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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 atom s (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 Bond formation always releases
energy; exothermic Bond dissociation always requires
energy; endothermic Kinetic Energy (energy associated
with motion): KE = 1/2mv2
THERMOCHEMISTRY
The study of heat changes during chemical reactions. Based on the net energy of bonds dissociating
and bonds breaking ∆H is the symbol representing “change in heat” Differences in bond dissociation energies allow us to
determine if heat is/will be released or needs to be absorbed during a reaction. ∆H = B.E.P B.E.R
If the products require less bond energy than the reactants, the excess energy is released (∆H = -) and vice versa (∆H = +)
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
KMnO4 + C3H8O3 --> K2CO3 + Mn2O3 + CO2 + H2O + heat Ba(OH)2 + NH4Cl + heat --> NH3 + BaCl2 + H2O
Energy graphs showing the difference between an exothermic and an endothermic reaction.
Rate of Reaction Rate = Speed The rate of a reaction depends on:
Temperature, Concentration of reactants, Catalysts
Reactions require a specific amount of “activation” energy (Ea) in order for reactants to react effectively.
Rubbing a match head against a rough surface provides the activation energy needed for the match to ignite.
Factors that Affect Rxn Rates
Rxn Rates & ConcentrationGraphs showing how reaction rates and reactant concentration vary with time.
Catalysts
Catalysts lower the activation energy for chemical reactions.
Equilibrium Many chemical reactions occur in two directions
- forward and reverse. Once the 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
Effect of Concentration Changes
Concentration changes that result when H2 is added to an equilibrium mixture.
Effect of Temperature
Equilibrium mixtures changing color with difference in temperatures.
Equilibrium Position A + 2B <==> C + D This position is defined by the amounts of
reactants and products If the equilibrium position shifts, equilibrium will
have to be reestablished with different amounts of reactants and products.
An equilibrium expression allows for a mathematical description of the position at equilibrium.
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
2NOCl(g) <==> 2NO(g) + Cl2(g)
If Keq = 1000, then the amount of products is
essentially 1000x greater than reactants.
Calculating Keq
A2 + B2 <==> 2AB Calculate Keq when [A] = 0.25 M; [B] = 0.35 M; [AB] =
2.50 M
If [A] increases to 0.55 M; [B] increases to 0.45 M, what would the new [AB] become?
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.
Example of Le Chatelier’s Principle
C6H6(g) + 3H2(g) <==> C6H12(g) + heat
Increase [C6H6]
Decrease [C6H12] Increase temperature
Reactions of Ionic Compounds (an important example)
Tooth Enamel Demineralization
Ca10(PO4)6(OH)2 <==> 10Ca2+ + 6PO43- + 2OH-