Question

Many oxidation- reductions can be balanced by inspection. Try to balance the following reactions by inspection. In each reaction, identify the substance reduced and the substance oxidized.

a) $\mathbf{}{\mathbf{Al}}_{\left(s\right)}\mathbf{+}{\mathbf{HCl}}_{\left(\mathrm{aq}\right)}\to {\mathbf{AlCl}}_{3\left(\mathrm{aq}\right)}\mathbf{+}{\mathbf{H}}_{2\left(g\right)}$

b) ${\mathbf{CH}}_{4\left(g\right)}\mathbf{+}{\mathbf{S}}_{\left(s\right)}\to {\mathbf{CS}}_{2\left(l\right)}\mathbf{+}{\mathbf{H}}_{2\left(g\right)}$

c) ${\mathbf{C}}_3{\mathbf{H}}_{8\left(g\right)}\mathbf{+}{\mathbf{O}}_{2\left(s\right)}\to {\mathbf{CO}}_{2\left(g\right)}\mathbf{+}{\mathbf{H}}_2{\mathbf{O}}_{\left(l\right)}$

d) $\mathbf{Cu}\mathbf{+}\mathbf{2}{{\mathbf{Ag}}^{+}}_{\left(\mathrm{aq}\right)}\to \mathbf{2}{\mathbf{Ag}}_{\left(s\right)}\mathbf{+}{\mathbf{Cu}}^{2+}$

Short answer

a) $2A{l}_{\left(s\right)}+6HC{l}_{\left(aq\right)}\to 2AlC{l}_{3\left(aq\right)}+3H_{2\left(g\right)}$

b) $C{H}_4+4S_{\left(s\right)}\to C{S}_{2\left(l\right)}+2H_{2}S$

c) $C_3{H}_{8\left(g\right)}+5O_{2\left(s\right)}\to 3C{O}_{2\left(g\right)}+4H_2{O}_{\left(l\right)}$

d) $C{u}_{}+2A{g}^{+}\to 2Ag+C{u}^{2+}$

Step-by-step solution

Subpart a)

The coefficientsof the reaction can be given as:

$2A{l}_{\left(s\right)}+6HC{l}_{\left(aq\right)}\to 2AlC{l}_{3\left(aq\right)}+3H_{2\left(g\right)}$

Here, the oxidized substance is Al. So, the oxidation number increases from$0\text{\hspace{0.17em}to\hspace{0.17em}}+3$

Here, hydrogen is reduced.So, the oxidation number increases from+1 to 0.

Subpart b)

$C{H}_{4\left(g\right)}+S_{\left(s\right)}\to C{S}_{2\left(l\right)}+H_{2\left(g\right)}$

The coefficients of the reaction can be given as: $C{H}_4+4S_{\left(s\right)}\to C{S}_{2\left(l\right)}+2H_{2}S$

$C{H}_4$is the substance which is oxidized and reduced.

Subpart c)

The coefficients of the reaction can be given as: $C_3{H}_{8\left(g\right)}+5O_{2\left(s\right)}\to 3C{O}_{2\left(g\right)}+4H_2{O}_{\left(l\right)}$

The substance$C_3{H}_8$ is oxidized and the substance is reduced.

Subpart d)

$\text{Oxidation\hspace{0.17em}}:Cu\to C{u}^{2+}+2e$

$\text{Reduction\hspace{0.17em}}:A{g}^{+}+e\to Ag/2$

$C{u}_{}+2A{g}^{+}\to 2Ag+C{u}^{2+}$

Here, Cuis the oxidizing and reducing agent; and $Ag$ is the oxidizing and reducing agent.

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