Page 29 - Spotlight A+ Form 4 & 5 Chemistry KSSM

Form
5 Chemistry Chapter 1 Redox Equilibrium

CHAP. CHAP.
1 8. Zinc, Zn strip is placed into silver nitrate, (a) Oxidation half equation 1
Ag(NO ) solution. Silver, Ag and zinc nitrate,
3 2 (b) Reduction half equation
Zn(NO ) are formed. (c) Overall ionic equation
3 2
Zn(s) + 2AgNO (aq) → Ag(s) + Zn(NO ) (aq) (d) Oxidising agent
3 3 2 (e) Reducing agent
Based on the reaction equation, determine the
following: C3
1.2 Standard Electrode Potential potential of an unknown half-cell with a
standard electrode. Hydrogen electrode was
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1. The strength of an oxidising agent and a chosen as a standard electrode.
reducing agent depends on the standard
electrode potential, E . Platinum surface promotes oxidation of
0
2. Standard electrode potential, E 0 is a hydrogen molecule, H to hydrogen ions,
2
H or reduction of hydrogen ions, H to
+
+
measurement of the potential for equilibrium. hydrogen gas, H .
2
3. If a metal is immersed in a solution containing
the metal ions, the metal atoms tend to release Atoms of platinum, Pt electrode
electrons to form metal ions and enter the
solution. The electrons released accumulate on H (g) at e – H +
2
1 atm
the metal, making the metal negatively charged. H 2
4. In a short time, the metal is surrounded by Platinum, e – H +
Pt wire
positive ions. A small number of metal ions H (g) e – H +
2
will receive electrons and become metal atoms Platinum, Pt e – H 2
again. electrode e – H +
Acidic solution,
M(s)  M (aq) + ne – H 1.0 mol dm –3 Half equation of the
n+
+
hydrogen half-cell:
–
5. Thus, an equilibrium is achieved between metal 2H (aq) + 2e  H (g)
+
2
n+
M atoms and metal M ions in a half-cell. Figure 1.12 Standard hydrogen electrode half-cell
This condition results in a potential difference 10. Electrode potential of standard hydrogen
between the metal (electrode) and its solution electrode, E 0 is assigned as 0.00 V.
reference
(electrolyte). The potential difference is called as 11. The electrode potential in a half-cell when
electrode potential. compared relative to the standard hydrogen
6. Hydrogen gas, H can also be dissolved in electrode, SHE under standard conditions is
2
+
a solution to form hydrogen ions, H .The called standard electrode potential, E .
0
+
hydrogen ions, H in the solution will also 12. Standard conditions for the measurement of
receive electrons to become hydrogen gas, standard electrode potential of a half-cell:
H . An equilibrium is achieved between the (a) Concentration of ions in aqueous solutions
2
hydrogen gas, H and the hydrogen ions, H in is 1.0 mol dm .
+
-3
2
the solution. (b) Gas pressure of 1 atm or 101 kPa.
o
H (g)  2H (aq) + 2e – (c) Temperature at 25 C or 298 K.
+
2
(d) Platinum is used as an inert electrode.
7. Because hydrogen is in gaseous state, an inert
electrode, namely platinum, Pt is used as a Determine the value of the standard electrode
conductor so that it can come into contact with potential
hydrogen gas, H as shown in Figure 1.12. 1. Standard hydrogen electrode consists of a
2
8. Since, there is a potential difference between the platinum electrode immersed in a 1.0 mol
electrode that is in contact with hydrogen gas, dm of strong acid solution, H through which
−3
+
H and the solution containing hydrogen ions, hydrogen gas, H at a pressure of 1 atm is
2
H , the electrode potential for hydrogen half- bubbled. Thus, the reference half reaction is as
2
+
cell is produced. follows:
9. It is impossible to measure the absolute value of
–
+
an electrode potential in a half-cell. Therefore, 2H (aq; 1M) + 2e  H (g, 1 atm) E 0 reference = 0.00 V
2
scientists compare the value of electrode
312 1.2.1