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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

Decomposition reactions are usually endothermic, whereas combination reactions are usually exothermic. Give a qualitative explanation for these trends.

Calculate the standard enthalpy of formation for diamond, given that $$\begin{array}{l} \text { C(graphite) }+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) \\ \qquad \begin{aligned} \Delta H^{\circ} &=-393.5 \mathrm{~kJ} / \mathrm{mol} \\ \mathrm{C}(\text { diamond })+\mathrm{O}_{2}(g) \longrightarrow & \mathrm{CO}_{2}(g) \\ \Delta H^{\circ} &=-395.4 \mathrm{~kJ} / \mathrm{mol} \end{aligned} \end{array}$$

Determine the standard enthalpy of formation of ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) from its standard enthalpy of combustion \((-1367.4 \mathrm{~kJ} / \mathrm{mol})\)

A 3.53-g sample of ammonium nitrate \(\left(\mathrm{NH}_{4} \mathrm{NO}_{3}\right)\) was added to \(80.0 \mathrm{~mL}\) of water in a constantpressure calorimeter of negligible heat capacity. As a result, the temperature of the water decreased from \(21.6^{\circ} \mathrm{C}\) to \(18.1^{\circ} \mathrm{C}\). Calculate the heat of solution \(\left(\Delta H_{\mathrm{soln}}\right)\) of ammonium nitrate.

A 4.117 -g impure sample of glucose \(\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right)\) was burned in a constant-volume calorimeter having a heat capacity of \(19.65 \mathrm{~kJ} /{ }^{\circ} \mathrm{C}\). If the rise in temperature is \(3.134^{\circ} \mathrm{C},\) calculate the percent by mass of the glucose in the sample. Assume that the impurities are unaffected by the combustion process. See Appendix 2 for thermodynamic data.
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