Problem 78 - Chemistry A Molecular Approach

Question

Predict the products and write a balanced molecular equation for each reaction. If no reaction occurs, write "NO REACTION." a. \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q)+\mathrm{HNO}_{3}(a q) \longrightarrow\) b. \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}(a q)+\mathrm{LiOH}(a q) \longrightarrow\) c. aqueous strontium sulfide and aqueous copper(II) sulfate

Step-by-Step Solution

Answer

a. NO REACTION. b. Balanced equation: \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}(\text{aq}) + 3\mathrm{LiOH}(\text{aq}) \longrightarrow \mathrm{Cr}\left(\mathrm{OH}\right)_{3}(\text{s}) + 3\mathrm{LiNO}_{3}(\text{aq})\). c. Balanced equation: \(\mathrm{SrS}(\text{aq}) + \mathrm{CuSO}_{4}(\text{aq}) \longrightarrow \mathrm{CuS}(\text{s}) + \mathrm{SrSO}_{4}(\text{s})\).

Step 1: Part A: Mixing Strong Acids

When mixing two strong acids like sulfuric acid \(\mathrm{H}_{2}\mathrm{SO}_{4}\) and nitric acid \(\mathrm{HNO}_{3}\), no reaction occurs because they are both fully ionized in solution and no new substances are formed.

Step 2: Part B: Double Replacement Reaction

For the reaction between chromium(III) nitrate \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}\) and lithium hydroxide \(\mathrm{LiOH}\), a double replacement reaction will occur. The products are chromium(III) hydroxide and lithium nitrate. Writing the unbalanced equation, we have: \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3} + 3\mathrm{LiOH} \longrightarrow \mathrm{Cr}\left(\mathrm{OH}\right)_{3} + 3\mathrm{LiNO}_{3}\). Balancing the reaction yields the same equation as it is already balanced.

Step 3: Part C: Double Replacement Reaction

For the reaction between strontium sulfide \(\mathrm{SrS}\) and copper(II) sulfate \(\mathrm{CuSO}_{4}\), another double replacement reaction occurs. The products are copper(II) sulfide and strontium sulfate. An unbalanced equation is: \(\mathrm{SrS} + \mathrm{CuSO}_{4} \longrightarrow \mathrm{CuS} + \mathrm{SrSO}_{4}\). This equation is also balanced as written.

Key Concepts

Double Replacement Reaction

Double replacement reactions are a type of chemical reactions where the anions and cations of two different reactants switch places, forming two new compounds. They are common in aqueous solutions where ionic compounds are involved. For such reactions to occur, at least one of the products must be a precipitate, a gas, or a weakly dissociated compound like a weak acid or weak base.

For example, when chromium(III) nitrate reacts with lithium hydroxide in water, it leads to the formation of chromium(III) hydroxide and lithium nitrate. This can be predicted by switching the anions \text{(NO\(_3\) and OH)} between the cations {Cr and Li). The discerning eye can see that this type of reaction typically involves compounds with two separate charged particles—cations and anions—and results in the exchange of these particles between reactants.

In the provided exercise, parts B and C exemplify double replacement reactions, showcasing the interchange of anions and cations between reactants to create new products.

Balancing Chemical Equations

Balancing chemical equations is the process of ensuring that there are equal numbers of each type of atom on both sides of a chemical equation. It's grounded in the law of conservation of mass, which says that mass cannot be created or destroyed in a chemical reaction. Thus, what goes into the reaction must come out in some form.

To balance a chemical equation, you must add coefficients, whole numbers placed before the chemical formula, to make the number of atoms of each element the same on both sides of the equation. It's crucial not to change the subscripts in a chemical formula as it would change the compound's identity.

In the exercises provided, the balanced equation for the double replacement reaction between chromium(III) nitrate and lithium hydroxide is a prime example. The chemical equation \text{Cr(NO\(_3\))\(_3\) + 3LiOH \text{→} Cr(OH)\(_3\) + 3LiNO\(_3\)} is balanced, as the numbers of chromium, nitrogen, oxygen, and hydrogen atoms are the same on both sides.

Strong Acids

Strong acids are substances that completely dissociate into their ions in aqueous solutions, releasing hydrogen ions, H+, which are responsible for the characteristic acidic properties. Examples of strong acids include sulfuric acid \text{(H\(_2\)SO\(_4\))}, nitric acid \text{(HNO\(_3\))}, and hydrochloric acid \text{(HCl)}. Because they dissociate fully in solution, mixing two strong acids together, like in part A of the exercise, typically does not result in a chemical reaction, as there's already a surplus of hydrogen ions and no new products can be formed.

Understanding the nature of strong acids is important in predicting whether a reaction will occur. If one of the reactants is a strong acid and the other a base, we can predict that they'll react to form water and a salt. However, when two strong acids are mixed, as in part A with sulfuric acid and nitric acid, we observe no reaction since the ionic species in the solution remain unchanged.