with it. The presence of double bond allows ethylene to readily undergo addition reactions by opening out the double bond to form two single bonds.
Therefore because of the high reactivity of the double bond of ethylene, it means that it can be easily converted into a range of very useful products and the starting materials for several important plastics (polymers).

Also would we need to know the reactions of ethylene to make other things such as ethanol

[RuiAce]

Also would we need to know the reactions of ethylene to make other things such as ethanol

That one - absolutely.

However, on top of it:
- Being able to write the equations for the polymerisation of ethene to polyethylene is looked highly upon.
- Most of the other ones can be worked out on the spot so long as you know the ethanol one (e.g. ethene -> ethane, bromination)

[cnimm2000]

Hey guys,
If you were asked to describe the dehydration of ethanol is this all you have to know?

Ethylene is made from ethanol by dehydration. Dehydration is a chemical reaction in which water is removed from a compound.
Ethanol is dehydrated by heating it with concentrated sulfuric acid which acts as a catalyst and a dehydrating agent and is used to remove water from the ethanol molecule.

[RuiAce]

Hey guys,
If you were asked to describe the dehydration of ethanol is this all you have to know?

Ethylene is made from ethanol by dehydration. Dehydration is a chemical reaction in which water is removed from a compound.
Ethanol is dehydrated by heating it with concentrated sulfuric acid which acts as a catalyst and a dehydrating agent and is used to remove water from the ethanol molecule.

When there’s chemistry involved, you should always specify the relevant chemical equation as well. Also, I’m not too keen on the idea of water “being removed”, because it’s not a reactant. Rather, water is just a product in the reaction.

Fairly sure your catalyst is correct and nothing else important is missing.

[cnimm2000]

Are we required to know both of the process of hydration:
- Indirect hydration
- Catalytic hydration
If a question popped up in an exam would we also be required to write both?
Also i wanted to clarify if I understood both processes correctly:
Indirect:
The reaction is facilitated by dilute sulfuric acid which acts as a catalyst.
The reaction of ethylene to produce ethanol occurs in two steps.
1.   Ethylene is reacted with 98% sulfuric acid in absorption columns held at 55-80C and 1-3.5 MPa pressure to produce ethyl hydrogen sulfate.
2.   The ethyl hydrogen sulfate is then reacted with water until the sulfuric acid concentration is about 50% and then heated to 70-100C. This generates ethanol, the sulfuric acid is reformed. 
The ethanol that forms is distilled off and has a concentration of 70% (v/v)

Catalytic:
Alternative method includes passing ethylene and steam over a silica gel or zeolite surface impregnated with phosphoric acid at 300C and at high pressure. The O—H bond of the water breaks, and a H atom and an OH group add across the double bond, causing it to break. The final product is ethanol and the double bond has been destroyed.

[itssona]

Are we required to know both of the process of hydration:
- Indirect hydration
- Catalytic hydration
If a question popped up in an exam would we also be required to write both?
Also i wanted to clarify if I understood both processes correctly:
Indirect:
The reaction is facilitated by dilute sulfuric acid which acts as a catalyst.
The reaction of ethylene to produce ethanol occurs in two steps.
1.   Ethylene is reacted with 98% sulfuric acid in absorption columns held at 55-80C and 1-3.5 MPa pressure to produce ethyl hydrogen sulfate.
2.   The ethyl hydrogen sulfate is then reacted with water until the sulfuric acid concentration is about 50% and then heated to 70-100C. This generates ethanol, the sulfuric acid is reformed. 
The ethanol that forms is distilled off and has a concentration of 70% (v/v)

Catalytic:
Alternative method includes passing ethylene and steam over a silica gel or zeolite surface impregnated with phosphoric acid at 300C and at high pressure. The O—H bond of the water breaks, and a H atom and an OH group add across the double bond, causing it to break. The final product is ethanol and the double bond has been destroyed.

As long as you understand indirect hydration well (which looking from what you wrote, you do ) The main thing I think, is that sulphuric acid acts as a catalyst and dehydrating agent (the fact that it is a dehydrating agent means it absorbs water and this is necessary to drive the equilibrium towards the ethylene product.
Hope this helped

[itssona]

just wondering if for Le Chatelier's Principle, it is enough to know the direction of equilibrium for pressure change, temp change, and concentration change? Is that pretty much the applications of knowing Le Chatelier's Principle?
Thank you guys

[MisterNeo]

just wondering if for Le Chatelier's Principle, it is enough to know the direction of equilibrium for pressure change, temp change, and concentration change? Is that pretty much the applications of knowing Le Chatelier's Principle?
Thank you guys

Hey
Depending on which option you do, Le Chatelier's Principle can also be used in Industrial questions where they ask to predict any changes in colour of a solution when a certain change occurs. It is also used in calculations involving the K constant. The questions in the core topics are pretty straight forward with LCP like in Haber process compromises, etc.
Knowing the changes in equilibrium would be enough however you should be able to explain why it shifts that particular direction such as when an exothermic reaction is cooled, the products are favoured to produce more heat to minimise that lowered temperature.