What Is Heat of Reaction Definition Formula Types and Calculation
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
ΔH = 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 in Thermochemistry
1. What is heat of reaction in chemistry?
Heat of reaction is the amount of heat energy absorbed or released during a chemical reaction at constant pressure. It is also called the enthalpy change (ΔH) of the reaction.
- If heat is released, ΔH is negative (exothermic reaction).
- If heat is absorbed, ΔH is positive (endothermic reaction).
- It is usually expressed in kJ/mol.
2. What is the formula for heat of reaction?
The formula for heat of reaction at constant pressure is ΔH = Hproducts − Hreactants.
- ΔH = enthalpy change of the reaction
- Hproducts = total enthalpy of products
- Hreactants = total enthalpy of reactants
3. What is the difference between exothermic and endothermic reactions?
The difference is that exothermic reactions release heat (ΔH < 0), while endothermic reactions absorb heat (ΔH > 0).
- Exothermic example: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
- Endothermic example: CaCO3(s) → CaO(s) + CO2(g)
- Exothermic reactions feel warm; endothermic reactions feel cold.
4. How do you calculate heat of reaction using Hess's Law?
You calculate the heat of reaction using Hess's Law by adding enthalpy changes of individual steps to get the overall ΔH.
- Write balanced equations for each step.
- Reverse equations if needed (change the sign of ΔH).
- Multiply equations if needed (multiply ΔH accordingly).
- Add all equations and their ΔH values.
5. What is the standard heat of reaction (ΔH°)?
Standard heat of reaction (ΔH°) is the enthalpy change when a reaction occurs under standard conditions (1 atm pressure and 298 K).
- All reactants and products must be in their standard states.
- It is expressed in kJ/mol.
- Often calculated using standard enthalpies of formation.
6. How do you calculate heat of reaction from standard enthalpies of formation?
The heat of reaction from standard enthalpies of formation is calculated using ΔH°reaction = ΣΔH°f(products) − ΣΔH°f(reactants).
- Multiply each ΔH°f value by its stoichiometric coefficient.
- Add values for products.
- Subtract values for reactants.
7. What are the units of heat of reaction?
The most common units of heat of reaction are kilojoules per mole (kJ/mol).
- kJ/mol expresses energy change per mole of reaction.
- In calorimetry, heat may also be measured in joules (J) or kilojoules (kJ).
- 1 kJ = 1000 J.
8. How is heat of reaction measured experimentally?
Heat of reaction is measured experimentally using a calorimeter to determine the temperature change during a reaction.
- Measure initial and final temperatures.
- Use the formula q = mcΔT, where m = mass, c = specific heat capacity, and ΔT = temperature change.
- Relate heat (q) to moles to find ΔH in kJ/mol.
9. Why is heat of reaction important in chemistry?
Heat of reaction is important because it shows whether a reaction releases or absorbs energy and helps predict reaction behavior.
- Determines if a reaction is exothermic or endothermic.
- Used in designing industrial processes (e.g., combustion, fertilizer production).
- Helps assess energy efficiency and safety.
10. Can you give an example of calculating heat of reaction?
Yes, the heat of reaction can be calculated using standard enthalpies of formation for a balanced equation.
- Reaction: C(s) + O2(g) → CO2(g)
- ΔH°f[CO2(g)] = −393.5 kJ/mol
- ΔH°f[C(s)] = 0, ΔH°f[O2(g)] = 0
- ΔH°reaction = (−393.5) − (0 + 0) = −393.5 kJ/mol
