Quartz 4

ElectroChemistry Main Note

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Note

Chapters to Refer to:

Definition

Electrochemistry is the process of using electricity to break down or decompose a compound into its separate components (usually ionic compound in the molten form or aqueous solution)

  • Compound is known as an electrolyte and electricity → is conducted through it → in order for decomposition to take place.

Important

Can be covalent compound with ionic character. These covalent substances → ionise (dissociate to form ions) e.g. in solution of hydrogen chloride → when dissolved in water/aqueous solution

Importance of Electrolysis

  • Extraction of useful pure elements → from their molten compounds. Example of useful pure elements include → very reactive metals such as group 1 metals and non metals such as chlorine

What is an electrolytic cell

Definition

electrolytic cell is a device that converts electrical energy into chemical energy

  • Electrolysis of compound → takes place in electrolytic cell
  • Electrolytic cell → uses electricity → drive non spontaneous chemical reactions (redox reaction). e.g. sodium chloride does not decompose on heating to produce sodium and chlorine gas. To obtain sodium, molten sodium chloride must be electrolysed
  • Hence energy conversion in electrolytic cell:
    • Electrical energy → chemical energy

Note

non spontaneous chemical reaction is one which does not occur on its own without an external input of energy.

  • requires continuous supply of energy (heat, electricity, light) to proceed

Parts of Electrolytic Cell

Parts of Electrolytic Cell

Questions to Understand for this Chapter

Note

  1. Explain in terms of bonding and structure, why water doesn’t conduct electricity
  2. Explain why graphite is a good conductor of electricity
  3. Explain why sodium chloride is found to be a good conductor of electricity in molten and aqueous state but not solid state
  4. Explain why dilute HCl is better conductor of electricity compared to dilute Ethanoic acid even if both concentrations and volumes are same

Important

Oxidation ALWAYS occurs at the anode and reduction ALWAYS occurs at the cathode

Electrolytic Cell which uses Inert (Unreactive) Electrodes.

Electrolytic Cell which Uses Inert (Unreactive) Electrodes

Writing Half Equations

Electrolysis of Molten Ionic Compounds Using Inert Electrodes

Electrolysis of Molten Ionic Compounds Using Inert Electrodes

Sample

  • always write down “anode or positive electrode” or “cathode or negative electrode”
  • Note the correct way to describe the reaction at the electrodes.
    • e.g.
      • chloride ions are discharged at the anode → as they undergo oxidation → by losing electrons to form chlorine molecules
      • sodium ions are discharged at the cathode as they undergo reduction to form sodium atoms
  • common incorrect way of describing reactions → is identifying particles wrongly.
  • e.g. → chlorine is discharged at anode and sodium is discharged at cathode.

Electrolysis of Aqueous Solutions Using Inert Electrodes

Electrolysis of Aqueous Solutions Using Inert Electrodes

Products of Electrolysis OF ANY SOLUTION can be predicted by Following the Steps:

  1. identify cations and anions present in solution

  2. For anions: i) oxygen is formed if solution

    • Has sulfates or nitrates (concentrated or dilute)
    • Is dilute (e.g. dilute HCl, dilute Sodium chloride etc)

    ii) Halogens () are formed if halides () → are present in high concentrations (e.g. concentrated sodium chloride, concentrated HCl)

  3. For cations → refer to Reactivity Series Main Note

    • ions of metals → above hydrogen → will not be discharged → ions will remain in solution → hydrogen gas is discharged instead.
    • ions of metals → below hydrogen → will be discharged → metal will be deposited on electrode

  4. Identify cation and anion → remaining in solution(this is the product formed in the solution) Solution may cause:

    • change in pH value (if or is discharged → resulting in change in concentration of ion)
    • Increase in concentration (if both or are discharged) (alkalinity and acidity respectively)
    • change in colour intensity (e.g. if is discharged form Cu, blue solution fades or turns colourless and pink/red-brown solid is formed)
Example!

Products of Electrolysis of following electrolytes → predicted using 4 steps above. Inert Electrodes are used for electrolysis of these substances Electrolysis of Concentrated Aqueous Sodium Chloride, Dilute Sulfuric Acid, Aqueous Copper(II) Sulfate

Important

equation for oxygen gas produced at the anode. 4e^-$

Sample

During electrolysis, it was observed that mass of anode decreases and white precipitate forms. suggest explanation

  • as electrolysis progresses, the use of electricity causes electrodes to heat up over time. At higher temperatures, oxygen gas produced at anode will react with carbon anode to produce carbon dioxide gas, resulting in the loss in mass of anode.
  • carbon dioxide gas in turn reacted with calcium hydroxide (electrolyte, limewater) to form a white precipitate of calcium carbonate (insoluble - Salts Main Note)

Electrolysis of Aqueous Solutions Using Reactive Electrodes

Electrolysis of Aqueous Solutions Using Reactive Electrodes

Industrial Applications of Electrolysis

Industrial Applications of Electrolysis

Simple Cells (Electric, Galvanic, Voltaic Cells)

Simple Cells (Electric, Galvanic, Voltaic Cells)

Sample

explain why theoretical ratio of hydrogen to oxygen is 2:1

  • 4 moles of electrons are transferred from anode to cathode, which results in the formation of 2 mol of H2 gas at cathode and 1 mol of O2 gas at the anode.
  • according to Avogadro’s law, 1 mol of any gas occupies 24dm3 at r.t.p. thus the mol ratio of H2 to O2 produced in reaction is the same as its volume ratio which is 2:1

Graph View

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