CHM116A Lecture 8-Student Slides
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Transcript of CHM116A Lecture 8-Student Slides
CHM 116 Spring 2012
Today’s Lecture
Section 17.1, 17.2, 17.3
Next Lecture
Read Sec. 17.3, 17.4, 17.5
In Silberberg
Exam 1, Mon. Feb 6, 6:30 – 7:30, Hall of Music
Exam I TONIGHT: Feb. 6. 6:30 PM Elliott Hall of Music
Plan to arrive by 6:15 pm. Bring your PU ID, your seat assignment, a simple calculator, and pencils.
Good Luck Tonight!
2
Bromocresol Green Equilibrium System
Indicator - substance whose solution changes color due to changes in pH
H+ + In- ⇌ HIn blue-green yellow
Bromocresol green structure
is abbreviated HIn.
HIn
Determination of K
H+ + In- ⇌ HIn
Determine [HIn] and [In-] using spectrophotometry at 2 λ’s.
Determine [H+] using pH meter.pH = -log[H+]
Is K constant when the pH (and concentrations) are varied?
+ -
[HIn]K =
[H ][In ]
Lab Preparation
Chapter 7Read the ENTIRE labReview Appendices C (Volumetric
Measurement) and D (Spectroscopy)
Do the prelab on pp 56-7. (For #1 you must show your work.)
Attend recitation.
Lecture 8: Learning Objectives
See Silverberg, pp. 709 – 710
Specifically concentrate your efforts on the eighteen (18) items in
“Master These Skills”
Kinetics vs. Equilibrium
Kinetics applies to the speed of a reaction, the concentration of product that appears (or of reactant that disappears) per unit time
Speed: dissociation of an acid in H2O
Strong Acid: virtually all HCl dissociates to ions
HCl H+ + Cl-
Weak Acid: less than 10% dissociates to ions
CH3COOH CH3COO- + H+
Speed of a reaction: both complete in <1 sec
Equilibrium applies to the extent of a reaction, the concentration of product that has appeared after an unlimited time in a closed system, or once no further change in concentration occurs
At equilibrium: rateforward = ratereverse
A system at equilibrium is dynamic on the molecular level; no further net change in concentration is observed because the rate of formation = the rate of disappearance of each reactant and product.
Kinetics vs. Equilibrium
Reversible Reaction: A reaction that can proceed in either direction.
Vocabulary & Convention
For the reversible reaction:
MgCO3 MgO + CO2
MgCO3 can give MgO and CO2
MgO and CO2 can give MgCO3
MgO + CO2 MgCO3
By convention: Reactants still on left, products on the right
Demo
2 NO2(g) N2O4(g)
BrownBrown ColorlessColorless
Equilibrium is reached at fixed temp (room temp), in closed system
Change in Temp alters equilibrium concentrations
Backward rxnForward rxn
rewarmed
Reversible Reactions
After the temperature stops changing in the
2 NO2 N2O4 reaction
The amounts of reactants and products stop changing. Equilibrium has been reached.
Equilibrium: the condition at which the amount, concentration, and/or pressure of reactants and products in a closed system do not change, no matter how long the wait
EquilibriumEquilibrium
EquilibriumEquilibrium
Product conc. increases and then becomes constant at equilibrium
Reactant conc. declines and then becomes constant at equilibrium
Equilibrium achieved
What is rateforward when [H2] stops changing?
Rate = k[H2][I2] if elementary rxn
3. Amounts of reactants and products stop changing, but the reactions do not stop!
2. Constant conditions (constant T, constant P no delta E, etc.)
Five important ideas about equilibrium:
1. Both reactants and products are present and the system must be closed
Equilibrium
5.
4. Reactants and products continue to interconvert
sometimes it takes a very long time to reach equilibrium (ex. Rusting metal)
Quantitative Description of Equilibrium
2 NO2(g) N2O4(g)
BrownBrown ColorlessColorless
This happens to be an elementary reaction:
Ratefwd = kfwd[NO2]2 Raterev = krev[N2O4]
At equilibrium: rateforward = ratereverse
kfwd[NO2]2 = krev[N2O4]
Keq the equilibrium constant. This is a particular ratio of equilibrium concentrations of products and reactants at a particular temperature.
= Keq
small K
K<<1
large K
K>>1
intermediate K
K = ~1
Implication of the Magnitude of the Equilibrium Constant
1) Small K N2 (g) + O2 (g) 2 NO(g) K = 1 x 10 -30
2) Large K 2 CO(g) + O2 (g) 2 CO2 (g) K = 2.2 x 1022
3) Intermediate K 2 BrCl(g) Br2 (g) + Cl2 (g) K = 5
The reaction quotient, Q, is a ratio of the concentrations and/or pressures of the products and reactants in a reaction mixture that may or may not be at equilibrium.
w A + x B y C + z D
[C]y[D]z PCy x PD
z
Qc = or, if gases Qp = [A]w[B]x PA
w x PBx
Note: Q is based on the balanced equation
Qc based on concentration
Qp based on partial pressure of gas
Quantitative Description of EquilibriumThe Reaction Quotient: Q
The reaction quotient, Q, is a ratio of the concentrations and/or pressures of the products and reactants in a reaction mixture that may or may not be at equilibrium.
w A + x B y C + z D
[C]y[D]z Qc = = Kc when system comes to
[A]w[B]x equilibrium
PCy x PD
z
Qp = = Kp when system comes to PA
w x PBx equilibrium
Quantitative Description of EquilibriumSpecial Case: Keq = Q
Reaction Quotients
Write reaction quotients: Qc for a. Qp for b.
a.Cu2+ + 4 NH3 = Cu(NH3)42+
b. 2 P2(g) + 5 O2(g) = P4O10 (g)
Doccam
The reaction quotient, Q, is a ratio of the concentrations and/or pressures of the products and reactants in a reaction mixture that may or may not be at equilibrium.
w A + x B y C + z D
[C]y[D]z Qc = = Kc when system comes to
[A]w[B]x equilibrium
PCy x PD
z
Qp = = Kp when system comes to PA
w x PBx equilibrium
Quantitative Description of EquilibriumSpecial Case: Keq = Q
K in Terms of Pressure
Kp = Kc(RT)n
∆n = Σ coeff prod – Σ coeff react
(If the total # of moles of gas do not change, then ∆n = 0 and Kp = Kc)
A consequence of the gas law:
Partial pressure Concentration (mol/L)
Doccam
Concentration of a Pure Liquid
What is the concentration of H2O (M.Wt. 18 g/mole) in 18 mL of water (18 g)?
What is the concentration of H2O (M.Wt. 18 g/mole) in 1,800 mL of water (mass 1,800 g)?
1 mole/0.018 L = 56 M
100 mole/1.8 L = 56 MThe concentration of water does not change as the volume of water changes. The concentration of a solid, a liquid, or a solvent does not change even if its volume changes. So
leave out of Q & K equations!
The reaction quotient for a heterogeneous system.
solids do not change their
concentrations
CaCO3(s) > CaO(s) + CO2 (g) Partial pressure of CO2 is the same in both jars
Because the concentration does not change, we do not
include the concentration of solids, pure liquids, or solvents in a reaction quotient.
[solid], [liquid], [solvent] = 1
Write reaction quotients for
a. CaCO3(s) CaO(s) + CO2(g) Qp =
a. 2 H2O2(aq) 2 H2O(l) + O2(g) Qc =
c. NH3(g) + HCl(g) NH4Cl(s) Qp =
The reaction quotient for a heterogeneous system.
Doccam
[NO]2 [O2][NO2]2
[NO]2
[N2] [O2]
Writing the Reaction Quotient for an Overall Reaction
Qc (overall) =[NO2]2
[N2][O2]2
For the individual steps:
(1) N2 (g) + O2 (g) 2 NO(g) Qc1 =
(2) 2 NO(g) + O2 (g) 2 NO2 (g) Qc2 =
For: N2 (g) + 2 O2 (g) 2 NO2 (g)
Qc1 x Qc2 = x =[NO]2
[N2] [O2][NO2]2
[NO]2 [O2][NO2]2
[N2][O2]2
Q1 x Q2 x Q3 … = Qoverall (Overall Q is product of all Q’s)
i.e. For the sum of reactions, the overall Q is the product of Qs
Q and K for an Overall Reaction
Qc (overall) =[NO2]2
[N2][O2]2For: N2 (g) + 2 O2 (g) 2 NO2 (g)
Q1 x Q2 x Q3 = Qoverall
likewise
K1 x K2 x K3 = Koverall
•If an overall reaction is the sum of two or more reactions, the overall Q (or K) is the product of the individual Q (or K)
•Keq is independent of mechanism!•Can use stoichiometry of equation to calculate Q (or K)
The Form of Q for a Forward and Reverse Reaction
Sulfur dioxide reacts with oxygen to form sulfur trioxide.
2 SO2 (g) + O2 (g) 2 SO3 (g)
Qc(fwd) =[SO3]2
[SO2]2[O2]
2 SO3 (g) 2 SO2 (g) + O2 (g)
This reaction is reversible. What is Q for the reverse reaction?
[SO2]2[O2]Qc(fwd)
Qc(rev) = =[SO3]2
1
The equilibrium expression for a reaction written in reverse is the reciprocal of that for the original reaction.