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? \(\left(\Delta H_{\mathrm{f}}^{\circ}\right.\) for \(\mathrm{NaNO}_{3}(a q)=-467 \mathrm{~kJ} / \mathrm{mol}\) ) 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 enthalpy change of the reaction is -1614 kJ/mol. The mass of sodium carbonate required for complete neutralization is 6.31 x 10^7 g, and the amount of heat evolved is -1.92 x 10^9 kJ. The major air pollution concern was the significant release of heat, which could have led to thermal air pollution and negative effects on local ecosystems and human health.

Step-by-step solution

Find values for enthalpy changes of formation

From the problem, we know the enthalpy change of formation for NaNO3(aq) is \(-467\,kJ/mol\). We don't have the other values provided, so we'll need to look up the enthalpy change of formation for the remaining compounds in a table. From a standard table, we find: \(\Delta H_f^\circ(HNO_3(aq))=-207\,kJ/mol\) \(\Delta H_f^\circ(H_2O(l))=-286\,kJ/mol\) \(\Delta H_f^\circ(CO_2(g))=-394\,kJ/mol\) We also know that there's no enthalpy change for Na2CO3(s) since it is a reactant in its standard state.

Calculate enthalpy change for the reaction

We can now calculate the enthalpy change for the reaction using the enthalpy changes of formation and the balanced stoichiometric coefficients: \[\Delta H^\circ = \sum \Delta H_f^\circ (products) - \sum \Delta H_f^\circ (reactants)\] \[\Delta H^\circ = [2 \times (-467) + (-286) + (-394)] - [2 \times (-207)]\] \[\Delta H^\circ = -1614\,kJ/mol\] Step 2: Calculate the mass of sodium carbonate needed

Calculate the mass of nitric acid spilled

Given the volume of the nitric acid spill (2.0 x 10^4\,gal) and its density (1.42\,g/cm^3), we can first convert it into mass: (2.0 x 10^4\,gal) x (3.78541\,L/gal) x (1000\,mL/L) x (1\,cm^3/mL) x (1.42\,g/cm^3) = 1.07 x 10^8\,g

Calculate the mass of pure HNO3 in the spilled solution

We know that the spilled nitric acid solution contains 70% HNO3, by mass. So, we can now calculate the mass of pure HNO3 in the spilled solution: Mass of HNO3 in solution = (1.07 x 10^8\,g) x (0.70) = 7.5 x 10^7\,g

Convert mass of HNO3 to moles

To find the mass of Na2CO3 required, convert the mass of HNO3 to moles first: Moles of HNO3 = Mass HNO3 / Molar mass HNO3 Moles of HNO3 = (7.5 x 10^7\,g) / (63.01\,g/mol) = 1.19 x 10^6\,mol

Calculate moles of Na2CO3 needed

Now, using stoichiometry and the balanced equation, we can determine the moles of Na2CO3 needed: Moles Na2CO3 = (1 mol Na2CO3 / 2 mol HNO3) x Moles of HNO3 Moles Na2CO3 = (1/2) x 1.19 x 10^6\,mol = 5.95 x 10^5\,mol

Calculate mass of sodium carbonate required

To calculate the mass of sodium carbonate required, we can now convert moles of Na2CO3 to mass using its molar mass: Mass of Na2CO3 = Moles Na2CO3 x Molar mass Na2CO3 Mass of Na2CO3 = (5.95 x 10^5\,mol) x (105.99\,g/mol) = 6.31 x 10^7\,g Step 3: Calculate the heat evolved

Calculate heat evolved

Since we know the enthalpy change for the reaction and the moles of nitric acid reacted, we can calculate the heat evolved: Q = \(\Delta H^\circ \) x Moles of HNO3 Q= (-1614\,kJ/mol) x 1.19 x 10^6\,mol = -1.92 x 10^9\,kJ a. Answer: The enthalpy change of the reaction is -1614\,kJ/mol. The mass of sodium carbonate required for complete neutralization is 6.31 x 10^7\,g, and the amount of heat evolved is -1.92 x 10^9\,kJ. b.

Determine major air pollution concern

The reaction releases a significant amount of heat, as indicated by the large negative value of \(\Delta H^\circ \) (-1614\,kJ/mol). This heat release can cause the temperature of the local environment to increase significantly, leading to thermal air pollution and potentially causing harm to local ecosystems and human health. b. Answer: The major air pollution concern was the significant release of heat, which could have led to thermal air pollution and negative effects on local ecosystems and human health.

Key concepts

Neutralization Reaction

In the context of chemistry, a neutralization reaction involves an acid and a base reacting to form water and salt. This is exactly what occurred during the nitric acid spill in Denver, where nitric acid (HNO_3aq) was neutralized with sodium carbonate (Na_{2}CO_{3}s). The chemical equation for this reaction is:\[2 HNO_{3}(aq) + Na_{2}CO_{3}(s) ightarrow 2 NaNO_{3}(aq) + H_2O(l) + CO_2(g)\]This reaction is important as it reduces the hazard associated with acid spills by converting the strong acid into less harmful products. The formation of water and carbon dioxide gas as by-products also indicates a complete and efficient neutralization process.
Utilizing stoichiometry is crucial to predict the quantities of reactants needed for complete neutralization. The balanced equation helps determine that two moles of nitric acid react with one mole of sodium carbonate. This reaction not only neutralizes the acid but also releases carbon dioxide, which, although non-toxic at standard concentrations, can pose risks in confined spaces.

Nitric Acid Spill

The incident on April 3, 1983, where nitric acid spilled in Denver, is an example of how industrial activities can result in environmental hazards. Nitric acid is a strong acid, capable of significant corrosion and acidification if not managed properly. When such a spill occurs, immediate containment measures are crucial to minimize environmental damage and personal injury.
The spilled amount, approximately \(2.0 \times 10^4\) gallons, would have posed a significant risk due to its potential to lower local pH levels drastically. Therefore, the importance of promptly neutralizing the spill with sodium carbonate lies in its effectiveness to mitigate the corrosive effects of nitric acid. Converting the acid into neutral substances like sodium nitrate (NaNO_{3}) helps in controlling the spread and impact of the spill.

• Containment efforts prevent the acid from entering water systems.
• Neutralization renders the spilled acid into safer forms.
• Public safety is maintained by reducing exposure to hazardous chemicals.

Stoichiometry

Stoichiometry is the branch of chemistry that deals with the calculation of reactants and products in chemical reactions. In the case of the nitric acid spill, stoichiometry was used to ascertain the amount of sodium carbonate (Na_{2}CO_{3}) required to neutralize the spill completely.From the balanced chemical equation,\[2 \mathrm{HNO}_{3} + \mathrm{Na}_{2} \mathrm{CO}_{3} \rightarrow \mathrm{NaNO}_{3} + \mathrm{H}_{2} \mathrm{O} + \mathrm{CO}_{2}\]it is clear that two moles of nitric acid react with one mole of sodium carbonate. By knowing the mass and concentration of the nitric acid spill (70% concentrated by mass and a density of 1.42 g/cm³), one can compute the exact amount of each substance involved.
The process involves steps like converting the volume of spilled acid to mass using density, then finding the moles of nitric acid from this mass, and applying stoichiometric ratios to determine the moles and mass of sodium carbonate required. Thanks to stoichiometry:

• It allows calculating precise quantities needed for complete neutralization.
• Facilitates using chemical equations to prevent wastage of neutralizing agents.
• Helps predict the outcome and side products of the neutralization reaction.

Heat Evolution

During the neutralization reaction, heat is commonly evolved, particularly in exothermic reactions like the one involving nitric acid and sodium carbonate. Calculating the heat evolved is vital for assessing the potential risks related to temperature changes in the environment where the reaction occurs.
For the nitric acid neutralization, the standard enthalpy change (\Delta H^\circ) is provided as \(-1614 \mathrm{~kJ/mol}\). This value indicates that a considerable amount of heat is released when the acid is neutralized. Heat evolution in this case presents several concerns:

• An increase in temperature can lead to thermal pollution.
• Potential release of gases may raise local carbon dioxide concentrations.
• There is the risk of heat-related injury to personnel managing the spill if not appropriately contained.

Proper evaluation of heat produced in such reactions assists in devising safety measures to prevent adverse environmental and health impacts. This includes controlling reaction rates and ensuring adequate ventilation to disperse any gases produced.