During the electrolysis of concentrated hydrochloric acid (HCl_{(aq)}) using inert graphit… — JAMB UTME

During the electrolysis of concentrated hydrochloric acid ( $HCl_{(aq)}$ ) using inert graphite electrodes, a gaseous product $X$ is evolved at the anode while gas $Y$ is evolved at the cathode. Which option correctly identifies gas $X$ , gas $Y$ , and the primary factor responsible for the preferential discharge that yields gas $X$ ?

Gas $X$ is $Cl_2$ , Gas $Y$ is $H_2$ , driven by the high concentration of chloride ions.Answer
B
Gas $X$ is $O_2$ , Gas $Y$ is $H_2$ , driven by the position of hydroxide ions in the electrochemical series.
C
Gas $X$ is $H_2$ , Gas $Y$ is $Cl_2$ , driven by the high concentration of hydrogen ions.
D
Gas $X$ is $O_2$ , Gas $Y$ is $Cl_2$ , driven by the chemical nature of the graphite electrodes.

Answer

Gas

X

Cl_2

, Gas

Y

H_2

, driven by the high concentration of chloride ions.

In concentrated hydrochloric acid, the ions migrating to the anode are

Cl^-

and

OH^-

. Because the solution is concentrated, the concentration of

Cl^-

ions is far higher than that of

OH^-

. Consequently, the concentration factor predominates over position in the electrochemical series, resulting in

Cl^-

being preferentially oxidized to chlorine gas (

Cl_2

). At the cathode,

H^+

ions gain electrons to form hydrogen gas (

H_2

Step-by-Step Solution

Identify the ions present in concentrated hydrochloric acid (

HCl_{(aq)}

The cations present are

H^+

(from

HCl

and

H_2O

) and the anions present are

Cl^-

and

OH^-

Hydrochloric acid ionizes completely in water into

H^+

and

Cl^-

, alongside minor autoionization of water.

Determine the reaction taking place at the anode (positive electrode).

Chloride ions (

Cl^-

) are discharged preferentially to form chlorine gas (

Cl_2

Although

OH^-

is higher in the electrochemical series than

Cl^-

, the significantly higher concentration of

Cl^-

in concentrated

HCl

overrides position in the series.

Determine the reaction taking place at the cathode (negative electrode).

Hydrogen ions (

H^+

) are discharged to form hydrogen gas (

H_2

H^+

is the only cation present in solution, gaining electrons at the cathode.

Key Concept

Concentration effect on preferential discharge of anions during electrolysis

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