Heat & Enthalpy Changes

Objectives:

1.    Define the terms enthalpy and enthalpy changes.

2.    Explain what information is given by the standard enthalpy change of a reaction.

Key Terms:

enthalpy         standard enthalpy change

Notes:  (12-2)

As we have seen in the previous chapters, mass is always conserved in a stoichiometrically balanced reaction.  But what about energy?  If a reaction gives off heat by forming bonds it is considered exothermic and if it absorbs heat energy by breaking bonds it is considered endothermic.  The difference in the energies related to reactants and products along with the conditions of the reaction is called enthalpy

 Enthalpy is represented by the capital letter H involves pressure, volume and temperature takes into account all of the kinetic and potential energies of the reactants and products at constant temperature (25oC) and pressure (1atm) Denthalpy (DH)= Denergy (DJ)

Interpreting enthalpy equations

C(s) + H2O(l) + 113kJ  g   CO(g) + H2(g)        DHo = + 113kJ

In this reaction 113kJ of heat energy is absorbed in the endothermic reaction.  Since the products have more energy than the reactants the products are said to have undergone positive enthalpy (a positive gain in chemical potential energy).

 endothermic reactions result in positive enthalpy (+H)

(www.bcpl.net/~kdrews/drivingforces/ endothermic.gif)

C3H8(g) + 5O2(g)   g   3CO2(g) + H2O(g) + 2043kJ            DHo = - 2043kJ

 exothermic reactions result in negative enthalpy  (-H)

(www.bcpl.net/~kdrews/drivingforces/ endothermic.gif)

Standard Enthalpy Change

Although many elements come in different forms, there is usually one form that is more stable than another.  Take the example of carbon.  Carbon occurs naturally as coal, graphite and as a diamond.  All are exclusively carbon but the diamond is by far the most stable.

 Standard enthalpy change is denoted by the symbol DHo Represents elements in their most stable form Values are given for 1 mole of a given substance

Solving stoichiometry problems using enthalpy do not differ from what we learned last chapter.  Consider this question.

C3H8(g) + 5O2(g)   g   3CO2(g) + H2O(g) + 2043kJ            DHo = - 2043kJ

If you begin with 2.5 moles of oxygen, how much heat is produced in the reaction?  Since you start out with 1/2 the quantity of O2 required for the reaction, the heat produced is also halved. (1021.5kJ)