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1
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- Chapter 8. Chemical and Physical Change: Energy, Rate, and Equilibrium
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2
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- Up to now we have worked problems assuming that reactants are totally
converted to products as
- H2 + I2 g 2HI
- If 4 moles of H2 reacts get 8 moles of HI, etc.
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3
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- Many rxns can occur in the reverse direction (products to reactants) as
well as in the forward direction (reactants to products).
- In a closed system for A(g) g B(g)
- at some point in time
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4
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- At this point it appears
- Equilibrium is a ________________; on the molecular level the rxns
continue but every time an A forms a B, a B turns into an A and the
concs of A and B seem
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5
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- A(g) g B(g)
- B(g) g A(g)
- At equilibrium the
- So
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6
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- or how to write an equilibrium constant expression.
- aA(g) +
bB(g)DcC(g) + dD(g)
- [A] means that conc of A is in moles/L
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7
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- Keq is equal to the product of the conc of the products
raised to their stoichiometric coefficients divided by the product of
the concs of the reactants raised to their stoichiometric coefficients.
- The value of Keq is
- determined
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8
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- 2NO(g) + O2(g) D 2NO2(g)
- 2NH3(g) D N2(g) + 3H2(g)
- 3O2(g) D2O3(g)
- 2NO2(g) +7H2(g) D2NH3(g) + 4H2O(g)
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9
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- So far all examples have been for all species in the gas phase:
homogeneous equilibria
- Now for 2HgO(s) D 2Hg(l) + O2(g)
heterogeneous equilibrium
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10
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- P4(g) + 5O2(g) D P4O10(s)
- CaCO3(s) D CaO(s) + CO2(g)
- CH3COOH(aq) + H2O(l) D CH3COO-(aq)
+ H+(aq)
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11
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- K>100 mainly have B at equilibrium, very little A; rxn is essentially
complete
- K<0.01 mainly have A at equil., very little B; essentially no rxn has
occurred
- K btn 1 and 100 more B than A at equil but have measureable amts of A
and B
- K btn 0.01 and 1 have more A than B at equil but have measurable amts of
A and B
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12
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- Predict whether reactants or products are favored at equil
- S2(g) + 2H2(g) D 2H2S(g) K=2.8x10-21
- CO(g) + 2H2(g) DCH3OH(g) K= 10.5
- Br2(g) + Cl2(g) D2BrCl(g) K =58.0
- I2(g) D 2I(g) K =
6.8x10-3
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13
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- Fluorine can react with oxygen to yield oxygen difluoride: 2F2(g) + O2(g) D
2OF2(g)
- What is the value of Keq if the following concentrations are
found at equilibrium? [O2] = 0.200mol/L; [F2]=0.0100mol/L [OF2]=0.0633mol/L
- What is [O2] at equil when [OF2]=0.22mol/L and [F2]=0.0300mol/L?
- What is [OF2] at equil when [F2]=0.080mol/L and
[O2] = 0.650mol/L?
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14
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- Le Chatelier’s Principle: when a stress is applied to a system at equil
(which removes the system from equil) the system shifts to relieve the
stress and reach a new equil state.
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15
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- Stresses are:
- 1. addition or removal of reactants and products at constant volume
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16
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- 2. changes in pressure : pressure is caused by gas molecules hitting the
container walls.
- If you increase the pressure, the rxn will shift
- If you decrease the pressure,
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17
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- If you have the same no of gas molecules on both sides of the rxn,
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18
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- 3. Temperature changes (remember the value of K changes when you change
the temp)
- exothermic
- endothermic
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19
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- 4. adding a catalyst: a catalyst
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20
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- Cl2(g) + H2(g) D 2HCl(g)
- K = 2.6 x 1033 and DH = -44kcal at 25oC
- Is the rxn endo or exothermic?
- Are the reactants or products favored at equil?
- What happens when:
- increase pressure
- inc conc of HCl
- inc conc of Cl2
- dec conc of H2
- add a catalyst.
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21
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- 7.59 Predict what happens when increase the pressure:
- C(s) + H2O(g) D CO(g) + H2(g)
- 2H2(g) + O2(g)D 2H2O(g)
- 2Fe(s) + 3H2O(g) D Fe2O3(s)
+ 3H2(g)
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22
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- 7.61 The rxn 3O2(g) D 2O3(g) has DH=68kcal. Does
the equil constant for the rxn increase or decrease when the temp
increases?
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23
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- NH3(g) + O2(g) D NO(g) + H2O(g) +heat
- Balance
- Explain the effect on equil of
- raising the pressure
- adding NO(g)
- decreasing the conc of NH3
- lowering the temp
- adding a catalyst
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Notes
