Hsc Marathon 2009 - Chemistry (2 Viewers)

[bored of sc]

woops, u forgot + water on the product side.

Oh yeah, thanks.

 

[shady145]

In the meantime: Give two features of a reversible reaction, when a dynamic equilibrium has been set up.

there is a consistancy in measurable properties
if reaction conditions are kept the same then the reaction rate slows down until the system is in equilbrium.

what does AAS stand for and describe how it works when performing its job

 

[Kaatie]

Atomic absorption spectroscophy detects minute concentrations of an metals
when it is in solution. Solution is sprayed into flame of burner n intensity of light absorbed is determined. Conc determined from calibration graph of diluted standards.

I have used one of these has anyone else?

Q: describe the effect of halons

 

[study-freak]

Atomic absorption spectroscophy detects minute concentrations of an metals
when it is in solution. Solution is sprayed into flame of burner n intensity of light absorbed is determined. Conc determined from calibration graph of diluted standards.

I have used one of these has anyone else?

Q: describe the effect of halons

Depending on the number of marks allocated, you will most probably not get a full mark.

You did not explain why the light is absorbed and why does it have to be absorbed for AAS to work? And what is a calibration curve (I know, but you didn't say)? Hence maybe 1 mark or if tough, 2 marks off.

Answer to your question: Halons are groups of hydrocarbons with all hydrogens substituted (or replaced) by halogens that contain at least one bromine and typically some fluorine atoms. They, when diffused into the stratosphere, cause ozone breakdown and so weakens the UV absorbing shield. (I would include equations depending on the number of marks, but cbb to write it here.)

Q: Explain the difference of AAS to normal flame tests and why it is superior to those flame tests. (3 marks)

 

[Revacious]

AAS (Atomic Absorbtion Spectroscopy) and flame tests both work on the premises that different metal ions absorb specific wavelengths of energy, causing excitation of the atom, before the atom returns to the ground state, releasing energy in the form of light. However, AAS can be used to quantify trace amounts of a particular element, whereas flame tests only verify the presence of a particular species. Furthermore, because AAS can detect such small concentrations of metal ions, it's application extends beyond the reach of flame tests, which cannot be used effectively to identify trace elements. It is superior to the flame test in this respect.

although i wouldnt go as far to call it superior...

Explain the need to monitor a named chemical reaction. (3 marks)

 

[rarefish]

Reaction - Haber process
Temperature must be monitored in the Haber process, because it is an exothermic reaction thus the lower the temperature, the higher the yield. BUT it would lower the rate of reaction, extending the time taken to produce the desirable amount of ammonia. Therefore the temperature must be compromised.
The pressure needs to be monitored, as the higher the pressure the greater the yield, but it will destory the vessel. Finally the ratio of N2:H2 should be monitored and maintained at 1:3 to maximise the amount of products.

I think that's all.......

My questions IS:
Explain the differences between the terms strong, weak, concentrated and dilute.

 

[Kaatie]

A strong acid is one that fully dissociates eg hcl
a weak acid is one that does not fully dissociate eg ch3cooh
a concentrated substance has a large amount of solute e.g 1m hcl
a dilute substance only contains a small amount of solute e.g 0.001m hcl

Q: describe the difference between the alkanol and alkanoic acid functional groups in carbon compounds (3marks )

 

[study-freak]

A strong acid is one that fully dissociates eg hcl
a weak acid is one that does not fully dissociate eg ch3cooh
a concentrated substance has a large amount of solute e.g 1m hcl
a dilute substance only contains a small amount of solute e.g 0.001m hcl

Q: describe the difference between the alkanol and alkanoic acid functional groups in carbon compounds (3marks )

Just some correction. Replace the highlighted word with 'ionise in water' and the underlined part with 'solution.'

Answer to Q: An alkanol has a hydroxyl group (OH) while an alkanoic acid has carboxyl group (COOH). Both of them contribute to strong hydrogen bonding, yet that of COOH groups is stronger. This can be attributed to their double hydrogen bonding capacity. COOH also has a higher molecular mass, leading to higher dispersion forces. This allows alkanoic acids to have higher boiling points. The carboxyl group may also give weakly acidic properties as H in COOH is ionised and separated. OH groups cannot do this.

Q: Explain why CFCs are considered harmful although it may use up ozone pollutants in photochemical smogs.

 

[rarefish]

CFCs are dangerous because they release an chlorine free radical, which in the troposphere reacts with the ozone (that is toxic) and save lives. But due to their inert nature, they float in to the stratosphere, where the ozone is NOT harmful but helpful in blocking harmful UV rays reaching earth. The chlorine free radical reacts with ozone turning it in to oxygen, destroying the useful ozone layer.

Next Question:
Explain the esterification process with use of chemical equation.

 

[Kaatie]

Esterification is the acid catalysed reaction between an alkanoic acid (RCOOH) and an alkanol (R'OH). In this reaction a molecule of water is released as a molecule of alkanoic acid joins with a molecule of alkanol. The H atom of the alkanol molecule joins with the OH group of the acid to form water. The OR' group from the alkanol then bonds to the C atom of the alkanoic acid group to form the ester e.g.
Ch3cooh(l) + ch3oh(l) <-> ch3cooch(l) + h2o(l)

oh and dissociate is a board accepted term but I will change the other word thanks

Q: 'the ozone in our atmosphere can be reguarded as both 'friend and foe'' discuss this statement with reguard to the location and properties of ozone

 

[shady145]

ill give a crap answer and not post a new one up, instead someone can finish it lol.
ozone in the lower atmosphere e.g. the troposphere is deadly to living organisms, since it is highly reactive it reacts with tissue in our body. also due to high reactivity nitrates and sulfates are produced, and when it rains these nitrates and sulfates produce acid rain, however in the upper atmosphere e.g. the stratosphere ozone blocks out harmful UV radiation, stopping it from reaching us preventing a possible cause of cancer.

 

[study-freak]

Esterification is the acid catalysed reaction between an alkanoic acid (RCOOH) and an alkanol (R'OH). In this reaction a molecule of water is released as a molecule of alkanoic acid joins with a molecule of alkanol. The H atom of the alkanol molecule joins with the OH group of the acid to form water. The OR' group from the alkanol then bonds to the C atom of the alkanoic acid group to form the ester e.g.
Ch3cooh(l) + ch3oh(l) <-> ch3cooch(l) + h2o(l)

oh and dissociate is a board accepted term but I will change the other word thanks

Q: 'the ozone in our atmosphere can be reguarded as both 'friend and foe'' discuss this statement with reguard to the location and properties of ozone

Well, dissociation, strictly speaking, only applies to ionic solids. When HCl is dissolved in water, it ionises rather than dissociates since it's HCl is originally molecular.

 

[rarefish]

Q: 'the ozone in our atmosphere can be reguarded as both 'friend and foe'' discuss this statement with reguard to the location and properties of ozone

Ozone is O3 which in the troposhere is dangerous because it irritates eyes and respiratory systens, in this case it is a foe. But in the stratosphere where their is an abundance of harmful UV rays O3 splits into O free radical and O2 thus stopping these harmful UV rays from reaching the earth. This helps to reduce skin cancer and lessen damages to retina of eyes.

My Question:
Explain the use and chemical principle behind AAS (Atomic Absorption Spectroscopy)

 

[Kaatie]

wow ill tell my teacher that one

 

[mz-stress]

Answer dat pplz

what group does glycerol fall under, a fat or an alcohol and why does it contain OH at the end ?

 

[Revacious]

Re: Answer dat pplz

is that in the syllabus? because i have no idea.

its probably an alcahol? and it has oh groups at the end cause... when you add water to the triester, thats just what happens? Maybe its so whoever is performing saponification on a wide scale can be really pissed off, cause they have to distil it, wasting their time and money getting it.

 

[clintmyster]

Explain the use and chemical principle behind AAS (Atomic Absorption Spectroscopy)

AAS is used to measure the ppm and possibly ppb concentration of metal cations, for example Pb and Zn that are present in soils and waterways. These metals need to be monitored as they can be heavy and toxic, causing damage to not only us but marine life and instigating many other environmental problems.
An aqueous solution containing known relevant metal cations is made up and sprayed into a flame where it is vaporised. A lamp of a characteristic wavelength to a specific metal being tested passes through the vaporised sample enabling the electrons to shift orbitals. The light then passes through a prism or monochromator. By rotating the prism or using a monochromator, light of different wavelengths can be focussed and the intensity of the light reaching the detector can be measured by the fact that absorbance is proportional to the concentration of ions.

Queston [Girra 09 H/Y]: As a research Scientist, you have been asked to design a battery that will produce the greatest possible voltage. Using your data sheet, justify your choice of anode and cathode reactions for such a battery. (4)

 

[jole1512]

there two methods of cracking through fractional distalation and catalytic cracking

fractional u know from the previous post

catalytic cracking is:
usualy used to break down long chain hydrocarbons
it uses zeolites as the catalys
and always produces alkane and an alkene

 

[Kaatie]

Next question?

 

[xxstef]

identify a radioactive element used in industry (not medical industry) and relate its uses to its properties.