Question

Lanthanum metal forms cations with a charge of \(3+\). Consider the following observations about the chemistry of lanthanum: When lanthanum metal is exposed to air, a white solid (compound A) is formed that contains lanthanum and one other element. When lanthanum metal is added to water, gas bubbles are observed and a different white solid (compound \(B\) ) is formed. Both \(A\) and \(B\) dissolve in hydrochloric acid to give a clear solution. When either of these solutions is evaporated, a soluble white solid (compound C) remains. If compound C is dissolved in water and sulfuric acid is added, a white precipitate (compound D) forms. (a) Propose identities for the substances A, B, C, and D. (b) Write net ionic equations for all the reactions described. (c) Based on the preceding observations, what can be said about the position of lanthanum in the activity series (Table 4.5)?

Short answer

Compound A is lanthanum oxide (La2O3), formed by the reaction of lanthanum with oxygen. Compound B is lanthanum hydroxide (La(OH)3), formed by the reaction of lanthanum with water. Compound C is lanthanum chloride (LaCl3), formed by the reaction of lanthanum oxide or hydroxide with hydrochloric acid. Compound D is lanthanum sulfate (La2(SO4)3), formed by the reaction of lanthanum chloride with sulfuric acid. The net ionic equations for these reactions are: 1. \( 4 \text{La}(s) + 3 O_{2}(g) \to 2 \text{La}_{2}O_{3}(s) \) 2. \( 2 \text{La}(s) + 6 H_{2}O(l) \to 2 \text{La(OH)}_{3}(s) + 3 H_{2}(g) \) 3. \( \text{La}_{2}O_{3}(s) + 6 \text{HCl}(aq) \to 2 \text{LaCl}_{3}(aq) + 3 H_{2}O(l) \) 4. \( 2 \text{La(OH)}_{3}(s) + 6 \text{HCl}(aq) \to 2 \text{LaCl}_{3}(aq) + 6 H_{2}O(l) \) 5. \( 2 \text{LaCl}_{3}(aq) + 3 H_{2}SO_{4}(aq) \to \text{La}_{2}(SO_{4})_{3}(s) + 6 HCl(aq) \) Based on lanthanum's reactivity with oxygen and water, it can be deduced that it has a high position in the activity series as it is above hydrogen and more active.

Step-by-step solution

(Identify Compound A)

(Lanthanum metal when exposed to air forms a white solid that contains lanthanum and one other element. The most common element in the air that reacts with metals is oxygen. So, Compound A is likely to be lanthanum oxide (La2O3).)

(Identify Compound B)

(When lanthanum metal is added to water, gas bubbles are observed and a different white solid is formed. The gas formed is likely to be hydrogen gas (H2), as most metals react with water to release hydrogen. The white solid (Compound B) is likely to be lanthanum hydroxide (La(OH)3).)

(Identify Compound C)

(Both Compound A (La2O3) and Compound B (La(OH)3) dissolve in hydrochloric acid (HCl) to give a clear solution. When this solution is evaporated, a soluble white solid (Compound C) remains. Compound C must be a soluble salt of lanthanum chloride (LaCl3), which is the product of the reaction between lanthanum hydroxide or lanthanum oxide and hydrochloric acid.)

(Identify Compound D)

(When Compound C (LaCl3) is dissolved in water and sulfuric acid (H2SO4) is added, a white precipitate (Compound D) forms. This precipitate is likely to be insoluble lanthanum sulfate (La2(SO4)3), which forms from the reaction between lanthanum chloride and sulfuric acid.)

(Write net ionic equations for all the reactions)

1. Reaction between lanthanum and oxygen \[ 4 \text{La}(s) + 3 O_{2}(g) \to 2 \text{La}_{2}O_{3}(s) \] 2. Reaction between lanthanum and water \[ 2 \text{La}(s) + 6 H_{2}O(l) \to 2 \text{La(OH)}_{3}(s) + 3 H_{2}(g) \] 3. Reaction between lanthanum oxide and hydrochloric acid \[ \text{La}_{2}O_{3}(s) + 6 \text{HCl}(aq) \to 2 \text{LaCl}_{3}(aq) + 3 H_{2}O(l) \] 4. Reaction between lanthanum hydroxide and hydrochloric acid \[ 2 \text{La(OH)}_{3}(s) + 6 \text{HCl}(aq) \to 2 \text{LaCl}_{3}(aq) + 6 H_{2}O(l) \] 5. Reaction between lanthanum chloride and sulfuric acid \[ 2 \text{LaCl}_{3}(aq) + 3 H_{2}SO_{4}(aq) \to \text{La}_{2}(SO_{4})_{3}(s) + 6 HCl(aq) \]

(Determine the position of lanthanum in the activity series)

(Lanthanum reacts with both oxygen and water, indicating that it is a highly reactive metal. Based on its reactivity with oxygen and water, we can infer that lanthanum is above hydrogen in the activity series and must be towards the more active end of the series. Comparing lanthanum's reactivity to the elements in Table 4.5, we can deduce that it is likely to have a high position in the activity series.)

Key concepts

Net Ionic Equations

Net ionic equations are a simplified way to look at chemical reactions. They focus only on the chemical species that are involved in the reaction, excluding spectator ions. Spectator ions do not change during the reaction and are not included. Net ionic equations help in understanding what actually happens during a chemical exchange by breaking the reaction down to the essentials.

For lanthanum, there are several reactions where net ionic equations are useful:

• Lanthanum with oxygen: This forms lanthanum oxide. The net ionic equation is \[ 4 \text{La}(s) + 3 O_{2}(g) \to 2 \text{La}_{2}O_{3}(s) \]
• Lanthanum with water: Lanthanum reacts with water to create lanthanum hydroxide and hydrogen gas: \[ 2 \text{La}(s) + 6 H_{2}O(l) \to 2 \text{La(OH)}_{3}(s) + 3 H_{2}(g) \]
• Lanthanum oxide with hydrochloric acid: This results in lanthanum chloride and water, \[ \text{La}_{2}O_{3}(s) + 6 \text{HCl}(aq) \to 2 \text{LaCl}_{3}(aq) + 3 H_{2}O(l) \]

Notice, in these net ionic equations, we only focus on substances that change during the reaction.

Understanding net ionic equations can greatly enhance the grasp of what occurs during a reaction, as it strips the equations down to showcase the core transformation.

Oxides and Hydroxides

In chemistry, oxides and hydroxides often form when a metal reacts with oxygen or water. They typically contain the metal element combined either with oxygen or hydroxide ions. This forms an important part of the corrosion process of metals.

For lanthanum:

• Oxide: When lanthanum reacts with oxygen, it forms lanthanum oxide \(\text{La}_{2}O_{3}\). This oxide appears as a white solid and illustrates lanthanum’s tendency to oxidize in air.
• Hydroxide: In water, lanthanum forms lanthanum hydroxide \(\text{La(OH)}_{3}\), another white compound. This is why bubbles are observed during its reaction with water—due to the release of hydrogen gas.

Both oxides and hydroxides of lanthanum dissolve in acids like hydrochloric acid, showcasing the typical behavior of metal oxides and hydroxides which often form salts in acidic environments.

Understanding these reactions can be crucial in industrial applications and in predicting how different substances will react under varied conditions.

Activity Series

The activity series is a list of metals ranked by their reactivity from highest to lowest. It helps predict how different metals will react, particularly with water, acids, and other substances.

In relation to lanthanum:

• Lanthanum’s ability to react with both oxygen and water implies a higher activity level, positioning it above hydrogen in the activity series. This means that lanthanum can replace hydrogen in reactions, a significant property for discerning its reactive nature.
• The observation that lanthanum forms new compounds effortlessly with common elements further supports its position in the higher segment of the activity series.

An activity series serves as a valuable tool in predicting how a particular metal will behave in a chemical process, aiding in the design of reactions and understanding properties of metallic elements like lanthanum.