Exercises: Oxidation numbers and the ion-electron method

When a sheet of metallic zinc is placed in an aqueous solution of hydrochloric acid, vigorous bubbling of hydrogen gas occurs according to the following process:

$$\mathrm{Zn(s) + 2\,HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)}$$

1. a) Calculate the oxidation number of all the atoms in the reactants and products.
2. b) Identify the species that is oxidised, the one that is reduced, the oxidising agent and the reducing agent, justifying your answer in terms of electron transfer.

In acidic aqueous medium, the nitrate ion can be converted into nitrogen monoxide, while the tin(II) ion can change to tin(IV).

1. a) Write and balance the reduction half-equation of the nitrate ion, $\mathrm{NO_3^-(aq)}$, to nitrogen monoxide, $\mathrm{NO(g)}$, in acidic medium, stating the number of electrons transferred.
2. b) Write the oxidation half-equation of the cation $\mathrm{Sn^{2+}(aq)}$ to $\mathrm{Sn^{4+}(aq)}$ and check that both half-equations balance simultaneously in mass and electric charge.

The diagram below illustrates the flow of electrons that occurs spontaneously when two redox half-cells interact through a conducting circuit. Observe the arrangement of the chemical species and the direction of the electron flow:

1. a) Write the balanced oxidation and reduction half-equations, deducing the numerical value that must replace the question mark in the number of electrons transferred, and write the overall net ionic equation.
2. b) Assign, with justification, the roles of oxidising agent and reducing agent from the changes in oxidation numbers.

In basic solution, the permanganate anion, $\mathrm{MnO_4^-}$, is able to oxidise the sulfite anion, $\mathrm{SO_3^{2-}}$, producing solid manganese dioxide, $\mathrm{MnO_2(s)}$, and the sulfate anion, $\mathrm{SO_4^{2-}}$.

1. a) Write and balance, using the ion-electron method in basic medium, the oxidation and reduction half-equations, detailing the balancing of atoms using $\mathrm{H_2O}$ and $\mathrm{OH^-}$, as well as the balancing of charges using electrons.
2. b) Obtain the overall net ionic equation and check explicitly that the total charge of the reactants equals that of the products.