Question

Complete and balance each of the following single-replacement reactions: (a) \(\mathrm{Mg}(s)+\mathrm{H}_{2} \mathrm{SO}_{4}(a q) \rightarrow\) (b) \(\mathrm{Al}(s)+\mathrm{H}_{2} \mathrm{SO}_{4}(a q) \rightarrow\)

Short answer

(a) Mg + H₂SO₄ → MgSO₄ + H₂ (b) 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂

Step-by-step solution

Understand Single-Replacement Reactions

In a single-replacement reaction, an element replaces another element in a compound. Typically, metals replace metals or hydrogen (in acids), while nonmetals replace nonmetals. Check which element is more reactive according to the activity series.

Predict the Products for Reaction (a)

When magnesium (Mg) reacts with sulfuric acid (H₂SO₄), it will replace hydrogen (H₂) because Mg is more reactive. The predicted products are magnesium sulfate (MgSO₄) and hydrogen gas (H₂). The unbalanced equation is: \( \mathrm{Mg}(s)+\mathrm{H}_{2}\mathrm{SO}_{4}(aq) \rightarrow \mathrm{MgSO}_{4}(aq)+\mathrm{H}_{2}(g) \).

Balance Reaction (a)

Balance the equation by ensuring the same number of each type of atom appears on both sides. The balanced equation is: \( \mathrm{Mg}(s)+\mathrm{H}_{2}\mathrm{SO}_{4}(aq) \rightarrow \mathrm{MgSO}_{4}(aq)+\mathrm{H}_{2}(g) \). No coefficients are needed, as it is already balanced.

Predict the Products for Reaction (b)

When aluminum (Al) reacts with sulfuric acid (H₂SO₄), it will replace hydrogen (H₂) because Al is more reactive. The predicted products are aluminum sulfate (Al₂(SO₄)₃) and hydrogen gas (H₂). The unbalanced equation is: \( \mathrm{2Al}(s)+\mathrm{3H}_{2}\mathrm{SO}_{4}(aq) \rightarrow \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3}(aq)+\mathrm{3H}_{2}(g) \).

Balance Reaction (b)

Ensure the number of atoms for each element is the same on both sides of the equation. The balanced equation is: \( \mathrm{2Al}(s)+\mathrm{3H}_{2}\mathrm{SO}_{4}(aq) \rightarrow \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3}(aq)+\mathrm{3H}_{2}(g) \).

Key concepts

Balancing Chemical Equations

Balancing chemical equations is a crucial skill in chemistry that ensures the law of conservation of mass is upheld in chemical reactions. This law states that in a chemical reaction, atoms are neither created nor destroyed. Therefore, we must have an equal number of each type of atom on both sides of the equation. This process involves adjusting coefficients, which are numbers placed before compounds or elements in the equation.

Let's consider an example with a single-replacement reaction. Suppose magnesium (\( \mathrm{Mg(s)} \)) replaces hydrogen in sulfuric acid (\( \mathrm{H}_{2}\mathrm{SO}_{4}(aq) \)). The products formed are magnesium sulfate (\( \mathrm{MgSO}_{4}(aq) \)) and hydrogen gas (\( \mathrm{H}_{2}(g) \)).

• Unbalanced reaction: \( \mathrm{Mg}(s)+\mathrm{H}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{MgSO}_{4}(aq)+\mathrm{H}_{2}(g) \)
• Balanced reaction: \( \mathrm{Mg}(s)+\mathrm{H}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{MgSO}_{4}(aq)+\mathrm{H}_{2}(g) \)

In this reaction, coefficients are not required because the number of atoms for each element is already balanced on both sides: one magnesium atom, two hydrogen atoms, and one sulfate ion. Balancing reactions like this ensures accuracy in representing chemical processes.

Single-Replacement Reactions

Single-replacement reactions are a category of chemical reactions where an elemental metal (or sometimes a nonmetal) replaces another element in a compound. For metals, these reactions are dictated by the reactivity of the involved elements. Reactive metals will replace less reactive metals or hydrogen in compounds, typically in an aqueous solution.

Here's how it works step-by-step:

• A metal is combined with a compound.
• The metal replaces a less reactive element within that compound.
• If the replacing element is more reactive, a new compound forms while the replaced element is released.

For instance, when aluminum (\( \mathrm{Al} \)) is placed in sulfuric acid (\( \mathrm{H}_{2} \mathrm{SO}_{4}(aq) \)), it replaces hydrogen to form aluminum sulfate (\( \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3} \)) and releases hydrogen gas (\( \mathrm{H}_{2(g)} \)). The equation for this can be seen as:

• Unbalanced: \( \mathrm{Al}(s)+\mathrm{H}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3}(aq)+\mathrm{H}_{2}(g) \)
• Balanced: \( \mathrm{2Al}(s)+\mathrm{3H}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3}(aq)+\mathrm{3H}_{2}(g) \)

In this type of reaction, the metal aluminum showcases its ability to replace hydrogen due to its higher reactivity, adhering to the principles of single-replacement reactions.

Activity Series

The activity series is an essential tool in predicting whether single-replacement reactions will occur. This series is a list of elements, typically metals, ordered by their reactivity. It tells us which elements can replace others in compounds based on their position in the series.

Here's how the activity series aids in understanding reactions:

• If a metal is higher than another metal or hydrogen in the activity series, it can replace it in a compound.
• For nonmetals, the same concept applies within their reactivity context.

When you have magnesium (\( \mathrm{Mg} \)) and aluminum (\( \mathrm{Al} \)), they are both above hydrogen on the activity series:

• Magnesium reacts with sulfuric acid to form hydrogen gas because Mg is more reactive than hydrogen.
• Aluminum will also react with sulfuric acid, releasing hydrogen gas and forming aluminum sulfate due to its higher reactivity than hydrogen.

Using the activity series helps chemists and students predict the outcomes of reactions with greater accuracy. It provides a systematic way to determine which metals can displace others in chemical reactions.