Tag Archives: school

Try the Most Difficult MCQ that VCAA Ever Asked (2021 edition)

Some multiple choice questions (approximately 2 out of every 30) are “tricky” – that is, they contain a distractor that students choose more frequently than the correct answer.

This collection of 10 multiple choice questions is entirely comprised of questions where students did worse than guessing (in other words, <25% of students chose the correct answer). One of these questions was so tricky that only 8% got it right.

Try these questions then scan the QR code at the end for the solutions.

VCE Chemistry Written Examination 2020 Analysis

The pandemic turned VCE Chemistry upside-down. Stoichiometry, traditionally a difficult topic, was the best-answered of all. Chromatography, traditionally an easy topic, was the most difficult for the class of 2020.

Most noticeable is the increase in “difficult” topics highlighted red in the chart above. (For a comparison with the 2013 & 2014 VCE Chemistry written examinations, click here.)

Unbelievably, the Victorian state average score in the 2020 VCE Chemistry written examination was a FAIL at just 47.9%.

Disruptions to learning caused by the pandemic could help explain why the VCAA is considering making the VCE Chemistry curriculum substantially easier from 2023 onwards. If the educational effects of the 2020 pandemic really do linger for most of this decade then making the curriculum easier fails to tackle the root of the problem, which is the loss of quality study-hours. I believe the only correct remedy is to provide current students with extra training and support to make up for the pandemic… not to drop the bar so low that our students cannot compete on the world stage.

Marks lost by topic. Chapter numbers refer to the Heinemann Chemistry 2 textbook.

What percentage of the highest achievers in each VCE subject have an Asian (Chinese) surname?

Data from VCE high achievers list 2018-2020 and the top 100 Asian surnames according to Wikipedia

This chart shows the percentage of students with a top 100 Asian surname among high-achieving VCE students (≥2 study scores ≥40) by subject with EAL students excluded from the analysis.

The proportion of high-achievers with Asian surnames was highest in the following subjects: Specialist Maths, Maths Methods, Physics, Chemistry, Accounting and English Language. Conversely, the least Asian subjects among high-achievers were Drama, Sociology and Theatre Studies.

All spelling variations of the top 100 Asian surnames listed on Wikipedia were included in the analysis, for example Li as well as Lee.

VCE Chemistry written examination paper analysis of marks allocated per chapter

Examination reports are very useful but most students don’t read them. I’ve scoured the examination reports from 2017, 2018 and 2019 and analysed how many marks were awarded for each topic of the VCE Chemistry course, and recorded what percentage of students got these right. As usual, this revealed that VCAA asks more questions on topics that students frequently get wrong.

Tip for students: focus more of your attention on the red topics in the chart above.

Chapter numbers refer to those used in the Heinemann Chemistry 2 textbook.

Focus on perfecting your written responses in VCE Chemistry… because that’s where most of the marks are awarded.

Students obsess over significant figures and mole calculations… but these are only worth 1 and 16 marks, respectively in the final written examination. Over two-thirds of the marks in the VCE Chemistry written examination are awarded for written responses where calculations are not necessary.

Tip for students: focus on perfecting your written responses such as explanations of bonding, chromatography, protein structures, and, most importantly, critiquing experimental designs.

I revamped this classic for the VCE Chemistry study design. Download it now: We Lied to You (2019 edition)

we lied to you cover 3d

This book is a collection of lies we taught to our Year 12 Chemistry students in their graduation year.

The lies include well-meaning simplifications of the truth, mistakes in the textbook, and, in a few extreme cases, blatant falsehoods.

This book isn’t a criticism of the VCE Chemistry course at all. In fact, I wrote this book to demonstrate the overwhelming complexity of Chemistry and the consequential need to make appropriate omissions and generalisations during our teaching as we tailor our lessons to the appropriate year level of students.

Rules taught as true usually work 90% of the time in this subject. Chemistry has rules, exceptions, exceptions to exceptions and so on. You’ll peel pack these layers of rules and exceptions like an onion until you reach the core, where you’ll find physics and specialist maths.

Click here to download We Lied to You (2019 edition).

Annotated VCE Chemistry data book for the 2019 study design

Each year, the VCAA subtly upgrades the VCE Chemistry data book. Each year, I print it and annotate it to show students the wealth of useful information hidden within it (most of which, is in plain sight).

This year, the VCAA has changed some “constants” and added some interesting functional groups to the spectroscopy tables. Smaller things are changed, too. All the protons in the 1H NMR table are now in bold; not just the ambiguous ones.

Free download: James Kennedy’s Annotated VCE Chemistry data book 2019 edition

Too big? Download the low resolution (4 Mb) version here

Start using this annotated version of the data book for your year 11 and year 12 chemistry homework exercises. While you can’t take this annotated version into the final examination (or into most SACs), seeing the annotations frequently throughout the two years will help you find things faster in the final examination.

Do you have feedback? Any comments? Do you require 1-to-1 chemistry tutoring? Email me at jameskennedymonash@gmail.com and I’ll get back to you personally.

making an annotated vce chemistry data book 2019 thumbnail

Get your Future-Proof VCE Chemistry Formula Book for just $55

James Kennedy's VCE Chemistry Formula Book 2017-2021Inspired by the formula booklets used by VCE Physics and VCE Maths Methods, here’s an 8-page Chemistry formula booklet you can use for your Year 11 and 12 Chemistry assignments. This custom-made booklet is a collection of reliable formulae that I have been using to answer VCE Chemistry questions while teaching and tutoring around Melbourne.

There are 76 formulae on 8 pages. At least 10 of these formulae aren’t in the three main chemistry textbooks. Orders are shipped in A4-sized booklet that resembles the VCAA Data Booklet.

Orders from schools, students and tutors are all welcome. Price includes free international delivery and a 10% voucher for the T-shirt store.

Order your copy now by clicking here

Learn from the best

James Kennedy achieved outstanding A-level results in 2006 in Maths, Chemistry, Physics and Biology. Those excellent grades (which equate to an ATAR of 99+) earned him a BA (Hons) degree and a Masters degree in Natural Sciences from the University of Cambridge.

Shortcut formulae were just one of the techniques James used to pass his A-level exams and get into Cambridge. Along with structured revision, revision guides, practice papers and study notes on wall-cards, James used shortcut formulae to save precious time in the examination hall. You can get your own copy of these original shortcut formulae – revised and updated for the 2017-2021 VCE Chemistry course – for just $55 including free international shipping. Click here to get your copy.

Here’s an preview of the inside

VCE Chemistry Formula Booklet, $55. Free, Fast Delivery Included.
VCE Chemistry Formula Booklet, $55. Free, Fast Delivery Included.

Click here to purchase your 8-page companion book with 76 formulae relevant for VCE Chemistry Units 1-4

Let’s add oxygen, fluorine and neon gases

Oxygen from Theodore Gray's amazing book, The Elements
Oxygen from Theodore Gray’s amazing book, The Elements

This post concludes the Periodic Table Smoothie experiment.

Recall that we’ve just finished adding one mole of nitrogen gas and created a bizarre boron polymer at the bottom of our vessel. The temperature was 350 °C and the pressure in our vessel was 891 kPa.

Today, we’re going to add 1.00 mole of oxygen gas, stand back and observe.

Nothing happens.

This is disappointing news.

Many of the substances in our vessel react (more accurately, explode) in the presence of oxygen but the ignition temperature for all of those explosions to take place is at least 500 °C. The temperature of our vessel is set at just 350 °C. At this temperature, nothing would actually happen.

There’s not enough activation energy to break bonds in the reactant particles in order to get the reaction started. We call this activation energy (EA) in chemistry. If we were to add a source of excessive heat (e.g. a matchstick), the vessel would explode.

Should we heat up the vessel to 500 °C and blow up the experiment right here?

If we did, the following reactions would happen:

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Enough of these reactions – particularly the first three – are sufficiently exothermic to trigger a chain reaction – at least up to the reaction of oxygen with beryllium carbide. The vessel would bang, explode, and shatter. The helium would float away, dangerous lithium amide would fly out sideways, and polyborazine powder, whatever that is, would land on the floor.

Let’s not ignite our experiment – not yet.

Conclusion after adding 1.00 mole of oxygen gas

Substance Amount in mol
He(g) 1.000
Be(s) 0.514
LiH(s) 0.000
Li2C2(s) 0.272
B2H6(g) 0.000
Be2C(s) 0.175
H2(g) 0.007
BeC2(s) 0.136
CH4(g) 0.009
N2(g) 0.552
NH3(g) 0.154
LiNH2(s) 0.277
polyborazine 12.194 grams

Pressure: 891 kPa (higher than before due to the addition of nitrogen gas)
Temperature: 350 °C (vessel is still being maintained at constant temperature)

Oxygen was relatively uneventful. Let’s add fluorine and see what happens.

Let’s add fluorine gas

elements110009
Elements by Theodore Gray

The following three reactions would all occur as 1.00 mole of fluorine gas is added:

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These two products are quite interesting:

  • HF, hydrogen fluoride, an aqueous solution of which was used by Breaking Bad’s Walter White to dissolve evidence (his victims)
  • NF3, nitrogen trifluoride, is used as an etching agent when making printed circuit boards (PCBs)

Let’s add neon gas

elements110010
Elements by Theodore Gray

When 1.00 mole of neon gas is added, the total pressure inside the vessel increases but no reaction occurs. The concentrations of all the other gases present are unaffected.

The End

That concludes our Periodic Table Smoothie experiment. The most interesting conclusion was the discovery of polyborazine, the bizarre solid that collected at the bottom of the vessel.

Also of interest was how easily we created ammonia, one of the simplest of biological compounds, just by mixing elements together. Could the compounds necessary for life be so easy to create that their existence is an inevitable consequence of the Big Bang? Is life inevitable? If the Big Bang were to happen all over again, would life occur? And would it look any different?

Possibly not.

We Lied To You

Click to download the book
click-here

This book contains 50 lies taught in the VCE Chemistry course.

These lies include well-meaning simplifications of the truth, mistakes in the textbook, and, in a few extreme cases, blatant falsehoods.

This book isn’t a criticism of the VCE Chemistry course at all. In fact, I just want to highlight the sheer complexity of Chemistry and the need to make sweeping generalisations at every level so it can be comprehensible to our students. This is a legitimate practice called constructivism in pedagogical circles. (Look that up.)

Many of these ‘lies’ taught at VCE level will be debunked by your first-year chemistry lecturers at university.

Here’s a preview of some of the lies mentioned in the book. Check out all 50 by clicking the download link at the bottom of the page.


lie-2


lie-15


lie-25


we-lied-to-you-cover-lightbulbs

click-here


Chemists need to speak the same language as the public

scientist-clipart-science-clip-art-school1
Chemists and the public need to be speaking the same language

Chemicals

The public uses the word ‘chemical’ to mean ‘synthetic substance’. Chemists have traditionally opposed this definition and stuck with ‘substance’ instead, responding with “everything is a chemical” in defence.

Arguing over definitions is futile and avoids the elephant in the room – that there’s been almost no public outreach to support the field of chemistry in the last few decades to counteract growing public skepticism of science (and of chemistry in particular).

Furthermore, it’s even more futile arguing over definitions when the Oxford English Dictionary provides a clear answer to this debate:

chemical (noun) – a distinct compound or substance, especially one which has been artificially prepared or purified

I ask all chemists to embrace the dictionary definition of ‘chemical’ and stop bickering with the public over definitions.

My main concern here is that if “everything is a chemical”, then it therefore follows that ‘chemophobia’ is the fear of everything, which is nonsensical. If we’re going to talk about chemophobia, we’re also going to have to accept the definition of chemical that the OED and the public have been using for a long time: that “chemical” = “artificially prepared substance”.

So what do we call non-synthetic chemicals? Try using a word with less baggage such as “molecule”, “compound”, “substance” or “element” where it’s relevant. By using these words, we avoid the natural=good/artificial=bad divide, which is the central assumption of chemophobia.

Chemophobia

‘Chemophobia’ is an irrational aversion to chemicals perceived as synthetic.

The word ‘chemophobia’ refers to a small subset of people who are not only disenfranchised by science, but who have subscribed to alternative sources of knowledge (either ancient wisdom or – sadly – Google). Many people with chemophobia are protesting against the establishment, and this is particularly evident in the anti-GMO movement. At the core of most people who oppose GMOs is a moral/political opposition to having their food supply controlled by giant corporations. No number of scientific studies concluding the safety and reliability of GMO crops will succeed in persuading them otherwise because the anti-GMO movement is founded on moral/political beliefs, not on science. By throwing science at them, we’re wasting our time.

More important than chemophobia

The Royal Society of Chemistry’s recent report on Public Perceptions of Science showed roughly a 20-60-20 range of attitudes towards chemistry.

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No matter how the RSC phrased the question, roughly 20% of the UK public who were surveyed indicated a negative attitude towards chemistry, and another 20% showed a positive attitude. The 60% in the middle felt disconnected from the subject – maybe disliked it in school – but felt neutral towards it when asked.

Chemophobia afflicts some people in the bottom 20%. They gave negative word-associations with ‘chemistry’ (e.g. ‘accidents’, ‘dangerous’ and ‘inaccessible’).That bottom 20% group is so vocal (e.g. Food Babe) that they distract chemists from the 60% in who are neutral. The ‘neutral’ crowd is a much larger audience that’s much easier to engage/persuade through outreach efforts. We should focus on talking to them.

Neil deGrasse Tyson has said in interviews that his huge TV hit show COSMOS was aimed at “people who didn’t even know they might like science”. That’s the middle 60%. Brian Cox’s amazing Wonders of the Universe was aimed at a similar audience – but chemistry has nothing similar to offer. We’re engaging those who are already interested (with academic talks and specialist journals) and we’re engaging with the bottom 20% via social media and comments on foodbabe.com… but why haven’t we started engaging the middle 60%, who gets most of their science information from TV? How many chemistry TV icons can you name? Where are the multi-channel launches of big-budget chemistry documentaries*? Chemistry is lagging far behind biology and physics in that regard.

*BBC Four’s Chemistry: A Volatile History (2010) doesn’t count – it was only three episodes long, got no further than ‘the elements’ and was presented by a PHYSICIST!

Focus on the 60% who are ‘neutral’

I ask chemists to focus on addressing the disinterested 60%. From an outreach perspective, this is much more fun and is positive rather than reactionary. By engaging those who feel neutral about chemistry, we might even empower enough of the public to fight chemophobia (online, at least) by themselves – without our direct intervention.

I urge chemists to tell the public what you do in simple terms. Describe your work to the public. Tweet about it. Participate in your university/faculty’s YouTube videos by explaining your work and its relevance. Offer advice as a science correspondent for local media outlets (many universities have ‘expert lines’ – get involved). Give your ‘talk’ at local schools – it make a HUGE difference to students’ perceptions of science. Devote 5% of your working time to doing outreach. As a teacher, I’m practically doing it full-time.

Plus, we urgently need a chemistry TV hero. Could someone do that, too, please?

Let’s add nitrogen gas

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‘Nitrogen’ page from Theodore Gray’s amazing book, ‘The Elements’

Initial conditions

Recall from last week that our Periodic Table Smoothie contains the following species:

Substance Amount present (moles)
He(g) 1.00000
Be(s) 0.51435
LiH(s) 0.27670
Li2C2(s) 0.27165
B2H6(g) 0.23300
Be2C(s) 0.17470
H2(g) 0.14267
BeC2(s) 0.13625
CH4(g) 0.00949

Pressure: 718 kPa
Temperature: 350 °C

Reactions of nitrogen in our 10-litre vessel

Our freshly-added 1.00 mol of nitrogen gas, N2(g), reacts with hydrogen gas to make ammonia in the following reversible (equilibrium) reaction. We will assume that the interior metal surface of the vessel is a suitable catalyst for this reaction (e.g. iron). 

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There are three other reactions below that might have occurred at higher temperature, but I’ve chosen not to raise the temperature of the vessel at this point. Rather, we’ll keep it at 350 °C to keep things manageable.*

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*I was tempted at this point to elevate the temperature of our vessel to 500 °C so that the second reaction could take place as well. This would produce copious amounts of smelly ammonia gas, which would allow for larger quantities of interesting organic compounds to be produced later on. To keep our simulation safe and (relatively) simple, I’ve decided to keep the vessel at 350 °C. Interesting compounds organic will still form – only in smaller amounts.

Equilibria

The ammonia reaction above (the first equation) is actually an equilibrium reaction. That means that the reactants are never completely used up, and the yield is not 100%.

Recall from Le Châtelier’s principle that removing product from an equilibrium reaction causes the position of equilibrium to shift to the right, forming more product. This is because:

“If an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of the change.” – Le Châtelier’s principle

There are three reactions that will remove ammonia from our vessel while it’s being produced, and I’ve put all three of these into the simulation. One of these is the reverse of the reaction above (producing hydrogen and nitrogen gases) and the other two are described below. Let’s take a look at those other two reactions.

With what will the ammonia react in our vessel?

Ammonia can undergo the following reactions with the other things in our vessel**

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**The ammonia does react with methane and beryllium as well, but only at temperatures of 1200 °C and 600 °C, respectively.

Two compounds will be formed: lithium amide and borazine.[1] Lithium amide reacts with nothing else in the vessel, so the reaction chain stops there. Borazine, on the other hand, is much more interesting.

We’ve made borazine!

Borazine is a colourless liquid at room at temperature. It boils at 53 °C and has a structure that resembles that of benzene.

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Borazine is isostructural with benzene
Because of the electronegativity difference of about 1.0 between the B and N atoms in the ring, borazine has a mesomer structure:

600px-borazin_mesomers

Like benzene, there is partial delocalisation of the lone pair of electrons on the nitrogen atoms.

Borazine polymerises into polyborazine!

Fascinatingly, borazine polymerises into polyborazine at temperatures above 70 °C, releasing an equal number of moles of hydrogen gas.[2] Polyborazine isn’t particularly well-understood or well-documented, but one recent paper suggested it might play a role in the creation of potential ceramics such as boron carbonitrides. Borazine can also be used as a precursor to grow boron nitride thin films on surfaces, such as the nanomesh structure which is formed on rhodium.[3]

Like several of the other compounds we’ve created in our Periodic Table Smoothie, polyborazine has also been proposed as a hydrogen storage medium for hydrogen cars, whereby polyborazine utilises a “single pot” process for digestion and reduction to recreate ammonia borane.

polyborazylene_polymer
Polyborazine’s chemical structure
The hydrogen released during the polymerisation process will then react further with a little bit of the remaining nitrogen to produce a little more NH3(g) – but not much. Recall from earlier that the ammonia reaction is an equilibrium one, and the yield of NH3(g) at pressures under 30 atmospheres is very low. Pressure in our vessel is still only around 7 atmospheres.

Simulation results

nitrogen smoothie.png

Once polymerised, this would form about 12 grams of polyborazine:

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As far as I’m aware, no further reactions will take place in the vessel this week.

Conclusion after adding 1.00 mole of nitrogen gas

Substance Amount in mol
He(g) 1.000
Be(s) 0.514
LiH(s) 0.000
Li2C2(s) 0.272
B2H6(g) 0.000
Be2C(s) 0.175
H2(g) 0.007
BeC2(s) 0.136
CH4(g) 0.009
N2(g) 0.552
NH3(g) 0.154
LiNH2(s) 0.277
polyborazine 12.194 grams

Pressure: 891 kPa (higher than before due to the addition of nitrogen gas)
Temperature: 350 °C (vessel is still being maintained at constant temperature)

Next week, we’ll add a mole of oxygen gas to the vessel. Warning: it might explode.

References

  1. Stock, Alfred and Erich Pohland. “Borwasserstoffe, VIII. Zur Kenntnis Des B 2 H 6 Und Des B 5 H 11”. Berichte der deutschen chemischen Gesellschaft (A and B Series) 59.9 (1926): 2210-2215. Web.
  2. Mohammad, Faiz. Specialty Polymers. Tunbridge Wells: Anshan, 2007. Print.
  3. Toury, Berangere and Philippe Miele. “A New Polyborazine-Based Route To Boron Nitride Fibres”. Journal of Materials Chemistry 14.17 (2004): 2609. Web. 4 May 2016.

Let’s add carbon (graphite) powder

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‘Carbon’ page from Theodore Gray’s amazing book: The Elements

Today, we’re going to add 1.00 mole of carbon to our vessel. After adding boron last week, we left our vessel locked at 350 °C and with a pressure of 638 kPa. These reactions are taking place at 350 °C and constant volume (exactly 10 litres). Pressure inside the vessel will therefore change over time.

Allotropes of carbon

Carbon has various allotropes (structural arrangements of an element). Diamond is extremely strong and highly unreactive, while graphite is soft and brittle. The differences are all due to the type of bonding between carbon atoms. In diamond, carbon atoms are bonded by four strong covalent bonds with the surrounding atoms in a strong, hard three-dimensional ‘network lattice’. Graphite owes its softness and brittleness to the fact that its carbon atoms are bonded by only three strong covalent bonds in a two-dimensional ‘layer lattice’. Individual layers are very strong, but the layers can be separated by just the slightest disturbance. Touching graphite lightly onto paper will remove layers of carbon atoms and place them onto the page (such as in a pencil). Using a diamond the same way would likely tear the paper instead.

For this reason, I’m going to put graphite into the vessel instead of diamond. Diamond is so strong and inert that it’s unlikely to do any interesting chemistry in our experiment. Graphite, on the other hand, will.

The following seven chemical reactions will take place after adding carbon (graphite) powder

As soon as the carbon powder enters the vessel, it will begin to react with the following three species as follows:

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The ethyne produced in the third reaction will then react with any lithium and beryllium remaining in the vessel as follows:

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The hydrogen gas produced by the above two reactions will then react with lithium and carbon (if there’s any left) as follows:

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These reactions have the potential to all occur at the same time. Tracking them properly would require calculus and lots of kinetics data including the activation energy of each reaction and the rate constant for each equation. Quick searches on the National Chemical Kinetics Database yields no results for most of these equations, which means we won’t be able to use a computer model to calculate exact quantities of each product. Instead, I’m going to run a computer simulation using Excel that makes the following three assumptions:

  • all these reactions occur at the same rate;
  • all these reactions are first-order with respect to the limiting reagent;
  • all these reactions are zeroth-order with respect to reagents in excess.

The results will be a close approximation of reality – they’ll be as close to reality as we can get with the data that’s available.

Here are the results of the simulation

Here’s a graph of the simulation running for 24 steps. Exactly one mole of carbon powder is added at step 5.

Periodic Table Smoothie - Let's Add Carbon Carbon quickly reacts to form lithium carbide, beryllium carbide and two organic molecules: methane and ethyne
Carbon quickly reacts to form lithium carbide, beryllium carbide and two organic molecules: methane and ethyne

Summary of results

The results are incredible! We’ve made ethyne and methane, both of which have the potential to do some really interesting chemistry later on. I’m hoping that we can make some more complex organic molecules after nitrogen and oxygen are added – maybe even aminoethane – let’s see.

Hydrogen has also re-formed. I’m hoping that this gas lingers for long enough to react with our next element, nitrogen: we might end up making ammonia, NH3(g).

You may have noticed that I removed the “boron-lithium system” from the vessel. The 0.178 moles we created are now stored separately and will not be allowed to react any further. With such little literature about the reactivity of B3Li, it’s impossible to predict what compounds it’ll form later on. B3Li is so rare that doesn’t even have a Wikipedia page.

Pressure in the vessel increases to 718 kPa after carbon is added
Pressure in the vessel increases to 718 kPa after carbon is added

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Mass of sludge in the vessel changes after adding carbon

Here’s what’s present in the vessel after adding carbon

Substance Moles present after 500 ‘steps’
He(g) 1.00000
Be(s) 0.51435
LiH(s) 0.27670
Li2C2(s) 0.27165
B2H6(g) 0.23300
Be2C(s) 0.17470
H2(g) 0.14267
BeC2(s) 0.13625
CH4(g) 0.00949

We also have 0.178 moles of B3Li stored separately in another vessel.

Next week, we’ll add nitrogen and see what happens.

Let’s add boron powder

elements110005
‘Boron’ page from Theodore Gray’s book, The Elements

Boron is a metalloid: an intermediate between the metals and non-metals. It exists in many polymorphs (different crystal lattice structures), some of which exhibit more metallic character than others. Metallic boron is non-toxic, extremely hard and has a very high melting point: only 11 elements have a higher melting point than boron.

British scientist Sir Humphrey Davy described boron thus:

“[Boron is] of the darkest shades of olive. It is opake[sic], very friable, and its powder does not scratch glass. If heated in the atmosphere, it takes fire at a temperature below the boiling point of olive oil, and burns with a red light and with scintillations like charcoal” – Sir Humphrey Davy in 1809

Initial condition

Before we add the 1.00 mol of boron into our reaction vessel, we need to recall what’s already in there from our experiments so far:

  • H2(g): 0.70 mol
  • He(g): 1.00 mol
  • Li(s): 0.40 mol
  • LiH(s): 0.60 mol
  • Be(s): 1.00 mol

The temperature of our vessel is 99 °C and the pressure of the gaseous phase is 525.5 kPa.

Now, let’s add our 1.00 mol of boron powder.

Which reactions take place?

Boron reacts with hydrogen gas to produce a colourless gas called borane, BH3(g), according to the following equation[1]:

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Boron also reacts with lithium in very complex ways. If we heat the vessel up to 350 °C, we’d expect to see the formation of a boron-lithium system with chemical formula B3Li according to this equation[2]:

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Notice that now we’ve heated up our vessel to 350 °C to allow this reaction to happen, the lithium at the bottom of the vessel has melted.

Boron reacts with lithium hydride as well, but only at temperatures around 688 °C. With our vessel’s temperature set at 350 °C, we won’t observe this particular reaction in our experiment.[3]

Some allotropes of boron – in particular, the alpha allotrope that was discovered in 1958 – is capable of reacting with beryllium to form BeB12. Because we’re using beryllium powder, which has semi-random  symmetry, we won’t see any BeB12 forming in our vessel. Alpha-boron only exists at pressures higher than around 3500 kPa. At our moderate pressure of only 525.5 kPa, powdered (semi-random) boron will prevail and no BeB12 will form.[4]

For simplicity’s sake, let’s assume that the two reactions above take place with equal preference.

Boron powder reacts with hydrogen gas

Let’s do an ice table to find out how much borane we make.

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A quick n/ratio calculation shows us that the hydrogen gas is limiting in this reaction:

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We can expect all of the hydrogen gas to react with the boron powder.

units are mol 2 B 3 H2 2 BH3
I 0.50 0.70 0
C -0.466 -0.70 +0.466
E 0.0333 0 0.466

Borane is very unstable as BH3, and it would probably dimerise into B2H6(g). This is still a gas at 350 °C and is much more stable than BH3. For the rest of this experiment we’ll assume that our 0.466 mol of BH3 has dimerised completely into 0.233 mol of B2H6.

Boron powder reacts with lithium

With the molar ratios present in our vessel, at 350 °C, we’d expect to witness the formation of a boron-lithium system, with chemical formula B3Li.

image030.png

A quick n/ratio calculation shows that in this reaction, the boron powder is limiting.

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All of the remaining boron therefore reacts with lithium. To calculate exactly how much B3Li we’ve created, let’s do another ice table:

units are mol 3 B Li B3Li
I 0.533 0.40 0
C -0.533 -0.178 +0.178
E 0 0.222 0.178

What’s in our vessel after adding boron?

We have the following gas mixture in our vessel:

Helium gas, He(g): 1.00 mol

Helium is an inert noble gas that will probably remain in the vessel until the end of the experiment. It’s used in party balloons.

Borane gas, B2H6(g): 0.233 mol

We made this today. Borane is used in the synthesis of organic chemicals via a process called hydroboration. An example of hydroboration is shown below.

250px-hydroboration-oxidation_of_1-methyl-cyclohex-1-ene

At the bottom of the vessel, there’s a sludge, which contains the following liquids and solids:

Molten lithium, Li(l): 0.22 mol

Lithium is used in the production of ceramics, batteries, grease, pharmaceuticals and many other applications. We’ve got 0.22 moles of lithium, which is about 1.5 grams.

Beryllium powder, Be(s): 1.00 mol

Beryllium is used as an alloying agent in producing beryllium copper, which is used in springs, electrical contacts, spot-welding electrodes, and non-sparking tools.

Lithium hydride, LiH(s): 0.60 mol

Lithium hydride is used in shielding nuclear reactors and also has the potential to store hydrogen gas in vehicles. Lithium hydride is highly reactive with water.

Boron-lithium system, B3Li(s): 0.178 mol

We made this today… but what is it? Not much is known about this compound – in fact, it doesn’t even have a name other than “boron-lithium system, B3Li”. It’ll probably decompose eventually in our experiment – maybe when we alter the pressure or temperature of the vessel at some later stage. We’ll need to keep an eye on this one.

The original H2(g) and B(s) have been reacted completely in our experiment.

What’s the pressure in our vessel now?

At the end of our reaction, the temperature of our vessel is still set at 350 °C and the pressure of the gaseous phase inside the vessel can be calculated to be a moderate 638 kPa as follows:

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*It should also be noted that some evidence exists for a reaction between LiH and BH3, forming Li(BH4). The reaction seems to take place stepwise with increasing temperature. A quick read of this paper suggests that in our vessel, which is at 350 °C, any Li(BH4) formed would actually break back down into boron powder and hydrogen gas, which would in turn react with each other and with lithium metal to form BH3 and LiH again. The net result would be a negligible net gain of LiH and a negligible net loss of boron powder. We will continue calculating this Periodic Table Smoothie under the assumption that if any Li(BH4) forms, it breaks down before we add the next element, and the overall effect on our system is negligible.

**Li(BH4) is an interesting compound: it’s been touted as a potential means of storing hydrogen gas in vehicles – it’s safer and releases hydrogen more readily than LiH, which was mentioned above.[5]

Next week, we’ll add element number 6, carbon, and see what happens.

References

  1. “Borane”. Wikipedia. N.p., 2016. Web. 14 Apr. 2016.
  2. Okamoto, H. “The B-Li (Boron-Lithium) System”. Bulletin of Alloy Phase Diagrams 10.3 (1989): 230-232.
  3. Matkovich, V. I. Boron And Refractory Borides. Berlin: Springer-Verlag, 1977. Print.
  4. Gaulé, G. K. Boron, Volume 2: Preparation, Properties And Applications. New York: Plenum Press, 1965. Print.
  5. Saldan, Ivan. “A Prospect For Libh4 As On-Board Hydrogen Storage”. Open Chemistry 9.5 (2011): n. pag. Web.