#### chm 121 exam 4 Flashcards

energy

the capacity to do work
can be in the form of radiant, thermal, chemical, nuclear and
potential energy

work (w)

work = -P?V
the result of a force acting through a distance
ex: pushing a box across the floor

heat (q)

the flow of energy caused by a temperature difference
- the exchange of thermal energy

kinetic energy

the energy associated with the motion of an object

thermal energy

the energy associated with with the temperature of an object
- is a type of kinetic energy because it arises from the
motions of atoms or molecules within a substance

chemical energy

the energy associated with the relative positions of electrons and
nuclei in atoms and molecules
- is a form of potential energy

potential energy

the energy associated with the position or composition of an object

law of conservation of energy

energy can be neither created or destroyed

but energy can:- be transferred from one object
to another- assume different forms
the total amount of energy in the universe is constant

system vs surroundings

system - the portion of the universe that is singled out for investigation
surroundings - everything in the universe that exists outside
the system under investigation; everything with which the system can
exchange energy

thermochemistry

the study of heat change in chemical reactions

units of energy

KE = 1/2 mv2
-where mass is in kg and velocity is in m/s
1 joule of energy is the amount of energy needed to move a 1 kg
mass at a speed of 1 m/s
1 joule (J) = 1 kg x m2 / s2

first law of thermodynamics (law of energy conservation)

energy can be converted from one form to another, but cannot be
created nor destroyed
the total energy of the universe is constant; because the universe
does not exchange energy with anything else, its energy content does
not change
- the sum of the energy changes in the system and the surroundings
must be 0

internal energy

the sum of the kinetic and potential energies of all of the particles
that compose the system

state functions

properties that are determined by the state of the system, regardless
of how that condition was achieved; the magnitude of the change
depends ONLY on the initial and final states of the system
- the potential energy of hiker 1 and hiker 2 are the same
because they arrived at the same place, even though they took
different paths

if energy enters the system, ?E will be...

positive, +
- energy flows into the system and out of the surroundings
- reactants have a lower internal energy than the products

if energy leaves the system, ?E will be...

negative, -
- energy flows out of the system and into the surroundings
- reactants have a higher internal energy than the products

internal energy change equations

?E = Efinal - Einitial
?E = Eproducts - Ereactants
?E = q + wwhere q = heat and w = work
?Esys = -?Esurr

heat (q) and work (w) are NOT state functions so...

their value depends on the process

thermal energy always flows from...

matter at higher temperatures to matter at lower temperatures
ex: a cup of hot water transfers heat to its surroundings as it
cools down

thermal equilibrium

when the thermal energy of the system is = to the thermal energy of
the surroundings
- at thermal equilibrium, there is no additional net transfer of heat

heat capacity (C)

the amount of heat (q) required to raise the temperature of a
given quantity (m) of the substance by 1 degree Celsius
q = mC?t
?E = q + w

specific heat capacity (Cs)

the amount of heat required to raise the temperature of 1
gram of the substance by 1 degree celsius
units = J/g x oC
q = mCs?t = ?H

pressure-volume work equation

w = -P?V
units of work = L x atm (if V is in L and P is in atm)
to convert, use 101.3 J = 1 L x atm

enthalpy (H)

used to quantify the heat flow into or out of a system in a process
that occurs at constant pressure
?H = ?E + P?V
?H = heat given off or absorbed during a reaction at constant pressure

endothermic reaction

a chemical reaction with positive ?H, that absorbs heat from its surroundings
- a process in which heat must be supplied to the system from
the surroundings

exothermic reaction

a chemical reaction with negative ?H, which releases heat to its surroundings
- a process that gives off heat; transfers thermal energy from
the system to the surroundings

bomb vs coffee-cup calorimetry

- bomb calorimetry occurs at constant volume and
measures ?E for a reaction
- coffee-cup calorimetry occurs at constant
pressure and measures ?H for a reaction

hess's law

the change in enthalpy for a stepwise process is the sum of the
enthalpy changes of the steps

standard enthalpy of formation (?H0 f)

the heat change that results when one mole of a compound is formed
from its elements at a pressure of 1 atm

standard enthalpy of reaction (?H0 rxn)

the enthalpy of a reaction carried out at 1 atm
?H0 rxn = sum [moles x ?H0
f]products - sum [moles x ?H0 f]reactants

spontaneous process

one that occurs without being driven by some outside force (without
ongoing outside intervention)
ex: sugar dissolves in coffee, water freezes below 0 degrees and
melts above it, heat flows from a hotter object to a colder object,
iron exposed to oxygen and water forms rust

what does/does not thermodynamics tell us

it tells us whether a reaction is spontaneous based only on the
properties of the reactants and products
it does not tell us about the pathway between the two

entropy (S)

a measure of the randomness or disorder of a system
when order increases, S decreaseswhen disorder increases, S increases
?S = Sf - Si when Sf >
Si then ?S >0
solid state is more ordered than liquid state, which is more
ordered than the gas stateSsolid <
Sliquid << Sgas

units of entropy

joules per kelvin (J/K)
entropy is a measure of energy dispersal (joules) per unit
temperature (kelvins)

second law of thermodynamics

for any spontaneous process, the entropy of the universe increases
(?Suniv > 0)
energy is dispersed, not concentrated (energy moves from an area of
greater energy to an area or lower energy)

which direction does a chemical system proceed

a chemical system proceeds in a direction that increases the entropy
of the universe

equation for entropy

S = k ln W?S = Sf - Si
?S = k ln (Wf / Wi)
W = number of microstatesWf > Wi
then ?S > 0Wf < Wi then ?S < 0

processes that lead to an increase in entropy (?S > 0)

changing from solid to liquid to gas increases entropy

an exothermic process ____ the entropy of the surroundings

increases

an endothermic process ____ the entropy of the surroundings

decreases

value of ?Ssurr at high and low temps

?Ssurr is:
positive and large at low temperature- if you are release
energy into surroundings that are cold (not a lot of energy), the
impact is great
positive and small at high temperature- if you release energy
into surroundings that are already hot (have a lot of energy), the
impact is small

the change in entropy of the surroundings depends on:

1. the amount of heat transferred into or out of the surroundings
2. the temperature of the surroundings

gibbs free energy (G)

the maximum amount of work energy that can be released to the
surroundings by a system (for a constant temperature and pressure system)
?G = ?Hsys - T?Ssys
- often called the chemical potential because it is analogous to
the storing of energy in a mechanical system

3rd law of thermodynamics

for a perfect crystal at absolute zero (0 K), the absolute entropy = 0
this is because in a perfect crystal, there is only one way to
arrange the components (W = 1)

standard entropy of reaction (?S0 rxn)

the entropy change for a reaction carried out at 1 atm and 25oC
?S0 rxn = [sum(n x
S0)products] - [sum(n x S0)reactants]

standard free-energy of reaction (?G0 rxn)

the free-energy change for a reaction when it occurs under standard
state conditions (25oC and 1 atm)
?G0 rxn = [sum(n x G0
f)products] - [sum(n x G0 f)reactants]

standard free energy of formation (?G0 f)

the free-energy change that occurs when 1 mole of the compound is
formed from its elements in their standard states

what does ?G0 rxn actually mean

the change in free energy for a chemical reaction represents a
theoretical limit as to how much work can be done
by the reaction
in a real reaction the amount of energy available
to do work is even less than ?G0 rxn because
some energy is lost to the surroundings as heat

reaction rate

the change in the concentration of a reactant or a product with time (M/s)

average vs instantaneous rate of reaction

avg: ?[element/compound] / ?t
instantaneous: the rate for a specific instance in time (can't do
w/o calc)

rate law

expresses the relationship of the rate of a reaction to the rate
constant and the concentrations of the reactants raised to some powers
- experimentally found
k[A]nk = rate constantn = reaction order

reaction order (n)

determines how the rate depends on the concentration of the reactant
- always defined in terms of reactant (not product)
concentrations- is not related to the stoichiometric coefficient
of the reactant
n = 0 -> zero order: rate is independent of the concentration of A
n = 1 -> first order: rate is directly proportional to the
concentration of A
n = 2 -> second order: rate is proportional to the square of the
concentration of A

first order reaction

rate is directly proportional to the concentration of A
k = rate / [A] = (M/s) / M units: s-1
- ?[A] / ?t = k[A]

half life (t1/2)

the time required for the concentration of a reactant to decrease to
half of its initial concentration
first order half life is independent of the initial concentration
t1/2 = t when [A] = [A]0 / 2
= ln2 / k

second-order reactions

rate is proportional to the square of the concentration of A
rate = -?[A] / ?t = k [A]2
k = rate / [A]2 = (M/s) / M2
units: 1/M2 = M-1 x s-1
t1/2 = 1 / k[A]0 where [A]0
is the concentration of A at t=0

integrated rate law

the relationship between the concentrations of the reactants and time
depends on the order of the reaction

zero order reactions

rate is independent of the concentration of A - same speed no matter
how much is put in
rate = -?[A] / ?t = k [A]0 = kunits: M x s-1
[A] = [A] - kt
t1/2 = [A]0 / 2k^ initial concentration matters

intermediates

species that appear in a reaction mechanism but not in the overall
balanced equation
an intermediate is always formed in an early elementary step and
consumed in a later elementary step (product then reactant)

molecularity of a reaction

the number of molecules reacting in an elementary step

unimolecular reaction - elementary step with 1 molecule
bimolecular reaction - elementary step with 2 molecules
termolecular reaction - elementary step with 3 molecules

reaction mechanism

the series of individual chemical steps by which an overall chemical
reaction occurs

rate-determining step

the slowest elementary step which determines the rate law for the
overall reaction

activation energy (Ea)

the minimum amount of energy needed to begin a chemical reaction
(turn reactants into products

transition state

the arrangement of atoms found at the top of the potential energy hill

collision model

a chemical reaction occurs after a sufficiently energetic collision
between two reactant molecules
ex: if you clap your hands in the wrong place/ wrong way / not hard
enough, it wont make a sound
there needs to be enough activation energy for the reaction to occur

catalyst

a substance that increases the rate of a chemical reaction but is not
consumed by the reaction

heterogeneous catalysis

catalyst + reactant are in different phases

homogeneous catalyst

reactants + catalyst in the same phase, usually liquid