hen magnesium metal is burned in air (Figure 3.5), two products are produced. One is magnesium oxide, MgO. The other is the product of the reaction of Mg with molecular nitrogen, magnesium nitride. When water is added to magnesium nitride, it reacts to form magnesium oxide and ammonia gas. e. The standard enthalpy of formation of solid magnesium nitride is −461.08 kJ/mol. Calculate the standard enthalpy change for the reaction between magnesium metal and ammon

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Identify the chemical reactions involved: (1) Magnesium reacts with nitrogen to form magnesium nitride, and (2) magnesium nitride reacts with water to form magnesium oxide and ammonia.

Write the balanced chemical equations for each reaction: (1) \( 3\text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2 \) and (2) \( \text{Mg}_3\text{N}_2 + 6\text{H}_2\text{O} \rightarrow 3\text{MgO} + 2\text{NH}_3 \).

Use the standard enthalpy of formation for magnesium nitride to find the enthalpy change for the first reaction. The enthalpy change for the formation of \( \text{Mg}_3\text{N}_2 \) is given as \(-461.08\, \text{kJ/mol}\).

Calculate the enthalpy change for the second reaction using the standard enthalpies of formation for \( \text{MgO} \) and \( \text{NH}_3 \). Use the formula: \( \Delta H = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants}) \).

Combine the enthalpy changes from both reactions to find the overall standard enthalpy change for the reaction between magnesium metal and ammonia.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Standard Enthalpy of Formation

The standard enthalpy of formation (ΔHf°) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It is a crucial concept in thermodynamics, allowing chemists to calculate the energy changes associated with chemical reactions. The values are typically given in kJ/mol and can be used to determine the overall enthalpy change for a reaction by applying Hess's law.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction, regardless of the pathway taken. This principle allows for the calculation of enthalpy changes for reactions that are difficult to measure directly by using known enthalpy values of related reactions. It is particularly useful in determining the enthalpy change for complex reactions involving multiple steps.

Reaction of Magnesium with Nitrogen

When magnesium reacts with nitrogen, it forms magnesium nitride (Mg3N2). This reaction is exothermic, releasing energy, and is significant in understanding the behavior of magnesium in different environments. The subsequent reaction of magnesium nitride with water produces magnesium oxide and ammonia, illustrating the importance of understanding both the formation and decomposition reactions of compounds in thermochemical calculations.

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Textbook Question

Potassium superoxide, KO₂, is often used in oxygen masks (such as those used by firefighters) because KO₂ reacts with CO₂ to release molecular oxygen. Experiments indicate that 2 mol of KO₂(s) react with each mole of CO₂(g). (c) What mass of KO₂(s) is needed to consume 18.0 g CO₂(g)? What mass of O₂(g) is produced during this reaction?

Textbook Question

When magnesium metal is burned in air (Figure 3.5), two products are produced. One is magnesium oxide, MgO. The other is the product of the reaction of Mg with molecular nitrogen, magnesium nitride. When water is added to magnesium nitride, it reacts to form magnesium oxide and ammonia gas. c. In an experiment, a piece of magnesium ribbon is burned in air in a crucible. The mass of the mixture of MgO and magnesium nitride after burning is 0.470 g. Water is added to the crucible, further reaction occurs, and the crucible is heated to dryness until the final product is 0.486 g of MgO. What was the mass percentage of magnesium nitride in the mixture obtained after the initial burning?

Textbook Question

When magnesium metal is burned in air (Figure 3.5), two products are produced. One is magnesium oxide, MgO. The other is the product of the reaction of Mg with molecular nitrogen, magnesium nitride. When water is added to magnesium nitride, it reacts to form magnesium oxide and ammonia gas. d. Magnesium nitride can also be formed by reaction of the metal with ammonia at high temperature. Write a balanced equation for this reaction. If a 6.3-g Mg ribbon reacts with 2.57 g NH3(𝑔) and the reaction goes to completion, which component is the limiting reactant? What mass of H2(𝑔) is formed in the reaction?