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Chapter 6: Problem 41

What is meant by the standard enthalpy of a reaction?

Short Answer

Expert verified
It is the heat change during a reaction under standard conditions.

Step by step solution

01

Define Enthalpy

Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system. It reflects the internal energy of the system plus the product of pressure and volume. It is represented as \( H = U + PV \), where \( H \) is enthalpy, \( U \) is internal energy, and \( PV \) is the product of pressure and volume.
02

Understand Standard Enthalpy

The standard enthalpy of a reaction refers to the change in enthalpy when one mole of a substance reacts under standard conditions. Standard conditions typically mean a pressure of 1 bar (or 1 atmosphere), and substances are in their stable states at 25掳C.
03

Recognize Role in Thermochemistry

In thermochemistry, the standard enthalpy change of a reaction provides information about the heat absorbed or released during a chemical reaction under standard conditions. This measurement allows chemists to predict how much heat will be involved in reaction scaling or temperature adjustments during experiments.
04

Apply to Reaction Equation

The standard enthalpy of a reaction can be calculated using the standard enthalpy of formation of the reactants and products. The formula is: \( \Delta H^\circ = \sum H^\circ_f \text{(products)} - \sum H^\circ_f \text{(reactants)} \), where \( H^\circ_f \) is the enthalpy of formation.

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Key Concepts

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

enthalpy
Enthalpy is a fundamental concept in chemistry, particularly in the study of energy changes during reactions. This thermodynamic property represents the total heat content of a system and combines the internal energy with the pressure-volume work that the system can perform on its surroundings. In mathematical terms, enthalpy (\( H \) is defined as \( H = U + PV \), where \( U \) is the internal energy and \( PV \) denotes the product of pressure and volume. This relationship makes enthalpy especially useful for understanding heat transfer in processes occurring at constant pressure, which is a common condition in laboratory and industrial settings.
thermochemistry
Thermochemistry is the branch of chemistry that examines the heat involved in chemical reactions and physical transformations. At its core, thermochemistry provides insights into the energy changes that accompany chemical changes, helping chemists to understand reaction energetics and feasibility.
  • Key concepts include enthalpy changes, heat capacity, and the conservation of energy as described by the first law of thermodynamics.
  • Through thermochemistry, we can predict whether a reaction will be exothermic (releasing heat) or endothermic (absorbing heat).
  • This information is crucial for applications such as designing chemical processes, developing new materials, and optimizing reactions for energy efficiency.
thermodynamics
Thermodynamics is a vast field that covers the principles governing energy and its transformations. In chemistry, thermodynamics helps in understanding how energy transfers affect matter at the molecular level.
  • The subject is built on four fundamental laws that describe how energy interacts with matter.

  • These laws can explain phenomena such as energy conservation, entropy, and how temperature changes can drive chemical reactions.

  • Thermodynamics provides the framework to quantitatively describe the conditions under which reactions occur and how variables like temperature and concentration affect them.
  • By applying thermodynamic principles, chemists can derive important properties like Gibbs free energy, which indicates the spontaneity of a reaction.
enthalpy of formation
The enthalpy of formation is a specific type of enthalpy change that occurs when one mole of a compound is formed from its elements in their standard states. Standard states typically mean conditions of 1 bar pressure and substances at 25掳C.
  • These values are crucial for calculating the overall enthalpy change of a reaction.
  • By using standard enthalpy of formation values, chemists can utilize the equation \( \Delta H^\circ = \sum H_f^\circ \text{(products)} - \sum H_f^\circ \text{(reactants)} \)
  • This equation helps determine the heat emitted or absorbed in reactions, aiding in both theoretical studies and practical applications.
  • The standard enthalpy of formation acts like a benchmark ensuring all calculations align with these typical conditions.
This concept is pivotal in designing processes where controlling the heat exchange is essential, such as in energy-efficient manufacturing or safer chemical reaction setups.

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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.

Lime is a term that includes calcium oxide \((\mathrm{CaO},\) also called quicklime) and calcium hydroxide \(\left[\mathrm{Ca}(\mathrm{OH})_{2},\right.\) also called slaked lime]. It is used in the steel industry to remove acidic impurities, in air-pollution control to remove acidic oxides such as \(\mathrm{SO}_{2}\), and in water treatment. Quicklime is made industrially by heating limestone \(\left(\mathrm{CaCO}_{3}\right)\) above \(2000^{\circ} \mathrm{C}\) : $$ \begin{aligned} \mathrm{CaCO}_{3}(s) \longrightarrow \mathrm{CaO}(s)+\mathrm{CO}_{2}(g) \\ \Delta H^{\circ}=177.8 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$ Slaked lime is produced by treating quicklime with water: $$ \begin{aligned} \mathrm{CaO}(s)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(s) \\ \Delta H^{\circ}=-65.2 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$ The exothermic reaction of quicklime with water and the rather small specific heats of both quicklime \(\left(0.946 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right)\) and slaked lime \(\left(1.20 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right)\) make it hazardous to store and transport lime in vessels made of wood. Wooden sailing ships carrying lime would occasionally catch fire when water leaked into the hold. (a) If a 500 -g sample of water reacts with an equimolar amount of \(\mathrm{CaO}\) (both at an initial temperature of \(25^{\circ} \mathrm{C}\) ), what is the final temperature of the product, \(\mathrm{Ca}(\mathrm{OH})_{2}\) ? Assume that the product absorbs all of the heat released in the reaction. (b) Given that the standard enthalpies of formation of \(\mathrm{CaO}\) and \(\mathrm{H}_{2} \mathrm{O}\) are \(-635.6 \mathrm{~kJ} / \mathrm{mol}\) and \(-285.8 \mathrm{~kJ} / \mathrm{mol}\), respectively, calculate the standard enthalpy of formation of \(\mathrm{Ca}(\mathrm{OH})_{2}\)

Calculate the work done when \(50.0 \mathrm{~g}\) of tin dissolves in excess acid at \(1.00 \mathrm{~atm}\) and \(25^{\circ} \mathrm{C}\) : $$ \mathrm{Sn}(s)+2 \mathrm{H}^{+}(a q) \longrightarrow \mathrm{Sn}^{2+}(a q)+\mathrm{H}_{2}(g) $$ Assume ideal gas behavior.

Methanol, ethanol, and \(n\) -propanol are three common alcohols. When \(1.00 \mathrm{~g}\) of each of these alcohols is burned in air, heat is liberated as shown by the following data: (a) methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right),-22.6 \mathrm{~kJ}\); (b) ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right),-29.7 \mathrm{~kJ}\) (c) \(n\) -propanol \(\left(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\right),-33.4 \mathrm{~kJ} .\) Calculate the heats of combus- tion of these alcohols in \(\mathrm{kJ} / \mathrm{mol}\).

(a) A snowmaking machine contains a mixture of compressed air and water vapor at about 20 atm. When the mixture is sprayed into the atmosphere it expands so rapidly that, as a good approximation, no heat exchange occurs between the system (air and water) and its surroundings. (In thermodynamics, such a process is called an adiabatic process.) Do a first law of thermodynamics analysis to show how snow is formed under these conditions. (b) If you have ever pumped air into a bicycle tire, you probably noticed a warming effect at the valve stem. The action of the pump compresses the air inside the pump and the tire. The process is rapid enough to be treated as an adiabatic process. Apply the first law of thermodynamics to account for the warming effect. (c) A driver's manual states that the stopping distance quadruples as the speed doubles; that is, if it takes \(30 \mathrm{ft}\) to stop a car traveling at \(25 \mathrm{mph}\) then it would take \(120 \mathrm{ft}\) to stop a car moving at 50 mph. Justify this statement by using the first law of thermodynamics. Assume that when a car is stopped, its kinetic energy \(\left(\frac{1}{2} m u^{2}\right)\) is totally converted to heat.
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