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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

Ice at \(0^{\circ} \mathrm{C}\) is placed in a Styrofoam cup containing \(361 \mathrm{~g}\) of a soft drink at \(23^{\circ} \mathrm{C}\). The specific heat of the drink is about the same as that of water. Some ice remains after the ice and soft drink reach an equilibrium temperature of \(0^{\circ} \mathrm{C}\). Determine the mass of ice that has melted. Ignore the heat capacity of the cup. (Hint: It takes 334 J to melt \(1 \mathrm{~g}\) of ice at \(\left.0^{\circ} \mathrm{C} .\right)\)

A 1.00 -mole sample of ammonia at 14.0 atm and \(25^{\circ} \mathrm{C}\) in a cylinder fitted with a movable piston \(\mathrm{ex}\) pands against a constant external pressure of 1.00 atm. At equilibrium, the pressure and volume of the gas are 1.00 atm and 23.5 L, respectively. (a) Calculate the final temperature of the sample. (b) Calculate \(q, w,\) and \(\Delta E\) for the process. The specific heat of ammonia is \(0.0258 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\)

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 \(21^{\circ} \mathrm{C} ?\)

If energy is conserved, how can there be an energy cricis?

The standard enthalpy change \(\Delta H^{\circ}\) for the thermal decomposition of silver nitrate according to the following equation is \(+78.67 \mathrm{~kJ}\) : $$ \mathrm{AgNO}_{3}(s) \longrightarrow \mathrm{AgNO}_{2}(s)+\frac{1}{2} \mathrm{O}_{2}(g) $$ The standard enthalpy of formation of \(\mathrm{AgNO}_{3}(s)\) is \(-123.02 \mathrm{~kJ} / \mathrm{mol}\). Calculate the standard enthalpy of formation of \(\mathrm{AgNO}_{2}(s)\).
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