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.