Problem 25 - Chemistry and Chemical Reactivity

Question

When magnesium burns in air, it forms both an oxide and a nitride. Write balanced equations for the formation of both compounds.

Step-by-Step Solution

Answer

The balanced equations are: \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \) and \( 3\text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2 \).

Step 1: Write the Reaction for Magnesium Oxide Formation

When magnesium burns in air, it reacts with oxygen from the air to form magnesium oxide. The word equation for this reaction is: Magnesium + Oxygen → Magnesium Oxide. To balance this chemical equation, we first write the unbalanced chemical equation: \[ \text{Mg} + \text{O}_2 \rightarrow \text{MgO} \].

Step 2: Balance the Magnesium Oxide Equation

In the unbalanced equation, there are two oxygen atoms on the left side and one oxygen atom on the right. To balance the oxygen, we place a coefficient of 2 in front of MgO: \[ \text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \]. Now there are two magnesium atoms on the right, so we add a coefficient of 2 in front of magnesium on the left: \[ 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \].

Step 3: Write the Reaction for Magnesium Nitride Formation

Magnesium also reacts with nitrogen in the air to form magnesium nitride. The word equation is: Magnesium + Nitrogen → Magnesium Nitride. We write the unbalanced chemical equation: \[ \text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2 \].

Step 4: Balance the Magnesium Nitride Equation

In the unbalanced equation, the right side has three magnesium atoms and two nitrogen atoms. We place a coefficient of 3 in front of magnesium on the left side: \[ 3\text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2 \]. Now the equation is balanced.

Key Concepts

Balancing Chemical Equations

Balancing chemical equations is an essential skill in chemistry that ensures the law of conservation of mass is respected. This law states that in a chemical reaction, matter cannot be created or destroyed. Thus, the number of atoms for each element must be the same on both sides of the equation. Let's start with the concept of balancing a chemical reaction:

• Write down the word equation first to identify the reactants and products.
• Next, translate this into a symbolic equation using chemical symbols and formulas.
• To balance, count the number of atoms for each element on both sides.
• If there is an imbalance, adjust the coefficients (the numbers in front of chemical formulas) to balance.

Balancing is often seen as trial and error, but with practice, you'll notice common strategies, such as starting with the element that appears in the least number of reactants and products. In our example, we balanced the equations for magnesium burning in air, resulting in both magnesium oxide and magnesium nitride.

Magnesium Oxide

When magnesium burns in the presence of oxygen, magnesium oxide is formed. This reaction is highly exothermic, meaning it releases a significant amount of heat and light, often seen as a bright white flame:

• Word Equation: Magnesium + Oxygen → Magnesium Oxide
• Unbalanced Equation: \( \text{Mg} + \text{O}_2 \rightarrow \text{MgO} \)
• Balanced Equation: \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \)

The balanced equation indicates that two atoms of magnesium react with one molecule of oxygen to form two units of magnesium oxide. This illustrates the importance of balancing, as it reflects the real proportions in which substances react. Magnesium oxide is a white powdery solid, commonly used as a refractory material and a fast-acting antacid.

Magnesium Nitride

Magnesium's reaction with nitrogen from the air yields magnesium nitride, which involves forming a bond between magnesium and nitrogen atoms:

• Word Equation: Magnesium + Nitrogen → Magnesium Nitride
• Unbalanced Equation: \( \text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2 \)
• Balanced Equation: \( 3\text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2 \)

Balancing this equation is a bit different because magnesium nitride contains three magnesium atoms and two nitrogen atoms. Therefore, placing a coefficient of three in front of magnesium on the reactant side balances the magnesium and nitrogen atoms on both sides of the equation. Magnesium nitride is less stable than magnesium oxide and can react with water to release ammonia—a useful method in chemistry labs to produce ammonia gas.