Chemistry: The Solvay Process

Hello, readers, welcome to another episode of 'The HSC with AB," after a somewhat belated hiatus (read: procrastination). Today marks the first post in almost two weeks, as well as the first one in the three weeks of study leading up to the HSC. This one is about Chemistry, since someone asked me to do one on the Solvay process (apparently I take requests now?), and so here it is.

The Solvay process is the process which is used to manufacture sodium carbonate, a weakly basic salt used in the manufacture of glass, ceramics, and paper, as well as a water softener and to remove sulfur dioxide. It is produced from the raw materials of sodium chloride, calcium carbonate and ammonia, although ammonia is recycled (more on that later). If you want to get down to it, the overall reaction is CaCO_{3 (s)} + 2NaCl _(aq) → Na₂CO_{3 (aq)} + CaCl_{2 (aq)}, but it's not as simple as that. Put calcium carbonate and sodium chloride in a beaker together and nothing will happen, so you need some intermediate steps. And that's where the tricky bit comes in.

There are four steps involved in the Solvay process, and a whole mess of equations. The syllabus states that you have to analyse a flow chart, and what I've found is that all flow charts for the Solvay process are either a) messy or b) incomplete, just due to the sheer amount of recycling that goes on. So I'll put up my flow chart, then go through the steps. This one might take a while to sink in, so don't be troubled if you don't get it straight away. It's not too difficult, but it's a fair amount to remember.

There are four steps to remember: Brine Purification, Hydrogen Carbonate Formation, Formation of Sodium Carbonate and Ammonia Recovery. Now I've never been a huge fan of mnemonics, but someone taught me this one and I haven't forgotten it, so obviously it must be effective, so when you're in the exam, just remember: Both Paige Hawkins and Corinne Fulford Failed Senior Chemistry and Are Raging. That's one that just stuck with me, and hopefully it'll do likewise with you. Anyway. I'll put up the flow chart now and then go step by step.

Brine purification pretty much just involves removing impurities from sea water to leave water and sodium chloride, also known as brine. Calcium salts are precipitated out by adding sodium carbonate, as per Ca²⁺_(aq) + CO₃^2–_(aq) → CaCO_{3 (s)}, and magnesium salts by adding sodium hydroxide, with Mg²⁺_(aq) + 2OH^–_(aq) → Mg(OH)_{2 (s)}.

So now we move on to step 2: Hydrogen carbonate formation, which if we look at our diagram is at the carbonating tower. But first, a small diversion in the form of our lime kiln, where CaCO_{3 (s)} → CaO_(s) + CO_{2 (g)}. This will become more important later, but just know for now that we get some of our carbon dioxide from here. But anyway. We collect our brine, our ammonia, and our carbon dioxide, and we form two chemicals: sodium hydrogen carbonate and ammonium chloride, which can be expressed as NaCl_(aq) + NH_{3 (g)} + H₂O_(l) + CO_{2 (g)} → NH₄Cl_(aq) + NaHCO_{3 (aq)}. A fairly long equation, yes, but it does have to be memorised. With that being said, we move onto Step 3 (I am trying to make this as interesting as I can, but this topic is a tad dry).

Now what we came here for: Step 3: Formation of sodium carbonate. We take our sodium hydrogen carbonate for step 3 and we heat it up (in the diagram, it's in the heater; that's probably not it's real name. Work with me here.). So we heat it up and we form sodium carbonate, water and carbon dioxide, the latter of two which we recycle. Our equation, because we need it, is 2NaHCO_{3 (s)} → Na₂CO_{3 (s)} + CO_{2 (g)} + H₂O_(l).

So now we have our sodium carbonate, are we finished? No! We have Step 4: Ammonia recovery. Remember our CaO from Step 2? We take that and we dissolve it in water, as per CaO_(s) + H₂O_(l) → Ca(OH)_{2 (aq)}. We then take this calcium hydroxide and we react it with the ammonium chloride from Step 2, giving us our ammonia back, as well as water and waste calcium chloride, with our magical chemical equation being 2NH₄Cl_(aq) + Ca(OH)_{2 (aq)} → 2NH_{3 (g)} + CaCl_{2 (aq)} + 2H₂O_(l). We recycle our ammonia and our water and we're left with that equation from the beginning: CaCO_{3 (s)} + 2NaCl _(aq) → Na₂CO_{3 (aq)} + CaCl_{2 (aq)}.

So now we've done the actual process, we're left with some final bits that the syllabus demands. Firstly, environmental problems. The main problem is the waste calcium chloride, which can be sold as a desiccant, or a drying agent (you know, those little packets you sometimes get in food packets that say do not eat?), but are mainly thrown away just because of the sheer amount that is produced. It can be discarded into the ocean without ill effects though, since all it would do is put salt into the ocean which really doesn't do much. If you want another environmental problem, then the process requires heat (see Step 3), which means burning coal, global warming: the standard drill. Really, all you need is the calcium chloride bit.

Two bits left: Locating a chemical industry for the Solvay process and the "carrying out of a chemical step involved in the Solvay process, identifying any difficulties associated with the laboratory modelling of the step," to quote the syllabus. For the first one: put it near the ocean. There is salt water there (seriously, that's it). Finally, the second bit, which is a little trickier to do in a blog considering it requires a prac, but that's why you have Youtube videos. This one does both the heating of calcium carbonate (Step 2) and the formation of calcium hydroxide (Step 4).

Well, that seems to be it for the Solvay process. I do hope soon to an English post soon, maybe one on Wilfred Owen's poetry, or one of his poems. We'll see. I also take requests, so if you want me to do a topic, I'll be happy to give it a shot. Finally, you'll see I have several sets of notes ready for download: all but Chemistry and English. Chemistry is hopefully coming soon, English is difficult to make study notes for; there's no real syllabus to work on, especially for related texts. That being said, post is now officially done.

Good luck with study,

AB

Physics: Doping and Photovoltaic Cells

Greetings all, and welcome to another episode of "The HSC with AB," and my second post on Physics. I'm doing another post on Physics before some other subjects because a) I don't think Maths and English lend themselves too well to this type of blog (that being said, I will try to do a post on English soon, emphasis on try), and b) Physics is my favourite subject as well as the one that can get the most difficult at times.

Today I'm going to be covering some work from the subject of semiconductors, and specifically, the areas of doping and photovoltaic cells, as you could probably guess from the title. Doping, which I'll talk about first, is when small quantities of other elements are added to the crystal lattice of silicon semiconductors. Photovoltaic cells are also known as solar cells, and they convert light to electricity.

To understand doping, you have to first understand about valence electrons. While that might sound frightening, all it is is that all atoms have a certain amount of outer electrons, the maximum being eight. Silicon, which semiconductors are mostly made of, have four.

That being said, we can now get onto doping. There are two types of doped semiconductors: n-type and p-type semiconductors. In n-type semiconductors, small amounts of silicon (about one atom in 200,000) are replaced by phosphorus, which has 5 valence electrons. What this means is that one electron for every atom of phosphorus becomes a free electron, as shown to the right. What this means is that the phosphorus atoms become positively charged (as the electrons are negatively charged). In p-type semiconductors, the silicon is replaced instead by boron, which has 3 valence electrons. In these semiconductors, there are free positively charged electron holes, and the boron atoms are negatively charged.

So that's doping. To sum up: n-type semiconductors have free negatively charged electrons, and p-type semiconductors have free positively charged electron holes. But what does this have to do with photovoltaic cells? As it turns out, quite a lot.

Photovoltaic cells, or solar cells as I'll call them (because that's just easier), consist of two layers: a p-type and an n-type semiconductor separated by a junction, called, creatively, a p-n type junction. The free electrons from the n-type layer then move into the p-type layer. That leaves the positively charged atoms in the n-type layer and the negatively charged atoms in the p-type layer.

Now let's take our solar cell outside into the sun. The sunlight hits the solar cell, creating electron-hole pairs at the p-n type junction (or, in other words, electrons get knocked out of the silicon lattice, leaving holes). The negatively charged electrons are then attracted to the positively charged atoms in the n-type layer. These electrons then move through a circuit attached to the n-type layer, generating electricity. These electrons then move back to the p-type layer, joining up with the positively charged electron-holes. if that seems a tad confusing, then the diagram should probably make a lot more sense.

So, the conclusion: solar cells are made up of two layers: the positively charged n-type layer and the negatively charged p-type layer. When light hits the junction between the layers, electrons move to the n-type layer, around a circuit, and back to the p-type layer, generating electricity.

Well, that's doping and photovoltaic cells, but before I go, I want to point out a new feature of my blog. Above this post you should see several buttons. Click these buttons, and you can download my study notes. As of when I'm writing this, there's only Economics and Physics for download, but when my notes are all completed, they all will be up there (and I will say the ones available for download are only tentatively done, and are liable to change, though not in great detail.). And now that's all I have to say.

Another Physics post done,

AB

Economics: Australia and the GFC

Hello everybody, and welcome again to another riveting episode of "The HSC with AB." Today, I'll be breaking the pattern of my last two posts, in that instead of a specific dot point, I'll be synthesising (ooh, I know my English key words) a couple of different points into one big overarching post: how Australia survived the GFC.

For those who don't do Economics, feel free to skip this one, but I will say that I think Economics is one of the topics that is a good thing to know at least a little about. I mean, no matter what you do, you will be getting money. The economy will affect you. Knowing the Haber process: probably not. But I digress. In this post, I'll be first going through the two arms of macroeconomics in monetary and fiscal policy, and then onto microeconomics, and finally onto our links to Asia. If none of that made any sense to you, don't worry: it will.

To those who do do Economics, I will point out that a) I'm not using too much terminology, as it can get rather confusing at times (though rest assured I do know it) and b) I very well may have missed some things - in fact I probably have: there's no real dot point I'm working from. In which case, feel free to comment.

Macroeconomics is the part of economics concerned with overall demand. It has two arms: monetary and fiscal policy (more on the latter later). Monetary policy is, more or less, the setting of interest rates by the RBA to alter inflation by changing demand (broadly, increasing demand causes prices to increase, also known as inflation). From 1996 to 2002, interest rates tended to go down, which boosted the economy. At around about 2002, however, the RBA decided to start increasing interest rates - slowly, at about a quarter percent per time, but they started to go up, in an attempt to slow the economy down. The reason is that if the economy starts expanding (as it was doing before the GFC), eventually the bubble can burst and there is a crash. For that reason, the RBA had interest rates at 7.25% when the GFC hit.

In August 2008, the cash rate was at 7.25%. Eight months later, in April, it was at 3%. By comparison, in the eight months before August '08, the cash rate had only gone up 1%. While 3% may seem low, other countries had their cash rates much lower to try and stimulate their economy. In April 2009, the US had their cash rate at 0.25%, the UK was at 0.5%, the EU had 1.25% and 1.5% (it is the EU, after all), and Japan's was at 0.1%. Our interest rate was somewhat high compared to Western economies (I've intentionally not mentioned China: more on that later), and that was a result of some of our other policies. While monetary policy played a large role, it wasn't alone.

Fiscal policy played a large role as well. From 1998 to mid-2008, the government ran a surplus, just because of how strong the economy was (as a rule of thumb, the better the economy is, the more the government receives in tax and the less it needs to spend). And it used that surplus to pay back some of the money it had borrowed over the years - so much so, that the government debt to GDP was about 8%. In comparison to 45% for the UK, 60% for the US, 65% for the EU and 190% for Japan (that was not a typo). So when the GFC rolled around, the government had enough money to actually go out and do something.

Which it did. The government started pumping money into the economy in an attempt to increase demand. It ran a huge deficit in an attempt to reverse the effects of the GFC. You may remember the $42 billion stimulus package in 2009, which was intended to increase spending which would increase aggregate demand, helping the economy. You will note that our economy didn't actually go into recession. This was at least partly due to fiscal policy.

Next we move onto microeconomics, which is the part of economics that deals with supply. What this mostly involves are things like infrastructure and training programs. When the GFC hit, the Rudd government put a lot of money into microeconomic reform, as it's called (a more detailed list here), including $2 billion in infrastructure such as road and rails, and almost $1.7 billion in training programs and higher education. The upshot was that improvement of infrastructure helped business make more and be more efficient (thus allowing them to hire more) and training programs allowed unemployed people gain more skills and re-enter the workforce.

This microeconomic reform meant that unemployment growth was slowed, and it reversed much quicker. Unemployment hit a maximum of 5.8% in Australia, while in the US and EU, it reached over 10%. And, on top of all that, employment helps with an increase in demand (the more people hired, the more people spending), which, as I've said, improves the economy.

Lastly, I'll mention our links to Asia, and specifically, China. China survived the GFC better than Australia did (yes, their growth rate did drop - to 6%, which was more than ours was before the GFC), and that was great news for our export sector. Australia mostly exports commodities, like coal and iron, and China imports a lot of commodities. Hence, when the GFC hit, we survived more or less intact, since China just kept buying our coal and iron and Australia just kept selling it to them. If we had mostly been selling to Europe and the US as we had done before we realigned our interests to Asia, we probably wouldn't have done nearly as well.

Well, in the words of Forrest Gump, 'That's all I have to say about that.' Before I go though, I will link you to some marvelous videos I found which Ebony would approve of, as they are by the vlogbrothers, which I've heard she likes just a little bit. The two videos I found were explanations for the Greek debt crisis and the American debt crisis, and they were very informative. Next post I'm thinking I might do an English post (the horror!). But we'll see.

Have a nice weekend,

AB

Chemistry: The Haber Process

Welcome, readers, to another exciting instalment of "The HSC with AB," where I'll be doing my first post on Chemistry, which is most likely my worst subject bar Extension 2. Today I'll be going through the Haber process, which is the process which synthesises ammonia (NH₃) from nitrogen and hydrogen. I decided to do a bit about the Haber process today because it's one that I'm having some trouble with, and there's a whole section in Chemical Monitoring and Management dedicated to it. It's an important one.

The Haber process was invented by a German chemist, Fritz Haber, in the 1900s (this is a dot point we need to know, and I'll try to make it as exciting as I can.). Before 1914, ammonia was mainly used in fertilisers, and Germany was the only one synthesising it, as Britain was having it imported. Then WWI rolled around, and ammonia needed to be used in explosives. Now, usually Germany would not have been able to produce ammonia, and might not have been able to fight in the war, but thanks to Fritz Haber, it could suddenly generate a lot of the stuff. So now it could fight in WWI.

Clearly, ammonia was very important in world history.

Now our history lesson is out of the way, we can get to the science involved. Ammonia, nowadays, is used in industry, most commonly in the production of fertilisers, plastics, cleaners, and non-ionic detergents, and as such, is pretty important in day-to-day life.

The Haber process can be summarised as N_{2 (g)} + 3H_{2 (g)} ⇌ 2NH_{3 (g)} [ΔH = —92 kJ/mol], but it's a lot more detailed than that (that was an equilbrium symbol, by the way. It doesn't show up brilliantly, but it's the best I can get). There's a lot of stuff to be absorbed here, but I don't think much of it is particularly difficult. So, with that in mind, let's get to the Haber process.

First, we need our nitrogen and our hydrogen. Nitrogen is easy to get: you breathe it. Air is mostly nitrogen, and so all that really needs to be done is getting the oxygen out of it, leaving the nitrogen. We do this by burning methane: CH₄ + 3O₂ → CO₂ + H₂O. Hydrogen is also, funnily enough, generated using methane, except we react it with steam: CH₄ + 2H₂O → CO₂ + 3H₂. And then we filter out the carbon dioxide and trace gases (more on why that is later), and we have our nitrogen and hydrogen.

So now we get into the Haber process itself. This can seem confusing at first, since textbooks like to have overly complex diagrams, so I found this one to the right on the net. It's just that simple. What the syllabus needs though, is an explanation of that diagram, so let's get to it.

Firstly, the simple stuff. The pressure is 200 atm because it favours the product side of the reaction. Le Chatelier's Principle 101. Moving along. The catalyst, Fe₃O₄ or magnetite, is there because it speeds the reaction up (by lowering the activation energy). Easy. The ratio of N₂:H₂ is kept at 1:3, so all the reactants are used up. Ammonia is also regularly removed, as this shifts equilibrium to the right, as per Le Chatelier's Principle.

Now, the tricky part. You'll notice that the temperature is 400°C. While that is fairly hot, it's not very hot (for industrial production). This is because an increase in heat does two things to this reaction. First, it speeds it up (that's basic chemistry), but it also favours the reactant side of the equation, due to the fact that this reaction is an exothermic one (Le Chatelier's Principle strikes again). Something's gotta give. So in order to balance out reaction rate and yield for maximum production, the process is kept at a moderate 400°C.

Finally, we need our monitoring part of the Haber process. This is fairly straightforward. Temperature, pressure, and the ratio of the reactants all have to be monitored for the reasons above. The produced ammonia needs to be monitored to ensure sufficient quality. The process also needs to be monitored to remove unwanted gases. Argon and methane will lower the efficiency of the process (if they're there, nitrogen and hydrogen aren't). CO, CO₂ and sulfur compounds will "poison" the catalyst. Oxygen can react explosively. We definitely don't want these gases in.

OK, I think that about does it for the Haber process. I think this went a little better than MRI yesterday, if only because MRI is very complex at times, and this isn't as much. I sure learnt something, and I hope you did too. Remember to comment!

As always,

AB

Physics: MRI

Kicking the study part off in "The HSC with AB" is fittingly, my favourite subject, Physics. While I have complained a small amount about doing Medical Physics instead of From Quanta to Quarks, I do understand that there are a lot of people (read: Asians) who want to be doctors, so they really don't have that much of a choice. Please note that I like to have a lot of paragraphs, to avoid a wall of text that a) won't look too appealing and b) will not help divide potentially long-winded explanations. Bite-sized chunks is what I'm aiming for.

Today I'm going to be looking at MRI, which is quite possibly one of the most complex things we've done in Physics so far, but I think seems a lot harder than it actually is. That's not to say it's not difficult, but it's not rocket surgery.

MRI is, in short, measuring the magnetic properties of hydrogen nuclei (or, protons) in water molecules in a patient. As different parts of the body have different water contents, each part of the body has different concentrations of hydrogen nuclei. This in turn means each part of the body has different magnetic properties, and as such, MRI can produce a three-dimensional image of the body. It's a little more complicated than that, obviously, but that's the nuts and bolts of it. If you take this much out of this post, I consider my work done (no, not really. But I figure it's a good introduction to a complex subject.).

I'm going to be using a lot of terminology in this, so allow me to preface with some definitions. Firstly, when I say antiparallel, I don't mean perpendicular. I mean parallel in the opposite direction. Secondly, precession is the rotation of an axis of a rotating object. While that may sound confusing, imagine a top, or, better yet, look to the right. That's precession. Lastly, a voxel is like a three-dimensional pixel. A cube instead of a square (if you play Minecraft, imagine every cube is a voxel.) OK, with that out of the way, let's jump into MRI.

Firstly, the patient lies inside the MRI machine, with an intense magnetic field parallel to the body (called the longitudinal direction, or the Z-axis). The protons in the water molecules will tend to align their spins with the field (imagine a top spinning. It tends to align its spin with gravity. A similar thing is happening here.). Most of the protons will align parallel, but some will align antiparallel.

Radio waves are then passed through the patient. This causes two things. One, it causes an equal number of protons to align parallel and antiparallel to the field. Two, the precession of the protons synchronises.

That might seem confusing (and if it doesn't, congratulations), but I'll try to explain better. I want you to stand up and move your arms in circles, almost like you are doing the butterfly (the swimming style that everybody hates). Your arms are now the protons. There is, as you can see, an equal number of protons parallel and antiparallel to the field (one arm each). Also, the precession of the protons is synchronised (your arms are moving together at the same rate).

OK, back to our MRI machine, and back to our protons. These synchronised, precessing protons produce a net rotating magnetic field. This is entirely in the XY plane (if you like, imagine a string connecting your hands and rotate your arms). This produces a changing flux in the detecting coils which, if we know our Motors and Generators, produces a current.

Now, in our MRI machine, the radio waves stop. This reverses what happened when we applied our radio pulse earlier. Firstly, the protons with antiparallel spins revert back to having parallel spins over time, causing a longitudinal component to emerge. The time it takes for this component to emerge is called T₁. Secondly, the precessing protons desynchronise, making the rotating magnetic field in the XY plane disappear. The time it takes for this to happen is called T₂. These will become much more important later, but for now, we need to look at one more part of the MRI machine: the gradient coils.

These gradient coils simply produce a weak magnetic field which slightly alters the strong magnetic field of the MRI machine from point-to-point inside the patient. This means that at every point in the patient, the magnetic behaviour of the protons is different. Now here's the good part: this changes the T₁ and T₂ values in each voxel (if you're confused, look back at the definitions). The detectors can measure these differences and so can differentiate between the properties of each voxel.

What does this mean? Every mm³ of space has slightly different properties which, when fed into the computer, can be calculated. This produces a high-resolution, three-dimensional image of the inside of a patient.

To summarise: a powerful magnetic field, combined with a pulse of radio waves, generates a rotating magnetic field. When a weaker, gradient field is applied, it slightly alters the rotating magnetic field from point-to-point, which can be detected and processed to produce a detailed 3D image.

This may seem very confusing, and you may not get it all in the one go. It introduces a lot of new ideas, and not simple ones either. I would suggest taking a break, letting your mind take in some of the info, and coming back when you're ready. There's not going to be a test tomorrow (unless you're reading this November 3, in which case, good luck!).

For those weirdos who need impacts on society, MRI is very safe since it doesn't use dangerous ionising radiation like X-rays. It is expensive though, since it uses superconductors and liquid helium.

OK, I think this about wraps up MRI. If I've missed anything important, or if I've got something wrong, or if you've found an easier way to explain something, or anything really, then please let me know in the comments below.

With MRI ticked off the list,

AB

Introduction and Thesis

Hello all, and welcome to The HSC with AB. In this, I'll be going through the HSC, or at least my courses.

But first, a little backstory. I have a real problem with studying. I have some trouble concentrating on one thing at a time, and if something is monotonous, so much the worse. During the month or so of study between the end of Term 3 and November 4 (my last exam), I need to buckle down and concentrate on two subjects a day, for three hours apiece, every day, according to my timetable.

You can see a problem here.

Hence, I need something that is productive but also doesn't feel like just reading from my study notes, compiling flashcards, doing practice exams, and so on. Thus, this blog is born.

So now we come to the purpose of the blog. I will be putting up interesting areas of study, picking out points of weakness, producing theses, analysing notes and exams: in short, this will be my own "study buddy" and record of work all rolled up into one big package. While it won't be my primary way of studying (that would be tantamount to suicide), I'm hoping it will be nonetheless an effective tool.

This, reader, is where you come in. This blog is more or less pointless without some sort of reader interface in which you can suggest ways to improve on notes I've made, introduce new lines of thinking, and, of course, criticise, criticise, criticise. In return, I'm hoping this blog will do the same for you. In the words of Mr. Gippel, 'A rising tide lifts all boats.' I'm hoping this blog will be at least a part of that rising tide.

We are no longer competing. So let us work together.

I'm hoping that through this blog, I will improve my study pattern, get better marks, and make it through the HSC. With any luck, this blog will make this coming month a more productive one. For me and for you.

To a brighter five weeks of study,
AB