recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

Electrochemical Cells and Ecell recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

Learning Outcomes recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

The Standard Cell Potential The standard cell potential E ө cell is the EMF between 2 half cells used measured under standard conditions. – All solutions of ions have a concentration of 1M – All gases are at 1atm – The system is at a stated temp, usually 298K. recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

Representing half cells: cell diagrams An electrochemical cell can be represented in a shorthand way by a cell diagram. The double vertical lines represents a salt bridge. The single lines represent a phase change between the solid metal and the aqueous metal ions. Zn (s) | Zn 2+ (aq) || Cu 2+ (aq) | Cu (s) The half cell with the greatest negative potential is on the left of the salt bridge, so E cell = E right cell – E left cell. In this case, E cell = – = V. E ө = VE ө = V The left cell is being oxidized while the right is being reduced. recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

The electrochemical series The electrochemical series is a list of standard electrode potentials (E ө ). The equilibria are written with the electrons on the left of the arrow, i.e. as a reduction Ag + (aq) / Ag (s) +0.34Cu 2+ (aq) / Cu (s) 02H + (aq) / H 2(g) -0.76Zn 2+ (aq) / Zn (s) -2.36Mg 2+ (aq) / Mg (s) E ө / VHalf equation Half cell Mg 2+ (aq) + 2e - Mg (s) Zn 2+ (aq) + 2e - Zn (s) 2H + (aq) + 2e - H 2(g) Cu 2+ (aq) + 2e - Cu (s) Ag + (aq) + e - Ag (s) Electrodes with negative values of E ө are better at releasing electrons (i.e. better reducing agents) than hydrogen. recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

REVIEW THE MORE NEGATIVE THE ELECTRODE POTENTIAL  The more reactive the metal  The more likely the metal to lose electrons (oxidised). recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

How do we measure the standard electrode potentials? As electrode potentials are relative to each other they need to be compared to a standard half cell (or reference electrode) to give individual cell potentials for the half cells. The common standard half cell to use is the Standard Hydrogen Cell. This has an EMF defined as 0V. recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

The standard hydrogen electrode

Combining half cells 2 recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

Combining half cells 1 recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

Calculating E cell The e.m.f of an electrochemical cell, E cell, is the difference between the standard electrode potentials of the two half cells. E cell = E ө (reduced) – E ө (oxidised) This can be worked out from the electrode potentials values in the electrochemical series. The positive electrode is taken to be the least negative half cell, and the negative electrode is the most negative half cell. recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential E cell = E ө (positive electrode) – E ө (negative electrode)

Calculating E ө cell : worked example An electrochemical cell is set up using the two half reactions below. What potential difference E cell would this cell generate? Give an overall cell reaction. E ө cell = E ө (positive electrode) – E ө (negative electrode) The zinc half cell has the more negative potential and so forms the negative electrode. Therefore: E ө cell = (+0.34) – (-0.76) Zn 2+ (aq) + 2e - Zn (s) E ө = VCu 2+ (aq) + 2e - Cu (s) E ө = V = V recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s)

Calculating E ө cell recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

Work to do Answer the summary questions on page 301 & 305 recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

Plenary recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

Jan The apparatus below can be used to measure the value of E ө cell for the reaction 2Fe 3+( aq) + Zn(s)  2Fe 2+ (aq) + Zn 2+ (aq) recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential

Jan Copper reacts with silver ions according to the reaction below. Cu(s) + 2Ag+(aq)  Cu2+(aq) + 2Ag(s) E cell for this reaction is A V B V C V D V Ag + +e -  Ag = +0.80v Cu e -  Cu = +0.34v recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential E ө cell = E ө (positive electrode) – E ө (negative electrode) E ө cell = 0.80– 0.34 = +0.46v

Jan Which answer corresponds to the correct value of Ecell for the oxidation of hydrogen peroxide by manganate(VII) ions? The half-reactions are 2H + + O 2 + 2e –  H 2 O 2 E = V MnO 4 – + 8H+ + 5e –  Mn2+ + 4H 2 O E = V The overall equation is 2MnO 4 – + 6H + + 5H 2 O 2  2Mn H 2 O + 5O 2 A E cell = V B E cell = – 0.83 V C E cell = – 0.38 V D E cell = V E ө cell = E ө (positive electrode) – E ө (negative electrode) E ө cell = 1.51– 0.68 = +0.83v recall the definition of standard cell potential understand the need for a reference electrode Calculate the standard cell potential