Question

For each of the following reactions, indicate which elements are oxidized and which are reduced: a. \(\mathrm{WO}_{3}+3 \mathrm{H}_{2} \longrightarrow \mathrm{W}+3 \mathrm{H}_{2} \mathrm{O}\) b. \(2 \mathrm{Na}+\mathrm{Cl}_{2} \longrightarrow 2 \mathrm{NaCl}\) c. \(2 \mathrm{Fe}_{2} \mathrm{O}_{3}+3 \mathrm{C} \longrightarrow 4 \mathrm{Fe}+3 \mathrm{CO}_{2}\)

Short answer

a. W is reduced (from +6 to 0); H is oxidized (from 0 to +1). b. Na is oxidized (from 0 to +1); Cl is reduced (from 0 to -1). c. Fe is reduced (from +3 to 0); C is oxidized (from 0 to +4).

Step-by-step solution

Identify the oxidation states of elements in each reactant and product for reaction a

Assign oxidation states to W, O, and H in each compound before and after the reaction. In WO3, W is +6 and O is -2. In H2, H is 0 because it is in its elemental state. In H2O, H is +1 and O is -2.

Identify the changes in oxidation states for reaction a

In WO3, W goes from an oxidation state of +6 to 0 in W, indicating a reduction. Each H atom in H2 goes from an oxidation state of 0 to +1 in H2O, indicating oxidation.

Identify the oxidation states of elements in each reactant and product for reaction b

Assign oxidation states to Na and Cl in each compound before and after the reaction. Na is 0 in its elemental state, while in NaCl, Na is +1. Cl2 is 0 in its elemental state, while in NaCl, Cl is -1.

Identify the changes in oxidation states for reaction b

In the reaction, Na goes from an oxidation state of 0 to +1, indicating oxidation. Cl goes from an oxidation state of 0 in Cl2 to -1 in NaCl, indicating reduction.

Identify the oxidation states of elements in each reactant and product for reaction c

Assign oxidation states to Fe, O, and C in each compound before and after the reaction. In Fe2O3, Fe is +3 and O is -2. C is 0 in its elemental state. In Fe, the oxidation state is 0 and in CO2, C is +4 while O is -2.

Identify the changes in oxidation states for reaction c

In Fe2O3, Fe goes from an oxidation state of +3 to 0 in Fe, indicating a reduction. C goes from an oxidation state of 0 to +4 in CO2, indicating oxidation.

Key concepts

Oxidation States

Understanding oxidation states is crucial to mastering redox reactions. In simple terms, the oxidation state tells us how many electrons an atom has gained or lost compared to its neutral state.

For example, in the reaction \(\mathrm{WO}_{3}+3 \mathrm{H}_{2} \longrightarrow \mathrm{W}+3\mathrm{H}_{2} \mathrm{O}\), tungsten (W) has an oxidation state of +6 in \(\mathrm{WO}_3\) because oxygen is more electronegative and takes two electrons for each of its atoms. When \(\mathrm{WO}_3\) turns to \(\mathrm{W}\), tungsten has an oxidation state of 0 because it's in its elemental form.

Understanding this concept is vital in determining which element is being oxidized or reduced during a reaction. An increase in oxidation state means oxidation, while a decrease means reduction.

Oxidation and Reduction

Oxidation and reduction always occur together in chemical reactions—this partnership is known as a redox reaction. When an element undergoes oxidation, it loses electrons, and its oxidation state increases. Conversely, reduction involves an element gaining electrons, which decreases its oxidation state.

In the reaction \(2 \mathrm{Fe}_{2} \mathrm{O}_{3}+3 \mathrm{C} \longrightarrow 4\mathrm{Fe}+3 \mathrm{CO}_{2}\), iron (Fe) is reduced as its oxidation state decreases from +3 to 0. Carbon (C), on the other hand, is oxidized since its oxidation state increases from 0 to +4. Remembering 'LEO the lion says GER' can help—'Lose Electrons Oxidation and Gain Electrons Reduction'.

Chemical Reactions

Chemical reactions involve the rearrangement of atoms to form new substances. Redox reactions are a type of chemical reaction where electrons are transferred between atoms, causing changes in oxidation states, resulting in the conversion of reactants to products.

Take the reaction \(2 \mathrm{Na}+\mathrm{Cl}_{2} \longrightarrow 2 \mathrm{NaCl}\) as an example. Sodium (Na) atoms lose an electron to chlorine (Cl) atoms, which is a classic redox process. Na becomes oxidized and Cl becomes reduced, leading to the formation of NaCl. These electron transfer processes are integral to the transformation of matter and occur in countless chemical reactions within and around us.