Problem 36 - 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: Identify the Reactants and Products for Part (a)

For part (a), calcium (Ca) reacts with hydrogen gas ( \( \text{H}_2 \) ) to form calcium hydride ( \( \text{CaH}_2 \) ). This forms a simple combination reaction with the metal and hydrogen gas.

Step 2: Write the Unbalanced Equation for Part (a)

Write the chemical equation with the reactants and the product: \( \text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2 \). Now move to balancing the equation.

Step 3: Balance the Equation for Part (a)

In the equation \( \text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2 \), we have 1 calcium, and 2 hydrogens on both sides, which means the equation is already balanced. Therefore, the balanced equation remains the same.

Step 4: Identify Reactants and Products for Part (b)

For part (b), calcium hydride ( \( \text{CaH}_2 \) ) reacts with water ( \( \text{H}_2\text{O} \) ) to produce calcium hydroxide ( \( \text{Ca(OH)}_2 \) ) and hydrogen gas ( \( \text{H}_2 \) ). This is a decomposing reaction.

Step 5: Write the Unbalanced Equation for Part (b)

Write the equation of the reaction of calcium hydride and water: \( \text{CaH}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2 \). Next, proceed to balance the equation.

Step 6: Balance the Equation for Part (b)

In the equation \( \text{CaH}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2 \), we balance the hydrogens by placing a coefficient of 2 in front of water (\(\text{H}_2\text{O}\)): \( \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \).This equation is now balanced with 1 calcium, 4 hydrogens, and 2 oxygens on both sides.

Key Concepts

Balancing Chemical Equations

Balancing chemical equations is a crucial skill in chemistry. It's about ensuring that the same number of each type of atom appears on both sides of the equation.
This is in accordance with the Law of Conservation of Mass, which states that matter cannot be created or destroyed.
When balancing equations, always start by writing the unbalanced equation with the given reactants and products.

• Count the number of each type of atom on both sides.
• Add coefficients in front of compounds to balance the atoms. Remember, you can only change coefficients, not subscripts.
• Continue adjusting until you have the same number of each type of atom on each side.

For example, in forming calcium hydride (CaH₂) from calcium (Ca) and hydrogen gas (H₂), the equation is already balanced as: \[ \text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2 \] In contrast, when calcium hydride reacts with water, we initially write the equation as: \[ \text{CaH}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2 \]Balancing this equation involves adding coefficients to ensure equal numbers of hydrogen and oxygen atoms on both sides. The balanced equation becomes:\[ \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \]

Calcium Hydride

Calcium hydride (CaH₂) is an inorganic compound with unique properties. It forms when calcium reacts with hydrogen gas at elevated temperatures (300-400°C).
Calcium hydride plays a pivotal role in various industrial processes because of its ability to act as a powerful hydrogen source.
It releases hydrogen gas when it reacts with water, which makes it useful in generating hydrogen on-demand.
Additionally, it's commonly used as a drying agent for organic solvents due to its effective water-absorbing ability. Properties to note about calcium hydride:

• It's a solid, ionic compound.
• It appears as a white to grey powder.
• While stable under dry conditions, it reacts rapidly with water.

When handling calcium hydride, it's essential to store it in a dry environment to prevent accidental reaction with moisture from the air.

Reaction with Water

The reaction of calcium hydride with water is an example of a double displacement reaction.
This kind of reaction typically involves two compounds reacting and exchanging components (ions/molecules) to form two new compounds.In this scenario, calcium hydride reacts with water to produce calcium hydroxide and hydrogen gas.
The unbalanced and balanced chemical equations providing this reaction demonstrate the decomposition process:Initial unbalanced equation:\[ \text{CaH}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2 \]Balanced equation:\[ \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \]During the reaction:

• Calcium hydride decomposes as it reacts with water.
• Hydrogen gas is released, making it suitable for hydrogen production applications.
• Calcium hydroxide, a useful industrial compound, is formed.

This reaction also showcases how compounds can transform, illustrating the dynamic nature of chemical interactions.