Tag Archives: electrolysis

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!

Kennedy Rainbow Cell

Kennedy Rainbow Cell electrolysis chemistry demonstration initial setup aerial view
Initial Kennedy Rainbow Cell setup

Demonstrate electrolysis with an electrolytic cell in a petri dish.

Materials

  • 1 × Large petri dish
  • 1 × DC Power pack
  • ~50 mL Distilled water dH2O(l)
  • ~3 g potassium nitrate powder KNO3(s)
  • 2 × Graphite electrodes
  • 2 × Wires with crocodile clips
  • 1 × Clamp and stand
  • 1 × Very strong static magnet
  • 1 × Roll of sticky tape (any type)
  • ~10 drops of universal indicator
  • ~50 mL dilute HNO3(aq)
  • ~50 mL dilute KOH(aq)
  • 1 × Spatula

Method

  1. Place petri dish on clean, light-coloured bench and add distilled water until it is two thirds full
  2. Add ~10 drops of universal indicator and observe the colour.
    Q: What pH is the distilled water? (You’ll be surprised!)
    Q: Why is/isn’t the colour green?
  3. Add ~3 g of potassium nitrate to the petri dish and stir using a spatula until completely dissolved
  4. Adjust the pH of the distilled water carefully using the nitric acid and potassium hydroxide as required. Try to make the universal indicator colour green (as pictured) ~pH 7
  5. Attach one electrode to each of two wires using crocodile clips
  6. Dip each graphite electrode into the green solution at opposite sides of the petri dish. Hold these electrodes (and wires) in position by in position by sticky-taping each wire to the surface of the workbench
  7. Demonstrate the strength of the magnet by attaching it to the clamp. Carefully, clamp the magnet into the clamp and position the magnet 2 mm above the surface of the green solution
  8. Ensuring the power is turned off, very carefully, attach the wires to the DC power pack according to the manufacturer’s instructions
  9. Turn the voltage to zero (or very low) and turn on the power pack
  10. Turn the voltage up slowly (12 volts worked well) and observe any changes you might see in the Kennedy Rainbow Cell

Extensions

  • Turn off the power pack and stir the solution. Explain why the colour goes back to being green. (If it’s not green, explain that, too!)
  • Turn the magnet upside-down (TURN OFF THE POWER FIRST)
  • Reverse the polarity of the wires
  • Use AC current instead of DC
  • Use different indicators
  • Why would using NaCl(aq) be dangerous in this cell?
  • How can you maximise the swirling?
  • How can you make this experiment much more epic?

Click to download Kennedy Rainbow Cell worksheet (PDF)

Safety considerations

  • Make your own risk assessment before carrying out this experiment
  • The strong magnet is capable of attracting both wires to itself. Don’t be touching the exposed parts of the crocodile clips when this happens. If this does happen, immediately turn off the power pack and fix the problem. Secure the wires with more tape. Don’t touch the electrodes while the Cell is operating.
  • Don’t use chloride salts or hydrochloric acid in this experiment. The voltages involved can cause the production of toxic chlorine gas if sodium chloride is used. Use nitric acid and potassium nitrate instead.
  • Make sure the wires don’t touch each other.
  • Again, make your own risk assessment before carrying out this experiment

Video

Disclaimer

This cell is potentially dangerous. I accept no responsibility for and loss, damage or injury caused by the operation of a Kennedy Rainbow Cell. If you’re under 18, always get adult permission before you make this type of cell.