The standard enthalpy of formation of liquid water is -285.8 kJ / mol a. What is the significanc

step 1 The reason the negative sign is present is because energy is being released. Hence the process i

The standard enthalpy of formation of liquid water is -285.8...

The standard enthalpy of formation of liquid water is $-285.8 \mathrm{kJ} / \mathrm{mol}$ a. What is the significance of the negative sign associated with this value? b. Why is the magnitude of this value so much larger than the enthalpy of vaporization of water $\left(\Delta H_{\mathrm{vap}}^{\circ}=40.67 \mathrm{kJ} /\right.$ mol $) ?$ c. Calculate the amount of heat produced in making $50.0 \mathrm{mL}$ of water from its elements under standard conditions.

The standard enthalpy of formation of liquid water is $-285.8 \mathrm{kJ} / \mathrm{mol}$
a. What is the significance of the negative sign associated with this value?
b. Why is the magnitude of this value so much larger than the enthalpy of vaporization of water $\left(\Delta H_{\mathrm{vap}}^{\circ}=40.67 \mathrm{kJ} /\right.$ mol $) ?$
c. Calculate the amount of heat produced in making $50.0 \mathrm{mL}$ of water from its elements under standard conditions.

Key Concepts

Standard Enthalpy of Formation

This is the energy change when one mole of a compound is formed from its elements in their standard states. It establishes a reference point in thermochemistry, allowing the energy changes of reactions to be compared and understood systematically. It also quantifies the stability of a compound compared to its constituent elements.

Exothermic Reaction

An exothermic reaction is one that releases energy, usually in the form of heat, to the surroundings. The negative sign in the enthalpy value indicates that energy is released during the formation process, signifying that the product (in this case, liquid water) is more stable than the separate elements.

Phase Change Enthalpy

Phase change enthalpy, such as the enthalpy of vaporization, is the energy required to change a substance from one phase to another without changing its temperature. In the case of water, the relatively smaller enthalpy of vaporization compared to the enthalpy of formation indicates that while vaporization requires energy to overcome intermolecular forces, the formation of water involves forming strong chemical bonds, leading to a much larger energy release.

Stoichiometric Calculations in Thermochemistry

This involves converting the physical quantity of a substance (such as volume or mass) into moles, using its density and molar mass, and then applying the molar enthalpy change to calculate the total energy change. This concept is essential for relating laboratory-scale measurements to the energy changes predicted by thermodynamic data.

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