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Farewell, sweet chemistry, for now

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Edited by Vicky Fraser, Thursday, 2 June 2011, 08:30

Well, I have reached the halfway point in my study of science with the Open University - farewell, Book 4. I have enjoyed you very much, and I do believe that we have surprised each other.

You surprised me by re-introducing me to The Mole, and making me love it. I surprised myself by not only enjoying chemistry, but understanding it too.

However, I must put in a complaint. Not about the chemistry, you understand; nor about the way in which it was taught (although really, some of the writers need to embrace the idea of "less is more"). No, my beef is with those who set the questions for the TMAs (tutor-marked assignments).

In this case, the person who required us to needlessly rearrange an equation, then arrange it back again, when we could find the answer quite easily using the original equation and the information in the graph - thereby confusing everyone on the course - should be punished by being locked in a room with Silvio Berlusconi and Celine Dion being played on a loop.


But apart from that little blip, the TMA is done and dusted, and is going through a checking process. I should have dispatched it by the end of the day today.

I feel I've achieved quite a lot from this module: I understand, do not fear, and in fact have grown to love Avogadro's mole; I am able to write balanced chemical equations; I understand acids, bases and equilibrium; I can find the hydrogen ion concentration of a substance from its pH; and I am beginning to understand how drugs work (and therefore, how enzymes and hormones work). It's really fascinating stuff.

Fuel, and evidence, is being added to my mini-crusade against quackery. Well, my own personal local crusade, partially inspired by Ben Goldacre (I had my doubts before I started studying science, and before I discovered his Bad Science writings).

I should clarify: the placebo effect is real, and documented, and I'm absolutely happy with that. What really grates my carrot is when people peddle something like homeopathy as "science". Some homeopathic remedies are sold at a dilution of 200C. That means that one drop of the "remedy" has been diluted in 200 drops of water - 100 times over. It has been diluted more than the number of atoms in the entire universe. (Thanks to Bad Science for this nugget!)

And that is only one of the ways in which homeopathy is quackery.

But as I said - the placebo effect is fine. I have no problem with people parting with their hard-earned cash for nonsense, or for a placebo. What I DO have a problem with is quacks encouraging seriously ill people to stop their medication, and start taking sugar pills. That is dangerous, arrogant and pretty close to evil. I saw a forum discussion, via a tweet from Le Carnard Noir, in which homeopaths were talking about how to encourage HIV and AIDS patients to stop taking their retrovirals in favour of taking sugar pills.

And then one of them demanded that everyone else stop making a link between HIV and AIDS. That's not just deluded, it's dangerous. And vulnerable people, who are desperate, will listen to them.

I've also learned that when people say that, "Natural is better; chemicals are bad, m'kay" they have not really thought about what it is that they're saying.

Salicylic acid - the active ingredient in willow bark

Take the example the OU gave us: aspirin was developed from willow bark, which has the active ingredient salicylic acid. In days of yore, willow bark was used to treat aches and pains, and was quite effective - except for the side effect of stomach irritation. Chemistry has enabled scientists to adapt the natural drug - salicylic acid - to acetylsalicylic acid, which does the same thing, but without the side-effects.

Acetylsalicylic acid - the active ingredient in modern aspirin

Another example is Ventolin (or to give it its proper name, salbutamol). It mimics adrenaline, a chemical released by our bodies in times of stress. As it happens, adrenaline is very effective at opening the airways, thus relieving asthma - but the last thing an asthmatic wants is increased heart rate, changed blood flow, and the jitters. Salbutamol was developed from adrenaline, but tweaked slightly so it only affects the lungs, without affecting the other organs.

The natural remedy was a great start; but most people forget (or likely don't think about it at all) that the plant evolved the chemicals for its own good; not for ours. Why would a natural remedy, "designed" to benefit the plant it came from, be ideal for use on humans with no tweaking?

Instead of bemoaning the work of modern chemistry, people should be celebrating it. It's an incredibly creative area of science, and has saved and improved countless lives.

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Blue hair, yellow sweater, big smile

The nature of acids, and a long string of hydrocarbons

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I am charging through Book 4: The Right Chemistry, and after a shaky start, I'm enjoying it very much.

Last week I undertook an experiment to measure the acidity or otherwise of common household substances. It was just like being back at school, and I got to Make A Mess in the kitchen. Win!

I tested washing up liquid, shampoo, stain remover, laundry powder, tonic water, cranberry juice, bleach, tap water - and resisted the urge to plunder the house for anything that could have a Universal Indicator paper stuck into it. Including the cats.

I was very surprised at how acidic tonic water is - it has a pH of 3. So, what is pH?

pH stands for "potential hydrogen". I didn't know that, and don't remember being told at school (although it is entirely possible that I was setting fire to a bunsen burner at the time). This makes sense, however, as I now also know that the pH is a convenient way of describing what a substance's hydrogen ion concentration is.

So, tonic water has a pH of 3, which means that its hydrogen ion concentration is about 1.0 x 10¯³ mol dm¯³. Handy. Saves using lots of very small numbers and scientific notation.

Acids yield hydrogen ions when they are dissolved in water - so the higher the concentration of hydrogen ions, the more acidic the substance. Bases yield hydroxide ions: so the more hydroxide ions contained in a solution, the more basic that solution is.

The strength of an acid is determined by how far it dissociates in solution - hydrochloric acid, for example, is a strong acid because it dissociates almost completely. Almost all the HCl molecues dissociate to give positive hydrogen and negative chloride ions; whereas vinegar (acetic acid - or ethanoic acid, to give it its proper name) is a very weak acid as it only partially dissociates in solution.

The book then took us through the method of calculating a substance's pH from its hydrogen ion concentration - or vice versa. And very simple it is too. I can imagine it will come in very useful to you all on a daily basis - if for no other reason than to impress people in the pub.

"See that pint? It has a pH of 4.5, which means it has a hydrogen ion concentration of 0.0000316 mol dm¯³."

Anyway. I'm pleased with my progress, and have moved onto hydrocarbons. Which are pretty cool.

I am a long string of hydrocarbons. As are you. And so is almost everything, in fact. Including crude oil.

Hydrocarbons are subdivided into alkanes and aromatics. Alkanes are further subdivided into linear-chain alkanes, branched-chain alkanes, and cycloalkanes. This are all pretty good descriptions of their molecular structures.

Carbon has a valency of four, meaning that it can hang onto four other atoms. Hydrogen has a valency of one, so it can only hang onto one other atom. Linear-chain alkanes are long strings of carbon atoms attached to a maximum of two other carbon atoms, and two or three hydrogen atoms. These alkanes can also be folded over; they needn't be long, straight strings.

Branched-chain alkanes are similar to linear-chain alkanes, but instead of having two hydrogens, a carbon atom will be attached to a third carbon, forming a "branch". Hence the name.

And cycloalkanes are rings of carbon atoms, with hydrogen atoms attached. These, too, can have branches.

Aromatics are also rings of carbon atoms, but some of them have double bonds, and some of them single bonds.

All this is useful for grading petrol, believe it or not. When you're filling up your vehicle, the unleaded nozzles have "95" or "97" printed on them. These are the octane numbers; and the higher the octane number, the better the performance of the petrol. Y'see, linear-chain alkanes don't make very good motor fuel - they burn unevenly, and cause the engine to "knock" (small explosions interrupting the burn). Branched-chain and cycloalkanes are much better; and if you can add an aromatic to the mix, then it's better still.

I'm not sure why yet; I'll get back to you when I've found out.

I'm particularly enjoying hydrocarbons as I get to draw molecular structures. This pleases me: they are very regular, and appeal to my sense of neatness. This is ethane:


And this is an aromatic - napthalene - note the double bonds, and pleasant circular structure:

napthalene.jpgI've downloaded a chemistry drawing package to use for my Tutor Marked Assignment. I'll see how I get on with that...

I'm looking forward to Book 5: Life - and am hoping it will give me more of an idea of my future studying direction. I love everything so far - but I think a focused four-directional future will be time-consuming to say the least...

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Blue hair, yellow sweater, big smile

A fear conquered; or, musings on moles

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mole.jpegI am no longer afraid of moles! No, not the little furry buggers that make a mess of your lawn. The once-frightening, but now benign, number used in chemistry so that your head doesn't explode due to excess zeros.

A mole - also known as Avogadro's number - is 6.02 x 10²³ "things". So, one mole of oxygen atoms contains 6.02 x 10²³ atoms. That's quite a large number. So large, that most people can't get their heads around it.

Here's an analogy: one mole of marshmallows would cover the United States of America to a depth of around 6,500 miles*. That is a LOT of marshmallows.

One mole of moles (the little furry buggers this time) would, if placed end-to-end, stretch 11 million light years, and weigh almost as much as the moon.*

Water flows over Niagara Falls at about 650,000 kL (172,500,000 gallons) per minute. At this rate it would take 134,000 years for one mole of water drops (6.02 x 1023 drops) to flow over Niagara Falls.*

Anyway, enough analogies. Suffice it to say, it's a remarkably large number. Far too large to do anything practical with. So, chemists use the mole as a form of shorthand. At school, I hated chemistry specifically because of moles; I just couldn't get my head around it.

So it was with a sense of trepidation that I approached Book 4: The Right Chemistry.

My fears, however, were unfounded. I'm really, really, enjoying this book! The maths tackled so far has really helped to beat back the terrors of Very Large Numbers, and the book is great at explaining difficult concepts in simple terms.

I do think it helps that I am reading We Need to Talk About Kelvin when I'm not studying. This, too, is a cracking book that manages to explain extremely complicated ideas in layman's terms. Doing a bit of reading around the subject definitely helps to seal ideas into your mind, and allows them to take hold.

Anyway - I digress. I was talking about the mole, and its eternal usefulness.

avogadro.jpegOne mole of any substance contains 6.02 x 10²³ atoms, molecules or ions (whichever is most appropriate) of that substance. So, one mole of marshmallows contains 6.02 x 10²³ marshmallows; one mole of water contains 6.02 x 10²³ water molecules; one mole of mercury contains 6.02 x 10²³ mercury atoms.

And, one mole of any substance has a mass equal to the relative mass of that substance, expressed in grams. So one mole of oxygen atoms has a mass of 16.0 g; one mole of oxygen molecules (it's a diatomic molecule, see) has a mass of 32.0 g. With me?

The Avogadro hypothesis (named after Amadeo Avogadro, an Italian physicist who died in 1856) asserts that this is true. Actually, it asserts that equal volumes of different gases, at the same temperature and pressure, contain equal numbers of molecules. Which is beautifully simple, and has the far-reaching consequences I mentioned above.

It enables the mass of any given substance to be translated directly into numbers of molecules (or atoms) using the Avogadro constant: the mole.

Thus: the number of moles of a substance is equal to the mass of that substance divided by the molar mass of the substance.

E.g. How many moles are in 52 g of water? Well, the molar mass of water is (2 x 1.01) + 16.0 = 18.02 g mol‾¹

So the number of moles in the water = 52 g divided by 18.02 g mol‾¹ = 2.89 mol (3 significant figures). There are 2.89 moles of water molecules in 52 g of water.


And the scariest thing? I'm quite enjoying it all! Next, I shall enthuse about covalent bonds. They are this: aces.

*I can't claim the credit for these analogies. They came from a rather cool chemistry site.

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