AP Chemistry Zumdahl Notes, 9th ed. A Brief Collection of notes, Chapter 6 Feel free to open these files and annotate as you feel the need…this is for your success.

Thermochemistry: the Nature of Energy Energy: the capacity to do work or to produce heat Law of Conservation of Energy: energy can be converted from one form to another but can be neither created nor destroyed Potential energy: due to position or composition (chem. Bonds) Kinetic energy: due to the motion of the object; KE = ½ mv2 Temperature: a measure of the avg. KE of something Heat: the transfer of energy, involving a change in temperature (measurable!) Work: force applied over a change in distance

Thermochemistry: the Nature of Energy, cont. State function or property: a function or property dependent upon the state of the system at a specified time Example: boiling point at a given altitude/location Energy is a state function, but heat or work are not

Thermochemistry: the Nature of Energy: chemical Energy System: that which we are focused on Surrounding: everything around the system Exothermic: energy flows out of the system Endothermic: energy flows into the system Reality: energy flowing in/out must equal put in/out…energy is conserved! Thermodynamics: the study of energy and its movement First law: the energy of the universe is constant ΔE = q + w change in energy = heat and work done If ΔE<0, exothermic; if ΔE>0, endothermic Almost always, if not specified, take view of system, not surroundings! May have changes in energy, work, or both, so pay attention to details! w= -PΔv; p always refers to external pressure, that causing a compression/resists an expansion

Thermochemistry: enthalpy and calorimetry Enthalpy: a state function, whose symbol is H, means the sum of the internal energy of a system and the product of pressure and volume H = E + PV Caveat: at constant pressure, H = q ΔH = Hproducts –Hreactants ΔH = ΔE + PΔV Calorimetry: the science/study of measuring heat Heat capacity: C= heat absorbed/increase in temperature Specific heat capacity: J/g°C or J/gK Could be molar, so J/°C or J/K Typically done as constant pressure (atmospheric) so no work done! Q = smΔT s is specific heat, m is mass…if delta T (+), q (+), endothermic! If done as constant volume, still no work done!

Thermochemistry: Hess’s Law Hess’s Law: the total enthalpy change of a reaction is the sum of the enthalpy changes for the steps which can make up the overall reaction; this means that you can solve for either the total enthalpy change or that of a single step, providing you have the data for the other steps Characteristics of enthalpy changes If a reaction is reversed, the sign of ΔH is also reversed The magnitude of ΔH is directly proportional to the quantities of reactants and products in a reaction; if the coefficients in a balanced reaction are multiplied by an integer, the value of ΔH is multiplied by the same integer

Thermochemistry: Std. enthalpies of formation Std. enthalpy of formation: the change in enthalpy that arises from the formation of one mole of a given compound from its componenet elements in their standard states; ΔH° Conventions for compounds: The std. state of a gaseous substance is that pressure = 1 atm For a pure substance that is a liquid or solid, that is its std state For a substance present in a solution, the std state is 1. molar Conventions for elements: The std state of an element is the form at has at 1 atm and 25°C ΔH°reaction = Σ npΔH°(products) – Σ nrΔH°(reactants) (n is moles)

Thermochemistry: enthalpy calculations