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Week 9 CHEM 1310 - Sections L and M 1

PRS Question from Wed

What must you do to increase the value of Kfor the following exothermic reaction?

2 H2(g) + O2(g) 2 H2O(g)

1. Increase the total pressure

2. Decrease the total pressure

3. Increase the temperature

4. Decrease the temperature

5. Two of these

K =[H2O]2

[H2]2 x [O2]

Thus anything that shiftsequilibrium to the RIGHT

will increase K

Week 9 CHEM 1310 - Sections L and M 2

PRS Question from Wed

What must you do to increase the value of Kfor the following exothermic reaction?

2 H2(g) + O2(g) 2 H2O(g)

PV = nRT

Stress is relieved by shifting equilibrium to direction where there are fewer molecules…

As P ↑, V↓

LEFT → RIGHT2 H2(g) + O2(g) 2 H2O(g)

3 mol 2 mol

Week 9 CHEM 1310 - Sections L and M 3

PRS Question from Wed

What must you do to increase the value of Kfor the following exothermic reaction?

2 H2(g) + O2(g) 2 H2O(g)

PV = nRT

“Stress” of lower pressure (i.e. greater volume)causes shift in equilibrium to side with more molecules!

Consequently, lowering P…

RIGHT → LEFT2 H2(g) + O2(g) 2 H2O(g)

3 mol 2 mol

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Week 9 CHEM 1310 - Sections L and M 4

PRS Question from Wed

What must you do to increase the value of Kfor the following exothermic reaction?2 Hs(g) + O2(g) H2O(g)

What impact would temperature changes have?Consider the analogy with H2O…

Ice → WaterExothermic

Increasing T shifts equilibrium LEFT to RIGHT

Decreasing T shifts equilibriumRIGHT to LEFT

Week 9 CHEM 1310 - Sections L and M 5

PRS Question from Wed

What must you do to increase the value of Kfor the following exothermic reaction?2 Hs(g) + O2(g) H2O(g)

1. Increase the total pressure

2. Decrease the total pressure

3. Increase the temperature

4. Decrease the temperature

5. Two of these

Both of these resultin more H2O(g)

K =[H2O]2

[H2]2 x [O2]

Week 9 CHEM 1310 - Sections L and M 6

Ch 7: Acids and Bases

Nature of Acids and Bases

Acid Strength

The pH Scale

Calculating pH Involving Strong Acid Solutions Involving Weak Acid Solutions

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Week 9 CHEM 1310 - Sections L and M 7

Classification of Acids & Bases

Arrhenius Definition Acids donate H+ or increase [H+] in solution Bases donate OH- or increase [OH-] in solution

Bronsted-Lowry Definition Acids donate H+

Bases accept H+

Lewis Definition (later…)

Week 9 CHEM 1310 - Sections L and M 8

Arrhenius acids can be any of the following:

Binary acids: HCl or H2S

Polyprotic acids: H2SO4 or H3PO4

Oxoacids: HNO3 or H2SO4 or H3PO4

In each case there is an H+ group to donate!

Examples of Arrhenius Acids

Week 9 CHEM 1310 - Sections L and M 9

Some Arrhenius acids do not contain H at all!Why are they considered acids?

SO3 + H2O H2SO4

Because they increase the concentrationof H+ in solution!

A non-metal oxide (or acid anhydride)

More Arrhenius Acids

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Week 9 CHEM 1310 - Sections L and M 10

NaOH is an Arrhenius base.

Some Arrhenius bases do not contain OH at all!Why are they considered bases?

NH3 + H2O NH4+ + OH-

Because they increase the concentrationof OH- in solution!

Examples of Arrhenius Bases

Week 9 CHEM 1310 - Sections L and M 11

Arrhenius Bronsted-Lowry

Acids donate H+ Acids donate H+

Bases donate OH- Bases accept H+

B-L Acids: H2S, HCl, H2SO4, HNO3

B-L Bases: NH3, CN-, HCO3-

Bronsted-Lowry Examples

Week 9 CHEM 1310 - Sections L and M 12

Conjugate Acid-Base Pairs

Conjugate Base Subtract an H+ from the acid

Conjugate Acid Add H+ to the base

Examples: OH- is the conjugate base of H2O H2O is the conjugate base of H3O+ (called the

hydrodium ion, sometimes abbreviated H+) H2O is the conjugate acid of OH-

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Week 9 CHEM 1310 - Sections L and M 13

CH3COOH donates H+ to H2O

CH3COOH + H2O CH3COO- + H3O+

Acetic Acid Base Conjugatebase

Acid

CH3COOH + NaOH CH3COONa + H2OAcetic Acid Base Conjugate

baseAcid

Conjugate Acid-Base Pairs

Week 9 CHEM 1310 - Sections L and M 14

H2O can acts as an acid or a base

CH3COOH + H2O CH3COO- + H3O+

Acetic Acid Base Conjugatebase

Acid

CH3COOH + NaOH CH3COONa + H2OAcetic Acid Base Conjugate

baseAcid

Amphoterism and Water

Week 9 CHEM 1310 - Sections L and M 15

Dissociation Constant, Ka

Ka =[H3O+] x [A-]

[HA]HA + H2O H3O+ + A-

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Week 9 CHEM 1310 - Sections L and M 16

Kw = [H3O+] x [OH-]2 H2O H3O+ + OH-

From this we can derive the equation

for pH…

Dissociation Constant, Kw

Week 9 CHEM 1310 - Sections L and M 17

The pH Scale

Week 9 CHEM 1310 - Sections L and M 18

Kw = [H3O+] x [OH-]2 H2O H3O+ + OH-

pH = -log [H3O+]

pH < 7 Acidic Solution [H3O+] > [OH-]

pH = 7 Neutral Solution [H3O+] = [OH-]

pH > 7 Basic Solution [H3O+] < [OH-]

Let’s consider some calculations with pH…

Interpreting pH

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Week 9 CHEM 1310 - Sections L and M 19

Kw = [H3O+] x [OH-]

Calculate the pH of a solution in which the[OH-] = 1.2 x 10-6M

pH = -log [H3O+] = - log (8.3 x 10-9) = 8.1

[H3O+] = Kw

[OH-]

[H3O+] = 1 x 10-14

1.2 x 10-6= 8.3 x 10-9 M

Calculations with pH

Week 9 CHEM 1310 - Sections L and M 20

Water from a soil sample that is rich in calciumcarbonate has a pH of 8.14. What is the [H+]? Is the

sample acidic or basic?

[H+] = 10-pH = 10-8.14 = 7.2 x 10-9M pH < 7 Acidic Solution [H3O+] > [OH-]

pH = 7 Neutral Solution [H3O+] = [OH-]

pH > 7 Basic Solution [H3O+] < [OH-]

Since pH > 7, the sample is basic.

Calculations with pH

Week 9 CHEM 1310 - Sections L and M 21

PRS Question

Which of the following is the conjugate baseof HPO4

2-?

1. H3PO4

2. H2PO4-

3. HPO42-

4. PO43-

5. None of these

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Week 9 CHEM 1310 - Sections L and M 22

PRS Question

Which of the following is the conjugate baseof HPO4

2-?

1. H3PO4

2. H2PO4-

3. HPO42-

4. PO43-

5. None of these

Subtract H+ from the acid

Week 9 CHEM 1310 - Sections L and M 23

PRS Question

What is the concentration of H3O+ in anaqueous solution with a pH of 9.25?

1. 2.5 x 10-10 M

2. 5.6 x 10-10 M

3. 2.5 x 10-9 M

4. 9.6 x 10-5 M

5. None of these

Week 9 CHEM 1310 - Sections L and M 24

PRS Question

What is the concentration of H3O+ in anaqueous solution with a pH of 9.25?

1. 2.5 x 10-10 M

2. 5.6 x 10-10 M

3. 2.5 x 10-9 M

4. 9.6 x 10-5 M

5. None of these

Antilog (-9.25) = [H3O+]