Question

The enthalpy change for the reaction of hydrogen gas with fluorine gas to produce hydrogen fluoride is \(-542 \mathrm{~kJ}\) for the equation as written: $$ \mathrm{H}_{2}(g)+\mathrm{F}_{2}(g) \rightarrow 2 \mathrm{HF}(g) \quad \Delta H=-542 \mathrm{~kJ} $$ a. What is the enthalpy change per mole of hydrogen fluoride produced? b. Is the reaction exothermic or endothermic as written? c. What would be the enthalpy change for the reverse of the given equation (that is, for the decomposition of HF into its constituent elements)?

Short answer

a. The enthalpy change per mole of hydrogen fluoride produced is \(-271 \mathrm{~kJ}\). b. The reaction is exothermic as written. c. The enthalpy change for the reverse reaction is \(+542 \mathrm{~kJ}\).

Step-by-step solution

Find the Enthalpy Change per Mole of Hydrogen Fluoride Produced

To calculate the enthalpy change per mole of hydrogen fluoride produced, we need to look at the balanced equation: \[ \mathrm{H}_{2}(g)+\mathrm{F}_{2}(g) \rightarrow 2 \mathrm{HF}(g) \] Since the reaction produces 2 moles of hydrogen fluoride, we can find the enthalpy change per mole by dividing the total enthalpy change by 2 moles: \[ \frac{\Delta H}{2} \] Use the given enthalpy change, \(-542 \mathrm{~kJ}\): \[ \frac{-542 \mathrm{~kJ}}{2} = -271 \mathrm{~kJ} \] The enthalpy change per mole of hydrogen fluoride is \(-271 \mathrm{~kJ}\).

Determine Reaction Nature: Exothermic or Endothermic

In a chemical reaction, if the enthalpy change is positive, the reaction is endothermic (absorbs heat energy). If the enthalpy change is negative, the reaction is exothermic (releases heat energy). Since the given enthalpy change (\(\Delta H = -542 \mathrm{~kJ}\)) is negative, the reaction is exothermic as written.

Calculate the Enthalpy Change for the Reverse Reaction

To find the enthalpy change for the reverse of the given reaction (the decomposition of HF into its constituent elements), we simply change the sign of the enthalpy change of the original reaction. If \(\Delta H_{\text{forward}} = -542 \mathrm{~kJ}\), then: \[ \Delta H_{\text{reverse}} = -(-542 \mathrm{~kJ}) = +542 \mathrm{~kJ} \] The enthalpy change for the reverse reaction is \(+542 \mathrm{~kJ}\). In summary, the enthalpy change per mole of hydrogen fluoride produced is \(-271 \mathrm{~kJ}\), the reaction is exothermic as written, and the enthalpy change for the reverse reaction is \(+542 \mathrm{~kJ}\).

Key concepts

Exothermic Reaction

Exothermic reactions are fascinating chemical processes where energy is released into the surrounding environment. This release of energy usually occurs in the form of heat. You can recognize an exothermic reaction by its negative enthalpy change, which means that energy is given off rather than absorbed.
For example, in our discussed reaction where hydrogen reacts with fluorine to produce hydrogen fluoride, the enthalpy change is \(\Delta H = -542 \, \mathrm{kJ}\). This negative value indicates that the reaction is exothermic.
Here are some key points about exothermic reactions:

• The temperature of the surroundings increases as the reaction proceeds.
• Exothermic reactions are often used in heat packs and combustion processes.
• They are characterized by energy being released when bonds are formed in the products.

Understanding exothermic reactions is crucial in numerous industrial and everyday processes, making them an essential part of chemical studies.

Endothermic Reaction

Endothermic reactions are quite the opposite of exothermic reactions. They require the absorption of energy from their environment to occur. This energy is typically absorbed in the form of heat, resulting in a positive enthalpy change.
In these reactions, the system gains heat from the surroundings, making them feel cooler. An endothermic reaction would occur if, for instance, the products of a reaction have higher energy than the reactants after absorbing heat.
Some important characteristics of endothermic reactions include:

• The temperature of the surroundings often decreases as the reaction absorbs energy.
• They are common in processes like photosynthesis and the melting of ice.
• In endothermic reactions, energy is absorbed to break the chemical bonds of the reactants.

Understanding endothermic reactions helps us explain phenomena like why certain reactions need heat to proceed and how energy transfers in chemical processes.

Chemical Reaction

Chemical reactions are processes in which substances, known as reactants, transform into one or more different substances, called products. These transformations involve the breaking and forming of chemical bonds, leading to changes in energy.
Energy changes are a core aspect of chemical reactions and can result in either energy release or absorption. This is measured through the enthalpy change (\(\Delta H\)), which signifies whether a reaction is exothermic (\(\Delta H < 0\)) or endothermic (\(\Delta H > 0\)).
Essential aspects of chemical reactions involve:

• Reactions can happen quickly or slowly, depending on various factors like temperature, concentration, and presence of catalysts.
• Reactants and products can exist in different states such as gases, liquids, and solids.
• Balanced chemical equations provide a way to represent the transformation of reactants to products while conserving mass.

Understanding the nature of chemical reactions aids in grasping how substances interact with each other, ensuring advancements in fields like chemistry, biology, and environmental science.