Download Ch17-2 Driving Forces of Reactions

Reaction Energy & Reaction Kinetics
Thermochemistry….ch.17 p. 511
Thermochemistry = study of the transfer of heat in chemical
reactions and physical changes.
Temperature = measurement of kinetic energy of molecules
(Ex) fire wood
Thermal energy = total energy of matter… measured in joules
Heat = transfer of thermal energy.
Follows 2nd law of thermodynamics…
heat always flow from higher to lower temperature.
Enthalpy change: change of energy absorbed
or released as heat during a chemical
reaction…
exothermic = got hot = released heat
endothermic = got colder = absorbed heat
Do NaOH & NH3NO3
Exothermic
Reaction
Endothermic
Reaction
Energy may be consumed or produced
(exothermic or endothermic)
in a chemical reaction.
Enthalpy: change in energy
delta
H j/mol or kj/mol
“throughout nature reactions proceed from..
Higher energy state
lower energy state
“most chemical reactions in nature are exothermic”
Activation Energy
p. 533-534
match
Wood log
paper
Activation Energy: Ea
“ the energy needed to get the reaction started”
Potential Energy Diagram / endothermic
Potential Energy diagram / exothermic
Lecture notes 1
p. 533-534
The line is the energy
Read graph 2 ways
just like heating/cooling
curves
Ea = activation Energy for exothermic will be –
Ea’ = activation Energy for endothermic will be +
E = change in energy also known as
delta
“ notice
delta
E or
delta
H.
delta
H is equal for either direction”
Bonds breaking is endothermic
Bonds forming is exothermic
Label each graph endo / exo in forward reading.
Label Ea forward, Ea reverse,
delta
Determine value for each.
Eforward,
delta
Ereverse
How is height of curve related to ease with
which a reaction will occur?
What would the activation curve look like for
a rapid reaction?
What would happen if amount <Ea were
supplied?
Reaction Rate: “How fast the reaction proceeds”
Reaction Rate depends on :
1. number of molecule collisions
2. efficiency of the collisions
5 factors can effect the rate of reaction:
l. Nature of reactants
2. surface area (big chunk vs. powder)
3. temperature ( hot or cold)
4. concentration
5. catalyst (only effects rate does not participate in
reaction…can recover.)
• Entropy: a measure of degree of randomness of the
molecules.
S
j/mol*K or kj/mol*K
Delta
“most reactions in nature are toward disorder”
Organized
disorganized
Gas has more entropy than liquid …liquid more entropy than solid
S
L
g
Increase in entropy + S …..more crazy random (favored)
delta
Decrease in entropy -
delta
S ……more organized
You can tell entropy by looking at the
phases of the reaction…….ws. 17.2
Free Energy ( G)
determines whether a reaction will occur taking into account
both enthalpy and entropy and temperature .
Reactions are driven to
lowest enthalpy, highest entropy
To determine if a reaction will occur….
G must be negative in order
for a reaction to be probable.
Spontaneity: means that a reaction will occurdoesn’t say fast or slow, just that it will happen.
Ex: rusting of iron is a spontaneous reaction( even though it will take awhile) because it will
happen.
How to determine if a reaction is Spontaneous or Not.
Will it work or not?
Free Energy G formula
G= H-T S
G has to negative to be spontaneous
H- exothermic
S+ more disorder
T
in Kelvin
1. Will the following reaction be spontaneous?
NH4Cl3(s)
NH3 (g) + HCl3(g)
H= 176 kj/mol
The positve value of G shows
this reaction will not occur at the
stated temp.
S= 0.285 kj/mol.K
T=298.15K
Br2 (l)
Br2(g)
H= 31.0kj/mol
S= 93.0J/mol.K
At what temp will this be spontaneous?
Hint: try standard room temp first….298K
Temp must be above 333K
P548 #24, #25,
Reaction Equilibrium
Le Chatelier’s Principle: if a system at equilibrium is
subjected to a stress then the reaction will be shifted in
the direction to relieve the stress.
We will learn 4 stress effects :
change in pressure
change in temperature
change in concentration
adding a catalysts
Change the pressure:
an increase in pressure will favor a shift toward
the side with lowest number of gas particles.
3H2 (g) + N2 (g)
2NH3 (g) + heat
C (s) + H2 (g) + heat
CO (g) + H2 (g)
Change the temperature:
increasing Temperature will drive the reaction toward the
reactants (shift to left) if exothermic.
increasing temperature will drive the reaction toward the
products (shift to right) if endothermic.
Change in Concentration:
adding more reactants always drives the reaction toward
the products (shift to the right)
adding more products always drives the reaction toward
the reactants ( shift to the left)
“see-saw ride principle”
“Remember reverse psychology”…if take away some product then shift toward the products.
3 H2 + N2
2NH3
CaCO3
CaO + CO2
Ca(OH)2
CaO + H20
Fe2O3 + 3CO
2Fe + 3CO2
Adding a Catalyst:
does not cause a shift in equilibrium
speed up or slow down only
Review lecture notes ch 18
Ws 18.2
Writing Equilibrium equations p.553-557
reversible reaction:
A chemical reaction in which the products
Can react to reform the the reactants.
When is a reaction that is reversible considered to have
reached chemical equilibrium?
When rate of forward reaction = rate of reverse reaction
Equilibrium constant: Keq
A numerical ratio
of the concentration of the products
to the concentration of the reactants
Writing the equilibrium expression: Keq
l.
Write the concentration of the products using their
coefficients as exponents.
2. Divide the concentration of the reactants using their
coefficients as exponents.
3.
N2O4
4.
CO + 3H2
5.
8H2S
2NO2
CH4 + H2O
8H2 + S8