AP Chem unit 6: thermochem

in a thermodynamically favorable process, a system…

will end in a lower energy state than which it started

first law of thermodynamics

energy states cannot be created or destroyed

exothermic

heat is released from system to surroundings

endothermic

heat is absorbed by the system from surroundings

exothermic processes have a … change in enthalpy

favorable

endothermic processes have a … change in enthalpy

unfavorable

enthalpy

total heat content in system

forming bonds or IMFs is …

exothermic

breaking bonds or IMFs is …

endothermic

if more energy is absorbed to break bonds than is released by forming bonds, reaction is …

endothermic, absorbs heat, H>0

if more energy is released by forming bonds than absorbed to break bonds, reaction is …

exothermic and releases heat, H<0

A catalyst creates less …

activation energy, so activated complex is shorter (aka graph’s bump is lower)

three methods to calculate ΔH

enthalpies of formation, bond enthalpies, hess’s law

enthalpy of formation is the change …

in energy that takes place when ONE mole of a compound is formed (ONE mole of product) from its pure elements (standard conditions)

substances with … are more stable than their elements and the formation is…

ΔH<0, exothermic (releases energy)

substances with … are less stable than their elements and their formation is …

ΔH>0, endothermic (absorbs energy)

combustion is always an … process

exothermic

… specifically describes the amount of energy released when ONE mole of a compound combusts

enthalpy of combustion

breaking a bond is a … process

endothermic

bond energy is always

a pos number

3 steps of the dissolution of a salt (enthalpy of solution)

dont happen in this order necessarily:

1. ionic bonds holding solute together must be broken - endothermic (ΔH>0)

2. solvent molecules must be separated to make room for the solute - endothermic (ΔH>0)

3. ion dipole forces form between the separated ions and water - exothermic (ΔH<0)

hydration energy

the amount of heat energy released when one mole of gaseous ions is dissolved in water, becoming surrounded by water molecules. this is always negative

magnitude of lattice energy > hydration energy

dissolution is endothermic, more energy is required to break the ionic bonds and stretch IMFs than released by forming new attractions between solute and solvent.

magnitude of lattice energy < hydration energy

dissolution is exothermic, more energy is required to form new attractions between solute and solvent than is to break the ionic bonds and stretch IMFs

solid to liquid

melting or fusion

liquid to solid

freezing

liquid to gas

vaporization

solid to gas

sublimation

gas to solid

deposition

IMFs are strongest in

solid phase

large specific heat

can absorb a lot of heat without undergoing much of a temp change

low specific heat

experiences big temp changes aka cant absorb a lot of heat without going a temp change

ΔH… is always greater than ΔH… because IMFs must only be stretched for a substance to … but completly broken for a substance to …

vaporization, fusion | melt, vaporize

substances with lower specific heats have … slopes because they require less heat to change temp (heating curve)

greater

specific heat is … of slope

reciprocal