Problem 20 - Chemistry The Central Science In Si Units

Question

Indicate whether the following balanced equations involve oxidation-reduction. If they do, identify the elements that undergo changes in oxidation number. $$ \text { (a) } 2 \mathrm{AgNO}_{3}(a q)+\mathrm{CoCl}_{2}(a q) \longrightarrow 2 \mathrm{AgCl}(s)+ $$ \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}(a q)\) $$ \begin{array}{l} \text { (b) } 2 \mathrm{PbO}_{2}(s) \longrightarrow 2 \mathrm{PbO}(s)+\mathrm{O}_{2}(g) \\ \text { (c) } 2 \mathrm{H}_{2} \mathrm{SO}_{4}(a q)+2 \mathrm{NaBr}(s) \longrightarrow \mathrm{Br}_{2}(l)+\mathrm{SO}_{2}(g)+ \end{array} $$ \(\mathrm{Na}_{2} \mathrm{SO}_{4}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)\)

Step-by-Step Solution

Answer

(a) Not a redox reaction. (b) Redox, Pb reduced, O oxidized. (c) Redox, S reduced, Br oxidized.

Step 1: Analyze Equation (a)

For the equation \( 2 \text{AgNO}_3 (aq) + \text{CoCl}_2 (aq) \rightarrow 2 \text{AgCl} (s) + \text{Co}(\text{NO}_3)_2 (aq) \), note that silver (Ag) is always +1 in AgNO₃ and AgCl. Cobalt (Co) stays +2 in both CoCl₂ and Co(NO₃)₂. Chlorine (Cl) remains -1 in both compounds, and nitrogen (N) is +5, while oxygen (O) is -2 in both NO₃⁻ ions. Since no elements change oxidation states, this is not a redox reaction.

Step 2: Analyze Equation (b)

For the equation \( 2 \text{PbO}_2 (s) \rightarrow 2 \text{PbO} (s) + \text{O}_2 (g) \), check the oxidation states: \( \text{Pb} \) in PbO₂ is +4, while in PbO it is +2, indicating that lead is reduced. Oxygen goes from -2 in PbO₂ to 0 in O₂ as it is oxidized. Thus, this is an oxidation-reduction reaction where lead is reduced and oxygen is oxidized.

Step 3: Analyze Equation (c)

For equation \( 2 \text{H}_2\text{SO}_4 (aq) + 2 \text{NaBr} (s) \rightarrow \text{Br}_2 (l) + \text{SO}_2 (g) + \text{Na}_2\text{SO}_4 (aq) + 2 \text{H}_2\text{O} (l) \), sulfur (S) in H₂SO₄ is +6 and in SO₂ remains +4, indicating sulfur is reduced. Bromine (Br) in NaBr is -1 and becomes 0 in Br₂, indicating oxidation of bromine. This confirms it is an oxidation-reduction reaction where sulfur is reduced and bromine is oxidized.

Key Concepts

Oxidation States

Understanding oxidation states is crucial for identifying redox reactions. An oxidation state is a number assigned to an element in a compound, reflecting the number of electrons lost or gained by an atom. This helps in determining whether an element is oxidized or reduced during a chemical reaction. In a nutshell, the more positive the oxidation state, the more electrons have been lost (oxidation), and vice versa for reduction where electrons are gained. For example, in \(\text{Step 1}\) of our original problem, we see that in the reaction of \(2 \text{AgNO}_3 (aq) + \text{CoCl}_2 (aq) \rightarrow 2 \text{AgCl} (s) + \text{Co(NO}_3)_2 (aq)\), nothing changes in the oxidation states of silver, cobalt, chlorine, nitrogen, or oxygen, hence no redox reaction occurs. On the other hand, in \(\text{Step 2}\), \(2 \text{PbO}_2 (s) \rightarrow 2 \text{PbO} (s) + \text{O}_2 (g)\), lead's oxidation state falls from +4 to +2 (reduction), while oxygen's state increases from -2 to 0 (oxidation). This shift indicates that a redox reaction is taking place. Remember, identifying changes in oxidation states in any chemical equation is the first step in recognizing an oxidation-reduction reaction.

Chemical Equations

Chemical equations represent chemical reactions using symbols and formulas. They consist of reactants (the starting materials) on the left and products (the substances formed) on the right, with an arrow pointing from reactants to products. Balancing these equations is essential because it ensures the conservation of mass, meaning the same type and number of atoms are present on both sides of the equation.When dealing with redox reactions, it is crucial to not only balance atoms, but also to account for changes in oxidation states. In equation (b) \(2 \text{PbO}_2 \rightarrow 2 \text{PbO} + \text{O}_2\), we see the balancing of atoms and charges, reflecting the transfer of electrons that defines a redox change: lead is reduced and oxygen is oxidized.Chemical equations become a powerful tool in analyzing redox processes by systematically showing these transformations and helping us understand the overall flow of electrons.

Redox Reaction Analysis

Redox reaction analysis involves identifying which elements in a reaction route undergo oxidation and which are reduced. This understanding is essential because oxidation-reduction reactions involve electron transfer between substances. A straightforward way to identify such reactions is to look for changes in oxidation states. As we saw in step \(\text{Step 3}\), involving \(2 \text{H}_2\text{SO}_4 + 2 \text{NaBr} \rightarrow \text{Br}_2 + \text{SO}_2 + \text{Na}_2\text{SO}_4 + 2 \text{H}_2\text{O}\), sulfur goes from +6 in \(\text{H}_2\text{SO}_4\) to +4 in \(\text{SO}_2\), thus being reduced. Bromine shifts from -1 in NaBr to 0 in Br₂, thus being oxidized.By systematically analyzing the changes in oxidation states, we can pinpoint which atoms donate electrons (oxidized) and which accept electrons (reduced). This method ensures we accurately identify and understand electron transfers in chemical reactions, enriching our comprehension of how reactions occur and interact.