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Enthalpy is the measurement of energy released in certain chemical reactions in thermodynamic systems. The variations in the reactions, along with the heat content of the materials are what result in Enthalpy (H). Enthalpy Change for a reaction is represented by ΔH (also called heat of reaction).
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Keyterms: Enthalpy, Energy, Heat, Chemical reaction, Thermodynamic systems, Joules, Internal energy, Conservation of energy, Vaporization
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Enthalpy Definition
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The amount of internal energy and the output of a thermodynamic system's pressure and volume are defined as enthalpy. Enthalpy is a state function or energy-like property that has energy dimensions (and is thus calculated in joules or erg units). The enthalpy H is equal to the sum of the internal energy E and the pressure P multiplied by the system's volume V, i.e.,
H = E + PV.
The shift in internal energy is equal to the heat transmitted to the device minus the work performed by it, according to the law of conservation of energy. If the only work done is a change in volume at constant pressure, the change in enthalpy is exactly equal to the heat transferred to the device. When energy is applied to a substance to change its phase from a liquid to a gas, the amount of energy is called the enthalpy (or latent heat of vaporization) and is expressed in joules per mole.
Enthalpy Formula
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Enthalpy Formula
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Enthalpy is a thermodynamic concept. It is impossible to calculate a system's total internal energy. However, the heat transfer changes will be calculated. As a result, the enthalpy of a reaction is written as ΔH, with the symbol Δ denoting the change.
When a process starts at constant pressure, heat is evolved, either absorbed or released, and this equals the change in enthalpy. As a result, the enthalpy change is equal to the sum of internal energy (E) and the product of volume and pressure (P x V)
E+PV = H
The state function based on the state functions P, T, and E is referred to as enthalpy. It is represented by the difference in enthalpy ΔH between the initial and final states of a process. According to the following equation, the flow of heat q at constant pressure in a process is equal to the change in enthalpy.
ΔH = q
Knowing whether q is endothermic or exothermic can help define the relationship between q and ΔH. An endothermic reaction absorbs heat and reveals that heat from the surroundings is consumed in the reaction, so q>0. In an exothermic reaction, heat is also released and given to the environment. As a result, q < 0.
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Enthalpy Formula Calculation
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Enthalpy formuIa ΔH can be calculated in a variety of ways:
Method 1: The volume of the container does not change if the work done by or on a system is zero. The heat transfer (q) will be equal to the change in enthalpy.
q = m x s x ΔT
The mass is m, the specific heat is s, and the temperature change is ΔT in this equation.
Method 2: If the reaction is already known, it can be calculated using a table of heat change values ΔHf. The heat of formation is referred to as ΔHf. It refers to the heat used to create a substance from its constituent elements.
As a result, ΔHreaction=\(\Sigma\)ΔHf(products)–\(\Sigma\)ΔHf (reactants)
Method 3: To calculate the enthalpy of a reaction, use Hess's Law.
Method 4: Using the bond energies of the reactants and products, the ΔHreaction can be calculated.
ΔHreaction=\(\Sigma\)ΔHbondsbroken–\(\Sigma\)ΔHf(bondsformed
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Things to Remember
- Enthalpy is a thermodynamic function. It is the sum of the total internal energy of the system plus the product of Pressure and Volume.
- Enthalpy can be represented by the following equation: H = E + PV.
- During exothermic reactions, the device's heat is lost to the surrounding atmosphere. For such reactions, H is negative. The system absorbs heat from the atmosphere during endothermic reactions. For such reactions, ΔH is positive.
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Sample Questions
Ques. Is enthalpy negative or positive? (4 marks)
Ans. The change in enthalpy in an exothermic reaction is negative because the total heat is lost ("Exo-Thermic" means "heat is leaving"). A favourable change in enthalpy during an endothermic reaction will be the inverse of H, which is the total decrease in enthalpy achieved by heat generation.
If H is positive, the reaction is endothermic, meaning the system absorbs heat because the reaction products have a higher enthalpy than the reactants. A positive change in enthalpy is required to break a bond, whereas a negative change in enthalpy is required to form a bond.
Ques. What is the importance of Enthalpy? (3 marks)
Ans. The following equation can be used to calculate the thermodynamic system enthalpy, denoted by H:
H = PV + U
Where U is the system's inner energy, P is the pressure, and V is the volume. According to the second law of thermodynamics, the entropy of an isolated system can never decrease, i.e. the entropy change of an isolated system can only be positive or zero. As a result, a mechanism is only possible when the universe's entropy shift is positive. This implies that any strategy that does not violate the second thermodynamic law will increase the universe's entropy.
Ques. What are the standard enthalpy conditions? (2 marks)
Ans. The change in enthalpy when one mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions is known as the standard enthalpy of formation.
Ques. What distinguishes enthalpy from energy? (2 marks)
Ans. Since the latter is the quantity of heat released or absorbed during a change, the terms energy and enthalpy are not interchangeable. Energy refers to a system's total kinetic and potential energy.
Ques. Extensive or intensive: what is the attribute that enthalpy possesses? (2 marks)
Ans. The term "enthalpy" refers to a wide range of properties. The mass of a substance determines the value of enthalpy.
Ques. What is the Real-World Application of Enthalpy? (3 marks)
Ans. Hand warmers and refrigerators can both benefit from the shift in enthalpy. Freon, for instance, is a refrigerant that evaporates in a refrigerator. The enthalpy of vaporisation equals the coldness of the food in this process.
Ques. What factors influence enthalpy? (3 marks)
Ans. Enthalpy is a broad concept that is influenced by the amount of material we work with. The enthalpy value of a system is influenced by the state of reactants and products (solid, liquid, or gas). The enthalpy value is affected by the reaction's direction.
Ques. What happens when enthalpy is increased? (2 marks)
Ans. The enthalpy of a system controls whether it can undergo a reaction by increasing (i.e. when energy is added) or decreasing (i.e. when energy is released).... The quantity of energy lost or gained during a reaction is generally comparable to its enthalpy change.
Ques. What does the change in enthalpy tell you? (2 marks)
Ans. The difference between the energy spent to break bonds in a chemical reaction and the energy acquired by the production of new chemical bonds in the process is called an enthalpy change. It describes a system's energy change at constant pressure. ΔH stands for enthalpy change.
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