Problem 141 - Chemistry The Science in Context

Question

Rocks in Caves The stalactites and stalagmites in most caves are made of limestone (calcium carbonate; see Figure 4.14 ). However, in the Lower Kane Cave in Wyoming they are made of gypsum (calcium sulfate). The presence of \(\mathrm{CaSO}_{4}\) is explained by the following sequence of reactions: $$ \mathrm{H}_{2} \mathrm{S}(a q)+2 \mathrm{O}_{2}(g) \rightarrow \mathrm{H}_{2} \mathrm{SO}_{4}(a q) $$ \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q)+\mathrm{CaCO}_{3}(s) \rightarrow \mathrm{CaSO}_{4}(s)+\mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{CO}_{2}(g)\) a. Which (if either) of these reactions is a redox reaction? How many electrons are transferred? b. Write a net ionic equation for the reaction of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) with \(\mathrm{CaCO}_{3}\) c. How would the net ionic equation be different if the reaction were written as follows? \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q)+\mathrm{CaCO}_{3}(s) \rightarrow \mathrm{CaSO}_{4}(s)+\mathrm{H}_{2} \mathrm{CO}_{3}(a q)\)

Step-by-Step Solution

Answer

Provide the net ionic equation for the reaction between H2SO4 and CaCO3 and describe how the net ionic equation would differ if the reaction was written with H2CO3 as a product instead of H2O and CO2. Answer: Reaction 1 is a redox reaction, and 4 electrons are transferred. The net ionic equation for the reaction between H2SO4 and CaCO3 is CaCO3(s) + 2H+(aq) + SO4^2-(aq) → CaSO4(s) + H2O(l) + CO2(g). If H2CO3 is a product instead of H2O and CO2, the net ionic equation would be CaCO3(s) + 2H+(aq) + SO4^2-(aq) → CaSO4(s) + CO3^2-(aq).

Step 1: Calculate the oxidation numbers of elements

Calculate the oxidation numbers of the elements in the reactants and products for both reactions. Here are the oxidation numbers: Reaction 1: H2S(aq): H = +1, S = -2 O2(g): O = 0 H2SO4(aq): H = +1, S = +6, O = -2 Reaction 2: H2SO4(aq): H = +1, S = +6, O = -2 CaCO3(s): Ca = +2, C = +4, O = -2 CaSO4(s): Ca = +2, S = +6, O = -2 H2O(l): H = +1, O = -2 CO2(g): C = +4, O = -2

Step 2: Identify the redox reaction

Compare the oxidation numbers of the elements in the reactants and products. In Reaction 1, the sulfur in H2S is -2, and in the product, H2SO4 is +6, and the oxygen in O2 is 0, and in the product, H2SO4 is -2. Thus, there is a change in the oxidation numbers of S and O, so Reaction 1 is a redox reaction. In Reaction 2, the oxidation numbers of all the elements in the reactants and products remain the same; thus, Reaction 2 is not a redox reaction. Therefore, only Reaction 1 is a redox reaction.

Step 3: Determine the number of electrons transferred

In Reaction 1, calculate the change in oxidation numbers and find the number of electrons transferred. For sulfur (S): -2 (reactant) to +6 (product), change = +8 For oxygen (O): 0 (reactant) to -2 (product), change = -2 Since there are two oxygen molecules in the reaction, the total change for oxygen is -2 x 2 = -4. To balance the charges, multiply the change of S by 1 (4 e-) and the change of O by 2 (4 e-) to get a balanced redox reaction (both changes will have the same number of electrons). Hence, 4 electrons are transferred in Reaction 1. Answer to part a: Reaction 1 is a redox reaction, and 4 electrons are transferred. #b. Writing the net ionic equation for H2SO4 and CaCO3 reaction#

Step 1: Write the molecular equation

The molecular equation is already given: H2SO4(aq) + CaCO3(s) → CaSO4(s) + H2O(l) + CO2(g)

Step 2: Write the complete ionic equation

Now, break all the soluble species into their corresponding ions: 2H+(aq) + SO4^2-(aq) + CaCO3(s) → CaSO4(s) + 2H+(aq) + H2O(l) + CO2(g)

Step 3: Write the net ionic equation

Cancel the spectator ions, which, in this case, are 2H+(aq) on both sides: CaCO3(s) + 2H+(aq) + SO4^2-(aq) → CaSO4(s) + H2O(l) + CO2(g) Answer to part b: The net ionic equation is CaCO3(s) + 2H+(aq) + SO4^2-(aq) → CaSO4(s) + H2O(l) + CO2(g). #c. Difference in the net ionic equation if H2CO3 is a product#

Step 1: Write the molecular equation with H2CO3

The new molecular equation is: H2SO4(aq) + CaCO3(s) → CaSO4(s) + H2CO3(aq)

Step 2: Write the complete ionic equation with H2CO3

Breaking all the soluble species into their corresponding ions: 2H+(aq) + SO4^2-(aq) + CaCO3(s) → CaSO4(s) + 2H+(aq) + CO3^2-(aq)

Step 3: Write the net ionic equation with H2CO3

Cancel the spectator ions, which, in this case, are 2H+(aq) on both sides: CaCO3(s) + 2H+(aq) + SO4^2-(aq) → CaSO4(s) + CO3^2-(aq) Answer to part c: The net ionic equation would be different if H2CO3 is a product: CaCO3(s) + 2H+(aq) + SO4^2-(aq) → CaSO4(s) + CO3^2-(aq).

Key Concepts

Redox Reaction

When exploring the fascinating world of chemistry in caves, a key concept is the redox reaction, which stands for reduction-oxidation reaction. This is a process where electrons are transferred between chemical species, with one species gaining electrons (reduction) and another losing electrons (oxidation). In the Lower Kane Cave scenario, the first reaction, where hydrogen sulfide reacts with oxygen to form sulfuric acid, is identified as a redox reaction because sulfur's oxidation number increases from -2 to +6, indicating it's being oxidized, and oxygen's oxidation number decreases from 0 to -2, showing it's being reduced. In a redox process like this, the movement of electrons is fundamental to converting reactants into products.

Net Ionic Equation

Understanding net ionic equations is crucial for students studying reactions in an aqueous solution. It simplifies the equation by omitting spectator ions that do not participate in the chemical change and shows only the entities that are involved in the reaction. The net ionic equation for the reaction of sulfuric acid with calcium carbonate, which leads to gypsum formation in caves, emphasizes the importance of the solid calcium carbonate reacting with sulfuric acid to produce calcium sulfate, water, and carbon dioxide, without including the hydrogen ions that remain unchanged.

Oxidation Numbers

To fully understand redox reactions, one must grasp the concept of oxidation numbers. These numerical values assigned to atoms within a compound or ion indicate the degree of oxidation (electron loss) or reduction (electron gain) the atom has undergone. When comparing the oxidation numbers of reactants and products, you can determine whether a redox reaction has occurred. For example, in caves, the transformation of calcium carbonate to gypsum involves comprehending these oxidation states to confirm that the initial reaction involves electron transfer, thus being a redox process.

Calcium Carbonate

Stalactites and stalagmites in caves often consist of calcium carbonate, a chemical compound that is the primary ingredient of limestone. When this compound reacts with sulfuric acid, an acid-base reaction occurs, forming gypsum and releasing water and carbon dioxide. This is a significant geological process in caves, as it changes the very structure and mineral compositions of these subterranean environments.

Gypsum Formation

In the context of cave chemistry, gypsum formation is particularly intriguing. Gypsum, or calcium sulfate, forms when calcium carbonate reacts with sulfuric acid. This sequence of reactions is responsible for creating the unique, white, chalky mineral deposits found in caves like the Lower Kane Cave in Wyoming. Students observing this transformation can appreciate the link between chemical reactions and the formation of geological features.

Electron Transfer

The electron transfer process is at the heart of redox reactions, and it's exemplified in the transformation of hydrogen sulfide to sulfuric acid, which occurs in some cave environments. This involves a shift in electrons from one molecule to another, effectively changing the oxidation states of the elements involved. In the example from the cave, four electrons are transferred, reshaping sulfur's oxidation number significantly. Grasping this concept is essential for students to understand the chemistry behind cave formations and the process of gypsum creation.