heat of reaction

(noun)

The enthalpy change in a chemical reaction; the amount of heat that a systems gives up to its surroundings so it can return to its initial temperature.

Related Terms

Examples of heat of reaction in the following topics:

  • Exothermic and Endothermic Processes

    • At constant pressure, the change in enthalpy is equal to the heat given off, or the heat absorbed, in a given chemical reaction:
    • Due to this relation, the change in enthalpy, $\Delta H$, is often referred to as the "heat of reaction."
    • Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light.
    • In the presence of water, a strong acid will dissociate quickly and release heat, so it is an exothermic reaction.
    • Endothermic reactions are reactions that require external energy, usually in the form of heat, for the reaction to proceed.

  • Constant-Volume Calorimetry

    • Constant-volume calorimeters, such as bomb calorimeters, are used to measure the heat of combustion of a reaction.
    • Bomb calorimetry is used to measure the heat that a reaction absorbs or releases, and is practically used to measure the calorie content of food.
    • A bomb calorimeter is a type of constant-volume calorimeter used to measure a particular reaction's heat of combustion.
    • The total heat given off in the reaction will be equal to the heat gained by the water and the calorimeter:
    • From the change in temperature, the heat of reaction can be calculated.

  • Internal Energy and Enthalpy

    • The enthalpy of reaction measures the heat released/absorbed by a reaction that occurs at constant pressure.
    • The first law of thermodynamics states that the energy of a closed system is equal to the amount of heat supplied to the system minus the amount of work done by the system on its surroundings.
    • In this equation, U is the total energy of the system, Q is heat, and W is work.
    • The enthalpy of reaction is defined as the internal energy of the reaction system, plus the product of pressure and volume.
    • Due to this relation, the change in enthalpy is often referred to simply as the "heat of reaction."

  • Changes in Temperature

    • Reactions can be classified by their enthalpies of reaction.
    • Reactions with positive enthalpies—those that absorb heat from their surroundings—are known as endothermic.
    • A diagram of the reaction coordinate for an exothermic reaction is shown in .
    • Applied to temperature, Le Chatelier's Principle predicts that the addition of heat to a system will cause an opposing reaction in the system to remove heat.
    • Endothermic reactions, on the other hand, will be shifted towards product formation as heat is removed from the reaction's surrounding environment.

  • Calorimetry

    • Calorimetry is the measurement of the heat of chemical reactions or physical changes.
    • Calorimetry is the science of measuring the heat of chemical reactions or physical changes.
    • Multiplying the temperature change by the mass and specific heat capacities of the substances gives a value for the energy given off or absorbed during the reaction:
    • Dividing the energy change by how many grams (or moles) of A were present gives its enthalpy change of reaction.
    • The inner cup holds a known amount of a solute, usually water, that absorbs the heat from the reaction.

  • Solving Problems with Calorimetry

    • Calorimetry is used to measure the amount of heat produced or consumed in a chemical reaction.
    • When an endothermic reaction occurs, the heat required is absorbed from the thermal energy of the solution, which decreases its temperature.
    • A different type of calorimeter that operates at constant volume, colloquially known as a bomb calorimeter, is used to measure the energy produced by reactions that yield large amounts of heat and gaseous products, such as combustion reactions.
    • The temperature increase is measured and, along with the known heat capacity of the calorimeter, is used to calculate the energy produced by the reaction.
    • The temperature change produced by the known reaction is used to determine the heat capacity of the calorimeter.

  • Energy Changes in Chemical Reactions

    • By the Law of Conservation of Energy, however, we know that the total energy of a system must remain unchanged, and that oftentimes a chemical reaction will absorb or release energy in the form of heat, light, or both.
    • This stored chemical energy, or heat content, of the system is known as its enthalpy.
    • Exothermic reactions release heat and light into their surroundings.
    • Excess energy from the reaction is released as heat and light.
    • Significant heat energy is needed for this reaction to proceed, so the reaction is endothermic.

  • Thermochemical Equations

    • Since enthalpy is a state function, the value of $\Delta H$ is independent of the path taken by the reactions to reach the products.
    • In an endothermic system, the $\Delta H$ value is positive, so the reaction absorbs heat into the system.
    • Notice that in an endothermic reaction like the one depicted above, we can think of heat as being a reactant, just like A and B.
    • In an exothermic system, the $\Delta H$ value is negative, so heat is given off by the reaction.
    • Notice that here, we can think of heat as being a product in the reaction.

  • Activation Energy

    • The source of the activation energy needed to push reactions forward is typically heat energy from the surroundings.
    • Heat energy (the total bond energy of reactants or products in a chemical reaction) speeds up the motion of molecules, increasing the frequency and force with which they collide.
    • Like these reactions outside of cells, the activation energy for most cellular reactions is too high for heat energy to overcome at efficient rates.
    • If cellular temperatures alone provided enough heat energy for these exergonic reactions to overcome their activation barriers, the essential components of a cell would disintegrate.
    • This figure implies that the activation energy is in the form of heat energy.

  • The First Law of Thermodynamics

    • Conversely, in an exothermic reaction, the heat that is released in the reaction is given off and absorbed by the surroundings.
    • We know that chemical systems can either absorb heat from their surroundings, if the reaction is endothermic, or release heat to their surroundings, if the reaction is exothermic.
    • However, chemical reactions are often used to do work instead of just exchanging heat.
    • If you've ever witnessed a video of a space shuttle lifting off, the chemical reaction that occurs also releases tremendous amounts of heat and light.
    • Another useful form of the first law of thermodynamics relates heat and work for the change in energy of the internal system: