Extracurricular Topics (3 Viewers)

[anomalousdecay]

Post anything that is out of scope with the HSC syllabus in here. Include any curiosities and any random thoughts or questions about chemistry you may have. Feel free to post whether you are past students, 2013'ers, 14'ers, 15'ers and beyond.

I'll start from this interesting topic that someth1ng brought up:

Why when you decrease the concentration of electrolytes in both half cells of a galvanic cell does the voltage increase? Both electrolytes were NaCl????

Are you sure you haven't made an error when you did the experiment? Because in theory, increase in concentration of electrolyte should increase voltage, and likewise, a decrease in concentration-i.e. via dilution with water should decrease the voltage(EMF) obtained.

Well it was a class examination and when over 20 groups did it, we all got the same result. The voltage went up everytime!

Also I have another question, can graphite be used as a cathode in a galvanic cell?

You need to have a look at the Nernst equation for this.

This is first year chemistry at tertiary level so you won't know it. You need to know the half reactions of the cell and the overall reaction to find Q and z.

I can't really help further without more details.

It's possible to get really high voltages with galvanic cells by making one half cell have a very low concentration and one with very high concentration such that entropy (the effect that shows that things get more disordered) will increase.

I will research a bit on this tomorrow and try to find a reason why this occurs. Really interesting.

 

[hawkrider]

Nice! I'm doing chemistry next year! Should be fun

I'll post some questions later on when I start the course.

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[xGhanem]

Nice! I'm doing chemistry next year! Should be fun

I'll post some questions later on when I start the course.

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It's awesome! Physics is the best though.

 

[anomalousdecay]

It's awesome! Physics is the best though.

I found Age of Silicon and Motors and Generators my favourite Physics modules and Production of Materials and Acidic Environment my favourites for Chemistry.

 

[hawkrider]

OK, I've got a couple of questions to ask in advance: how abstract are the concepts to understand and how should I prepare for assessments and exams in terms of study, making notes etc.

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[anomalousdecay]

OK, I've got a couple of questions to ask in advance: how abstract are the concepts to understand and how should I prepare for assessments and exams in terms of study, making notes etc.

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Year 11 Chemistry can get a bit tricky, hence the reason why I found it harder than Year 12. Basically:

In Year 11 you learn about what Chemistry is.

In Year 12 you learn about how to apply Chemistry to real life problems.

So in terms of abstract, the preliminary course is quite abstract, but HSC is quite straight forward.

In terms of exam preparation, past papers and your previously written notes are your best friends. This goes for most subjects.
In saying that, the only exams I barely had read my notes were for final externals (HSC) because I had already known the content before, and I found that the most accurate and best notes were the HSC sample answers and marking guidelines.

 

[strawberrye]

OK, I've got a couple of questions to ask in advance: how abstract are the concepts to understand and how should I prepare for assessments and exams in terms of study, making notes etc.

The most effective way to determine how abstract the concepts are is to borrow a preliminary chemistry book over the holidays and have a read, and likewise, you will found with experimenting with different methods in year 11-you will eventually come up with the best way for you to prepare for assessments and exams. One more tip is to try to connect knowledge across modules as often as you can so that even abstract concepts doesn't seem so isolated and integrated body of knowledge is always helpful in understanding and enjoyment of the subject in general.

Essentially in year 11-you will complete 4 main modules:

1)The Chemical Earth-main concepts includes physical (e.g. gravimetric analysis) and chemical separation techniques(e.g. Adding a solvent, then filtration)different properties used to identify pure substances, categorising matter into homogenous/heterogeneous/pure/mixtures/elements/compounds etc

2)Metals-main concepts includes: properties, uses and reactions of metals, relating physical properties of metals to their uses, history of metallurgy, extraction and refining of metal from ores(e.g. copper), the activity series, redox-reaction, quantitative aspects of formulae and equations(i.e. learn about moles and how to use it in calculation questions, relative atomic mass, chemists: Dalton, Gay-Lussac and Avogadro's postulates and laws), trends of the periodic table

3)Water- main concepts includes: uses(consumption, recreation etc), properties(e.g. surface tension, viscosity, specific heat capacity etc) and structure of water(i.e. as a polar molecule), learn about different intermolecular forces such as dispersion, dipole-dipole forces and hydrogen bonding, learn about the structure of different substances and relate it to their solubility in water(e.g. ionic, polar/non-polar molecular, covalent lattices), precipitation reactions, molarity and concentration and application in solving questions, thermal pollution

4)Energy-main concepts include:allotropes of carbon, structural formulae and molecular models of simple carbon molecules, fractional distillation of crude oil, homologous series, naming alkanes and alkenes and their physical properties, isomers, functional groups, composition of fossil fuels and uses, safety precautions for storing alkanes, exothermic/endothermic reactions, enthalpy, heat of combustion, activation energy, factors influencing the rate of chemical reactions

My top 3 tips for preparing for chemistry assessments and exams(can be applied to science subjects in general):

1)Practical assessments-make sure you know reliability, validity, accuracy, sources of error, possible areas for improvement, independent/dependent/controlled variables, risk assessment for all the experiments you have done in class, make sure you can master basic science skills such as drawing line of best fit, drawing experimental set up accurately, using numbered points in writing a logical procedure, understand the requirements of the exam(I.e. how much time, how many parts) and allocate your time accordingly, make sure you understand the chemical principle/theory behind why you did each experiment

2)Theory assessments and exams-make sure you PERSONALLY write a set of summarised notes according to each dot point of the syllabus in a concise but detailed manner-make sure you include appropriate diagrams as well, after you wrote the notes, make sure to start practising questions and CHECKING YOUR ANSWERS in textbooks-i.e. Roland Smith's Conquering Chemistry has an exam question section which is quite good, ask your teacher for practice questions and make links between dotpoints and across modules to reinforce your understanding. Throughout the year, you should be progressively cutting down your notes as you store more things into your long term memory-revise regularly. Before your final trial exam-you should try to get your hands on as many preliminary exams as possible to practice-and look at the marking guidelines, remember to practice under EXAM CONDITIONS. Make sure when you make notes you are at least referring to 3 different sources to synthesise your information. (colour-code and use mind-maps to make your notes engaging and easy to remember). In your notes, don't neglect to include all your experiments and second-hand investigations.

3)For other types of assessments such as group presentations, individual research assignments, second-hand investigations-you should strive to always include a comprehensive bibliography that indicates you have sourced your information not only from websites, but from journals/books as well, you should understand how to evaluate accuracy/reliability/validity in relation to the sources of information you are using, like other types of assessment, pay close attention to the marking criteria and make sure you fulfil it as much as possible, make sure your information is comprehensive, non-repetitive and answers the verb of the question: i.e. discuss, evaluate, compare, contrast, assess, examine (so for that matter, memorise what the main verbs used in your exam questions means, it is included in the link: http://www.boardofstudies.nsw.edu.au/syllabus_hsc/glossary_keywords.html)

Basically, when you want to know about year 12 chemistry-just check out the syllabus as well as the relevant textbooks, and the principles of study and making notes are essentially the same-the key difference is now you can start using trial papers a lot earlier because there will be many more of them available with also official HSC exam papers which you can practice on as soon as you finish a particular topic. Essentially, to succeed in chemistry, or indeed any sciences, you need to understand, constantly apply your understanding through practising questions and adopting feedback from your assessments to do better in year 12-with repetition and consistent application you will achieve your desired marksHope this helps and I hope you will enjoy the preliminary and HSC chemistry course

 

[timeflies]

The most effective way...

+1 for strawberrye's amazingly helpful essays all over BoS

 

[anomalousdecay]

+1 for strawberrye's amazingly helpful essays all over BoS

Hahahaha. She is doing some good work for the 2014'ers.

 

[hawkrider]

Hahahaha. She is doing some good work for the 2014'ers.

And 2015'ers as well. I tried to convince her to become mod cause of her band 6 responses, but she refused

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[anomalousdecay]

And 2015'ers as well. I tried to convince her to become mod cause of her band 6 responses, but she refused

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Give it time. She could be one in a couple of months.

 

[hawkrider]

Give it time. She could be one in a couple of months.

The thing is... She said she's going to be busy once her university degree starts which means she won't be able to contribute as much. She said she'd be better as a normal member.

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[anomalousdecay]

Hey I read some site that said ethanols low heat of combustion means that you need more of it in order to go the same distance as petroleum. I thought wouldnt it be the other way around? can someone plz esplain. Thanks

Heat of combustion means the amount of energy that is released when a substance is burnt. So as ethanol as a lower heat of combustion it means it will produce less energy when burnt ( combusted) by oxygen. As you're using a vehicle as an example, if you fill a 40L tank with ethanol, less energy will be created when that in completely combusted when compared to a 40L rank of petroleum or octane. Less energy = less distance travelled. Hopefully this makes sense haha

This.

The reason why this happens can be explained by simple Chemistry.

Unleaded Petrol's main component for releasing energy is Octane. This molecule has more of a molecular mass than ethanol. As a result, there is more potential energy in the Petrol than Ethanol. Hence, you get more energy from the higher heat of combustion of Petrol.

This is also comparable to Diesel, which is made up of Hydrocarbons of twice the molecular mass of Petrol, thus giving Diesel a higher heat of Combustion.

However this can lead to a compromise in efficiency as it is harder to completely combust fuels higher molecular mass. (This is out of syllabus scope, will post in extracurricular).

I brought this up in another thread. It gave me the thought that quite possible there is a maximum you can have on the chain of the Hydrocarbon.

Also, it led me to the thought that Polymers would have a low heat of combustion, as they would be inefficient in energy conversion.

So, this would produce a heat of combustion graph like this:

Pretty interesting. I wonder why it happens though. Any ideas anyone???

I could also be wrong and it would be good if anyone could spot it.

 

[anomalousdecay]

The thing is... She said she's going to be busy once her university degree starts which means she won't be able to contribute as much. She said she'd be better as a normal member.

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That's why I said give it time.

 

[gwilymprice]

Hey anomalous are you suggesting there is a peak in energy potentials as the carbon chain increases and then drops off? Sorry if it's an obvious answer from your above post, was just a bit confused

 

[anomalousdecay]

Hey anomalous are you suggesting there is a peak in energy potentials as the carbon chain increases and then drops off? Sorry if it's an obvious answer from your above post, was just a bit confused

No need to be sorry. Thanks for joining the conversation.

Yeah that's what I was thinking about. I'm not sure if it is true or not but think about it:

Does burning polyethylene or burning octane release more energy?

I just thought now it might not be the case, and the reason could be behind the rate of reaction. Remember that the surface area of the substance affects its overall reaction rate.


So if you melt polyethylene into a liquid form similar to octane, what result would we get?


Interesting stuff Chemistry.

 

[gwilymprice]

Well I theoretically (assuming complete combustion) there should be no peak and decrease in the potential energy as the carbon chain increases. As with each -CH2- link added, more bonds with more energy potential is gained, this should allow for a greater heat of combustion, as it is the breaking of bonds which release the energy, so I feel logically the more bonds the more energy. However greater molecular mass means less chance of complete combustion I think.

With the combustion of polyethylene, you can achieve a liquid state that would combust, however there would be great amounts of degradation to the polymer in the process of achieving the combustion, this would cause reactions of it's own which may not be classified as combustion. Also the state changes of the polyethylene may be a problem. But if the liquid polyethylene and octane had completely the same structure and bonds I can't see why it wouldn't yield a higher heat of combustion.

I will do some further investigating and talk to some materials chemists.

 

[anomalousdecay]

Well I theoretically (assuming complete combustion) there should be no peak and decrease in the potential energy as the carbon chain increases. As with each -CH2- link added, more bonds with more energy potential is gained, this should allow for a greater heat of combustion, as it is the breaking of bonds which release the energy, so I feel logically the more bonds the more energy. However greater molecular mass means less chance of complete combustion I think.

With the combustion of polyethylene, you can achieve a liquid state that would combust, however there would be great amounts of degradation to the polymer in the process of achieving the combustion, this would cause reactions of it's own which may not be classified as combustion. Also the state changes of the polyethylene may be a problem. But if the liquid polyethylene and octane had completely the same structure and bonds I can't see why it wouldn't yield a higher heat of combustion.

I will do some further investigating and talk to some materials chemists.

Yeah that's true. I was trying to find a reason behind this though.

The less chance of complete combustion also ties in with the rate of reaction as I stated before.

 

[anomalousdecay]

From the combustion of hydrocarbons, I had a thought of how energy is released.

Now, in combustion of hydrocarbons, you need to break bonds. This process requires an activation energy. As an example, I will model the use of Octane.



In an internal combustion engine, you have a source of electrical energy from the spark plugs. This allows for the activation energy to be surpassed, at the compression stage of the gases, where the spark spreads over to all hydrocarbon molecules.

So what we get is enough energy to break the bonds in the hydrocarbon and oxygen molecules. Now this process involves the breaking of dispersion forces, which are generally weak, so little energy is required to do so. Then the bond formation occurs, where the water and carbon dioxide form.

When this occurs, hydrogen bonds form to obtain water molecules. These hydrogen bonds are very strong and hence release a lot of energy.
Also, when forming carbon dioxide, a considerable amount of energy is produced.

Now in the compression stage, this becomes an uncontrollable reaction, that causes an explosion.

Hence, the piston will receive a thrust as the energy is released. Also, the gases expand the chamber, as the original mixture was of liquid petrol. The energy released is then used to provide a torque on the crank shaft. The crank shaft will provide a rotation for the wheels.

Also, the crank shaft works as an input transducer, and hence a generator, converting mechanical energy into electrical through the alternator, which is fed into the battery. This is the same electrical energy required to provide that activation energy at the spark plugs.

So in the end, this process does not only involve the combustion of hydrocarbons.

You also need to consider the use of equilibrium measures, to prevent a loss in energy. These measures do NOT only include the reaction.

For example, the use of pressurisation maximises the efficient complete combustion of the mixture. If the spark plugs provide energy when there is little fuel in the chamber, or if there is low pressure, then most of the hydrocarbon will not ignite, hence providing a loss of fuel.

Le Chatelier's Principle comes into play, however since this is a continual process, you not only lose energy, but also lose fuel.

Just my 2 cents on an example of how combustion takes place in an everyday use.

 

[strawberrye]

This is a very detailed explanation, anomalousdecay, your explanation was certainly much more than 2 cents worth