Post on 21-Jan-2016
Flashcards for Unit 12
Equilibrium
Rate of forward process=
Rate of reverse process
Conditions for Equilibrium
2-way processClosed System
Dynamic Equilibrium
Macroscopic level – looks constant.
Microscopic level – lots of action
Types of Equilibria
1) Physical: Phase & Solution2) Chemical
A solution at equilibrium must be
Saturated
Phase Equilibrium
H2O(l) H2O(g)
Solution EquilibriumIonic Cmpd
NaCl(s) Na+(aq) + Cl-
(aq)
Solution EquilibriumCovalent Cmpd
C6H12O6(s) C6H12O6(aq)
H2O(g) H2O(l)
Phase Equilibrium
NH4Cl(s) NH4+(aq) + Cl-
(aq)
Solution Equilibrium
Chemical Equilibrium – Haber Synthesis
N2(g) + 3H2(g) 2NH3(g) + heat
LeChatelier’s Principle
A system at equilibrium will respond to remove a
stress
What’s a stress for a chemical system?
Change in temperature, pressure, or
concentration
Catalyst
Substance that increases the rate of a chemical reaction without itself
being consumed.
Catalyst
Does not shift the equilbrium point – the equilibrium
concentrations are the same. You just get to equilibrium
quicker.
At what temperature can H2O(s) and H2O(l) exist in equilibrium?
0C or 273K
At what temperature can H2O(g) and H2O(l) exist in equilibrium?
100C or 373K
Pressure Changes
Only affect systems that have a gas in them
An increase in pressure favors which side?
CO2(g) CO2(aq)
When P , more CO2 dissolves!
N2(g) + 3H2(g) 2NH3(g) + heat
Equilibrium shifts to right.Concentration of H2 .
Concentration of NH3 . Temperature
StressStress What happens?What happens?
N2(g) + 3H2(g) 2NH3(g) + heat
Shifts to the side with fewer moles of gas, in this
case the right.
How does this system respond How does this system respond to an to an increaseincrease in pressure? in pressure?
4 moles 2 moles
N2(g) + 3H2(g) 2NH3(g) + heat
Shifts to the side with more moles of gas, in this
case the left.
How does this system respond How does this system respond to a to a decreasedecrease in pressure? in pressure?
H2(g) + I2(g) + heat 2HI(g)
This system has the same # of moles on each side.
It cannot respond to pressure changes.
How does this system respond How does this system respond to a to a decreasedecrease in pressure? in pressure?
2 moles 2 moles
NaCl(s) Na+(aq) + Cl-(aq)
This system has no gases. It does NOT respond to
pressure changes.
How does this system respond How does this system respond to a to a changechange in pressure? in pressure?
N2(g) + 3H2(g) 2NH3(g) + heat
Equilibrium shifts to the left.
Concentration of N2 & H2 . Temperature .
Stress
What happens?What happens?
N2(g) + 3H2(g) 2NH3(g) + heat
Equilibrium shifts to the right. Concentration of N2 & H2 .
Temperature .
Stress
What happens?What happens?
N2(g) + 3H2(g) 2NH3(g) + heat
Equilibrium shifts to the left. Concentration of N2 . Concentration of NH3 .
Temperature .
Stress
What happens?What happens?
Exothermic Reaction
Energy is released: It flows from system to
environment. Temperature of environment increases.
Endothermic Reaction
Energy is absorbed: It flows from environment to system. Temperature of environment
drops.
Heat of reaction, H
Hproducts - Hreactants
The difference between the potential energy of the products & the potential energy of the reactants?
Hproducts – Hreactants or H
H for exothermic rxn
H is negative. System has net loss in energy.
H is negative?
Exothermic reaction. Products have less
potential energy than reactants.
H is positive?
Endothermic reaction. Products have more potential
energy than reactants.
H for endothermic rxn
H is positive. System has net gain in energy.
A + B C + D + heat
Exothermic Reaction. Heat term is on product
side.
A + B + heat C + D
Endothermic Reaction. Heat term is on reactant
side.
Which phase change is endothermic?
a) Gas to liquidb) Gas to solidc) Solid to gasd) Liquid to solid
Which phase change is exothermic?
a) Solid to gasb) Liquid to gasc) Solid to liquidd) Liquid to solid
A reaction occurs in water & the
temperature of the water increases.
Endo or Exo?
Exothermic
A reaction occurs in water & the
temperature of the water decreases.
Endo or Exo?
Endothermic
A + B C + D. H = -45 kJ.
What is H for C + D A + B?
H = +45 kJ
A + B C + D. H = -45 kJ.
What is H for: 2A + 2B 2C + 2D?
H = -90 kJ
Entropy
Degree of disorder or chaos in a system.
Entropy
The more degrees of freedom a system has, the higher
the entropy.
S
Entropy
H
Enthalpy or
Chemical Potential Energy
0 entropy?
A perfect crystal at 0K
S
Change in entropySproducts – Sreactants
Which phase has the greatest entropy?
Gas phase – the molecules have more ways of moving
& more places to be!
Which phase has the greatest entropy?
Sgas >> Sliquid > Ssolid
Did the entropy increase or decrease?
NaCl(s) Na+(aq) + Cl-(aq)
Increase!
Did the entropy increase or decrease?
CO2(g) CO2(aq)
Decrease!
Did the entropy increase or decrease?
2H2O(l) 2H2(g) + O2(g)
Increase!
Spontaneous Process
Physical or Chemical change that occurs with no outside
intervention. (Supplying activation energy does not
count!)
Nature is lazy & disorganized.
Nature favors:Negative value for H: Net lossPositive value for S: Net gain
When will a change never occur?
When both factors go against nature:Positive value for H: Net gain in
energy.Negative value for S: Net loss in chaos
Endo: starts low, ends high
Endo or Exo?
Exo: starts high, ends low
Endo or Exo?
Reactants
Identify reactants, products, activated complex
Activated Complex
Products
Potential energy of reactants
Identify the arrow
Activation energy of forward rxn
Identify the arrow
Heat of reaction
Identify the arrow
Activation energy of reverse reaction
Identify the arrow
Potential energy of activated complex
Identify the arrow
Potential energy of products
Identify the arrow
Collision Theory
Effective collisions between reacting particles must
occur for a reaction to take place.
Effective Collisions
EnergeticFavorable Orientation
Rate of reaction
The speed of the reaction. The faster it occurs, the
greater the rate.
Breaking Bonds …
Absorbs Energy
Making Bonds …
Releases Energy
6 Factors that influence the rate of reaction
Nature of reactants (ionic or covalent)TemperatureConcentrationPressure (for gases)Surface area (for heterogeneous
reactions)Presence of a catalyst
Raising the temperature increases the reaction rate by
Increasing both the number of collisions and the efficiency of
the collisions.
As the concentrations of the reactants , the rate of rxn …
increases
As the pressure on gas-phase reactants , the rate of rxn …
Increases.
Which 4.0-gram Zn sample will react faster with acid,
and why?
a)Lumpb)Barc)Powderedd)Sheet metal
More surface area
How does a catalyst work?
Provides an alternate reaction pathway with a lower activation energy.
Does a catalyst change H?
NO.
Which reacts faster: ions in solution or covalent
molecules?
Ions in solution.
Which reacts faster in water: 1 cm3 of Na or 1 cm3 of Ca?
1 cm3 of Na - Nature of the metals
Which is usually slower, an organic reaction or an
inorganic reaction?
Organic, because organic particles contain covalent
bonds.
Lowers the activation energy.
Effect of a catalyst?
Potential energy of reactants
Identify the arrow
Activation energy of forward rxn
Identify the arrow
Activation energy of reverse rxn
Identify the arrow
Potential energy of activated complex
Identify the arrow
Heat of reaction
Identify the arrow
Potential energy of products
Identify the arrow