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Chapter 6: Problem 41

What is meant by the standard enthalpy of a reaction?

Short Answer

Expert verified
The standard enthalpy of a reaction is the heat change that occurs during a specific chemical reaction, measured under standard conditions (1 atmosphere pressure and 25 degrees Celsius temperature), considering one mole of reactants and products in the reaction.

Step by step solution

01

Understanding What Enthalpy is

Enthalpy is a measure of total energy in a thermodynamic system. It includes the internal energy, which is the energy required to create the system, and the amount of energy required to make room for the system by displacing its environment and establishing its volume and pressure.
02

Comprehending 'Reaction Enthalpy'

The reaction enthalpy, also known as heat of reaction, refers to the enthalpy change during a chemical reaction. For an exothermic reaction, the enthalpy change is negative as heat is released. For an endothermic reaction, the enthalpy change is positive as heat is absorbed.
03

Understanding 'Standard Enthalpy of Reaction'

'Standard Enthalpy of Reaction' or 'Heat of Reaction at standard state' is the heat change that occurs during a chemical reaction, specifically measured under standard conditions i.e., at 1 atmosphere pressure and 25 degrees Celsius temperature. The amount of reactants and products are considered as one mole in the reaction. It is symbolized as \( 螖H掳 \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Thermodynamic System
A thermodynamic system is a defined portion of the universe that we set aside to study or observe, specifically focusing on energy changes and exchanges. It can be as simple as a glass of water or as complex as a car engine. The system is separated from the rest of the universe, referred to as the surroundings, by a boundary.
This boundary can be physical, like a container wall, or imaginary. We commonly classify thermodynamic systems into three types based on their interactions with the surroundings:
  • **Open Systems**: These allow the exchange of both energy and matter with their surroundings.
  • **Closed Systems**: Energy can be exchanged, but not matter.
  • **Isolated Systems**: Neither energy nor matter is exchanged with the surroundings.
A clear understanding of how these systems work is crucial in studying their energy transformations, such as changes in enthalpy during a reaction.
Enthalpy Change
Enthalpy change refers to the difference in a system鈥檚 enthalpy before and after a process occurs. In the context of chemical reactions, it is the amount of heat absorbed or released. This concept is central for understanding how energy plays a role in reactions.
For any given reaction, enthalpy change can be represented as \( \Delta H \). Depending on whether the system absorbs heat or releases it, this change can be negative or positive:
  • **\( \Delta H < 0 \) (Negative):** Represents an exothermic reaction where heat is released.
  • **\( \Delta H > 0 \) (Positive):** Indicates an endothermic reaction where heat is absorbed.
This quantitative measure helps chemists understand the heat changes involved in chemical processes and predict reaction behaviors under different conditions.
Exothermic Reaction
An exothermic reaction is characterized by the release of heat to the surroundings. This type of reaction results in a negative enthalpy change \( (\Delta H < 0) \).
During exothermic reactions, chemical bonds in the reactants break, and new bonds form in the products. In this process, the system releases more energy than it absorbs, often resulting in a temperature increase of the surroundings.
Common examples of exothermic reactions include:
  • Combustion reactions, like burning fossil fuels.
  • Neutralization reactions between acids and bases.
  • The rusting of iron.
Understanding these reactions is vital in fields like engineering and environmental science, where controlling or harnessing energy efficiently is important.
Endothermic Reaction
In contrast to exothermic processes, endothermic reactions absorb heat from their surroundings, resulting in a positive enthalpy change \((\Delta H > 0) \).
These reactions require an input of energy to proceed, as the formation of products includes bond breaking that consumes more energy than the new bonds release.
Examples of endothermic reactions include:
  • Photosynthesis in plants, where sunlight is absorbed to synthesize glucose from carbon dioxide and water.
  • Melting ice, which requires heat to change state from solid to liquid.
  • Evaporation of water, which absorbs heat as it changes from liquid to gas.
Understanding endothermic reactions is crucial in biological processes and chemical manufacturing, where energy input needs to be managed carefully.

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Most popular questions from this chapter

After a dinner party, the host performed the following trick. First, he blew out one of the burning candles. He then quickly brought a lighted match to about 1 in above the wick. To everyone's surprise, the candle was relighted. Explain how the host was able to accomplish the task without touching the wick.

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

Consider the following data: $$ \begin{array}{lcc} \text { Metal } & \text { Al } & \text { Cu } \\ \hline \text { Mass (g) } & 10 & 30 \\ \text { Specific heat (J/g* } \left.^{\circ} \mathrm{C}\right) & 0.900 & 0.385 \\\ \text { Temperature }\left({ }^{\circ} \mathrm{C}\right) & 40 & 60 \\ \hline \end{array} $$ When these two metals are placed in contact, which of the following will take place? (a) Heat will flow from Al to Cu because Al has a larger specific heat. (b) Heat will flow from \(\mathrm{Cu}\) to \(\mathrm{Al}\) because \(\mathrm{Cu}\) has a larger mass. (c) Heat will flow from Cu to Al because Cu has a larger heat capacity. (d) Heat will flow from Cu to Al because Cu is at a higher temperature. (e) No heat will flow in either direction.

Define calorimetry and describe two commonly used calorimeters. In a calorimetric measurement, why is it important that we know the heat capacity of the calorimeter? How is this value determined?

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.
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