Specific Enthalpy vs Enthalpy
Enthalpy is a fundamental concept in thermodynamics that measures the total heat content of a system. It is often used to describe the energy changes that occur during chemical reactions or phase transitions. However, there is another term closely related to enthalpy known as specific enthalpy. While both terms are used to quantify energy changes, they have distinct differences in their definitions and applications. In this article, we will explore the differences between specific enthalpy and enthalpy, and understand how they are used in various fields of science and engineering. So, let’s dive in and unravel the mysteries behind these two important concepts in thermodynamics.
Key Takeaways
- Enthalpy is a thermodynamic property that represents the total heat content of a system, while specific enthalpy is the enthalpy per unit mass of a substance.
- Specific enthalpy is useful in engineering applications as it allows for more accurate calculations and comparisons between different substances.
- Enthalpy is typically measured in units of energy, such as joules or calories, while specific enthalpy is measured in units of energy per unit mass, such as joules per kilogram.
- The specific enthalpy of a substance can be calculated by dividing the enthalpy by the mass of the substance.
- Both enthalpy and specific enthalpy are important concepts in thermodynamics and are used to analyze and design various processes and systems.
Enthalpy vs Specific Enthalpy
Enthalpy: Definition and Components (Internal Energy and Pressure-Volume Work)
Enthalpy is a fundamental concept in thermodynamics that helps us understand the energy content of a system. It is denoted by the symbol “H” and is defined as the sum of the internal energy (U) and the product of pressure (P) and volume (V) of the system. In other words, enthalpy accounts for both the heat energy and the work done by the system.
The internal energy represents the total energy stored within the system, including the kinetic and potential energies of its particles. On the other hand, the pressure-volume work accounts for the energy transferred to or from the system due to changes in its volume under constant pressure.
Specific Enthalpy: Definition as Enthalpy per Unit Mass
Specific enthalpy, denoted as “h,” is the enthalpy per unit mass of a substance. It is obtained by dividing the enthalpy (H) of the system by its mass (m). Specific enthalpy allows us to analyze the energy content of a substance on a per-unit-mass basis, which is particularly useful in engineering and thermodynamic calculations.
By expressing enthalpy in terms of mass, specific enthalpy enables us to compare the energy content of different substances or different amounts of the same substance.
Comparison between Enthalpy and Specific Enthalpy
The main difference between enthalpy and specific enthalpy lies in their units and the way they are used. Enthalpy is an extensive property, meaning it depends on the size or amount of the system. On the other hand, specific enthalpy is an intensive property, which remains constant regardless of the system size.
Enthalpy allows us to analyze the total energy content of a system, taking into account both the internal energy and the work done. It is commonly used in thermodynamic calculations, such as determining the heat transfer in chemical reactions or phase changes.
Specific enthalpy, on the other hand, provides a more detailed analysis by considering the energy content on a per-unit-mass basis. It is particularly useful in engineering applications, such as calculating the energy required for heating or cooling a specific mass of a substance.
Units of Measurement for Enthalpy and Specific Enthalpy
Enthalpy is typically measured in units of energy, such as joules (J) or calories (cal). The SI unit for enthalpy is joule, while the calorie is commonly used in the field of chemistry.
Specific enthalpy is expressed in the same units as enthalpy but divided by mass. Therefore, the units for specific enthalpy are joules per kilogram (J/kg) or calories per gram (cal/g).
To summarize, enthalpy and specific enthalpy are both important concepts in thermodynamics. Enthalpy provides a comprehensive analysis of the energy content of a system, while specific enthalpy allows for a more detailed examination on a per-unit-mass basis. Understanding these concepts is crucial for engineers, scientists, and anyone working with energy systems.
Specific Enthalpy of Steam Water System
The specific enthalpy of a steam water system is a crucial thermodynamic property that helps us understand the energy content of the system. It is defined as the amount of heat energy per unit mass of a substance, and it plays a significant role in various engineering applications, particularly in the field of thermodynamics.
Specific Enthalpy Values for Vaporization and Saturation of Water
When water undergoes a phase change from a liquid to a vapor state, it requires a certain amount of energy. This energy is known as the specific enthalpy of vaporization. It represents the heat energy needed to convert one unit mass of liquid water into vapor at a given temperature and pressure.
Similarly, the specific enthalpy of saturation refers to the energy content of water when it exists in a saturated state. In this state, water is at the boiling point corresponding to a specific pressure. The specific enthalpy of saturation is the sum of the specific enthalpy of liquid water and the specific enthalpy of vaporization.
Calculation of Specific Enthalpy for Saturated Steam at Atmospheric Pressure
To calculate the specific enthalpy for saturated steam at atmospheric pressure, we need to consider the specific enthalpy of liquid water and the specific enthalpy of vaporization. The specific enthalpy of liquid water can be determined using tables or equations based on temperature. The specific enthalpy of vaporization can also be obtained from tables or correlations.
By adding the specific enthalpy of liquid water and the specific enthalpy of vaporization, we can determine the specific enthalpy of saturated steam at atmospheric pressure. This value represents the energy content of steam in its saturated state.
Increase in Specific Enthalpy with System Pressure
As the pressure of a steam water system increases, the specific enthalpy also increases. This is because higher pressures require more energy to convert water into steam. The specific enthalpy of steam at a given pressure is higher than that at atmospheric pressure due to the additional energy required for vaporization.
The increase in specific enthalpy with system pressure is an important consideration in various industrial processes, such as power generation and steam-driven machinery. Understanding the relationship between pressure and specific enthalpy allows engineers to design efficient systems and optimize energy usage.
Calculation of Specific Enthalpy for Superheated Steam
Superheated steam refers to steam that has been heated beyond its saturation point. To calculate the specific enthalpy for superheated steam, we need to consider the specific enthalpy of saturated steam and the specific enthalpy of superheating.
The specific enthalpy of saturated steam can be determined as mentioned earlier. The specific enthalpy of superheating represents the additional energy required to heat the steam beyond its saturation point. This value can be obtained from tables or correlations based on the temperature and pressure of the superheated steam.
By adding the specific enthalpy of saturated steam and the specific enthalpy of superheating, we can determine the specific enthalpy of superheated steam. This value represents the total energy content of steam in its superheated state.
Molar Enthalpy vs Specific Enthalpy
Definition of Molar Enthalpy and Specific Enthalpy
Enthalpy is a fundamental concept in thermodynamics that measures the total energy of a system. It is denoted by the symbol “H” and is often used to describe heat transfer and energy changes in chemical reactions. Molar enthalpy and specific enthalpy are two related but distinct concepts that are commonly used in different contexts.
Molar enthalpy, also known as molar heat capacity, is the amount of heat required to raise the temperature of one mole of a substance by one degree Celsius. It is represented by the symbol “Cp” and has units of joules per mole per degree Celsius (J/mol·°C). Molar enthalpy takes into account the mass of the substance and is used to calculate the enthalpy change in reactions.
On the other hand, specific enthalpy, also known as specific heat capacity, is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius. It is represented by the symbol “cp” and has units of joules per kilogram per degree Celsius (J/kg·°C). Specific enthalpy is used for heat balance calculations in engineering and is independent of the mass of the substance.
Differences between Molar Enthalpy and Specific Enthalpy
The main difference between molar enthalpy and specific enthalpy lies in the units of measurement and the quantity being considered. Molar enthalpy is expressed in terms of moles of a substance, while specific enthalpy is expressed in terms of kilograms of a substance.
Molar enthalpy takes into account the mass of the substance and is used to calculate the enthalpy change in reactions. It is particularly useful in chemical reactions where the quantities involved are often measured in moles. Specific enthalpy, on the other hand, is used for heat balance calculations in engineering, where the focus is on the energy required to heat or cool a given mass of a substance.
Molar Enthalpy used for calculating enthalpy change in reactions
Molar enthalpy is commonly used in chemical reactions to calculate the enthalpy change, also known as the heat of reaction. The enthalpy change is the difference in enthalpy between the products and the reactants in a chemical reaction. By knowing the molar enthalpies of the reactants and products, it is possible to calculate the enthalpy change using the principle of conservation of energy.
For example, in the combustion of methane (CH4), the molar enthalpy of methane is -890.3 kJ/mol, and the molar enthalpy of carbon dioxide (CO2) is -393.5 kJ/mol. By subtracting the molar enthalpy of the reactants from the molar enthalpy of the products, the enthalpy change of the reaction can be determined.
Specific Enthalpy used for heat balance calculations in engineering
Specific enthalpy is widely used in engineering for heat balance calculations. Heat balance calculations involve determining the amount of heat energy required to heat or cool a given mass of a substance. Specific enthalpy allows engineers to calculate the energy requirements for various processes, such as heating water or cooling air.
For example, in HVAC (Heating, Ventilation, and Air Conditioning) systems, specific enthalpy is used to calculate the energy required to heat or cool air as it flows through the system. By knowing the specific enthalpy of the incoming and outgoing air, engineers can determine the amount of energy needed to achieve the desired temperature change.
Units of measurement for Molar Enthalpy and Specific Enthalpy
Molar enthalpy is measured in joules per mole per degree Celsius (J/mol·°C), while specific enthalpy is measured in joules per kilogram per degree Celsius (J/kg·°C). These units reflect the different quantities being considered – moles for molar enthalpy and kilograms for specific enthalpy.
Calculation of total enthalpy change using Molar Enthalpy and Specific Enthalpy
To calculate the total enthalpy change in a system, both molar enthalpy and specific enthalpy can be used. The molar enthalpy is used to determine the enthalpy change at the molecular level, while the specific enthalpy is used to determine the enthalpy change at the mass level.
The total enthalpy change can be calculated by multiplying the molar enthalpy by the number of moles involved in the reaction and adding it to the product of the specific enthalpy and the mass of the substance. This allows for a comprehensive understanding of the energy changes occurring in a system.
Different Types of Enthalpy Change
Enthalpy change refers to the amount of heat energy transferred during a chemical reaction or physical process. There are various types of enthalpy changes that occur in different scenarios. Let’s explore some of the most common types of enthalpy changes.
Explanation of Various Types of Enthalpy Changes
Enthalpy of Reaction: The enthalpy change associated with a chemical reaction is known as the enthalpy of reaction. It represents the difference in enthalpy between the products and the reactants. This value can be positive or negative, depending on whether the reaction is exothermic (releases heat) or endothermic (absorbs heat).
Enthalpy of Combustion: The enthalpy change that occurs during the combustion of a substance is called the enthalpy of combustion. It measures the heat energy released when a substance reacts with oxygen to form carbon dioxide and water.
Enthalpy of Formation: The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states is known as the enthalpy of formation. It provides information about the stability and energy content of the compound.
Enthalpy of Neutralization: The enthalpy change that occurs when an acid reacts with a base to form a salt and water is called the enthalpy of neutralization. It represents the heat energy released or absorbed during the neutralization process.
Enthalpy of Solution: The enthalpy change associated with the dissolution of a solute in a solvent is known as the enthalpy of solution. It measures the heat energy released or absorbed when a solute dissolves to form a solution.
Enthalpy of Fusion: The enthalpy change when a substance changes from a solid to a liquid state at its melting point is called the enthalpy of fusion. It represents the heat energy required to overcome the intermolecular forces holding the solid together.
Enthalpy of Sublimation: The enthalpy change when a substance changes directly from a solid to a gaseous state without passing through the liquid state is known as the enthalpy of sublimation. It measures the heat energy required to overcome the forces holding the solid together and convert it into a gas.
Enthalpy of Vaporization: The enthalpy change when a substance changes from a liquid to a gaseous state at its boiling point is called the enthalpy of vaporization. It represents the heat energy required to overcome the intermolecular forces holding the liquid together.
Enthalpy of Mixing: The enthalpy change that occurs when two or more substances are mixed together is known as the enthalpy of mixing. It measures the heat energy released or absorbed during the mixing process.
By understanding these different types of enthalpy changes, we can gain insights into the energy transformations that occur during chemical reactions and physical processes. These values play a crucial role in various scientific and engineering applications, such as designing efficient energy systems and understanding the thermodynamics of chemical reactions.
Favorable Enthalpy
Enthalpy is a fundamental concept in thermodynamics that helps us understand the energy changes that occur during chemical reactions. It plays a crucial role in determining whether a reaction is favorable or not. In this section, we will explore the relationship between enthalpy change and reaction favorability, as well as introduce the concept of Gibbs free energy.
Favorability of a reaction based on enthalpy change
The enthalpy change of a reaction, often denoted as ΔH, is a measure of the heat energy absorbed or released during a chemical reaction. It represents the difference in enthalpy between the products and the reactants. By analyzing the sign and magnitude of ΔH, we can determine whether a reaction is exothermic or endothermic.
An exothermic reaction releases heat energy to the surroundings, resulting in a negative ΔH value. On the other hand, an endothermic reaction absorbs heat energy from the surroundings, leading to a positive ΔH value. The magnitude of ΔH indicates the amount of heat energy involved in the reaction.
Relationship between enthalpy change and entropy change
Entropy, denoted as ΔS, is a measure of the disorder or randomness in a system. It accounts for the number of ways in which the particles of a system can be arranged. A positive ΔS value indicates an increase in disorder, while a negative ΔS value suggests a decrease in disorder.
The relationship between enthalpy change (ΔH) and entropy change (ΔS) is described by the equation:
ΔG = ΔH – TΔS
where ΔG is the Gibbs free energy change and T is the temperature in Kelvin. The Gibbs free energy change determines the spontaneity of a reaction. If ΔG is negative, the reaction is spontaneous and favorable. If ΔG is positive, the reaction is non-spontaneous and unfavorable. If ΔG is zero, the reaction is at equilibrium.
Gibbs free energy (ΔG) is a thermodynamic potential that combines the effects of enthalpy change (ΔH) and entropy change (ΔS) to determine the spontaneity of a reaction. It provides a more comprehensive understanding of reaction favorability compared to enthalpy alone.
The equation ΔG = ΔH – TΔS tells us that for a reaction to be favorable, the enthalpy change must be negative (exothermic) and the entropy change must be positive (increase in disorder). However, the temperature also plays a crucial role. At low temperatures, a negative enthalpy change dominates, making the reaction favorable. At high temperatures, a positive entropy change dominates, making the reaction favorable.
Frequently Asked Questions
1. Why are enthalpy values different?
Enthalpy values can vary depending on the specific process or reaction being considered. Factors such as temperature, pressure, and the presence of other substances can influence the enthalpy of a system, leading to differences in the measured values.
2. Is enthalpy the same as specific heat?
No, enthalpy and specific heat are different properties. Enthalpy is a measure of the total energy of a system, including both its internal energy and the work done on or by the system. Specific heat, on the other hand, is a measure of the amount of heat required to raise the temperature of a unit mass of a substance by a certain amount.
3. What is the difference between specific heat capacity and enthalpy?
Specific heat capacity refers to the amount of heat required to raise the temperature of a unit mass of a substance by a certain amount. Enthalpy, on the other hand, is a measure of the total energy of a system, including both its internal energy and the work done on or by the system. While specific heat capacity is a property of a substance, enthalpy is a property of a system.
4. Why is the experimental enthalpy higher than the theoretical value?
There can be several reasons for the experimental enthalpy to be higher than the theoretical value. Experimental errors, incomplete reactions, side reactions, and impurities in the reactants can all contribute to the deviation between the experimental and theoretical enthalpy values.
5. What is the difference between standard enthalpy and enthalpy?
Standard enthalpy refers to the enthalpy change that occurs under standard conditions, which typically include a temperature of 25°C and a pressure of 1 bar. Enthalpy, on the other hand, is a more general term that refers to the total energy of a system, including both its internal energy and the work done on or by the system.
6. What is specific enthalpy?
Specific enthalpy is the enthalpy per unit mass of a substance. It is a measure of the total energy of a unit mass of a substance, including both its internal energy and the work done on or by the substance.
7. What is the difference between enthalpy change and standard enthalpy change?
Enthalpy change refers to the difference in enthalpy between the reactants and the products in a chemical reaction. It can be positive (endothermic) or negative (exothermic). Standard enthalpy change, on the other hand, refers to the enthalpy change that occurs under standard conditions, typically at a temperature of 25°C and a pressure of 1 bar.
8. What is the difference between specific enthalpy and enthalpy?
Specific enthalpy refers to the enthalpy per unit mass of a substance, while enthalpy is a more general term that refers to the total energy of a system. Specific enthalpy takes into account the mass of the substance, whereas enthalpy does not.
9. What is the difference between enthalpy of formation and enthalpy of reaction?
Enthalpy of formation refers to the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states. Enthalpy of reaction, on the other hand, refers to the enthalpy change that occurs in a chemical reaction, regardless of the formation of new compounds.
10. What is the difference between enthalpy of mixing and enthalpy of solution?
Enthalpy of mixing refers to the enthalpy change that occurs when two or more substances are combined to form a mixture. Enthalpy of solution, on the other hand, refers to the enthalpy change that occurs when a solute is dissolved in a solvent to form a solution.
I am Sangeeta Das. I have completed my Masters in Mechanical Engineering with specialization in I.C Engine and Automobiles. I have around ten years of experience encompassing industry and academia. My area of interest includes I.C. Engines, Aerodynamics and Fluid Mechanics. You can reach me at
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