Problem 143 - Chemistry The Science in Context

Question

Which of the following reactions of calcium compounds is \(/\) are redox reactions? a. \(\quad \mathrm{CaCO}_{3}(s) \rightarrow \mathrm{CaO}(s)+\mathrm{CO}_{2}(g)\) b. \(\mathrm{CaO}(s)+\mathrm{SO}_{2}(g) \rightarrow \mathrm{CaSO}_{3}(s)\) c. \(\mathrm{CaCl}_{2}(s) \rightarrow \mathrm{Ca}(s)+\mathrm{Cl}_{2}(g)\) d. \(3 \mathrm{Ca}(s)+\mathrm{N}_{2}(g) \rightarrow \mathrm{Ca}_{3} \mathrm{N}_{2}(s)\)

Step-by-Step Solution

Answer

Reactions (b), (c), and (d) are redox reactions. This is because there are changes in the oxidation numbers of at least one element involved in these reactions.

Step 1: Reaction (a) Analysis

Firstly, let's analyze the oxidation numbers of each element in reaction (a):\(\mathrm{CaCO}_{3}(s) \rightarrow \mathrm{CaO}(s)+\mathrm{CO}_{2}(g)\). For this reaction, the oxidation numbers are as follows: Ca: +2 C: +4 O: -2 Since the oxidation numbers do not change for any element during the reaction, reaction (a) is not a redox reaction.

Step 2: Reaction (b) Analysis

Now let's analyze the oxidation numbers of each element in reaction (b): \(\mathrm{CaO}(s)+\mathrm{SO}_{2}(g) \rightarrow \mathrm{CaSO}_{3}(s)\). For this reaction, the oxidation numbers are as follows: Ca: +2 O: -2 S: From +4 to +6 Since the oxidation number of Sulphur changes from +4 to +6, reaction (b) is a redox reaction.

Step 3: Reaction (c) Analysis

For reaction (c): \(\mathrm{CaCl}_{2}(s) \rightarrow \mathrm{Ca}(s)+\mathrm{Cl}_{2}(g)\), the oxidation numbers are as follows: Ca: From +2 to 0 Cl: From -1 to 0 As the oxidation numbers of both Calcium and Chlorine change during the reaction, reaction (c) is a redox reaction.

Step 4: Reaction (d) Analysis

Lastly, let's analyze the oxidation numbers of each element in reaction (d): \(3 \mathrm{Ca}(s)+\mathrm{N}_{2}(g) \rightarrow \mathrm{Ca}_{3} \mathrm{N}_{2}(s)\). For this reaction, the oxidation numbers are as follows: Ca: From 0 to +2 N: From 0 to -3 As the oxidation numbers of both Calcium and Nitrogen change during the reaction, reaction (d) is a redox reaction. In conclusion, reactions (b), (c), and (d) are redox reactions.

Key Concepts

Oxidation Numbers

Understanding oxidation numbers is key to grasping redox reactions. An oxidation number, often referred to as oxidation state, is a value assigned to an element in a compound that reflects its degree of oxidation or reduction. The rules for calculating oxidation numbers include assigning a value of zero to a pure element and using known values for elements in standard compounds, like hydrogen (+1) and oxygen (-2) in most scenarios.

In a redox reaction, the transfer of electrons is represented by changes in oxidation numbers. An increase in oxidation number indicates oxidation (loss of electrons), while a decrease signifies reduction (gain of electrons). To analyze a reaction to see if it's a redox reaction, we compare the oxidation numbers of each element before and after the reaction. No change in oxidation numbers, no redox reaction; but if there's a change, redox is at play.

• To find the oxidation number of an element, you should first refer to any known values for common elements, such as oxygen, which is usually -2 except in peroxides where it's -1.
• For neutral molecules, the sum of oxidation numbers must equal zero, leading to assessments over what the unknown oxidation values could be based on known ones.
• In ions, the sum of oxidation numbers should reflect the charge on the ion.

Chemical Reactions

Chemical reactions are transformations that convert reactants into products through the breaking and forming of chemical bonds. Understanding how different substances interact in a chemical reaction is foundational to chemistry. Redox reactions, a category of chemical reactions, involve the movement of electrons between atoms. The gain or loss of electrons alters the properties of the substance involved.

When teaching or learning about chemical reactions, breaking them down into types, such as synthesis, decomposition, single replacement, and double replacement, is useful. Redox reactions often fall into either the single or double replacement categories. Reaction (c) in the exercise, for example, is a single replacement reaction where calcium displaces chlorine to form elemental calcium and chlorine gas—a clear redox scenario shown by the exchange of electrons.

Key Points to Remember in Chemical Reactions

• Conservation of mass: In a chemical reaction, the mass of the reactants equals the mass of the products.
• Energy changes: Reactions may release or absorb energy in the form of light, heat, or electricity.
• Reaction rate: Factors such as concentration, temperature, and surface area can influence how fast a reaction proceeds.

Calcium Compounds

Calcium is a versatile element that forms a wide variety of compounds, which have prominent roles in construction, biology, and industry. Calcium compounds often include ions of calcium combined with other elements, resulting in materials like calcium carbonate (CaCO₃), a component of limestone and marble, and calcium oxide (CaO), known as quicklime.

In the context of the exercise, calcium compounds participate in different types of reactions. Reactions (a) and (b) feature calcium carbonate and calcium oxide undergoing chemical transformations. Despite being part of these processes, (a) is not a redox reaction as there is no electron transfer with respect to calcium or any other element involved.

Key Characteristics of Calcium Compounds

• Calcium Carbonate (CaCO₃): A common substance found in rocks, it's used in building materials and dietary calcium supplements.
• Calcium Oxide (CaO): When mixed with water, it forms calcium hydroxide (slaked lime); it's utilized in cement and to adjust the pH of soil.
• Calcium Sulphate (CaSO₄): In its dihydrate form, known as gypsum, is a key ingredient in drywall and plasters.

Analyzing how calcium interacts with other elements in its compounds and the resulting reaction types is fundamental in understanding the nature of chemical processes it's involved in.