Calculating the Heat Produced

Oct 13, 2025 by miodeya.com 127 views

The Standard Enthalpy of Formation: Understanding the Heat Produced in Chemical Reactions

In the world of chemistry, understanding the standard enthalpy of formation is crucial in determining the heat produced or consumed during chemical reactions. The standard enthalpy of formation, denoted by $\Delta H _{ f }^{\circ}$ , is a measure of the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states. In this article, we will delve into the concept of standard enthalpy of formation and use it to calculate the heat produced when 10 grams of hydrogen gas is consumed with an excess of solid iodine.

What is Standard Enthalpy of Formation?

The standard enthalpy of formation is a thermodynamic property that is used to describe the energy change that occurs during the formation of a compound from its constituent elements. It is defined as the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states. The standard enthalpy of formation is denoted by $\Delta H _{ f }^{\circ}$ and is typically expressed in units of kJ/mol.

Calculating the Heat Produced

To calculate the heat produced when 10 grams of hydrogen gas is consumed with an excess of solid iodine, we need to first determine the number of moles of hydrogen gas that is consumed. The molar mass of hydrogen gas is 2 g/mol, so we can calculate the number of moles of hydrogen gas as follows:

\text{Number of moles of hydrogen gas} = \frac{\text{Mass of hydrogen gas}}{\text{Molar mass of hydrogen gas}}

\text{Number of moles of hydrogen gas} = \frac{10 \, \text{g}}{2 \, \text{g/mol}} = 5 \, \text{mol}

Next, we need to determine the heat produced when 5 moles of hydrogen gas is consumed with an excess of solid iodine. The standard enthalpy of formation of HI(g) is -25 kJ/mol, so we can calculate the heat produced as follows:

\text{Heat produced} = \text{Number of moles of hydrogen gas} \times \text{Standard enthalpy of formation of HI(g)}

\text{Heat produced} = 5 \, \text{mol} \times (-25 \, \text{kJ/mol}) = -125 \, \text{kJ}

Therefore, the heat produced when 10 grams of hydrogen gas is consumed with an excess of solid iodine is -125 kJ.

Understanding the Sign of the Enthalpy Change

The sign of the enthalpy change is important in understanding the direction of the reaction. A negative enthalpy change indicates that the reaction is exothermic, meaning that heat is released during the reaction. In this case, the heat produced when 10 grams of hydrogen gas is consumed with an excess of solid iodine is -125 kJ, indicating that the reaction is exothermic.

In conclusion, the standard enthalpy of formation is a crucial concept in understanding the heat produced or consumed during chemical reactions. By using the standard enthalpy of formation, we can calculate the heat produced when 10 grams of hydrogen gas is consumed with an excess of solid iodine. The heat produced is -125 kJ, indicating that the reaction is exothermic.

Atkins, P. W., & De Paula, J. (2010). Physical Chemistry (9th ed.). Oxford University Press.
Chang, R. (2010). Chemistry: The Central Science (11th ed.). McGraw-Hill.

Enthalpy of formation: A comprehensive review of the concept and its applications.
Chemical reactions: A detailed explanation of the different types of chemical reactions and their thermodynamic properties.

Standard enthalpy of formation: The change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states.
Exothermic reaction: A reaction that releases heat.
Molar mass: The mass of one mole of a substance.
Thermodynamic property: A property that is used to describe the energy change that occurs during a chemical reaction.
The Standard Enthalpy of Formation: Q&A

In our previous article, we discussed the concept of standard enthalpy of formation and how it is used to calculate the heat produced or consumed during chemical reactions. In this article, we will answer some of the most frequently asked questions about the standard enthalpy of formation.

Q: What is the standard enthalpy of formation?

A: The standard enthalpy of formation is a thermodynamic property that is used to describe the energy change that occurs during the formation of a compound from its constituent elements. It is defined as the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states.

Q: How is the standard enthalpy of formation calculated?

A: The standard enthalpy of formation is typically calculated using the following equation:

\Delta H _{ f }^{\circ} = \sum \Delta H _{ f }^{\circ}(\text{products}) - \sum \Delta H _{ f }^{\circ}(\text{reactants})

Where $\Delta H _{ f }^{\circ}$ is the standard enthalpy of formation of a compound, and the sums are taken over all the products and reactants in the reaction.

Q: What is the unit of the standard enthalpy of formation?

A: The unit of the standard enthalpy of formation is typically kJ/mol.

Q: Is the standard enthalpy of formation a state function?

A: Yes, the standard enthalpy of formation is a state function. This means that it depends only on the initial and final states of the system, and not on the path taken to get from one state to the other.

Q: Can the standard enthalpy of formation be negative?

A: Yes, the standard enthalpy of formation can be negative. This means that the reaction is exothermic, and heat is released during the reaction.

Q: Can the standard enthalpy of formation be positive?

A: Yes, the standard enthalpy of formation can be positive. This means that the reaction is endothermic, and heat is absorbed during the reaction.

Q: How is the standard enthalpy of formation used in real-world applications?

A: The standard enthalpy of formation is used in a wide range of real-world applications, including:

Chemical engineering: The standard enthalpy of formation is used to design and optimize chemical processes, such as the production of fuels and chemicals.
Materials science: The standard enthalpy of formation is used to understand the properties of materials, such as their melting points and boiling points.
Biology: The standard enthalpy of formation is used to understand the energy changes that occur during biological processes, such as the metabolism of nutrients.

Q: What are some common mistakes to avoid when working with the standard enthalpy of formation?

A: Some common mistakes to avoid when working with the standard enthalpy of formation include:

Not using the correct units: Make sure to use the correct units for the standard enthalpy of formation, which is typically kJ/mol.
Not considering the temperature: The standard enthalpy of formation is typically measured at a specific temperature, so make sure to consider this when using the value.
Not accounting for the pressure: The standard enthalpy of formation is typically measured at a specific pressure, so make sure to account for this when using the value.

In conclusion, the standard enthalpy of formation is a crucial concept in understanding the energy changes that occur during chemical reactions. By understanding the standard enthalpy of formation, we can design and optimize chemical processes, understand the properties of materials, and understand the energy changes that occur during biological processes.

Atkins, P. W., & De Paula, J. (2010). Physical Chemistry (9th ed.). Oxford University Press.
Chang, R. (2010). Chemistry: The Central Science (11th ed.). McGraw-Hill.

Enthalpy of formation: A comprehensive review of the concept and its applications.
Chemical reactions: A detailed explanation of the different types of chemical reactions and their thermodynamic properties.

Standard enthalpy of formation: The change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states.
Exothermic reaction: A reaction that releases heat.
Endothermic reaction: A reaction that absorbs heat.
Thermodynamic property: A property that is used to describe the energy change that occurs during a chemical reaction.