enthalpy change

Examples of enthalpy change in the following topics:

• Change in Enthalpy

  • Enthalpy changes are associated with chemical processes, and are important in understanding how the reaction has changed a chemical system.
  • 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).
  • Since enthalpy is a state function, or pathway independent, the route that the reaction takes does not change the enthalpy value.
  • 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.

• 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.
  • A thermochemical equation is a balanced stoichiometric chemical equation which includes the enthalpy change.

• Bond Enthalpy

  • Bond enthalpy is defined as the enthalpy change when a covalent bond is cleaved by homolysis.
  • Thus, the change in enthalpy, $\Delta H$, is a more useful quantity than its absolute value.
  • The change in enthalpy is negative in exothermic processes, because energy is released from the system into 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.
  • 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.

• 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.
  • Negative enthalpy change for a reaction indicates exothermic process, while positive enthalpy change corresponds to endothermic process.
  • The change in enthalpy for this reaction cannot be determined experimentally.
  • However, because we know the standard enthalpy change for the oxidation for these two substances, it is possible to calculate the enthalpy change for this reaction using Hess's law.
  • 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.

• 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.
  • The change in enthalpy does not depend upon the particular pathway of a reaction, but only upon the overall energy level of the products and reactants; enthalpy is a state function, and as such, it is additive.
  • In order to calculate the standard enthalpy of a reaction, we can sum up the standard enthalpies of formation of the reactants and subtract this from the sum of the standard enthalpies of formation of the products.
  • Next, we sum up our standard enthalpies of formation.

• Exothermic and Endothermic Processes

  • 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."
  • For this reason, the change in enthalpy, $\Delta H$, for an exothermic reaction will always be negative.
  • As such, the change in enthalpy for an endothermic reaction is always positive.
  • Therefore, the change in enthalpy is negative, and heat is released to the surroundings.

• Internal Energy and Enthalpy

  • 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.
  • Let's look once again at the change in enthalpy for a given chemical process.
  • Thus, at constant pressure, the change in enthalpy is simply equal to the heat released/absorbed by the reaction.
  • Due to this relation, the change in enthalpy is often referred to simply as the "heat of reaction."

• Comparison of Enthalpy to Internal Energy

  • Internal energy and enthalpy are both measurements that quantify the amount of energy present in a thermodynamic system.
  • Simply put, enthalpy accounts for heat flow within a system.
  • Sometimes, measuring the internal energy of a system may be an inaccurate gauge of the change in energy.
  • So, even if the heat change can be measured, $\Delta U\neq q$ because some work has been performed.
  • Therefore, to account for both the possible volume change at constant pressure and the internal energy, enthalpy is used.

• Energy Changes in Chemical Reactions

  • 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.
  • Describe the types of energy changes that can occur in chemical reactions