Question

(a) Explain Quinonoid theory of indicators.
(b) Write notes on IUPAC conventions of representations of a cell.

Answer 1

(a) Quinonoid theory of indicators:
(a) According to this theory, Acid base indicators are organic compounds with at least one benzene ring.
(b) The indicators exhibits tautomerism. It has two tautomeric structures such as Benzenoid and Quinonoid form which are in equilibrium with each other.
(c) One form exists in acidic solution and has one colour and the other form exists in basic solution and it has different colour.
(d) As the pH of the solution containing the indicator is changes, the solution shows a change of colour. The colour change is due to the fact that one tautomer changes over to the other.
(e) For e.g Phenolphthalein has two tautomeric forms.
Cell diagram (or) representation of a cell is abbreviated symbolic depiction of an electrochemical cell. A cell consists of two half cells. Each half cell is made of a metal electrode in contact with metal ion in solution.
(b) IUPAC conventions:
Let us consider zinc-copper cell.
(i) A single vertical line ($|$) represents a phase boundary between metal electrode and ion solution. Thus, the two half cells in a voltaic cell are indicated as
shown. In that the metal electrode in anode half cell is on the, left, while in cathode half cell is on the right of the metal ion.
(ii) A double vertical line represents the salt bridge, porous partition or any other means of permitting ion flow while preventing the electrolyte from mixing.
(iii) Anode half cell is written on the left and cathode half cell on the right.
(iv) In the complete cell diagram, the two half cells are separated by double vertical line (salt bridge) between them. The zinc-copper cell can now be written as given.
(v) The symbol for an inert electrode, like the plaitnum electrode is often enclosed in a bracket. for e.g as given
(vi) The value of emf of a cell is written on the right of the cell diagram, Thus, a zinc-copper cell has emf $1.1V$ and is represented as
$Zn|Zn{SO}_4\parallel Cu{SO}_4|Cu$ $E=+1.1V$
If the emf acts in the opposite direction through the cell circuit it is denoted as a negative value.
$Cu|Cu{SO}_4\parallel Zn{SO}_4|Zn$
$⟵$ Direction of circiut.
(vii) The negative sign also indicates that the cell is not feasible in the given direction and reaction will take place in the reverse direction only. The overall cell reaction for $E=-1.1V$ of the Daniel cell is
${Cu}_{\left(s\right)}+{Zn}_{aq}^{2+}⟶{Cu}_{aq}^{2+}+{Zn}_{\left(s\right)}$
(viii) The reversal of the cell current is accompanied by the reversal of direction of the cell reaction. Thus, a reversible cell is defined as that operates by reversal of cell reactions when the direction of flow of current is reversed.