Question

For each reaction, identify the substance being oxidized and the substance being reduced. (a) \(2 \mathrm{Sr}(s)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{SrO}(s)\) (b) \(\mathrm{Ca}(s)+\mathrm{Cl}_{2}(g) \rightarrow \mathrm{CaCl}_{2}(s)\) (c) \(\mathrm{Ni}^{2+}(a q)+\mathrm{Mg}(s) \longrightarrow \mathrm{Mg}^{2+}(a q)+\mathrm{Ni}(s)\)

Short answer

In reaction (a), Sr is oxidized and O is reduced. In reaction (b), Ca is oxidized and Cl is reduced. In reaction (c), Mg is oxidized and Ni^2+ is reduced.

Step-by-step solution

- Assign oxidation numbers

Identify the oxidation states of each element in the reactants and products. Elements in their elemental form have an oxidation state of 0. In compounds, oxygen generally has an oxidation state of -2, halogens have -1, and metals have positive oxidation states corresponding to their charges.

- Determine changes in oxidation states

Look for changes in oxidation states to identify which elements are oxidized and which are reduced. Oxidation involves an increase in oxidation number, whereas reduction involves a decrease.

- Analyze reaction (a)

For 2Sr + O2 -> 2SrO, Sr goes from 0 to +2; it loses electrons and is oxidized. O2 goes from 0 to -2 (in SrO); it gains electrons and is reduced.

- Analyze reaction (b)

For Ca + Cl2 -> CaCl2, Ca goes from 0 to +2; it loses electrons and is oxidized. Cl2 goes from 0 to -1 (in CaCl2); each Cl atom gains electrons and is reduced.

- Analyze reaction (c)

For Ni^2+(aq) + Mg(s) -> Mg^2+(aq) + Ni(s), Ni^2+ goes from +2 to 0; it gains electrons and is reduced. Mg goes from 0 to +2; it loses electrons and is oxidized.

Key concepts

Oxidation and Reduction in Chemistry

Oxidation and reduction, often referred to as redox reactions, are processes that involve the transfer of electrons between substances. In a redox process, one substance donates electrons, undergoing oxidation, while another substance accepts electrons, undergoing reduction. These two processes always occur together; when one species is oxidized, another is reduced.

In a redox reaction, the substance that loses electrons is said to be oxidized, and the one that gains electrons is reduced. For instance, in the reaction of strontium with oxygen (2Sr + O2 -> 2SrO), strontium (Sr) atoms lose electrons and are therefore oxidized, while the oxygen (O2) molecules gain electrons and are reduced. Similarly, in the reaction between calcium and chlorine (Ca + Cl2 -> CaCl2), calcium (Ca) is oxidized and chlorine (Cl2) is reduced.

A helpful mnemonic to remember this concept is 'OIL RIG', which stands for 'Oxidation Is Loss, Reduction Is Gain' of electrons. Understanding the principles of oxidation and reduction is crucial for analyzing and balancing chemical reactions and for grasping the flow of electrons in chemical processes.

Assigning Oxidation Numbers

Assigning oxidation numbers to atoms within molecules or ions is a foundational skill for analyzing redox reactions. Oxidation numbers, or oxidation states, help chemists keep track of electron transfer and are used to determine whether an atom is being oxidized or reduced in a chemical reaction.

To assign oxidation numbers, several rules must be followed:

• Pure elements have an oxidation number of 0.
• The oxidation number of a monatomic ion is equal to its charge.
• Oxygen typically has an oxidation number of -2, and hydrogen is usually +1.
• The sum of oxidation numbers in a neutral compound must be zero.
• In polyatomic ions, the sum of oxidation numbers must equal the ion's charge.

For example, in the reaction Mg + Ni^2+ -> Mg^2+ + Ni, magnesium (Mg) starts in its elemental form with an oxidation number of 0 and ends as a Mg^2+ ion with an oxidation number of +2. This change signifies that Mg is oxidized. Conversely, the nickel ion (Ni^2+) is reduced to elemental nickel (Ni), with its oxidation number changing from +2 to 0.

Chemical Reactions Analysis

Analyzing chemical reactions involves more than just observing the reactants turning into products. It's about understanding how the atoms and electrons are rearranged. This involves examining the steps of a reaction, identifying the role of each reactant and product, and determining which substances are oxidized and which are reduced — foundational aspects of redox reactions.

Step-by-step analysis allows us to delve deep into the process, identifying the loss or gain of electrons and changes in oxidation states. For instance, taking the reaction Ni^2+(aq) + Mg(s) -> Mg^2+(aq) + Ni(s), step-by-step analysis revealed the oxidation of Mg from 0 to +2 and the reduction of Ni^2+ from +2 to 0. Scientifically accurate descriptions of the electron transfers and changes in oxidation states are essential in predicting the behavior of substances in reactions, understanding their energetics, and applying this knowledge in broader contexts, such as in electrochemical cells or corrosion processes.