AP Chemistry 2019 Review Study Guide
Goal of the chapter is to determine if a reaction is spontaneous or not
Review of Terms
System
Part under study- the reaction
Surrounding
Everything else: the reaction flask, the atmosphere
Enthalpy Change(ΔH)
Heat gained or lost by the reaction
Exothermic: reaction that gives off heat The heat goes from the system into the surrounding
ΔH is negative
Endothermic:heat is being gained by the reaction; heat is acquired, no longer cool, now its hot ;) ohhh yaaaaaaaaaaaaaaaaaa
The heat goes from the surrounding into the system
ΔH is positive
First Law of Thermodynamics
Energy can’t be created or destroyed, it can only be eaten, yummy
Spontaneous Process
A change that occurs without outside help
Example: Opening a bottle of perfume, the fragrance will mix with the air molecules and spreads without doing anything
Example dos: Placing a piece of metal into cold water, heat spontaneously goes from the metal to the water
Non spontaneous process
A change that only proceeds with outside help
Example: Crack an egg into a frying pan, nothing will happen to the egg until you apply heat and start to cook. THe egg then becomes yummy mmmmm
Entropy(S)
Study of spontaneity
A measure of how energy disperses from a localized state to
Second Law of Thermodynamics
All processes that occur spontaneously move in a direction of increasing entropy
ΔS is greater than 0
Energy goes from being concentrated to being more dispersed
Predictions about Entropy
Entropy increases with an increase in temperature, adding heat (q)
Entropy increases in the conversion of a liquid to a gas
Entropy increases in the conversion of a solid to a liquid
Entropy increases when the number of molecules increases during a reaction
Example:
NH4Cl(s) → NH3 (g) + HCl(g) (increase)
1 mole 2 moles (1+1)
Knowing the change in entropy (Delta S) for a reaction can be important, when delta S is positive the reaction has a greater likelihood of being spontaneous
Formula: Delta Srxw = Delta SProducts - Delta S Reaction
Example:
C3H8 + 5O2 -> 3CO2 + 4 H2O
If the change is positive, entropy is increases, if negative it is decreasing
GIbbs Free energy
A method used to determine whether a process is spontaneous based on two values
Delta S and Delta H
Gibbs free energy ties these two values into one equation
Delta G = Delta H - T Delta S
ORRRRRRR
Delta GReaction = Delta GProducts - Delta GReactants
Some things:
Delta H is the enthalpy(heat) of reaction
Delta S is the Entropy amount
Delta G is the free energy of reaction
T is the temperature in Kelvin
Delta G < 0 reaction is spontaneous
Delta G = 0 the reaction is at equilibrium
Delta G > 0 the reaction is not spontaneous
Predicting Spontaneity
Delta H prefers to be negative (exothermic)
Delta S prefers to be positive (energy disperses)
Hax (basically want Delta G to be negative)
A negative Delta H(Enthalpy) and Positive Delta S(Entropy) means always spontaneous
A positive Delta H(Enthalpy) and negative Delta S(Entropy) means never spontaneous
A negative Delta H(Enthalpy) and negative Delta S(Entropy) means yes spontaneous if there is a low temperature
A positive Delta H(Enthalpy) and Positive Delta S(Entropy) means yes spontaneous if there is a high temperature