Chapter 5: Problem 60
How are the standard enthalpies of an element and of a compound determined?
Short Answer
Expert verified
Standard enthalpies are determined through calorimetry and Hess's Law, using known enthalpies of elements and compounds.
Step by step solution
01
Define Standard Enthalpy
The standard enthalpy, often denoted as \( \Delta H^\circ \), is the change in enthalpy when one mole of a substance is formed from its elements in their standard states under standard conditions (1 atm pressure and 298 K temperature). Standard enthalpies are typically reported in kJ/mol.
02
Standard Enthalpy of Elements
For an element in its most stable form at standard conditions, the standard enthalpy of formation is defined as zero. This is because no change is involved when an element converts into itself.
03
Measure Heat Change of Compounds
The standard enthalpy of a compound is determined through calorimetry experiments where the heat exchanged during a reaction is measured. This typically involves a combustion reaction in a calorimeter, where the heat released is used to calculate enthalpy.
04
Use Hess's Law
Hess's Law states that regardless of the pathway, the total enthalpy change for a given chemical reaction is the same. By using known enthalpies of formation from tables and combining chemical reactions, unknown enthalpies can be determined. This is critical for finding standard enthalpies when direct measurement is challenging.
05
Calculate Using Known Values
Once the heat values are measured, use the balanced chemical equation to relate these measurements to the standard enthalpy of formation. This involves calculating the sum of the standard enthalpies of the products minus the sum of the standard enthalpies of the reactants, all taken in their standard states.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Calorimetry
Calorimetry is a method used to measure the amount of heat absorbed or released during a chemical reaction or a physical change. This process is vital in determining the standard enthalpy change, particularly for chemical reactions. By understanding the heat involved, we gain insights into the energetics of reactions.
A device called a calorimeter is used to perform calorimetry experiments. Here's how it works:
A device called a calorimeter is used to perform calorimetry experiments. Here's how it works:
- The reaction occurs within the calorimeter, which is ideally an insulated container.
- The temperature change occurs due to the reaction.
- This temperature change, along with the calorimeter’s specific heat capacity and the mass of the substances involved, allows us to calculate the amount of heat exchanged.
Hess's Law
Hess's Law is a foundational principle in chemistry that enables the calculation of enthalpy changes in reactions where direct measurement is difficult. It states that the total enthalpy change for a given reaction is the same, regardless of the path taken between reactants and products.
This is particularly useful when direct calorimetric measurements are not feasible.
This is particularly useful when direct calorimetric measurements are not feasible.
- Use known enthalpy changes of related reactions.
- Combine these reactions to form a pathway equivalent to the desired reaction.
- Sum the enthalpy changes according to the reactions' stoichiometry to find the unknown enthalpy change.
Enthalpy of Formation
The enthalpy of formation, or standard enthalpy change of formation, refers to the heat change when one mole of a compound is formed from its elements in their standard states. This is critical for gauging the stability and energy requirements of compounds.
To comprehend this concept:
To comprehend this concept:
- The enthalpy of formation for any element in its most stable form is defined as zero. Examples include diatomic oxygen \(O_2\) and graphite carbon \(C\).
- For compounds, the enthalpy of formation is experimentally determined via calorimetry.
- Using tabulated enthalpy values, calculations can determine the energy changes in chemical processes.