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Heat of Reaction

Last updated date: 26th Feb 2024
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What is Heat of Reaction?

Heat of Reaction Definition - The quantity of heat that must be provided or removed during a chemical reaction in order to maintain the same temperature for all of the components present. If the pressure in the vessel containing the reacting system remains constant, the measured heat of reaction also represents the change in the thermodynamic quantity known as enthalpy or heat content, that occurs throughout the reaction—that is, the difference between the enthalpy of the substances present at the end of the reaction and the enthalpy of the substances present at the start of the reaction. 

As a result, the enthalpy of reaction, denoted by the symbol, is also known as the heat of reaction measured under constant pressure.

This page will discuss heat of reaction, heat definition chemistry, enthalpy of reaction and symbol for heat.

Enthalpy of Reaction

The ability to predict and measure the heat effects that occur as a result of chemical changes is critical to the understanding and application of chemical reactions. The heat effect that follows the transformation may be indicated by an increase or decrease in temperature, as the case may be, of the substances present if the vessel containing the reacting system is so insulated that no heat flows into or out of the system (adiabatic condition). For the right design of equipment for use in chemical processes, accurate temperatures of reactions values are required.

Symbol for Heat

The change in energy (ΔU) equals the sum of heat produced and work done. The pressure-volume work performed by an expanding gas is known as pressure-volume work (or just PV work). Consider the reaction of dissolving a piece of copper in concentrated nitric acid, which generates a gas. This reaction's chemical equation is as follows:

Cu(s) + 4HNO3(aq) → Cu(NO3)2(aq) + 2H2O(l) + 2NO2(g)

Heat of Reaction Formula

The reaction takes place in a closed system with a moveable piston that keeps the pressure constant. As nitrogen dioxide gas is generated, the piston will rise. By elevating the piston against the downward force exerted by the atmosphere, the mechanism accomplishes its task (i.e., atmospheric pressure). By multiplying the external pressure P by the volume change induced by piston move), we can calculate the amount of PV work done. When external pressure is constant (here, atmospheric pressure),

w = −PΔV

The sum of all of a system's components' kinetic and potential energy is the system's internal energy U. Heat and work are produced by a change in internal energy. Chemists commonly employ a related thermodynamic number termed enthalpy ( H ) (from the Greek enthalpy, meaning "to warm") to measure the energy changes that occur in chemical reactions. The sum of a system's internal energy U plus the product of its pressure P and volume V is known as its enthalpy:

H = U + PV

When a chemical change occurs at constant pressure (i.e., for a given P, P=0 ), the change in enthalpy (Δ H ) is proportional to the change in pressure.

ΔH = Δ(U + PV)

= ΔU + ΔPV

= ΔU + PΔV


= qp + w − w

= qp

This equation is only true for a process that happens at constant pressure, as indicated by the subscript p. We can see from Equation that the change in enthalpy, H of the system, is equal to the heat received or lost at constant pressure.

ΔH = ΔHfinal ΔHinitial = qp

Enthalpy of Reaction (Exothermic and Endothermic)

Because enthalpy is a state function, its magnitude is determined only by the system's beginning and ultimate states, rather than the path travelled. Most importantly, even if the process does not take place under constant pressure, the enthalpy change is the same.

Measure qp to find H for a reaction.

When studying energy changes in chemical reactions, the enthalpy of reaction (Hreaction ), or the change in enthalpy that occurs during a reaction, is usually the most essential number (such as the dissolution of a piece of copper in nitric acid). The enthalpy of a system decreases when heat flows from it to its surroundings, hence Hrxn is negative. When heat is transferred from the environment to a system, the enthalpy of the system increases, and Hrxn becomes positive. thus

ΔHreaction<0  for an exothermic reaction, and

ΔHreaction>0 for an endothermic reaction.

When Hrxn is negative, the enthalpy of the products is smaller than that of the reactants, implying that an exothermic process is energetically downward. If Hreaction is positive, the enthalpy of the products is larger than the enthalpy of the reactants, resulting in an endothermic reaction that is energetically uphill. The following discussion summarises two important enthalpy properties and changes in enthalpy.

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Standard Heat of Reaction

When one mole of a compound is generated from its constituent elements at 25° C (77° F) and one atmospheric pressure, the standard heat of formation is defined as the amount of heat absorbed or evolved when each material is in its usual physical state (gas, liquid, or solid). An element's heat of creation is arbitrarily assigned a value of zero. The quantity of heat released when one mole of a substance is burned in excess oxygen at 25° C and one atmospheric pressure is known as the standard heat of combustion. The approach known as Hess's law of heat summation is used to calculate heats of reactions using recorded values of temperatures of production and combustion.

Did You Know?

To report the heat received or emitted, a vast set of reference tables listing the enthalpy changes for all potential chemical reactions would be required, which would take an enormous amount of time and work. Fortunately, because enthalpy is a state function, we only need to know the reaction's start and ultimate states. This allows us to compute the enthalpy change for almost any chemical reaction with a modest amount of tabulated data, such as the following:

Enthalpy of Combustion - During a combustion reaction, there is a change in enthalpy. For the combustion of practically any chemical that will burn in oxygen, enthalpy changes have been recorded; these values are commonly reported as the enthalpy of combustion per mole of a substance.

Hfus is the Fusion Enthalpy - The enthalpy change that occurs when 1 mole of a substance melts (fuses). The enthalpy change occurs when 1 mole of a substance melts or fuses; these values have been measured for nearly all elements and most simple compounds.

Hvap (Enthalpy of Vaporisation) - The enthalpy shift that occurs when 1 mole of a material is vaporised. Enthalpy of solution: The enthalpy change that occurs when 1 mole of a substance is vaporised; these values have been the Enthalpy of solution. It is measured for almost all elements and most volatile substances.

The change in enthalpy that happens when a particular amount of solute dissolves in a defined amount of solvent (Hsolution). When a given amount of solute dissolves in a given amount of solvent, the enthalpy changes.

FAQs on Heat of Reaction

1. Does the Heat of Reaction Change?

Ans. The change in the enthalpy of a chemical reaction that occurs at constant pressure is known as the Heat of Reaction (also known as Enthalpy of Reaction). It is a thermodynamic unit of measurement that can be used to calculate the amount of energy released or created per mole in a reaction.

2. Is the Heat of Reaction Positive or Negative?

Ans. When the solution absorbs heat, q for the solution becomes positive. This indicates that the reaction generates heat for the solution to absorb, and the reaction's q is negative. When heat is absorbed from a solution, the solution's q value becomes negative.

3. Why is the Heat of Reaction Important?

Ans. The heat of reaction, also known as reaction enthalpy, is a critical component for scaling up chemical processes safely and successfully. When chemicals are converted, in a chemical reaction, the heat of reaction is the energy that is emitted or absorbed.