AP Chemistry
th
Zumdahl Notes, 9 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