Question

Indicate whether the following conversions represent an oxidation, a reduction or none (neither oxidation \(\begin{aligned}&\text { nor reduction). } \\\&\mathrm{HClO}_{4} & \text { (ii) } \mathrm{NH}_{4}^{+} \text {to } \mathrm{NH}_{3}\end{aligned}\) (i) \(\mathrm{HClO}_{3}\) to \(\mathrm{HClO}_{4}\) (iii) \(\mathrm{NO}_{2}\) to \(\mathrm{N}_{2} \mathrm{O}_{4}\) (iv) \(\mathrm{HSO}_{3}^{-}\)to \(\mathrm{SO}_{4}^{2-}\) (v) \(\mathrm{H}_{2} \mathrm{O}_{2}\) to \(\mathrm{H}_{2} \mathrm{O}\) (i) (ii) (iii) (iv) (v) (a) Oxida- Reduc- None None Oxidation tion tion (b) Oxida- None None Oxida- Reduction tion tion (c) Reduc- Oxida- Reduc- None Reduction tion tion tion (d) Oxida- Reduc- None Reduc- Reduction tion tion tion

Short answer

(i) Oxidation, (ii) None, (iii) None, (iv) Oxidation, (v) Reduction

Step-by-step solution

- Analyzing Oxidation States for HClO3 to HClO4

Examine the change in oxidation state of chlorine. In HClO3, chlorine has an oxidation state of +5. In HClO4, the oxidation state of chlorine is +7. An increase in oxidation state is indicative of oxidation.

- Analyzing Oxidation States for NH4+ to NH3

Examine the change in oxidation state of nitrogen. In NH4+, nitrogen has an oxidation state of -3. In NH3, nitrogen also has an oxidation state of -3. There is no change in the oxidation state, which means neither oxidation nor reduction has occurred.

- Analyzing Oxidation States for NO2 to N2O4

Examine the oxidation state of nitrogen in both compounds. In NO2, the oxidation state of nitrogen is +4. In N2O4, which is essentially two NO2 molecules bonded together, the nitrogen remains at an oxidation state of +4. No change in oxidation state indicates no oxidation or reduction.

- Analyzing Oxidation States for HSO3- to SO42-

Examine the change in the oxidation state of sulfur. In HSO3-, sulfur has an oxidation state of +4. In SO42-, the oxidation state of sulfur is +6. An increase in oxidation state signifies oxidation.

- Analyzing Oxidation States for H2O2 to H2O

Examine the change in the oxidation state of oxygen. In H2O2 (hydrogen peroxide), oxygen has an oxidation state of -1. In H2O (water), the oxidation state of oxygen is -2. A decrease in oxidation state is indicative of reduction.

Key concepts

Oxidation States

Understanding the oxidation state of an atom in a compound is crucial in identifying redox reactions. Oxidation states are essentially bookkeeping numbers that allow chemists to keep track of electron movement during chemical reactions. Each element in a compound is assigned an oxidation state that reflects its potential to lose, gain, or share electrons.

For example, in the compound HClO3, chlorine has an oxidation state of +5. When HClO3 is transformed into HClO4, the oxidation state of chlorine changes to +7. This increase in oxidation state is what we call oxidation. It signals that an element has lost electrons. Conversely, if an oxidation state decreases, as is the case when hydrogen peroxide (H2O2) becomes water (H2O), this signals a gain of electrons, or reduction.

Properly identifying oxidation states in exercises can be tricky due to varying rules, such as knowing the typical oxidation states of different elements and understanding that the sum of oxidation states in a neutral compound must be zero, or equal to the charge of the compound if it's an ion. A helpful tip for students is to memorize the common oxidation states of frequently encountered elements and to consistently apply the rules for assigning oxidation states.

Oxidation and Reduction

Oxidation and reduction, often referred to together as redox reactions, are two types of chemical processes that are mirror images of each other. Oxidation is the process in which an atom or a molecule loses electrons, while reduction is the gain of electrons. In the context of a redox reaction, you cannot have one without the other – they always come in pairs.

Oxidation and reduction can be identified by changes in oxidation states, as demonstrated in the provided textbook exercise. When chlorine's oxidation state increases from +5 in HClO3 to +7 in HClO4, we identified this as oxidation. In contrast, when the oxidation state of oxygen decreases from -1 in H2O2 to -2 in H2O, we recognize reduction.

A handy mnemonic to remember is 'OIL RIG'– which stands for 'Oxidation Is Loss, Reduction Is Gain' of electrons. It's also important for students to remember that the element which undergoes oxidation acts as a reducing agent, and the one that undergoes reduction acts as an oxidizing agent.

Chemical Reactions

Chemical reactions are the processes through which substances convert into new materials, often with entirely different properties. Redox reactions are a specific type of chemical reaction, characterized by the flow of electrons from one substance to another. This electron transfer often results in observable changes, such as color change, heat production, or the formation of gases or a precipitate.

In exercises like the one provided, analyzing the chemical reactions involves looking for these electron transfers, which are signaled by changes in the oxidation states of the elements involved. When no change in oxidation states occurs, such as from NH4+ to NH3, or from NO2 to N2O4, it means that there has been no net electron transfer, and thus no oxidation or reduction has taken place.

For students aiming to master the concept of chemical reactions, a fundamental understanding of the underlying principles, like conservation of mass and energy, as well as the ability to balance chemical equations, will significantly enhance their ability to grasp the concept of redox chemistry and apply it to various chemical scenarios.