the 'c' changes to 'zc' if the conjugation gives the stem an 'a' or an 'o'

aborrecer - to abhor, to loathe
adolecer de - to suffer from
adormecerse - to nod off to sleep {student in library who didn't sleep night before}
agradecer - to thank
amanecer - to dawn
compareceer - to appear
aparecer - to appear, show up
crecer - to grow
decrecer - to decrease, to decline
desaparecer - to disappear
desfallecer - to be exhausted
desmerecer - to be unworthy, to lose value
desvanecer - to dissipate, to dispel
embravecer - to enrage
enaltecer - to praise
encallecer - to become calloused, to become hard
encanecer - to go grey
endurecer - to harden, to strengthen
engrandecer - to exalt, to increase, to enlarge
enloquecer - to drive mad
enmudecer - to silence
enrarecer - to pollute
enriquecer - to bring wealth to
enrojecer - to redden
fallecer - to die, to pass away
entorpecer - to hinder (movement)
entristecer - to make sad
envilecer - to debase
fortalecer - to strengthen
escarnecer - to mock, to ridicule
esclarecer - to clear up, to shed some light on
estremecer - to shake
fenecer - to pass away, to die
florecer - to flower, to flourish
guarecer - to protect
humedecir - to moisten
oscurecer - to darken, to overshadow
palidecer - to turn pale
prevalecer - to prevail
conocer - to know
lucir - to shine