Thermochemistry, Hess’s Law and Driving Forces Review p Define: temperature, heat, specific heat, calorie and joule, exothermic, endothermic, enthalpy, Hess’s law, entropy and free energy

Some review terms for this unit: TTTTemperature HHHHeat SSSSpecific heat capacity AAAA measure of the average kinetic energy of random motion of the particles in a sample of matter TTTThe transfer of energy from one system at high temperature to another system at low temperatures TTTThe amount of heat energy required to increase the temperature of one gram of a substance by one degree Celsius

Distinguishing endo and exo-  Exothermic rx.:  Release energy  Has a –ΔH value  Has a smaller value with a decreasing slope on an enthalpy diagram  Has + heat as a product in a rx.  Endothermic rx.:  Absorbs energy  Has a +  H value  Has a larger enthalpy value after the rx.  Has + heat as a reactant

Enthalpy:  The amount of energy absorbed by a system as heat during a process at constant pressure   H = H products – H reactants  Thermochemical equation:  H 2(g) + ½ O 2(g)  H 2 O (g) kJ  * notice that heat is a product = exothermic  Most enthalpy of formation reactions are exothermic. Compounds produced with a large negative enthalpy are stable  Elements in their standard states are defined as  H = 0  Compounds produced with a high positive enthalpy change are relatively unstable. They can react violently.

Hess’s Law: the overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process.  Use Hess’s law to calculate the enthalpy of reaction for the combustion of nitrogen monoxide gas, NO, to form nitrogen dioxide gas, NO 2, as given in the following thermochemical equation: NO (g) + ½ O 2(g)  NO 2(g) NO (g)  ½ N 2(g) = ½ O 2(g)  H= kJ ½ N 2(g) + O 2(g)  NO 2  H=  H = -57.1

Driving Forces:  The change in energy is one of two factors that allow chemists to predict whether a reaction will occur spontaneously and to explain how it occurs. The randomness of the particles or the disorder termed entropy is the second.  Entropy is an actual measure of the degree of randomness in a system,  S  There is a tendency in nature towards more entropy or randomness  So, processes in nature are driven:  toward least enthalpy and  toward largest entropy.

Free energy  This combination enthalpy-entropy function is called free energy, G, of a system.   G =  H - T  S  The H and S can be positive or negative, so there are four possible combinations of terms:  1. -H (exo) and +S (more random) = always – G (spontaneous)  2. -H (exo) and –S (less random) = - G at low temperature  3. +H (endo) and +S (more random) = - G at high pressure  4. +H (endo) and –S (less random) = never negative or spontaneous