Tag Archives: redox

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!

Chapter 27/28: Six Universal Principles of Redox Reactions

Redox can be a confusing topic for VCE Chemistry students. It’s also taught right at the end of the year, when students are tired and some teachers are rushing their lessons so they can finish the course before the end of Term 3. Student motivation levels are at their lowest time of the year, which means that students often finish the course with an incomplete understanding of Redox.

Fortunately, there are six universal principles that are always true in Redox no matter what type of cell is being studied.

First, here’s a reminder of the types of cells you need to have studied in this course.

Galvanic Cells

  • Primary (can’t be recharged)
  • Secondary (can be recharged)
  • Fuel Cells (reactants are supplied continuously)

Electrolytic Cells

  • Electroplating Cells (no overall reaction)
  • Electrolytic Cells (non-spontaneous reaction)
  • Commercial Cells (usually molten electrolytes)
  • Recharge reaction of a secondary cell (non-spontaneous)

Now, here are the six universal Redox principles.

1. The strongest oxidant at the cathode reacts with the strongest reductant at the anode (SOC SRA)

To predict which species will react with each other, circle all the species present at the cathode on the electrochemical series. The highest species on the left will always react. Now, circle all the species present at the anode… the lowest species on the right will react.

2. The half-reaction with the highest E° value is always positive

In all cells, the half-equation with the highest electrode potential (also called ‘reduction potential’ or E° value) always occurs at the positive electrode. Similarly, the half-equation with the lowest electrode potential (E°) will always occurs at the negative electrode.

3. OIL RIG

Oxidation is loss of electrons. Reduction is gain of electrons.

4. ←AN OIL RIG CAT→

Anode reaction (oxidation reaction) is whichever reaction is happening to the left in the electrochemical series.

Cathode reaction (reduction reaction) is whichever reaction is happening to the right in the electrochemical series.

5. Electrons always flow in this order (RACO)

Reductant → anode → cathode → oxidant

6. In the internal circuit, cations always flow to the cathode, and anions always flow to the anode.

The internal circuit might be an electrolyte or a salt bridge that contains soluble weak oxidants and reductants such as KNO3(aq) (potassium nitrate). Either way:

  • cations always flow to the cathode; and
  • anions always flow to the anode.

Keep practicing redox questions by completing past papers, Checkpoints and Lisachem questions. If you need more help, contact me via the Get a Tutor button in the site’s menu bar. Students learn much faster with a tutor than on their own.

For more Study Tools for Year 12 students, click here.

Image courtesy of Annenberg Learner

Bilingual Chemistry Classroom Posters

English-Chinese Chemistry Posters Classroom Set. Click to download editable Word document version.
Click to download editable Word document version

Decorate your Chemistry classroom with these 40 free bilingual Chemistry posters.

Topics include:

  • lab equipment;
  • redox;
  • ions;
  • organic nomenclature; and
  • molecular geometry.

Feel free to edit or share them.

Also… Get the famous ‘all-natural banana’ poster prints here.

Poster Selection 3

Remember to check out our T-Shirt Store with T-shirts in 7 languages!

Visit the T-Shirt Store for Chemistry T-Shirts Made in Australia in 7 Languages. Buy online.

Get your My First Physics Alphabet poster set here, in both Pink and Blue editions

My Physics Alphabet Poster Set of 4 in BLUE
My Physics Alphabet Poster Set of 4 in PINK

For more posters and free infographics, visit the Posters section of the site here.