Explain the meaning of this thermochemical equation: $$\begin{array}{r}4 \mathrm{NH}_{3}(g)+5 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{NO}(g)+6 \mathrm{H}_{2} \mathrm{O}(g) \\\\\Delta H=-904 \mathrm{~kJ} / \mathrm{mol}\end{array}$$

Enthalpy change, denoted as \( \Delta H \), is a measure of the total energy change in a chemical reaction. In the context of a thermochemical equation, such as \( 4 \mathrm{NH}_{3}(g) + 5 \mathrm{O}_{2}(g) \rightarrow 4 \mathrm{NO}(g) + 6 \mathrm{H}_{2} \mathrm{O}(g) \) with \( \Delta H = -904 \mathrm{~kJ} / \mathrm{mol} \), the enthalpy change indicates the amount of heat released or absorbed during the reaction.

Positive \( \Delta H \) values mean that the reaction absorbs heat from the surroundings and is therefore endothermic, while negative \( \Delta H \) values indicate that heat is released, making the reaction exothermic. In our specific exercise, the negative \( -904 \mathrm{~kJ} / \mathrm{mol} \) signifies that energy is exuded into the surroundings as the reaction proceeds, which in turn implies that the reactants have more stored chemical energy than the products.

Exothermic reactions are chemical processes that release energy, often in the form of heat or light, into their surroundings. They are identified by a negative \( \Delta H \) in the thermochemical equation. The equation from our exercise is a classic example of an exothermic reaction, with the release of \( -904 \mathrm{~kJ} \) for every mole of \( \mathrm{NH}_{3}(g) \) reacted.

In everyday life, exothermic reactions are prevalent and can range from simple processes like the rusting of iron to complex biological reactions within our bodies. They are also essential in industrial applications where harnessing the energy released can be utilized for heating or driving other chemical processes.

Chemical stoichiometry is the quantitative relationship between the amounts of reactants and products in a chemical reaction. It is based on the balanced chemical equation and the conservation of mass principle. Stoichiometry involves using molar ratios from the balanced equation to calculate the amount of reactants required or products formed.

In our example equation, stoichiometry indicates that 4 moles of \( \mathrm{NH}_{3}(g) \) react with 5 moles of \( \mathrm{O}_{2}(g) \) to produce 4 moles of \( \mathrm{NO}(g) \) and 6 moles of \( \mathrm{H}_{2} \mathrm{O}(g) \). Understanding stoichiometry is critical for predicting yields in chemical reactions and for scaling reactions up or down in both laboratory and industrial settings.

State symbols in chemical reactions provide essential information about the physical state of the reactants and products—denoted as (s) for solids, (l) for liquids, (g) for gases, and (aq) for aqueous solutions. These symbols are important because the physical state of substances can influence the rate of the reaction and also affect thermodynamic properties such as enthalpy.

In our thermochemical equation, all reactants and products are gases, indicated by the (g) symbol. This is significant because the behavior of gases, especially in terms of volume and pressure changes, must be considered when analyzing or conducting a reaction under certain conditions. Moreover, the inclusion of state symbols helps in predicting the direction of energy transfer in cases where physical state changes are part of the reaction.