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

[pLuvia]

Isn't it:
NaOH + H₂O ---> Na⁺ + OH^-

What's B-L base?

 

[tristambrown]

Re cobalt 60 ---it's decomposition/stabilisation attempt releases Gamma rays which destroy cells on contact hence its use in "zapping" cancers and Gamma irradiating foodstuffs. I dont think we need to know what it decays into, just that it does and how it does it (ie by releasing gamma radiation)

Re this q: - pluvia is right. I think the point of this is to be sure we realise that the H2O is acting as the acid though.

Also a B-L base = bronsted lowry base = ie species acts as a proton acceptor in "X" particular reaction.

Part 1
NaOH(Aq) + H2O(L) -->NaOH + H+ +OH-
B-L Base + B-L Acid --> Base + Ionised Acid

Part 2
NaOH + H+ +OH- --->Na^+ + H2O + OH-
Base + Ionised Acid ---> Anion + Acid/base + Ahhrenius base

The base OH ion is basic because it is accepting a proton in this reaction

The acid (H2O) is B-L acid because it donated a proton in this reaction (funnily enough to form H2O again with the OH it is reacting with)



Look at the CO2 equilibrium for more detail on what this question is intending to get at .. why they picked NaOH and water for this q is beyond me.


CAN SOMEONE CONFIRM THAT despite being considered an acidic oxide CO2 is actually considered a B-L base below ? I think it is as it is accepting 2 H+ ions but am not 100% sure due to the involvement of O in the reaction 1

Reaction 1
CO2(Aq) + H2O(L) <--->H2CO3(Aq)
B-L Base + B-L Acid <----> Acid (by both definitions)

Reaction 2
H2CO3(Aq) + H20 (L) <-----> H3O^+(Aq) + HCO3^-(Aq)
Acid + B-L Base <-----> B-L Acid + B-L base

---------------------------------------------------------------------------------
Note above (H+ + H20 would be ahhrenius)

Also note the conjugate acid base pairs - These are reversible reactions and as such interchange with each other which is the acid & which the base by the donation/acceptance of a proton.

H2CO3(Aq) (CONJUGATE ACID)<-----> HCO3^-(Aq) (CONJUGATE BASE)
H20 (L) (CONJUGATE BASE) <-----> H3O^+(Aq) (CONJUGATE ACID)

--------------------------------------------------------------------------------------------

Reaction 3
HCO3^-(Aq) + H2O(L) <--------> H3O^+(Aq) + CO3^2-(Aq)
B-L Acid + B-L Base <------> Acid + Base



the prevoius working definition was that of ahhrenius.
As defined by Ahhrenius (or is it Arrhenius?) as a base anything that can react with water to form OH^1 ions is a base & an acid as anything that reacts with water to form H+ ions

 

[Sparcod]

A b-l base is one which is a proton acceptor, relative to a b-l acid.


Identify three atmospheric pollutants and explain where they came from.

 

[tristambrown]

SO2 NOx COx

SO2 - Byproduct of Al smelting / venting from volcanoes (is that spelled correctly ?)

NOx - Result of high temp combustion oxidising atmospheric Nitrogen gas

COx - Byproduct of combustion - eg cars, fossil feul power generation etc

All are toxic to humans and most form acids on reaction with water (contributing to acid rain)

 

[pLuvia]

I was actually asking what B-L stood for Should have phrased it properly

 

[KYeh]

Bronsted-Lowry

 

[Tim035]

Question: Describe the components/chemical principle behind atomic absorbtion spectroscopy and justify its usefulness in monitoring pollution.

 

[Petinga]

Atomic Absorbtion spectroscopy (AAs) is used to detect minute concentrations of metal cations ions in solutions. This involves the use of an atomic aborbtrion spectrophotometer composed of the metal to be anaylesd which produces wavelengths of light unique to that metal. A sample of solution is sprayed into a flame producing a vapour of atoms with light passing theogh the flame. The atoms absorb specific wavelenthgs of light. The monochroimator foccusses the light onto a photomultiplieer which gives an absorbance readout. The more metal cations the less light that reaches photomultiplier hence indicationin of greater concentration

AAS has been important in the monitoring of pollution becasue it allows minute concentrations of metal cations to be detected. Thus allows toxic cations in small concentrations which are harmful such as arsenic and mercury to be detected. This has allowed the detection of small concentrations of arsenic in Bangladesh waters and hence shows importance of AAS in pollution control.

Question: Describe the composition ansd layered structure of the atmosphere

 

[UzurOger]

A: Describe the composition and layered structure of the atmosphere

Spoiler

Troposphere (0-15km)
− 90% of atmosphere
− Temperatures down to -50°C
− Mostly nitrogen, oxygen, argon

Stratosphere (15-50km)
− 9.99% of atmosphere
− temperatures from -50°C up to 0°C
− Mostly oxygen and ozone

Mesosphere (50-85km)
− 0.009% of atmosphere
− temperatures from 0°C down to -100°C
− Mostly oxygen

There is this layered structure due to the temperature changes
− the temperature changes cause inversions layers which locks gasses in their respective
layers
− gasses from the stratosphere almost doesn’t filter down to the troposphere
− gasses from the troposphere filter very slowly up to the stratosphere

The temperature changes are created by the various chemical reactions that occur in the
layers

Q: Describe 03 as a molecule that acts as both a UV radiation sheild in the upper atmosphere and a lower atmosphere pollutant. Use Equations.
5 Marks

 

[pLuvia]

Spoiler

In the stratosphere ozone acts as a barrier that prevents harmful UV radiation from entering into the earth's surface, which these radiation can cause mutations, risk of eye cataracts, skin cancer etc. It does this by absorbing the UV radiation hence preventing the access of this radiation

In the lower atmosphere, ozone can cause photochemical smog which is a type of air pollution when sunlight acts upon car emissions. Since the [NO₂ are high the sunlight photodissociates it into NO and O and the O radical reacts with existing oxygen molecules to form ozone.

NO₂+sunlight --> NO + O
O+O₂--> O₃

And ozone is poisonous to us humans and has a very strong odour.

 

[haque]

Calculate the expected vapour pressure at 25 degrees celsius for a solution prepared by dissolving 158g of common table sugar sucrose molar mass=342.3g/mol in 643.5 cm^3 of water. At 25 degrees celsius the density of water is 0.9971 g/cm^3 and the vapour pressure is 23.76 torricelli

 

[onebytwo]

Calculate the expected vapour pressure at 25 degrees celsius for a solution prepared by dissolving 158g of common table sugar sucrose molar mass=342.3g/mol in 643.5 cm^3 of water. At 25 degrees celsius the density of water is 0.9971 g/cm^3 and the vapour pressure is 23.76 torricelli

wtf?

 

[kazan]

Calculate the expected vapour pressure at 25 degrees celsius for a solution prepared by dissolving 158g of common table sugar sucrose molar mass=342.3g/mol in 643.5 cm^3 of water. At 25 degrees celsius the density of water is 0.9971 g/cm^3 and the vapour pressure is 23.76 torricelli

yeh

what the fuck?

 

[Roobs]

haque keep it to the sylibus.....

 

[Ioup]

A 100l sample of air was collected and found to have a concentration of 15ppb.
-Calculate the volume of sulfur dioxide.
-Calculate the mass of so2 given the conditions are 25'c and 101.3Kpa.

 

[UzurOger]

I'm guessing no is really bothered to do any calculations, especially on the computer so...New Question if you wish to answer instead of other 2 calculations

Q: Analyse the benefits and problems associated with the use of radioactive isotopes in industry or medicine
5 Marks

 

[Petinga]

Cobalt-60 is involved in industrial radiography to inspect metal parts and welds for defects such as hairline cracks, metal fatigue and poor welds.
Some of the benefits of cobalt-60 in industrial radiography is that is highly penatritive gamma ray emmitter which allows metal objects such as pipelines far underground to be anaylsed for cracks instead of the costly procedure of digging up entire pipelines.It also has a long hjalf life of 5.37 years which means minimal maintence and replacement reducing exposure to workers and technicians. Another advantage is that it can be used in a chemicallt indert form inside a sealed container further reducing the exposure of radiation to workers. An environmenatal advantage is that it is insolube and thus no risk of underground water contamination and reverts back to nickel.
Hoever the disadvantages of cobalt-60 is that it requires a nuclear reactor fopr production and hence requires the costly disposal of radioactive wastes.Antother disadvantage is the exposure of radaition causing cancers

Question: Outline the historical deveolpment of ideas about acids

 

[Sparcod]

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

 

[Harry Seacombe]

Question: Outline the historical deveolpment of ideas about acids

Lavoisier proposed that acids were substances that contained oxygen. However, this was soon disproved as many oxygen-containing substances such as sodium oxides were basic and some acidic substances such as hydrochloric acids were distinctly acidic but contained no oxygen

Humphrey Davy suggested that acids were substances that contained replacable hydrogen - this hydrogen could be replaced partly or totally by metals. He also suggested that when acids reacted with metals they formed salts.

Arrhenius proposed that acids were substances that ionised in solution to produce hydrogen ions: acids were strong if they ionised completely and weak if they only ionised slightly in solution.

In the 1920s, two chemists, Bronsted and Lowry, independantly proposed that acids were proton donors and bases were proton acceptors

Question: Describe an example of a chemical reaction such as combustion were reactants form different products under different conditions and would thus need monitoring

 

[abbashussein]

Question: Describe an example of a chemical reaction such as combustion were reactants form different products under different conditions and would thus need monitoring


Combustion of Pentane:
Combustion in Excess Oxygen:
- Pentane burns purely to produce carbon dioxide and water

C5H12(g) + 9O2(g) * 5CO2(g) + 6H2O (g)

Incomplete Combustion:
- Without sufficient oxygen products may include carbon monoxide and with even lower levels of oxygen pure carbon is produced (soot)

C5H12(g) + 7O2(g) * 3CO2(g) + 2CO(g) + 6H2O (g)

C5H12(g) + 5O2(g) * CO2(g) + 2CO(g) + 2C(s) + 6H2O (g)

Monitoring:
- Car emissions and power stations need to monitor waste gases to ensure that products are produced at acceptable levels to avoid production of toxic carbon monoxide or particulates of soot


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)