Monthly Archives: September 2016

Redox Rules

Click to download REDOX RULES posters for VCE Chemistry
Click to download REDOX RULES posters for VCE Chemistry

What’s redox? We never learned that!

Yes, you did. I use the term “redox” to refer to all of the following chapters in Heinemann Chemistry 2, which you will have learned at the end of Term 3 (September).

  • Chapter 26: Redox (revision of Year 11)
  • Chapter 27: Galvanic Cells
  • Chapter 28: Electrolytic Cells

Don’t underestimate redox

The VCAA has consistently used redox to discriminate which schools and students have the self-discipline required to keep studying at the end of the year. Studies show that redox is taught at a time when student motivation is at its minimum: energy levels are low, emotions are high, and graduation is just over the horizon. Many schools and students gloss over these topics because they’re running out of time, any many students think they’ve grasped the topic – when they’ve actually grasped misconceptions instead.

VCAA VCE Chemistry how difficult is each topic
Notice how chapters 26, 27 and 28 are consistently the most difficult and the most frequently askedClick to download PDF version

Here are some popular redox lies (misconceptions)

LIE #1: The polarities switch during recharge
Nope. The polarities never switch. It’s the labels of ‘anode’ and ‘cathode’ that switch because the electrons are flowing the other way through the external circuit. Polarity is permanent.

LIE #2: Hydrogen fuel cells don’t emit any greenhouse gases
Wrong. They emit H2O, which is a powerful greenhouse gas. If you don’t believe that the VCAA can be this pedantic, think again. Read their 2015 Examiners Report here.

LIE #3: Each mole of electrons forms 1 mol Ag, 2 mol Cu or 3 mol Al in a cell
Wrong again. If you look at the half-equations, you’ll see that each mole of electrons actually forms 1 mol Ag, 12 mol Cu or 13 mol Al. That’s why I teach “1, 12 and 13 moles” instead of the typical “1, 2, 3 moles” rule.

LIE #4: Temperature increases the rate of reaction in electroplating
Wrong! Remember that Faraday’s first law states that m ∝ Q. Because Q = I×t, only those two things – current and time – can affect the mass deposited at the cathode.

LIE #5: Electrons always leave the anode and go towards the cathode
Wrong again. Electrons go RACO: to see what that means, download the posters above. This question appears in recent versions of Chemistry Checkpoints. Give it a try.

LIE #6: The cathode is always positive
Ask your teacher.

LIE #7: Ions flow one way in the salt bridge
Nope. Anions always migrate to the anode; and cations always migrate to the cathode.

LIE #8: KOHES always works for balancing half-equations
KOHES only works for cells with acidic electrolytes. For cells with alkaline electrolytes, which sometimes appear in VCAA papers despite not being in the study design (see page 46 here), you’ll need to use KOHES(OH). Here’s KOHES(OH) explained:

  1. Do KOHES as normal
  2. Add the same number of OH(aq) ions to each side of the half-equation to balance out the H+(aq)
  3. Cancel and simplify. Remember that H+(aq) + OH(aq) makes H2O(l). Remember also to cancel out any remaining H2O(l).

LIE #9: I can balance an unbalanced redox equation by putting numbers in the equation
Don’t be fooled by this one! The ONLY way to balance an unbalanced redox equation successfully is to do the following:

  1. Separate it into two half equations
  2. Balance them using KOHES or KOHES(OH) as appropriate
  3. Multiply them and recombine
  4. Cancel and simplify
  5. Done!

That’s a lot of work but it’s the  only way to do it successfully. If you try to ‘cheat’ by just writing numbers (molar coefficients) in front of the reactants and products, you’ll find that the charges don’t add up, and you’ll get zero marks for the question.

LIE #10: I can break up polyatomic ions to make balancing half-equations easier
Nope! You’re only allowed to separate aqueous species in a half equation or an ionic equation. Because the Mn and O are actually bonded together in a polyatomic ion, you’ll need to write this:

  • MnO4(aq) + 8H+(aq) + 5e → Mn2+(aq) + 4H2O(l)  2/2 marks

Instead of this:

  • Mn7+(aq) + 5e → Mn2+(aq)  0/2 marks

If in doubt, keep it intact and it’ll cancel out by the end if it’s a spectator ion.

LIE #11: The two reactants that are closest together on the electrochemical series react
Not always true. Use SOC SRA instead, which is explained in the posters above. Still struggling? Ask your teacher or tutor for help.

LIE #12: Oxidants are all on the top of the electrochemical series
They’re actually on the left, and all the reductants can be found on the right side of each half equation in the electrochemical series. There is no top/bottom divide on the electrochemical series: only a left/right divide of oxidants/reductants.

Decorate your school/bedroom/hallway

Surround yourselves with truthful redox revision using these 17 free Redox posters. I’ve had these up around the whiteboard for a few weeks now – they’re a constant reminder to students that redox has many ideas that are always true.

One more tip: print and laminate an electrochemical series (available here) so you can annotate it during dozens of practice dozens without wasting paper. Good luck!

Mystery supervolcano is at the root of the ‘mad scientist’ stereotype

The Mad Scientist stereotype was caused ultimately by a supervolcano that nobody can locate to this day
The Mad Scientist stereotype was caused ultimately by a supervolcano that nobody can locate to this day

In 1808, a massive volcano erupted somewhere on Earth. So large was the eruption that it bellowed sulfate particles into the atmosphere that caused significant global cooling in the years that followed (Guevara-Murua 2014). Despite its gargantuan size, nobody to this day has been able to locate the volcano or find any direct eyewitness accounts of its eruption. The volcanic eruption of 1808 remains an unresolved scientific mystery to this day.

How do we know this mystery volcano ever erupted at all? The first piece of evidence is an increase in sulfuric acid concentration found in Greenland ice cores, which are a characteristic ‘chemical signature’ of sulfur-rich volcanic eruptions (Dai 1991). The only major spike in sulfuric acid concentration in Greenland ice that doesn’t align with a real volcanic eruption observed somewhere on Earth is the spike found around 1808, suggesting the existence of this mysterious volcano.

The second piece of evidence is called the ‘sulfur isotope anomaly’. Deposits of sulfur buried deep underground have a different isotopic composition compared with sulfur sources on the planet’s surface. In the same way that we can monitor the effects of fossil fuel combustion on atmospheric concentrations of carbon dioxide, we can quantify the amount of sulfur emitted from volcanoes by measuring changes in the relative quantity of sulfur-33. A huge spike in Δ33S suggests an enormous volcanic eruption occurred – and that’s exactly what we see when we study samples from the year 1808.

The third piece of evidence comes from trees. Trees grow at different rates depending on the climate. In particular, trees grow faster when it’s warmer (but not too hot, of course, which inhibits their growth somewhat), and they grow more slowly when it’s cold. Counting tree rings can reveal not only the age of the tree, but measuring the thickness of each tree ring allows researchers to estimate the amount of growth the tree accomplished in a given year. By measuring different trees in the same region, researchers can gain insight into the past climate of that particular region. Analysis of tree rings has shown that bristlecone pine trees had drastically decreased growth rates in the summer of 1809, suggesting the climate cooled significantly around that time (Salzer 2007). Cooling might have been caused by a giant volcano.

While none of this evidence amounts to a direct observation that the mystery supervolcano ever erupted, we do have eyewitness accounts of volcanic ejecta from exactly the same time. All the evidence, taken together, definitely points to the fact that the supervolcano did in fact exist. Scientists, in fact, are certain.

The first eyewitness account was written a highly respected Colombian scientist called Francisco José de Caldas, who described “a transparent cloud that obstructs the sun’s brilliance” over Colombia for several months from December 1808 to February 1809. The second eyewitness was a physician named José Hipólito Unanue who wrote about seeing “sunset afterglows” over Peru in the same time period. Both these observations are characteristic of large volcanic eruptions.

The fact that atmospheric haze was observed in both Colombia and Peru, which are in the southern and northern hemispheres respectively, suggest that this volcano was located somewhere in the tropics. These observations imply that ash was cast 2,600 km in all directions but the effect on the climate was global. One researcher is quoted as saying the mystery volcano “blanketed the planet in ash”. (Cole-Dai n.d.)

Vulcanologists rate volcanic eruptions on a scale called VEI (volcanic explosivity index), which is similar to the Richter scale for earthquakes. It’s a logarithmic scale that approximates the volume of ash that’s ejected by a particular eruption. The logarithmic nature of the scale means that while a VEI-3 eruption is called “severe”, a VEI-4 event is called “cataclysmic”. In 2010, Eyjafjallajökull erupted in Iceland, resulting in ash cloud so large that it caused severe delays to air traffic across Europe, Greenland, Russia and eastern Canada. The Eyjafjallajökull eruption was a VEI-4 (“cataclysmic”) event.

When Mount Saint Helens erupted in 1908, killing 57 people and causing $1.1 billion of damage across Canada and the US, it was classified by vulcanologists as a VEI-5 (“paroxysmic”) event. Alarmingly, the mystery volcano in 1808 was at least 10 times more devastating than Mount Saint Helens in terms of the volume of ash ejected. The mystery volcano was a VEI-6 event, and it’s described by vulcanologists as “colossal”.

Volcanic ash acts “like a giant window shade, reflecting sunlight and lowering temperatures on the ground for years afterward” (Cole-Dai n.d.). Temperatures across Europe were measurably lower in the years that followed as the ash cloud obscured incoming rays from the sun. Trees grew more slowly (as evidenced by tree ring data), harvests were diminished and the climate cooled for several years afterwards.

This cooling came at a very inconvenient time. Temperatures were already lower than usual in the northern hemisphere due to the Little Ice Age. In a further devastating blow, a second, much larger volcano erupted on April 10, 1815. It was located on Mount Tambora in Indonesia and had an intensity of VEI-7 or “super-colossal” (this is just one level away from VEI-8, which is named rather horrifyingly, “apocalyptic”). Mount Tambora’s eruption was so ‘super-colossal’ that 90% of the islanders on Tambora were killed by lava flowing down from the sky. Downpours of hot ash killed trees and fish for miles around, covering them with inches of grey dust. Hot ejecta was propelled eighteen miles into the air above the volcano producing a ‘boom’ that could be heard a thousand miles away. People across Indonesia mistook the volcanic ‘boom’ for a ship’s rescue signal or a bomb detonation. Some army officials across Indonesia’s vast archipelago even dispatched troops to defend their islands after mistaking the ongoing volcanic roar for the sound of an invading army.

The sulfur dioxide released from the super-colossal Mount Tambora explosion reacted with gases in the stratosphere to produce 100 million tons of sulfuric acid, H2SO4. The sulfuric acid condensed and remained suspended in an ‘aerosol cloud’ (basically a cloud) that was accelerated by stratospheric jet streams (basically very strong winds) until the entire globe was smeared with a thin layer of H2SO4. This is a rare event, and only happens following truly colossal volcanic eruptions. Interestingly, H2SO4 reflects incoming rays from the sun, and temperatures, which were already low as a result of the mystery supervolcano in 1808, were lowered yet again. The year 1815 was, as some writers put it, “the year without a summer”. Temperatures that year were about three degrees lower than usual across Europe, which is incredible considering that both volcanoes erupted near the equator.

If the Mount Tambora volcano was a little smaller, the sulfuric acid would have formed in the atmosphere instead, and would have rained back down to the surface as acid rain. But at stratospheric altitudes, far above the clouds, the sulfuric acid haze stayed there for years acting as a kind of sunscreen for our planet.

How does this relate to chemophobia? The combination of the Little Ice Age, the 1808 mystery eruption and the super-colossal eruption of 1815 had cooled the climate to such an extent that the weather in Lake Geneva was terrible in the summer of 1815. Who was there at the time? Mary Shelley, of course, who was staying indoors drinking because the weather was too bad to go boating. Cold, bored and disappointed at the lack of a ‘summer’ holiday, Shelley and her companions set about writing ghost stories instead. Among them was Frankenstein, which featured the original, quintessential stereotype of a mad scientist. The cliché lives on to this day.

Thanks, volcano.