Starter: Hydrogen sulfide gas is bubbled through iodine solution. The orange colour slowly fades. The final solution is cloudy. The liquid contains a yellow solid. 1. Write a couple of sentences to link the observations to the species 2. Write two half equations and combine to a balanced full equation 3. Which species is oxidised and which is reduced. Give reasons.

Answer The aqueous iodine is orange and becomes colourless iodide ions. The colourless hydrogen sulphide gas becomes sulfur, a yellow solid. H2S(g) → S(s) + 2H+(aq) + 2e– I2(aq) + 2e– → 2I–(aq) H2S(g) + I2(aq) → S(s) + 2H+(aq) + 2I–(aq) The H2S is oxidised because it has lost electrons and the oxidation number of S has increased from -2 to 0. The I2 is reduced as it has gained electrons and the oxidation number of I has decreased from O to -1.

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Electrochemistry 301 Chemistry

Electrochemistry Electrochemistry is the chemistry of reactions that involve the transfer of electrons. In spontaneous reactions, electrons are released and used in electrochemical cells. In non spontaneous reactions, electrons have to be supplied to produce chemicals that are wanted in electrolytic cells or electrolysis.

Electrochemical Cells Copper Sulfate solution Zinc

Electrochemical Cells This is a REDOX reaction Copper Sulfate solution fades Zinc becomes coated with Cu

Electrochemical Cells If zinc is put into copper sulfate solution, a redox reaction will occur: Zn(s) + Cu(NO3)2 (aq)  Zn(NO3)2 (aq) + Cu(s) This redox reaction can be written as 2 half equations: Zn(s)  Zn 2+(aq) + 2e- Cu 2+(aq) + 2e -  Cu(s) These two half equations can occur in separate beakers provided there is a path for the electrons to travel (a wire) and a path for the ions to travel (a salt bridge)

Anode (-) Cathode (+) Salt Bridge The cathode is the electrode at which reduction occurs. The anode is the electrode at which oxidation occurs. Salt Bridge

> > > > Zn(s) → Zn2+(aq) + 2e- Cu2+(aq) + 2e- → Cu(s) The two half equations tell us which way the electrons will flow. i.e. from Zn to Cu. The Copper electrode will increase in mass and the solution will get lighter blue. The Zinc electrode will be eroded. The SALTBRIDGE completes the circuit. Anode (-) Cathode (+) > > > > Salt Bridge

Zn(s) /Zn 2+(aq)//Cu 2+(aq)/Cu(s) Cell Diagrams Instead of drawing pictures to show the set-up, a shorthand system has been devised The Zinc – Copper or Daniell cell is shown as: Zn(s) /Zn 2+(aq)//Cu 2+(aq)/Cu(s)

Zn(s) /Zn 2+(aq)//Cu 2+(aq)/Cu(s) // means a salt bridge. / means a physical boundary between reactants (eg between a solid and a gas or solution A comma is used to separate 2 reactants in the same phase (eg 2 solutions)

Rules Write the LHE first Write the left-hand cell as oxidation (ie reduced form first then the oxidised form). Write the right-hand cell as reduction (ie oxidised form first then the reduced form). Write the RHE last Example: Set up an electrochemical cell with the Mg2+/Mg half-cell and the Cr2O72-/Cr2+ half-cell Mg(s)/

Rules Write the LHE first Write the left-hand cell as oxidation (ie reduced form first then the oxidised form). Write the right-hand cell as reduction (ie oxidised form first then the reduced form). Write the RHE last Example: Set up an electrochemical cell with the Mg2+/Mg half-cell and the Cr2O72-/Cr2+ half-cell Mg(s)/Mg2+(aq

Rules Write the LHE first Write the left-hand cell as oxidation (ie reduced form first then the oxidised form). Write the right-hand cell as reduction (ie oxidised form first then the reduced form). Write the RHE last Example: Set up an electrochemical cell with the Mg2+/Mg half-cell and the Cr2O72-/Cr2+ half-cell Mg(s)/Mg2+(aq)//

Rules Write the LHE first Write the left-hand cell as oxidation (ie reduced form first then the oxidised form). Write the right-hand cell as reduction (ie oxidised form first then the reduced form). Write the RHE last Example: Set up an electrochemical cell with the Mg2+/Mg half-cell and the Cr2O72-/Cr2+ half-cell Mg(s)/Mg2+(aq)//Cr2O72-(aq)

Rules Write the LHE first Write the left-hand cell as oxidation (ie reduced form first then the oxidised form). Write the right-hand cell as reduction (ie oxidised form first then the reduced form). Write the RHE last Example: Set up an electrochemical cell with the Mg2+/Mg half-cell and the Cr2O72-/Cr2+ half-cell Mg(s)/Mg2+(aq)//Cr2O72-(aq),Cr3+(aq)

Rules Write the LHE first Write the left-hand cell as oxidation (ie reduced form first then the oxidised form). Write the right-hand cell as reduction (ie oxidised form first then the reduced form). Write the RHE last Example: Set up an electrochemical cell with the Mg2+/Mg half-cell and the Cr2O72-/Cr2+ half-cell Mg(s)/Mg2+(aq)//Cr2O72-(aq),Cr3+(aq)/Pt(s)