enthalpy of reaction

Examples of enthalpy of reaction in the following topics:

• Standard Enthalpy of Reaction

  • The standard enthalpy of reaction is the enthalpy change that occurs in a system when a chemical reaction transforms one mole of matter under standard conditions.
  • The standard enthalpy of reaction, $\Delta H^\ominus _{rxn}$, is the change in enthalpy for a given reaction calculated from the standard enthalpies of formation for all reactants and products.
  • Calculate the standard enthalpy of reaction for the combustion of methane:
  • In order to calculate the standard enthalpy of reaction, we need to look up the standard enthalpies of formation for each of the reactants and products involved in the reaction.
  • A calculation of standard enthalpy of reaction (∆H°rxn) from standard heats of formation (∆H°f)

• Internal Energy and Enthalpy

  • The enthalpy of reaction measures the heat released/absorbed by a reaction that occurs at constant pressure.
  • The enthalpy of reaction is defined as the internal energy of the reaction system, plus the product of pressure and volume.
  • Most often, we are interested in the change in enthalpy of a given reaction, which can be expressed as follows:
  • We will examine the change in enthalpy for a reaction at constant pressure, in order to see why enthalpy is such a useful concept for chemists.
  • Due to this relation, the change in enthalpy is often referred to simply as the "heat of reaction."

• Change in Enthalpy

  • Since the enthalpy of a system cannot be directly measured, we often concern ourselves with the change in enthalpy after a reaction has taken place.
  • By determining the value of the change of enthalpy, you can determine whether the reaction is endothermic (positive change) or exothermic (negative change).
  • Hess's law states that the standard enthalpy change of the overall reaction is the sum of the enthalpy change of all the intermediate reactions that make up the overall reaction.
  • By remembering and employing Hess's Law, the change in enthalpy for the overall reaction can be determined by adding up the enthalpies of the intermediate reactions.
  • We discuss where the energy in chemical bonds comes from in terms of internal energy and enthalpy, as well as how to approximate the overall heat of reaction using bond enthalpies.

• 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.
  • In contrast, reactions with negative enthalpies—those that release heat into their surroundings—are known as exothermic.
  • A diagram of the reaction coordinate for an exothermic reaction is shown in .
  • Evaluate the effect of temperature on the equilibrium state of a chemical reaction

• 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.
  • In exothermic reactions, the products have less enthalpy than the reactants, and as a result, an exothermic reaction is said to have a negative enthalpy of reaction.
  • In endothermic reactions, the products have more enthalpy than the reactants.
  • Thus, an endothermic reaction is said to have a positive enthalpy of reaction.

• Thermochemical Equations

  • Thermochemical equations are chemical equations which include the enthalpy change of the reaction, $\Delta H_{rxn}$ .
  • Enthalpy (H) is a measure of the energy in a system, and the change in enthalpy is denoted by $\Delta H$.
  • Since enthalpy is a state function, the value of $\Delta H$ is independent of the path taken by the reactions to reach the products.
  • 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.
  • Notice that here, we can think of heat as being a product in the reaction.

• Hess's Law

  • Hess's Law sums the changes in enthalpy for a series of intermediate reaction steps to find the overall change in enthalpy for a reaction.
  • This law states that if a reaction takes place in several steps, then the standard reaction enthalpy for the overall reaction is equal to the sum of the standard enthalpies of the intermediate reaction steps, assuming each step takes place at the same temperature.
  • Since enthalpy is a state function, the change in enthalpy between products and reactants in a chemical system is independent of the pathway taken from the initial to the final state of the system.
  • First it looks at combining reactions according to Hess's law and their heats of reaction, and then it discusses using standard heats of formation of the reactants and products to find the overall heat of reaction.
  • By Hess's law, the net change in enthalpy of the overall reaction is equal to the sum of the changes in enthalpy for each intermediate transformation: ΔH = ΔH1+ΔH2+ΔH3.

• Exothermic and Endothermic Processes

  • We will explore these concepts in more detail after introducing the concept of enthalpy.
  • Enthalpy (signified as H) is a measure of the total energy of a system and often expresses and simplifies energy transfer between systems.
  • Since the total enthalpy of a system cannot be measured directly, we most often refer to the change in enthalpy for a particular chemical reaction.
  • Due to this relation, the change in enthalpy, $\Delta H$, is often referred to as the "heat of reaction."
  • In an exothermic reaction, the total energy of the products is less than the total energy of the reactants.

• Bond Enthalpy

  • The total enthalpy, H, of a system cannot be measured directly.
  • The change ($\Delta H$) is positive in endothermic reactions because the products of the reaction have a greater enthalpy than the reactants, and heat is absorbed by the system from its surroundings.
  • Generally, a positive change in enthalpy is required to break a bond, while a negative change in enthalpy is accompanied by the formation of a bond.
  • Bond enthalpy, also known as bond dissociation energy, is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K (absolute zero).
  • Describe the changes in enthalpy accompanying the breaking or formation of a bond

• Standard States and Standard Enthalpy Changes

  • The standard enthalpy of formation refers to the enthalpy change when one mole of a compound is formed from its elements.
  • The standard enthalpy of formation, or standard heat of formation, of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements in their standard states.
  • For example, the standard enthalpy of formation for carbon dioxide would be the change in enthalpy for the following reaction:
  • Note that standard enthalpies of formation are always given in units of kJ/mol of the compound formed.
  • Graphite is the most stable state of carbon and is used in thermochemistry to define the heat of formation of carbon compounds.