Problem 26 - Chemistry and Chemical Reactivity

Question

Calcium reacts with hydrogen gas at \(300-400^{\circ} \mathrm{C}\) to form a hydride. This compound reacts readily with water, so it is an excellent drying agent for organic solvents. (a) Write a balanced equation showing the formation of calcium hydride from Ca and \(\mathrm{H}_{2}\) (b) Write a balanced equation for the reaction of calcium hydride with water (Figure 21.7 ).

Step-by-Step Solution

Answer

(a) \( \text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2 \) (b) \( \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \)

Step 1: Balance Equation for Formation of Calcium Hydride

The compound formed is calcium hydride, which has the chemical formula \( \text{CaH}_2 \). To form calcium hydride, calcium (\( \text{Ca} \)) reacts with hydrogen gas (\( \text{H}_2 \)). The balanced chemical equation is shown below:\[ \text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2 \]This means one mole of calcium reacts with one mole of hydrogen gas to form one mole of calcium hydride.

Step 2: Balance Equation for Reaction with Water

Calcium hydride reacts with water to form calcium hydroxide (\( \text{Ca(OH)}_2 \)) and hydrogen gas (\( \text{H}_2 \)). The balanced equation for this reaction is:\[ \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \]In this reaction, one mole of calcium hydride reacts with two moles of water to produce one mole of calcium hydroxide and two moles of hydrogen gas.

Key Concepts

Understanding Calcium Hydride

Calcium hydride is a chemical compound with the formula \( \text{CaH}_2 \). It consists of calcium (Ca) and hydrogen (H) and is known for its ability to react vigorously with water. This makes it a valuable component in various chemical applications including its use as a drying agent for organic solvents. The compound is typically formed by the direct combination of calcium with hydrogen gas at high temperatures, specifically between 300-400 degrees Celsius.

Calcium hydride appears as a grey powder and is highly reactive due to the alkaline earth metal, calcium. When it combines with hydrogen gas, it forms a stable ionic compound where calcium gives up electrons to hydrogen, forming hydride ions \( \text{H}^- \). This ionic property contributes to its distinct reactivity patterns, particularly its aggressive interaction with water.

Balancing Chemical Equations

In chemistry, balancing chemical equations is crucial as it obeys the law of conservation of mass. This law states that matter cannot be created or destroyed in any chemical reaction. Hence, the number of each type of atom must be the same on both sides of a chemical equation. Let's delve deeper into this concept using the chemical equations for calcium hydride.

For the formation of calcium hydride:

• The equation is \( \text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2 \).
• Here, one molecule of calcium reacts with one molecule of hydrogen gas to produce one molecule of calcium hydride.

Balancing this equation is straightforward since the number of calcium and hydrogen atoms is already the same on both sides.

For its reaction with water:

• The balanced equation is \( \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \).
• In this equation, one molecule of calcium hydride reacts with two molecules of water to form one molecule of calcium hydroxide and two molecules of hydrogen gas.

It's important to ensure both sides of the equation have the same amount of each type of atom, maintaining the balance.

Calcium Hydride Reaction with Water

Calcium hydride's interaction with water is a classic and prominent chemical reaction. This process is highly exothermic, which means it releases heat and is favored in situations where water needs to be eliminated from a chemical environment, such as during the drying of organic solvents.

When calcium hydride comes into contact with water, it leads to the formation of calcium hydroxide \( \text{Ca(OH)}_2 \) and hydrogen gas \( \text{H}_2 \). This reaction is represented by the following balanced equation:
\( \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \).

Key points to understand include:

• Calcium hydride starts this reaction by breaking its bond with hydride ions, allowing these ions to interact with the hydrogen atoms in water molecules.
• This interaction leads to the formation of hydrogen gas, which is released as bubbles during the reaction.
• The leftover calcium ions bond with hydroxyl groups from water molecules, yielding calcium hydroxide.

This whole process is highly efficient for producing hydrogen gas or for eliminating moisture from specific environments.