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Chapter 9: Problem 81

Why must the stoichiometry of a reaction be known in order to estimate the enthalpy change from bond energies?

Short Answer

Expert verified
Answer: Stoichiometry is important for estimating the enthalpy change from bond energies because it determines the correct quantitative relationship between reactants and products in a chemical reaction. It ensures the accurate calculation of the total bond energies of both reactants and products, which plays a significant role in finding the enthalpy change. Without accurate stoichiometry, we might not consider the correct number of moles of each reactant and product, leading to incorrect estimates of enthalpy change.

Step by step solution

01

Define enthalpy change and bond energy

Enthalpy change (鈭咹) is the heat energy absorbed or released during a chemical reaction. Bond energy is the amount of energy required to break a bond between two atoms, or the energy released when a new bond is formed.
02

Importance of stoichiometry in a chemical reaction

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It's used to balance chemical equations, ensuring the law of conservation of mass is obeyed. In stoichiometry, we consider the moles of different reactants and products involved in the reaction.
03

Calculating enthalpy change from bond energies

To calculate the enthalpy change of a reaction from bond energies, we need to: 1. Identify the bonds involved in the reactants and products. 2. Determine the bond energies of each bond. 3. Calculate the total bond energies of reactants (using stoichiometry). 4. Calculate the total bond energies of products (using stoichiometry). 5. Find the difference between the total bond energies of reactants and products to determine enthalpy change. \[\Delta H = \text{Total bond energies of reactants} - \text{Total bond energies of products}\]
04

Role of stoichiometry in estimating enthalpy change

Knowing the stoichiometry of a reaction is crucial in estimating the enthalpy change from bond energies because it affects the calculations of the total bond energies of reactants and products. Without accurate stoichiometry, we might not consider the correct number of moles of each reactant and product, leading to incorrect estimates of enthalpy change.
05

Conclusion

The stoichiometry of a reaction must be known in order to estimate the enthalpy change from bond energies because it allows us to determine the correct quantitative relationship between reactants and products in a chemical reaction. By understanding the stoichiometry of a reaction and balancing the chemical equation accurately, we can calculate the enthalpy change using bond energies correctly.

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Most popular questions from this chapter

Use the information in thermochemical equations (1) through ( 3 ) to calculate the value of \(\Delta H_{\mathrm{rxn}}^{\circ}\) for the reaction in equation (4). (1) \(\mathrm{Pb}(s)+\frac{1}{2} \mathrm{O}_{2}(g) \rightarrow \mathrm{PbO}(s) \quad \quad \Delta H_{\mathrm{rxn}}^{\circ}=-219 \mathrm{kJ}\) (2) \(\mathrm{C}(s)+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g) \Delta H_{\text {rxn }}^{\circ}=-394 \mathrm{kJ}\) (3) \(\mathrm{PbCO}_{3}(s) \rightarrow \mathrm{PbO}(s)+\mathrm{CO}_{2}(g) \quad \Delta H_{\text {rxn }}^{\circ}=86 \mathrm{kJ}\) (4) \(2 \mathrm{Pb}(s)+2 \mathrm{C}(s)+3 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{PbCO}_{3}(s) \quad \Delta H_{\mathrm{rxn}}^{\circ}=?\)

Use the following data to sketch a heating curve for one mole of methanol. Start the curve at \(-100^{\circ} \mathrm{C}\) and end it at \(100^{\circ} \mathrm{C}\). $$\begin{array}{ll}\hline \text { Boiling point } & 65^{\circ} \mathrm{C} \\\\\hline \text { Melting point } & -94^{\circ} \mathrm{C} \\\\\hline \text { Heat of vaporization } & 35.3 \mathrm{kJ} / \mathrm{mol} \\\\\hline \text { Heat of fusion }\left(\Delta \mathrm{H}_{\text {fus }}\right) & 3.18 \mathrm{kJ} / \mathrm{mol} \\\\\hline \text { Molar heat capacity }(\ell) & 81.1 \mathrm{J} /\left(\mathrm{mol} \cdot^{\circ} \mathrm{C}\right) \\\\\hline \text { Molar heat capacity }(g) & 43.9 \mathrm{J} /\left(\mathrm{mol} \cdot^{\circ} \mathrm{C}\right) \\\\\hline \text { Molar heat capacity }(\mathrm{s}) & 48.7 \mathrm{J} /\left(\mathrm{mol} \cdot^{\circ} \mathrm{C}\right) \\\\\hline\end{array}$$

What happens to the internal energy of a gas when it expands (with no heat flow)?

At high temperatures, such as those in the combustion chambers of automobile engines, nitrogen and oxygen form nitrogen monoxide: $$\mathrm{N}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}(g) \quad \Delta H_{\mathrm{comb}}^{\circ}=+180 \mathrm{kJ}$$ Any NO released into the environment may be oxidized to \(\mathrm{NO}_{2}:\) $$2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g) \quad \Delta H_{\mathrm{comb}}^{\circ}=-112 \mathrm{kJ}$$ Is the overall reaction, $$\mathrm{N}_{2}(g)+2 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g)$$ exothermic or endothermic? What is \(\Delta H_{\text {comb }}^{\circ}\) for this reaction?

During a strenuous workout, an athlete generates \(233 \mathrm{kJ}\) of thermal energy. What mass of water would have to evaporate from the athlete's skin to dissipate this energy?
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