Chapter 6 Energy Changes, Reaction Rates & Equilibrium
Chapter 6 Energy Changes, Reaction Rates & Equilibrium
CHEMICAL REACTIONS
When chemical reactions occur OLD bonds (in the reactants) are broken and NEW bonds (in the products) are formed.
The energy needed to break old bonds and form new ones can be studied through THERMOCHEMISTRY.
Bonds
Ionic - electrostatic forces of attraction between ions
Covalent - forces of mutual attraction of electrons between NONMETAL atoms (most NM atoms want an OCTET)
Hydrogen - weak forces of attraction
– between water molecules
– within DNA, holding the 2 strands together
Energy
Potential Energy (stored energy) - the energy of Chemical Bonds
Kinetic Energy (energy associated with motion): KE = 1/2mv2
∆E
∆E - represents the change in internal energy for a system
∆E = q + w
q: heat (- heat lost by the system); (+ heat gained by the system)
– Units: calories (cal), joules (J), etc.
w: work (- work done by the system); (+ work done to the system)
THERMOCHEMISTRY: The study of heat changes during chemical reactions.
∆H is the symbol representing “change in heat”
Like fingerprints, each substance can hold/release a specific amount of heat.
Example: for the same mass, water holds heat better than steel.
Endothermic Reactions: The reacting chemicals absorb heat from their surroundings
(Heat In!)
∆H = +
Ba(OH)2 + NH4Cl + heat --> NH3 + BaCl2 + H2O
Exothermic Reactions: The reacting chemicals release heat into their surroundings
(Heat Out!)
∆H = -
KMnO4 + C3H8O3 --> K2CO3 + Mn2O3 + CO2 + H2O + heat
C12H22O11 H2SO4 > C + H2O
Specific heat capacity (C): The capacity of a particular amount of a substance to hold heat.
Water: C = 1.0 cal/g˚C (4.18 J/g˚C)
Al: C = 0.21 cal/g˚C (0.90 J/g˚C)
Silver: C = 0.057 cal/g˚C (0.24 J/g˚C)
The change of heat of a substance/reaction is a function of three things:
Specific heat capacity
Mass
Temperature change: Tfinal - Tinitial
∆H = (C) (m) (∆T)
Calorimetry: determining Heat change (∆H) in the lab
Measuring mass and temperature change allows you to determine the heat change
during a change (chemical or physical)
In a controlled situation the heat lost by one substance can be equal to the heat
gained by another
Example: place a hot coin into cold water
In an ideal calorimeter, energy is conserved!! Heatabsorbed = Heatlost
(Cp . m . ∆T)in = (Cp . m . ∆T)out
Rate of Reaction
Rate = Speed
The rate of a reaction depends on:
Temperature, Concentration of reactants, Particle Size/Surface Area, Catalysts
Reactions require a specific amount of “activation” energy (Ea) in order for reactants to react effectively.
Molecular Orientation
Catalysts
Rxn Rates & Concentration
Equilibrium
Many chemical reactions occur in two directions - forward and reverse.
Once the initial reaction is established an equilibrium can develop.
Rate of forward reaction = Rate of reverse reaction
A(aq) + B(aq) <==> AB(aq)
3 factors affect equilibrium
Concentration (substances must be in aqueous or gaseous form)
Temperature (exo vs. endo)
Pressure - affects gases only (look at the # of moles of gases)
A(aq) + B(aq) <==> AB(aq) + heat
Equilibrium Position
This position is defined by the amounts of reactants and products (equilibrium expression = Keq).
If the equilibrium position shifts, equilibrium will have to be reestablished with different amounts of reactants and products.
Tooth Enamel Demineralization
Ca10(PO4)6(OH)2 <==> 10Ca2+ + 6PO43- + 2OH-
Equilibrium Expression
A ratio of [products] over [reactants]
Each [ ] is raised to the power equal to its coefficient in the balanced equation
The ratio is set equal to a constant (Keq)
A2(aq) + 2 B(aq) <==> 2AB(aq)
Keq =
Ex.: 2NOCl(g) <==> 2NO(g) + Cl2(g)
BaCl2(aq) + Na2SO4(aq) <==> 2NaCl(aq) + BaSO4(s)
Significance of Keq
Chemical stress effects
Le Chatelier’s Principle: A system in equilibrium which is stressed tries to return to equilibrium by shifting the reaction in a direction to relieve the stress
So, if we increase the concentration of some participant in the equilibrium, the system will try to react away that substance.
If we decrease the concentration of some participant in the equilibrium, the system will try to produce more of that substance.
If we increase the temperature or pressure of the system, the system will try to reduce the temperature or pressure by shifting the equilibrium position in a direction that changes the temp. or press..
Example of Le Chatelier’s Principle
C6H6(g) + 3H2(g) <==> C6H12(g) + heat
Increase [C6H6]
Decrease [C6H12]
Increase temperature
Effect of Temperature