Electrode potentials

Cards (17)

  • When a metal is dipped into a solution containing its ions an equilibrium is established between the metal and its ions. this is the basis of a half cell in an electrochemical cell.
  • What do electrochemical cells do?
    They produce electricity
  • What is an electrochemical series ?
    Electrochemical series ranks the reducing power of half equations.
    • reading each half equation forward - shows the ion Acting as an oxidising agent
    • reading each half equation backwards - shows the metal acting as a reducing agent.
  • what does the electrochemical series allows us to predict ?
    allows us to predicts the direction of redox reactions.
    • stronger reducing agents undergo a displacement reaction with weaker reducing agents (lower down the series)
  • define half cell (electrode)
    a metal dipped into a solution of its own ions
  • where does the position of equilibrium lie for a metal that is a stronger reducing agent ?
    The position of the reducing agents equilibrium lies to the left of the oxidising agents equilibrium. Therefore there will be more delocalised electrons in the metal that acts a reducing agent.
  • why does the stronger reducing agent have a more negative charge?
    the position of equilibrium lies to the left (of the oxidising agents equilibrium) therefore has more delocalised electrons
  • when two half cells are connected electrons flow away from reducing agent (which is more negatively charged) and towards the oxidising Agent (More positively charged).
    • as the concentration of reactants changes, the equilibrium in each half cell is disturbed:
    1. Reducing agent - equilibrium position shifts to the left. Therefore more electrons flow through the wire to the oxidising agent.
    2. Oxidising agent - equilibrium position shifts to the right. Therefore more ions come out of the solution.
  • How do you measure the electricity produced by the two half cells ?
    Using a voltmeter.
  • define electromotive force
    the maximum voltage possible between two electrodes
  • if a cells electromotive force is negative what does that tell us about the half cell?
    That the reducing agent is on the right. (the negative charge is on the right)
  • give the equation for what happens at the negative and positive electrode in a zinc-carbon cell
    Negative electrode - Zn (s) →   Zn2+ (aq)  +  2e- 
    positive electrode - 2NH4+ (aq) + 2e- → 2NH3 (g) + H2 (g)        
    overall equation - 2NH4+ (aq) + Zn (s)  → 2NH3 (g) + H2 (g)  + Zn2+ (aq)
  • Give the equations for the lead-acid batteries
    Negative electrode - Pb (s) +  SO42- (aq)  →   PbSO4 (s)  +  2e- 
    positive electrode - PbO2 (s) +  4H+ (aq) +  SO42- (aq) +  2e- →  PbSO4(s)  + 2H2O (l) 
    overall - PbO2 (s) +  4H+ (aq) +  2SO42- (aq) +  Pb (s) →  2PbSO4 (s)  + 2H2O (l)
  • give the equations for a NiCad cells
    negative electrode - Cd (s) +  2OH-  (aq) → Cd(OH)2 (s)  +  2e-   
    positive electrode - NiO(OH) (s) + H2O (l) + e- → Ni(OH)2 (s) +   OH-  (aq)   
    overall equation - 2NiO(OH) (s) + 2H2O (l) + Cd (s) → 2Ni(OH)2 (s) + Cd(OH)2 (s)   
  • Give the equations for the lithium ion cell
    Negative electrode - Li (s) →   Li+ (s)  +  e–     
    positive electrode - Li+ (s)  + CoO2 (s)  +  e– →   Li + (CoO2) – (s)
    overall equation - Li (s)  + CoO2 (s)  →   Li + (CoO2) – (s)       
  • equations for the hydrogen fuel cell
    negative electrode - 2H2 (g) + 4OH– (aq)  →  4H2O (l) +  4e–
    positive electrode - O2 (g) +  2H2O  +  4e– →  4OH– (aq)         
    overall equation - 2H2 (g) + O2 (g)  →   2H2O (l)   
  • Suggest why sodium carbonate cannot be used in the salt bridge for this cell.
    Al(s) | Al3+(aq) || H+ (aq) | H2(g) | pt(s)
    The carbonate ion will react with the acid from the standard hydrogen electrode :
    H+ + CO32- —> HCO3-
    this will change the ion concentration in the cell which changes the EMF of the cell.