Question

On Easter Sunday, April 3,1983, nitric acid spilled from a tank car near downtown Denver, Colorado. The spill was neutralized with sodium carbonate: \(2 \mathrm{HNO}_{3}(a q)+\mathrm{Na}_{2} \mathrm{CO}_{3}(s) \longrightarrow 2 \mathrm{NaNO}_{3}(a q)+\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{CO}_{2}(g)\) a. Calculate \(\Delta H^{\circ}\) for this reaction. Approximately \(2.0 \times\) \(10^{4}\) gal nitric acid was spilled. Assume that the acid was an aqueous solution containing \(70.0 \% \mathrm{HNO}_{3}\) by mass with a density of \(1.42 \mathrm{~g} / \mathrm{cm}^{3} .\) What mass of sodium carbonate was required for complete neutralization of the spill, and what quantity of heat was evolved? ( \(\Delta H_{\mathrm{f}}^{\circ}\) for \(\left.\mathrm{NaNO}_{3}(a q)=-467 \mathrm{~kJ} / \mathrm{mol}\right)\) b. According to The Denver Post for April 4,1983 , authorities feared that dangerous air pollution might occur during the neutralization. Considering the magnitude of \(\Delta H^{\circ}\), what was their major concern?

Short answer

The standard enthalpy change (∆H°) for the reaction is -327.4 kJ/mol. The mass of sodium carbonate required for complete neutralization is 6.30 x 10^7 g, and the heat evolved during the process is -1.94 x 10^8 kJ. The major concern regarding air pollution would be the release of a significant amount of heat, potentially causing the evaporation of water and creating high concentrations of airborne particulates and gases, which could be detrimental to public health.

Step-by-step solution

Find the enthalpy values for the reactants and products

To find ∆H° for the reaction, we need the standard enthalpy values for each substance involved. The problem only supplies ∆Hf° for NaNO3(aq), but for the other species, we know that the enthalpy of formation for elements in their standard state is zero. Thus, we have: ∆Hf° for \(HNO_3(aq)\) = -174.1 kJ/mol (from a data source) ∆Hf° for \(Na_2CO_3(s)\) = -1131.2 kJ/mol (from a data source) ∆Hf° for \(NaNO_3(aq)\) = -467 kJ/mol (given) ∆Hf° for \(H_2O(l)\) = -285.8 kJ/mol (from a data source) ∆Hf° for \(CO_2(g)\) = -393.5 kJ/mol (from a data source) #Step 2: Calculate the standard enthalpy change#

Calculate ΔH° for the reaction using Hess's law

Using Hess's law, we can calculate the reaction enthalpy change as: ∆H° = [Σ (production moles × production enthalpy)] - [Σ (reactant moles × reactant enthalpy)] ∆H° = [(2)(-467) + (1)(-285.8) + (1)(-393.5)] - [(2)(-174.1)+(1)(-1131.2)] ∆H° = -327.4 kJ/mol #Step 3: Calculate the required mass of sodium carbonate#

Calculate the moles of nitric acid

Given that there was 2.0 x 10^4 gallons of 70% nitric acid solution spilled. Convert the spill volume and mass into moles of nitric acid: Volume of spill = 2.0 x 10^4 gallons × 3.785 liters/gallon × 1000 cm³/liter = 7.57 x 10^7 cm³ Mass of spilled nitric acid = 1.42 g/cm³ × 7.57 x 10^7 cm³ × 70% = 7.51 x 10^7 g of HNO₃ Now, we will find the moles of nitric acid: Moles of nitric acid = mass / molar mass = (7.51 x 10^7 g) / (63.01 g/mol) = 1.19 x 10^6 mol #Step 4: Calculate the required mass of sodium carbonate#

Calculate the moles of sodium carbonate required

Using the stoichiometry of the balanced reaction (1 mol Na₂CO₃ neutralizes 2 mol HNO₃), we can find the needed moles of sodium carbonate: Moles of sodium carbonate = (1/2) × 1.19 x 10^6 mol = 5.94 x 10^5 mol Now, we find the mass of sodium carbonate required: Mass of sodium carbonate = 5.94 x 10^5 mol × 105.99 g/mol = 6.30 x 10^7 g #Step 5: Calculate the heat evolved during neutralization#

Calculate the total heat evolved

Now that we have the moles of sodium carbonate, we can find the total heat evolved during the neutralization: Total heat evolved = moles of sodium carbonate × ∆H° Total heat evolved = 5.94 x 10^5 mol × -327.4 kJ/mol = -1.94 x 10^8 kJ #Step 6: Explain the concern about air pollution#

Major concern about air pollution

Since the ∆H° is a large negative value, it indicates that a significant amount of heat would be released during the neutralization. This large heat release could cause the evaporation of water from the aqueous products, creating a high local concentration of airborne particulates such as `NaNO3(aq)`. Additionally, the `CO2`(g) produced might also contribute to air pollution. The major concern would likely be the potential release of high-concentration particulates and gases in the nearby atmosphere, which could be detrimental to public health.

Key concepts

Nitric Acid Neutralization

Neutralization is a chemical reaction between an acid and a base, resulting in the formation of a salt and water. In the case of the nitric acid spill in Denver, sodium carbonate (a base) was used to neutralize the spilled nitric acid (an acid). This reaction can be described by the balanced equation:

• \(2 \mathrm{HNO}_{3}(a q)+\mathrm{Na}_{2} \mathrm{CO}_{3}(s) \rightarrow 2 \mathrm{NaNO}_{3}(a q)+\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{CO}_{2}(g)\).

This equation tells us that two moles of nitric acid react with one mole of sodium carbonate to produce two moles of sodium nitrate, one mole of water, and one mole of carbon dioxide gas. The products of this reaction are harmless, which is why sodium carbonate is often used for neutralizing acid spills. However, the reaction's effectiveness also heavily depends on the accurate stoichiometric calculation, ensuring the complete and safe conversion of the acid into non-hazardous products.
When considering such reactions, it is fundamental to understand the conditions under which they occur and the potential hazards they might pose, such as heat release or production of gases, which are discussed further.

Stoichiometry

Stoichiometry is the part of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. It allows us to calculate the amounts of substances consumed and produced. In the context of the nitric acid neutralization spill, stoichiometry was essential in determining the amount of sodium carbonate needed to neutralize the acidic spill completely.
First, we need to convert physical quantities, like volume and concentration, into moles since chemical reactions happen on a molecular level. For the spill, one has to calculate the total moles of nitric acid present in the 2.0 x 10^4 gallons of the solution.
The calculation steps are:

• Convert volume of the spill into mass using the density and percentage concentration of the solution.
• Calculate moles of nitric acid using its molar mass.
• Using the 1:2 stoichiometry from the chemical equation, calculate the necessary moles of sodium carbonate.

These calculations ensure that all the acid is neutralized, preventing any leftover acid from causing harm. Using stoichiometry, safety measures are numerically grounded, providing the means to deal with chemical spills effectively.

Heat of Reaction

The heat of reaction, denoted as \( \Delta H^{\circ} \), is a measure of the heat energy change during a chemical reaction. For the neutralization reaction between nitric acid and sodium carbonate, the heat of reaction is calculated using the standard enthalpies of formation for the reactants and products. These values provide insight into whether the reaction releases (exothermic) or absorbs (endothermic) heat.
With the given reaction, calculation shows:

• The reaction has an \( \Delta H^{\circ} \) of -327.4 kJ/mol, indicating it is exothermic.
• This means heat is released into the environment as the products are formed.

The significant amount of heat released during this reaction poses potential risks, as noticed in the Denver spill. Exothermic reactions can increase the temperature of the surroundings drastically, leading to further chemical hazards if not handled cautiously. It is essential to manage the heat produced efficiently to minimize any secondary reactions that might lead to air pollution or other environmental concerns.

Air Pollution Concerns

Air pollution concerns were significant during the neutralization of the nitric acid spill in Denver. As the reaction is highly exothermic, releasing -1.94 x 10^8 kJ of heat, it can result in localized hot spots. Such heat can lead to rapid evaporation of liquids, potentially causing secondary air pollution issues such as:

• Forming airborne particulates from the evaporated sodium nitrate solution.
• Releasing carbon dioxide gas, a by-product of the reaction.

The rapid release of heat may significantly increase the concentration of these substances in the air, posing an immediate health risk to nearby populations and ecosystems. Using adequate containment and ventilation practices, alongside neutralizing agents, can mitigate these concerns. Additionally, assessing the air quality before and after such incidents ensures a safe environment for both emergency responders and local inhabitants.