Stopsign's Chemistry Marathon (2006) (2 Viewers)

[Sparcod]

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

-concentration
-type of ions

....I need some help.

 

[UzurOger]

whats EMF??

 

[angmor]

difference in potential on the chart

 

[sando]

im stumped on this calculation.. can sum1 please help me:

pH of 0.01 molL HCl acid = 2
1. Calculate the pH after 10.0mL of 0.01 molL hydrochloric acid solution is diluted by the addition of 90.0mL of distilled water ?

 

[Nodice]

c1V1=c2V2
pH means c1=0.01, v1 = 0.01, v2=0.1
thus, 0.01x0.01 = c2 x 0.1
c2 = 0.001
pH = 3

 

[UzurOger]

difference in potential on the chart

how does that say EMF???

 

[sando]

c1V1=c2V2
pH means c1=0.01, v1 = 0.01, v2=0.1
thus, 0.01x0.01 = c2 x 0.1
c2 = 0.001
pH = 3

how does the distilled water impact on the working out ? .. thats the bit im having trouble understanding

 

[Nodice]

Dont worry, I had this problem last week Basically, pH depends on both the strength of the acid AND the concentration of the acid. Think about it this way: Lets say you have 0.1 mol/L HCl fully ionised. Its pH is 1. But when you add more water, the concentration of the acid reduces. For instance, it drops to concentration 0.001 mol/L because there are a lot of water molecules for every acid now. So the pH has changed to 3 by adding the water and making the concentration less.
Get it?

 

[sando]

thanks a bunch. ur more helpful then my teacher and textbook put together ! haha.


not sure if i should post up a new question. but heres one for someone who is extremely bored or nerdy lol:

glucose --> mixture containing ethanol --> pure ethanol --> ethyl butanoate

Describe the chemistry and procedure involved in each of these steps (7marks)

 

[Petinga]

The conversion of glucose into ethanol-water mixture involves yeast fermentation. This involves crushed sugar wastes such as mollasses being added to water with yeast, in the absence of oxygen at 37 degrees, in a fermneter vessel. The enzymes in the yeast cells covert starch and sucrose into glucose and or fructose. Fermentation can then occur with other enzymes in the yeast cells converting glucose into a ethanol-water mixture with carbon dioxide gas.
The next step, the conversion of ethanol mixture into pure ethanol involves the use of fractional distillation. THis involves heating the mixture ina round flat bottomed flask causing the alcohol (i.e ethanol) to boil off first due to a lower melting point and hence being condensed as it passes through a condenser tube into a beaker producing a pure ethanol of 95%.
The last step, inolves esterifcation reaction involving ethanol and butanooic acid. This involves heating 10ml of ethanol and 5ml of butanoic acid in a round flat bottomed flask with a few drops of concentrated sulfurc acid and few boiling chips. The reaction mixture is then allowed to cool for a futrter 5 minutes then added to a separating funnel with 10ml of distilled water. the bottom layer is run off and discarded whilst top layer is run off into a beaker containing 10ml of sodium carbonate. Thus the ester ethyl butanoate is produced.

Question: Describe the conditions neccesary to produce a galvanic cell

Keep posten da questions galore. its practice it doesnt matter if they are past paper question like sando jus post post post

 

[Naylyn]

some equations for the ethanol ect. one
fermentation
C6H12O6(aq) ---> 2CH3CH2OH(aq) + 2CO2(g)

i think the ethanol to ethyl butanoate is:
CH3CH2OH(ethanol) + CH3CH3CH2COOH(butanoic acid) ---> CH3COOCH2CH3CH3CH3(ethyl butanoate) + H2O

 

[Naylyn]

Question: Describe the conditions neccesary to produce a galvanic cell

You need the basic components which are
-electrolytes
-electrodes usually either the metals which are in the solution or inert metals such as graphite or platimun
-a salt bridge and external circut for the electrons to flow

Question:Why is the hole in the ozone layer concentrated over the antartic in the spring?

 

[Petinga]

There is an increase in ozone depletion over Antartica during spring due to an increase in solar UV radiation folowing a long winter. During winter, the solid partilces (i.e ice) catalyse the reaction between HCL and chrlorine nitrate producing molecular chlorine.AS the sun rises in early spring, there is an increase in solar radiation which causes molecular chrorine to broke down into 2 chlorine free radicals which attack ozone molecules hence causing depletion of ozone layer. By summer the supply of chlorine has been exhausted and ozone layer returns to normal concentration. This has been far worsened through the increased use of CFC's and halons.

Question: Define molar heat of combustion of a compound and desribe the procedure used to determine molar heat of combustion

 

[Naylyn]

Molar heat of combustion is the heat liberated when one mole of a substance if combusted assuming no heat loss and complete combustion.

The method that real labs use to determine the molar heat of combustion is called a bombcolorimeter:
-a known amount of the substance and the oxygen that is needed to combust the sunstance is placed in a sealed bag in the middle of a container of water
-the substance is exploded and the temperature rise of the water is measured,
-the formula q=MCT is used to calculate the raw heat of combustion
-this is then adjusted to account for the heat capacity of the container that the water is in, the thermometer and the bag that was used to hold the substance that was being measured,
-the molar heat of combustion is then calculated,

The other way is to burn some the substance over a container of water, then measure the heat rise and use q=mCt to get a very very rough value.

Question: What chemical tests could be used to identify copper in a waterway and what concentration could be considered problematic.

 

[Petinga]

Question: What chemical tests could be used to identify copper in a waterway and what concentration could be considered problematic.

To identify the prescence of copper in a waterway, a few drops of sodium hydroxide can be added to a sample of water. If a blue precipitate forms then copper is present in the waterway.

It is not important to know the concentration of partiular ions at which they cause problems.

Question: Discuss the effectiveness of AAS in pollution control and impacts it has had on our understanding of the effects of trace elements

 

[Petinga]

Question: What chemical tests could be used to identify copper in a waterway and what concentration could be considered problematic.

To identify the prescence of copper in a waterway, a few drops of sodium hydroxide can be added to a sample of water. If a blue precipitate forms then copper is present in the waterway. Another method would be Atomic Absorbtion Spectroscopy to determine the concentration of coppper ions but this involves the use of an atmoic aborbtion spectrophotometer which is not ver portable.

It is not important to know the concentration of partiular ions at which they cause problems.

Question: Discuss the effectiveness of AAS in pollution control and impacts it has had on our understanding of the effects of trace elements

 

[Naylyn]

I would say that it was important to know that copper is a trace element and as such very small concentrations (a few ppm) of it in waterways is important and that anything larger than this can have dangerous consequences

Question: Discuss the effectiveness of AAS in pollution control and impacts it has had on our understanding of the effects of trace elements

Before AAS was developed the primary method for measuring the concentration of substances in waterways was titration, however titration is not accurate for very small concentrations.

AAS is aaccurate for very small concentrations and has allowed us to develop our understanding of pollutants which can be damaging at concentrations of 1ppm or less, which has allowed us to combat them it has had a large effect.

AAS has had a very positive effect on our understanding of trace elements. Trace elements (such as Cu and Zn) must be present in very small amounts or health probelms can occur, however if trace elements are present in large amount they can have adverse health effects. Before AAS was developed it was not possible to monitor or even detect trace elements and as such their importance was not realised.

Question: How is a back titration performed?

 

[Petinga]

I know u need to know how to calculate questions from back titrations but i never knew u had to describe back titrations. They dont even specifcally show it any of the 3 study guides for chem i have so i believe it is not important to knwo other than calulate problems

Question: Describe the role of a named chemist and branch involved in

JUst post question ppl. even 2 to 5 questuins at a time to keep me busy. I dotn care if there from past papers or evn dirent syllabus points

 

[Naylyn]

If you are going to calculate for a back titration it helps to know how one is performed. The steps involved are actually quite simple:
1- React the solution being measured with an excess of as appropriate standard (generally NaOH for an acid and HCl for a base) and leave for a time to allow the reaction to go to completion.
2- Titrate the solution normally to calculate how much of the origonal solution is left,
3- Subtract the remaining amount of the origonal standard from the amount that was origonally added to calculate the amount used, which can then be used to find how much of the substance analysed reacted.

Question: Describe the role of a named chemist and branch involved in

This is a good one, I haddn't prepared an answer to this yet so i'll just memorise the one that is in my textbook:
An analytical chemist is concerned with determining what aubstances and how much of each are present in materials. One job that an analytical chemist can do is a plant chemist. A plant chemists job is not concerned with the day to day running of the plant but in ensuring that the equipment is functioning properly and that the end product is what it should be, the plant chemists job includes:
-Ensuring that the materials that are produced in each stage of the production meet the requirements if the next stage
-Monitoring the wastes to ensure they meet the environmental requirements
-Collaborating with engineers to ensure that the equipment is operating at maximum efficiency

I'll post a few questions that will keep you busy for a while.
Describe, with equations the process of ozone depletion by free radicles
Describe and assess the effectiveness of the measures taken to control ozone depletion.
Describe the process by which ozone smog in cities occurs, including in your answer the origon of the pollutants, when the problem is at its worst and why.
Explain the processes which produce acid rain inclusing in your answer the sources of the pollutants
In your course you performed an investigation to cpncentration of a domestic acid, describe the process that you undertook, did you results differ from what you expected, if so why?
Classify the following common household substances as acidic basic or neutral: Garden lime, antacid tablets, aspirin, soft drinks, drain cleaner and swimming pool water, tap water.
Why is it important to monitor cations and anions in our waterways.

Theres a few off the top of my head, they should all be on the syllabus

 

[Petinga]

1. cant be bothered with typing equations so just describe.
CFCS undergo photodissociatyion with ultra violet radiation producing chrlorine free radicals. the chlorine free radicals then attack ozone molecuels forming chrlorine oxide molecule. the chlorine oxide molecule then reacts with oxygen free radicals regenting another chlorine free radiucal which attacks another ozone molecule. Thus CFCS cause depletion of ozone layer in stratophere.
2. some steps taken to alleviate the problems of ozone depletion are:
-montral and kyoto protocol which set target for reducing ozone destrying chemicals
-alternatives to cfcs such as HCFS and HFCs. these reduce ozone depletion as HCFCS release less chlorine free radicals and HFCS release none hence resultinmg in zero ozone depletion.
However these methods can oly be effective if all contries and antions agee tpo protocols and replacemnt chemicals. Even so, it will take until 2050 for ozone layer to returnm to expected 1970s level if such steps are undertaken now. In meanwhile, some temporary solution for ozone depeltiuon are new suncreessns and uv stabilisers in polymers such as pvc.
6.
Garden lime - basic
antacid tablets - basic
aspirin - acidic
soft drinks - acidic
drain cleaner - basic
swimming pool water - neutral or slighly acidic to prevent microbe deveolpemnt
tap water - neutral

ill do the rest 2morrow - i need a rest for da night