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Chapter 6: Problem 40

How are the standard enthalpies of an element and a compound determined?

Short Answer

Expert verified
The standard enthalpy of an element in its standard state is always zero by definition and not determined experimentally. The standard enthalpy of a compound is determined experimentally by measuring the heat change when one mole of the compound is formed from its constituent elements in their standard states.

Step by step solution

01

Understanding Enthalpy

Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.
02

Understanding Standard Conditions

Standard conditions for measuring the enthalpy of an element or a compound refers to the state of a material under specific preset conditions at standard pressure (1 atm) and standard temperature (25 degrees Celsius or 298K). The standard enthalpy is also known as 'enthalpy of formation.', and all elements in their standard states (e.g., O2 gas, H2 gas, etc.) have a standard enthalpy equal to zero.
03

Measuring Enthalpy Changes

Enthalpy changes for elements and compounds are measured using calorimetry. In an experiment, the heat energy transferred in a chemical reaction is used to raise the temperature of water in a calorimeter. By calculating the amount of heat absorbed or released, the change in enthalpy can be determined.
04

Standard Enthalpy of an Element

As all elements in their standard states (e.g. O2 gas, H2 gas, etc.) have a standard enthalpy equal to zero, we don't determine it, it is set by definition.
05

Standard Enthalpy of a Compound

The standard enthalpy of a compound refers to the energy change when 1 mole of a compound is formed directly from its elements in their standard states. For example, the standard enthalpy of formation of Carbon Monoxide (CO) is the enthalpy change when one mole of CO is formed from C in its standard state (solid graphite) and O2 in its standard state (a gas).

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Most popular questions from this chapter

What is the difference between specific heat and heat capacity? What are the units for these two quantities? Which is the intensive property and which is the extensive property?

Which of the following standard enthalpy of formation values is not zero at \(25^{\circ} \mathrm{C} ? \mathrm{Na}(s), \operatorname{Ne}(g)\) \(\mathrm{CH}_{4}(g), \mathrm{S}_{8}(s), \mathrm{Hg}(l), \mathrm{H}(g)\)

A 46-kg person drinks 500 g of milk, which has a "caloric" value of approximately \(3.0 \mathrm{~kJ} / \mathrm{g}\). If only 17 percent of the energy in milk is converted to mechanical work, how high (in meters) can the person climb based on this energy intake? [Hint: The work done in ascending is given by \(m g h,\) where \(m\) is the mass (in kilograms), \(g\) the gravitational acceleration \(\left(9.8 \mathrm{~m} / \mathrm{s}^{2}\right),\) and \(h\) the height (in meters).]

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

The first step in the industrial recovery of zinc from the zinc sulfide ore is roasting, that is, the conversion of \(\mathrm{ZnS}\) to \(\mathrm{ZnO}\) by heating: $$\begin{array}{r}2 \mathrm{ZnS}(s)+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{ZnO}(s)+2 \mathrm{SO}_{2}(g) \\ \Delta H=-879 \mathrm{~kJ} / \mathrm{mol}\end{array}$$ Calculate the heat evolved (in kJ) per gram of ZnS roasted.
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