Stopsign's Chemistry Marathon (2006) (1 Viewer)

[Harry Seacombe]

Question:Compare the properties of the oxygen allotropes O2 and O3 and account for them on the basis of molecular structure and bonding.

O2 is a colourless, odourless gas condensing to a pale blue liquid. It is essential for all living matter. O3 is a colourless gas that condenses to a distinctly blue liquid. It has a strong and distinctive odour. It is poisonous.

Due to O3s increased molecular mass it has higher dispersion forces than O2. Also, ozone's co-ordinate covalent bond gives it a higher boiling point than O2 (-111 to -183 degrees C).

O2 has the roughly the same density as air whereas O3 has density of about 1.5 times that of air.

O2 is sparingly soluble in water as it is non-polar. O3 is considerably more soluble in water than O2.

When moolecular oxygen reacts, the double bond between the O atoms in the molecule has to be broken e.g. in C + O2 --> CO2. The splitting of the O=O bond requires considerable amounts of energy.

However, when ozone reacts, it generally just splits off one O atom, leaving behind a stable O2 molecule e.g. NO(g) + O3(g) --> NO2(g) + O2(g). The energy required to do this is much less than that needed to break a double bond and so ozone is much more reactive than molecular oxygen.

Question: Describe and assess the effectiveness of methods used to purify and sanitise mass water supplies

 

[Forbidden.]

.... Question:Compare the properties of the oxygen allotropes O2 and O3 and account for them on the basis of molecular structure and bonding. (5 marks)

Spoiler

Naturally occuring oxygen is a diatomic molecule which supports life and is odourless and is fairly reactive and used as an oxidising agent. Ozone exists in the atmoshpere, it is continually broken down by UV rays but reforms eventually. Ozone is a triatomic molecule which can cause eye and respiratory problems and has a slightly pungent odour and can be used to sterlise water supplies.

Because Oxygen is a diatomic molecule, and its short bond distance suggests it has a double bond. Diatomic oxygen has a high bond energy because of its covalent double bonds, where as ozone posesses a double and single bond, hence has a lower bond energy of approximately 103kJ/mol^-1 compared to the bond energy of 495kJ/mol^-1 for life-supporting diatomic oxygen.

Lol I still remember the values. Ya ?

 

[Petinga]

Answer: Describe and assess the effectiveness of methods used to purify and sanitise mass water supplies

There are a number of steps involved in the purification and sanitisation of mass water supplies. Some of the steps involved are:
-Screening. This involves mass water supplies being passed through large metal grates and screens. This removes large debris such as plastics containing large amounts of pathogens.
-Flocculation. This involves the removal of particulate matter. This involves the addition of Alum and Iron III chloride causing small particulate matter to coagulate into larger particles which can be filtered.
-Sedimentation. This involves water passing through settling tanks were flocs settle to bottom and are removed as a sludge.
-Filtration. This involves water supples being passed tjhrough coarse sand, gravel and anthracite filter removing the remaining particulate matter.
-Chlorination. This involves chlorine gas and hypochlorite compounds being added destroying paqthogens and microbes.
-Fluorination. This involbes flouride compounds in concentrations of 1ppm being added to strengthn the enaml of teerth and prevent tooth decay.

This is a cost effective method but doesnt remove all micro-oganisms such as giardia and cryptosporidium. A more effectiuve but costly method is ozone or uv sterillisation.

 

[abbashussein]

whats the next question gonna be

 

[Petinga]

Answer: Explain how refluxing of the esterification process works and why an acid is used

Esterification involves the a condensation reaction between an alkanoic acid and alkanol producing an ester and a water molecule. This is a relatively slow process and is often carried out at temps close to the boiling point of the alcohol. This involves heating of a reaction mixture in a reaction vessel with a cooling condeser attached. This prevents any voilatile compounds being lost such as ester or alcohol. This ensures maximum contact of reactants and increases the yeild of the ester. Also reduces explosiveness as it allows boiling of reaction mixture to occur in a non closed system and hence reduces explosiveness. Also speeds up the rate of reaction.

An acid such as concentrated sulfuric acid is used as a catalyst which lowers the activation energy and increases speed of reaction. It is a good dehyrating agent and hence absorbs water pushing reaction to right increasing the yield of ester. <!-- google_ad_section_end -->

 

[Petinga]

Question: Describe the procedure used to measure the sulphate content of a particular lawn fertilliser and the chemistry involved

 

[abbashussein]

Question: Describe the procedure used to measure the sulphate content of a particular lawn fertilliser and the chemistry involved

Procedure:

(1) Weigh out about 0.5g of ammonium sulfate. Record the exact mass.
(2) Add 10 ml of water to the (NH4)2SO4 sample and dissolve. Add 2ml of 1M HCl and 100ml distilled water. Heat on a hot plate to near boiling.
(3) Add 25ml of 0.20M BaCl2 solution dropwise without stirring. Heat (‘digest’) to near boiling for as long as possible. Add dropwise with stirring 10ml of 0.1% agar solution.
(4) Filter the mixture using a preweighed filter paper. Test the filtrate for complete precipitation with BaCl2. Wash the precipitate thrice using a wash bottle.
(5) Dry the filter paper and residue to constant weight. Record the finaly weight
(6) Set up a table to record all relevant experimental data.

Complete precipitation was judged for a clear solution by adding several drops of BaCl2 into the filtrate solution and seeing if it turns cloudy white. If it does, it means that the reaction was not complete and the filtrate would have to be treated with excess BaCl2 and filtered through again after adding agar and redigesting.

Question: Explaint the need for a catalyst and identify the catalyst used in the Haber Process

 

[bboyelement]

Question: Explaint the need for a catalyst and identify the catalyst used in the Haber Process

the catalyst is used to increase the rate of reaction, as lowering the temperature for high yield would decrease the rate of reaction. the catalyst is magnetite Fe₃O₄.


How does AAS work?

 

[angmor]

AAS is the abbreviation for Atomic Absorption Spectroscopy. it depends on the fact that different elements emit specific wavelengths of light depending on their energy level.
firstly a cathode lamp containing the element to be anaylsed is turned on. this lamp emits specific wavelengths of light corresponding to the element. next, a flame containing the sample vaporizes it. as the light from the lamp passes through this vapour, some light is absorbed due to the presence of the element in the sample.

the intensity of the light later detected by a photomultiplier is therefore proportional to the concentration of the target element.

NEXT Q: what effect does a catalyst have on the Haber Process, and state the name of this cataylst.

 

[Petinga]

The catalyst lowers the activation enrgy required for reaction and thus speeds up rate of reaction and loweres the temperatures required for the Haber process.
The catalyst used in the haber process is a porous iron/iron oxide catalyst containing small amouts of aluminuim oxide and potassium oxide.

Question: Discuss the importance of the maintenance of the conditions in the Haber process

 

[Harry Seacombe]

Question: Discuss the importance of the maintenance of the conditions in the Haber process

Because many different conditions must be maintained for efficient and safe operation of the Haber process, monitoring is essential. The conditions that need to be monitored include:

• Temperature and total pressure in the reaction vessel to keep them in the range for optimum conversion of reactants to products
• Ratio of H2 to N2 in the incoming gas stream: we need to ensure that this ration is kept at 3:1 to avoid a build-up of one reactant
• Concentrations of oxygen, carbon monoxide and carbon dioxide and sulphur conpounds in the incoming gas: we must ensure that oxygen is absent to avoid risk of explosion, and that oncentrations of carbon monoxide and dioxide and sulphur-containing compounds are sufficiently low to prevent poisoning of the catalyst
• Concentrations of argon and methane in the reaction vessel, because any build-up of these gases will lower the efficiency of the conversion
• Purity of the product ammonia, to ensure that no impurities contamilate the product of the process

Question: Describe the differences between the alkanol and alkanoic acid functiona groups in carbon compounds

 

[Sparcod]

An alkanol has a -OH functional group whereas alkanoic acid has a carboxylic functional group consisting of a double-bonded oxygen and an -OH.

 

[Ioup]

Question:How did the Bronsted Lowry theory of acids and bases improve upon Arrhenius's theory?

 

[rickem]

Arrhenius said in 1884 that acids produced hydrogen ions when dissolved in water and bases produced hydroxide ions when dissolved in water. This was a rather narrow definition and excluded basic metallic oxides, didn't recognise the role of the solvent ionising the acid/base as a chemical reaction, and doesn't take into consideration acid-base reactions where there is no ionisation.

Bronsted and Lowry both said that an acid was a proton donor and a base was a proton acceptor, which incorporatetd all the substances and ideas excluded by Arrhenius' theory. This was an improvement on Arrhenius' definition and also is widely accepted as the proper definition today. This also helped with defining conjugate acids and bases.

 

[sando]

Galvanic cell. one half cell made by putting a copper electrode in a copper (II) nitrate solution; the other half-cell made by putting a silver electrode in a silver nitrate solution. Electrodes connected to a voltmeter and KNO3 salt bridge used.

Explain, using equation, why copper electrode will increase in mass when a electrical generator replaces the voltmeter

 

[Petinga]

By replacling the voltmeter with a generator, the cell acts in reverse as an electrolytic cell converting electrical energy into chemical energy. The anode is positve and cathode is negative. Copper will be reduced and silver is oxidised.

CU^2 + 2e^-1 ->Cu(s)
Ag(s) -> Ag^+2 + 2e^-1

Thus copper increases in mass.

Question 1: Assess potential of ethanol as alternative fuel and pros and cons of use

Question 2: In relation to oxidant conc. and reductant conc., How can the emf of a cell be increase?

Question 3: Outline the use of ethanol as a fuel and explain why it can be callled a rebeweable resource

Question 4: summaris use of etahnol as an alternative car fuel and evalute current sucess of usage

 

[Tim035]

cmon... atleast 4 ppl in this post have already asked/ been answered questions about ethanol. If just a slab of information you have to learn. And I think that other question is ship wrecks and corrosion, I did forensic chemistry much more interesting

 

[sando]

new question then :

Evaluate the use of sodium hdroxide as a primary standard

 

[Petinga]

For a substance to be suitable for a primart standard it must be stable in air, have a high molar mass, have accurately known formula, a water soluble solid and have a high purity.
Sodium hydroxide is hygroscopic and absorbs water from the atmosphere thus not a good primary standard. As well as this sodium hydroxide reacts with the glass brurrete froming sodium silicate thus causing the galss to freeze. If used as a primary standard burerete must be rinsed immedicately to prevent the freezing of the tap. A more suitble primary standard is sodium carbonate.

Question: List factors that affect the emf genrated from a past experiment with galavainc cells.

 

[abbashussein]

cmon... atleast 4 ppl in this post have already asked/ been answered questions about ethanol. If just a slab of information you have to learn. And I think that other question is ship wrecks and corrosion, I did forensic chemistry much more interesting

we did Industrial

i wanted to do Forensic Chem, my school is so boring and the teachers probly dont know how to teach it