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Chapter 6: Problem 22

In writing thermochemical equations, why is it important to indicate the physical state (that is, gaseous, liquid, solid, or aqueous) of each substance?

Short Answer

Expert verified
It is significant to indicate the physical state of the substances in a thermochemical equation because the amount of energy that a substance absorbs or releases during a reaction can change significantly based on its physical state. This impacts the total heat change for the reaction and aids in accurately describing the energy changes that occur during the course of the reaction.

Step by step solution

01

Understanding thermochemical equations

Thermochemical equations are the chemical equations that include the enthalpy change of the reaction. They provide information about energy changes during a chemical reaction. They are written as: A + B -> C + D 螖H = X, where A and B are reactants, C and D are products, and 螖H is the enthalpy change. The equation should ideally represent the physical stages (gas, liquid, solid, or aqueous) of the reactants and products.
02

The role of the physical state

The physical state of a substance can impact how a substance reacts and thus, the enthalpy change of the reaction. The amount of energy that is absorbed or released during a chemical reaction can be drastically different depending on the states of reactant and product molecules. For example, the enthalpy of vaporization for water, which is the energy needed for water to transform from a liquid state into a gaseous state at a constant pressure, is considerably greater than the enthalpy of fusion, the energy needed to transform ice (solid water) into liquid water.
03

Conclusion

By indicating the physical state of each substance in a thermochemical equation, one can accurately describe the energy changes that occur during the course of reaction. This is because the energy required to change the state of a substance (like from solid to liquid, or liquid to gas) is included in the total heat change for the reaction.

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Most popular questions from this chapter

The \(\Delta H_{\mathrm{f}}^{\circ}\) values of the two allotropes of oxygen, \(\mathrm{O}_{2}\) and \(\mathrm{O}_{3}\), are 0 and \(142.2 \mathrm{~kJ} / \mathrm{mol}\), respectively, at \(25^{\circ} \mathrm{C}\). Which is the more stable form at this temperature?

Consider two metals A and B, each having a mass of \(100 \mathrm{~g}\) and an initial temperature of \(20^{\circ} \mathrm{C}\). The specific heat of \(A\) is larger than that of \(B\). Under the same heating conditions, which metal would take longer to reach a temperature of \(21^{\circ} \mathrm{C} ?\)

(a) A snowmaking machine contains a mixture of compressed air and water vapor at about 20 atm. When the mixture is sprayed into the atmosphere it expands so rapidly that, as a good approximation, no heat exchange occurs between the system (air and water) and its surroundings. (In thermodynamics, such a process is called an adiabatic process.) Do a first law of thermodynamics analysis to show how snow is formed under these conditions. (b) If you have ever pumped air into a bicycle tire, you probably noticed a warming effect at the valve stem. The action of the pump compresses the air inside the pump and the tire. The process is rapid enough to be treated as an adiabatic process. Apply the first law of thermodynamics to account for the warming effect. (c) A driver's manual states that the stopping distance quadruples as the speed doubles; that is, if it takes \(30 \mathrm{ft}\) to stop a car traveling at \(25 \mathrm{mph}\) then it would take \(120 \mathrm{ft}\) to stop a car moving at 50 mph. Justify this statement by using the first law of thermodynamics. Assume that when a car is stopped, its kinetic energy \(\left(\frac{1}{2} m u^{2}\right)\) is totally converted to heat.

From a thermochemical point of view, explain why a carbon dioxide fire extinguisher or water should not be used on a magnesium fire.

Suggest ways (with appropriate equations) that would enable you to measure the \(\Delta H_{\mathrm{f}}^{\circ}\) values of \(\mathrm{Ag}_{2} \mathrm{O}(s)\) and \(\mathrm{CaCl}_{2}(s)\) from their elements. No calculations are necessary.
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