Define oxidation and reduction. Identify oxidizing and reducing agents and determine oxidation states in given chemical reactions.

Here's the solution to question 3:

a) Define

• i) Oxidation in terms of oxygen: Oxidation is the gain of oxygen by a substance.
• ii) Reduction in term of Hydrogen: Reduction is the gain of hydrogen by a substance.

b) Identify

• i) The reducing agent in equation I: In $2\text{Cu(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{CuO(s)}$, Copper (Cu) gains oxygen, so it is oxidized. The substance that is oxidized is the reducing agent.
  • The reducing agent is $\boxed{\text{Cu}}$.
• ii) The oxidizing agent in equation II: In $\text{CuO} + \text{H}_2\text{(g)} \rightarrow \text{Cu} + \text{H}_2\text{O(l)}$, Copper oxide (CuO) loses oxygen, so it is reduced. The substance that is reduced is the oxidizing agent.
  • The oxidizing agent is $\boxed{\text{CuO}}$.

c) Determine the oxidation state of

• i) Copper in Cu: Copper in its elemental form has an oxidation state of 0.
  • The oxidation state is $\boxed{0}$.
• ii) Oxygen in H$_2$O: In water, hydrogen has an oxidation state of +1. Since there are two hydrogen atoms and the molecule is neutral, the oxygen atom must balance the charge. $$2(+1) + x = 0$$ $$2 + x = 0$$ $$x = -2$$
  • The oxidation state is $\boxed{-2}$.

d) State whether each of these reactions involves oxidation or reduction or both.

• i) $\text{Al}^{3+} + 3\text{e}^- \rightarrow \text{Al}$: Aluminum ion gains electrons to become neutral aluminum. The gain of electrons is reduction.
  • This reaction involves $\boxed{\text{reduction}}$.
• ii) $2\text{O}^{2-} \rightarrow \text{O}_2 + 4\text{e}^-$: Oxide ions lose electrons to form oxygen gas. The loss of electrons is oxidation.
  • This reaction involves $\boxed{\text{oxidation}}$.
• iii) $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$: Hydrogen goes from an oxidation state of 0 to +1 (oxidation), and oxygen goes from an oxidation state of 0 to -2 (reduction). Since both processes occur, it is a redox reaction.
  • This reaction involves $\boxed{\text{both}}$.