The salt ammonium nitrate can follow three modes of...

The salt ammonium nitrate can follow three modes of decomposition: (a) to $\mathrm{HNO}_{3}(g)$ and $\mathrm{NH}_{3}(g)$, (b) to $\mathrm{N}_{2} \mathrm{O}(g)$ and $\mathrm{H}_{2} \mathrm{O}(g)$, and (c) to $\mathrm{N}_{2}(g), \mathrm{O}_{2}(g)$, and $\mathrm{H}_{2} \mathrm{O}(g)$. Calculate $\Delta G_{\mathrm{rxn}}^{\circ}$ for each mode of decomposition at $298 \mathrm{~K}$. Explain in light of these results how it is still possible to use ammonium nitrate as a fertilizer and the precautions that should be taken when it is used.

The salt ammonium nitrate can follow three modes of decomposition: (a) to $\mathrm{HNO}_{3}(g)$ and $\mathrm{NH}_{3}(g)$, (b) to $\mathrm{N}_{2} \mathrm{O}(g)$ and $\mathrm{H}_{2} \mathrm{O}(g)$,
and (c) to $\mathrm{N}_{2}(g), \mathrm{O}_{2}(g)$, and $\mathrm{H}_{2} \mathrm{O}(g)$. Calculate $\Delta G_{\mathrm{rxn}}^{\circ}$ for each
mode of decomposition at $298 \mathrm{~K}$. Explain in light of these results how it is still possible to use ammonium nitrate as a fertilizer and the precautions that should be taken when it is used.

Key Concepts

Gibbs Free Energy

Gibbs free energy (?G) is a thermodynamic quantity that combines enthalpy and entropy, determining whether a chemical reaction is spontaneous under constant temperature and pressure. A negative ?G indicates a reaction that can proceed spontaneously, whereas a positive value implies non-spontaneity. In the context of decomposition reactions, calculating ?G for each pathway helps evaluate which are thermodynamically favorable and under what conditions they may occur.

Standard State Thermodynamics

Standard state thermodynamics involves evaluating reactions under defined conditions, typically 298 K and 1 atm pressure. Using standard Gibbs free energies of formation for the reactants and products allows one to compute the ?G for each reaction pathway. This approach is essential in comparing different modes of decomposition and understanding the stability of a chemical under normal conditions.

Decomposition Reaction Mechanisms

Decomposition reactions describe the breakdown of a compound into simpler substances, and many compounds can decompose by multiple pathways. Each mode of decomposition has a distinct set of products and associated energy changes. Analyzing these mechanisms helps in predicting which pathway will dominate under a given set of conditions, which is crucial for compounds that have both industrial and agricultural applications.

Thermodynamic Stability

Thermodynamic stability refers to how energetically favorable a chemical compound is under standard conditions. A compound that has multiple decomposition pathways with varying ?G values may be kinetically stable under normal circumstances yet could become unstable under specific conditions like high temperature or pressure. Evaluating the Gibbs free energy changes for different pathways provides insights into the operational safety and handling requirements of the compound.

Chemical Safety and Hazard Precautions

Understanding the energetics of various decomposition pathways is critical for implementing safety measures. For substances like ammonium nitrate, which are used in fertilizers, a decomposition mode with a very negative ?G under certain conditions may present an explosion hazard. Therefore, even when a compound is useful agriculturally, strict controls on storage conditions, handling procedures, and environmental factors are necessary to prevent unintended and dangerous decompositions.

Transcript

00:01 In this question we've been asked to determine the standard gives energy change of a reaction and it is important to know that this is a state function so we can calculate this using the standard gives energy change of formation of the species that are involved in each reaction.

00:19 So if we are to look at this, this is going to be the sum of the standard gives energy of formation of the products minus the sum of the standard gives energy of formation of the reactants and it is also important to know that this has to be multiplied by the respective stoichiometric coefficients of the species because these values that is the standard gives energy of formation is given in per more when we are retrieving these values from our booklets or our of indices they are reported under standard conditions and per more of substance so we have to multiply by the moles and these are given as the stoichiometric coefficients in the reaction so if we are to look at the first scenario where we have ammonium nitrate decomposing to form nitric acid h4 n -o -3 to form hno3 plus nh3.

01:29 These are our products, these are our reactants.

01:33 So we say products minus reactants.

01:37 So it gives energy, the standard gives energy associated with this reaction.

01:43 This is going to be equal to negative 73 .5.

01:52 Plus negative 16 .4.

01:56 These are the products minus the reactants, negative 183 .9.

02:02 So delta g of this reaction, this is going to be 9 4 kilojoules.

02:11 Then moving on to the next reaction where ammonium nitrate nh4, n -o3 is decomposing to form n2o plus 2h2o.

02:23 H2o so the standard gives energy of this reaction this is going to be equal to 103 .7 plus two moles of h2o negative 228 .6 all of this minus minus 183 .9 so the standard gives energy change of this reaction is going to be negative 1 6 .9 .6 and this is in kilojoules.

03:04 Moving on to the other reaction where we have nh4, n -o -3, decomposing to form n2 plus half -o2 plus h2o...

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