Today’s Objectives - Webb...

Today’s Objectives

Describe Brønsted–Lowry acids as proton donors and bases as proton acceptors

Write Brønsted–Lowry equations, including indicators,and predict whether reactants or products are favoredfor acid-base equilibrium reactions for monoprotic andpolyprotic acids and bases

Identify conjugate pairs and amphiprotic substances

Section 16.2 (pp. 722-736)

1

Brønsted-Lowry Acid-Base Concept modified Arrhenius theory comprehensively describes,

explains, and predicts acid-base rxn

includes hydronium ions, rxn with water, rxnequilibrium, & water ionization equilibrium

limited, however, b/c no provision for rxn not in (aq) solutions, nor explains substances that have both acid and base properties

Example: baking soda (sodium hydrogen carbonate)

alkaline (aq) solution and will partially neutralize lye solution (sodium hydroxide)

rxn discussed on p. 722

2

Proton Transfer Concept Focus on the chemical species in an acid-base rxn instead

of acidic or basic properties of their (aq) solution

upon dissolving in water, the substance reacts with a water molecule:

acids donate a proton to a water molecule

acid has hydrogen ions to give away

base accepts a proton from a water molecule

attracts hydrogen ions better than water

proton = p+ = H+ = hydrogen ion

3

Brønsted-Lowry Definitions:

acid proton donor

base proton acceptor

Proton Transfer Concept

4

Proton Transfer Concept Molecules compete for protons

water doesn’t need to be a reactant for p+ transfer to occur

hydronium ions in solution can react directly with a base

Example: ammonia reacting with hydrochloric acid, which quantitatively produces H3O

+(aq)

H3O+

(aq) + NH3(aq) ⇌ H2O(l) + NH4+

(aq)

ammonia (base) removes a proton from the hydronium ion (acid)

water is present as a solvent, but not the primary reactant

5

A B

H+

Proton Transfer Concept Concept extends beyond (aq) solutions;

water doesn’t even need to be present.

Consider a reaction in a system with gaseous entities:

HCl(g) + NH3(g) ⇌ NH4Cl(s)

6

A B

H+

acid is a proton donor and a base is a proton acceptor according to Brønsted-Lowry Concept

Focus on the idea of a base attracting a proton and an acid not attracting a proton

Only one proton transferred at a time


7

Weak acids attract proton more strongly than water molecule in solution

Conclusion inferred b/c at equilibrium pH evidence indicates few WA molecules have lost p+ in their collisions with water molecules

stronger base attracts protons more


8

Brønsted-Lowry Acid-Base Concept Brønsted-Lowry equation shows acid-base rxn

(neutralization) with proton transfer from the strongest acid present to the strongest base

not necessarily a SA or SB mentioned by Arrhenius definition

now acid or base defined as entity involved in the proton transfer for a particular rxn

9

Brønsted-Lowry Acid-Base Concept amphoteric (empirical term)

substance able to react as an acid or a base

Example: water

Greek word amphi meaning “both” – like amphibian (survives on land and in water)

ambiguous with amphiprotic (theoretical term)

entity (ion or molecule) able to accept or donate a proton.

hydrogen polyatomic ions are amphiprotic and with experimental evidence determined equilibrium shift direction favored – see p. 723i.e. amphiprotic entity causes amphoteric nature of a chemical substance

10

Brønsted-Lowry Acid-Base Concept Example: bicarbonate ions

in (aq) solution, some react with the water molecules by acting as an acid, and some react by acting as a base.

Kc values given for these reactions show that one predominates

The number of ions acting as a base is over 2000x more than the number reacting as an acid ⇒ alkaline solution (basic)

11

Brønsted-Lowry Acid-Base Concept Example:

baking soda – common substance with propertieseasy to explain but very difficult to predict.

amphiprotic nature of hydrogen carbonate ion(bicarbonate ions) can partly neutralize any acidor base spill

12

Brønsted-Lowry Acid-Base Concept

Food or drink spilled on fabric stain partially b/c of acidicor basic properties of the spill, but amphoteric property ofbaking soda will help reduce staining without knowingwhether substance is acidic or basic.

13

Brønsted-Lowry Acid-Base Concept Criteria used to test scientific theories:

ability to describe, explain, and predict phenomena that are observed

Brønsted-Lowry concept is considered a theoretical definition.

Fails to explain why p+ donated or accepted.

Also doesn’t predict theoretically which rxn occurs for a given entity in any given new situation.

14

Brønsted-Lowry Acid-Base Concept Advantages:

defines acid / base in terms of chemical rxn instead of as a substance that forms acidic or basic (aq) solutions

describes, explains, and predicts many more rxn in (aq) or other solutions with different solvent or btwnundissolved entities in a pure chemical state

15

Acid-Base Conjugate Pairs two acids and two bases present in any acid-base rxn

equilibrium (fwd & rev rxn)

16

Acid-Base Conjugate Pairs substances with formulas that differ only by a proton

are conjugate pairs

17

Acid-Base Conjugate Pairs Example: hydrochloric acid ionizes in water

When HCl reacts with water, the water wins the competition against the Cl- for the proton.

This is why at equilibrium essentially all of the HClmolecules have lost protons to water.

18

>99%

Acid-Base Conjugate Pairs Example: hydrochloric acid ionizes in water

Consider possibility of rev rxn

even though known that SA quantitatively ionize

19

Acid-Base Conjugate Pairs conjugates are the acid and base considered

for the reverse rxn

conjugate products of acids and bases:

A → CB and B → CA

Also consider WA examples, such as acetic acid and carbonic acid ionizing

20

Acid-Base Conjugate Pairs

General Case:

21

Relative Strength of Acids & Bases table indicates ability to ionize in water

see data booklet – pp. 8-9

Strengths determined by extent of proton transfer btwn with the water solvent

SA at top, very WA at bottom Empirically rank WA strength by comparing

ionization of solutions with the same concentration to determine acidity (pH)

22

Relative Strength of Acids & Bases Large Ka indicates strong acid (six on list)

quantitative ionization (complete p+ transfer)

produce hydronium ions in solution

proton weakly held by strong acids

weak / negligible p+ attraction

Inverse relationship between conjugate strength

i.e. stronger acid has a weaker conjugate base

23

Conjugate Pairs Strength

The stronger the base, the more it attracts a proton

proton acceptor

The stronger the acid, the less it attracts its own proton

proton donor

24

Conjugate Pairs Strength in terms of conjugate pairs:

stronger acid ⇒ weaker conjugate base If you are good at donating a proton, this means the

conjugate base is not good at competing for it

weak attraction for protons

stronger base ⇒ weaker conjugate acid If you are good at accepting a proton, this means the

conjugate acid is not good at giving it up

strong attraction for protons

25

Homework

Practice Qs – p. 724 #1-6; p. 726 #7-8

INV 16.1 Prelab – p. 727

27

INVESTIGATION 16.1 – p. 727

Discuss Pre-lab prepare prediction, procedure, and observation table

28

Predicting Acid-Base Rxn Equilibria Brønsted-Lowry excludes theoretical explanation of

why entities attract protons at varying strengths

Rely on empirical evidence (experimental observations) to predict the outcome

Predictions are restricted to pre-tested acid-base combinations with data already obtained

Simple generalizations allow for predictions of approximate equilibrium position in the proton transfer of acid-base mixtures

29

Predicting Acid-Base Rxn Equilibria Predict an acid-base rxn by combining concepts to predict

the products and the extent of the acid-base rxn

Collision Rxn Theory protons transfer b/c of collision btwn acid and base molecules

collisions are constantly occurring

each time a p+ is transferred to a stronger proton attractor

Many random collisions in mixture of several acid-base entities countless different acid-base rxns possible that all occur all the

time, to some extent

evidence indicates that a certain rxn dominates btwn the strongest acid and the strongest base in system

other rxns have negligible effect on equilibrium ∴ are ignored

30

Predicting Acid-Base Rxn Equilibria Theoretically, protons could transfer several times

each time to a stronger proton attractor

However, once a proton is transferred to the strongest base present, it will remain there as nothing outcompetes it for proton attraction

Likewise, once the strongest acid has given up its proton, its conjugate base cannot gain one back

CB is the weakest proton attractor in the whole system

p+ transfer occurs btwn the strongest acid and base

all other transfers are negligible so are ignored31

Predicting Acid-Base Rxn Equilibria consider relative position of the SA and SB to

determine equilibrium position

compare strength of entities in the rxn

see Learning Tip – p. 729

SA > SB rxn > 50% (products favored)

SB > SA rxn < 50% (reactants favored)

32

Predicting Acid-Base Rxns List all entities present initially as they exist in (aq) solution

ions, atoms, molecules, H2O(l)

33

Predicting Acid-Base Rxns List all entities present initially as they exist in (aq) solution

ions, atoms, molecules, H2O(l)

H3O+

(aq) is the SA that can exist

if a stronger acid is dissolved, it reacts instantly and completely with water to form H3O

+(aq) so all strong acids are written as H3O

+(aq)

OH-(aq) is the SB that can exist.

if a stronger base is dissolved, it reacts instantly and completely with water to form OH-

(aq)

The only example of this is soluble ionic oxides

write the cation and the oxide ion is written as OH-(aq)

34

Predicting Acid-Base Rxns NO entity can react as a base if it is weaker than water

For this reason, the conjugate bases of the strong acids are not considered bases in aqueous solutions

35

Sample Problem 16.1 – p. 729What will be the predominant reaction if spilled drain cleaner (sodium hydroxide) solution is neutralized by vinegar?

List entities present:

36

Na+(aq) OH-

(aq) CH3COOH(aq) H2O(l)


Identify and list all possible (aq) acids and bases, using the

Bronsted-Lowry definitions.

use Relative Strengths of Acids and Bases (see data booklet – p. 8)

amphiprotic entities are labeled for both possibilities.

conjugate bases on SA’s are not included.

metal ions are treated as spectators.

37

Na+(aq) OH-


A

B

A

B


Identify the SA and SB in the system

use Relative Strengths of Acids and Bases (see data booklet – p. 8)

SA – highest on table

SB – lowest on table

38

Na+(aq) OH-


A

B

A

B


Write an equation showing a transfer of one proton from the SA to the SB and predict the conjugate base and the conjugate acid to be the products.

39

Na+(aq) OH-


SA

SB


Predict the approximate position of equilibrium

see Learning Tip – p. 729

40

Na+(aq) OH-


SA

SB

NOTE: The reaction of

H3O+

(aq) and OH-(aq) is

always quantitative

(>99%) so a single

arrow can be used

Predicting Acid-Base Rxn Equilibria Specific restrictions dictate proton transfer rxn in

(aq) solutions

H3O+ is the strongest acid entity that exists in (aq)

solutions

Six strong acids (listed above hydronium) in water react instantly and quantitatively (completely) with water molecules to produce H3O

+

therefore all six SA can be written as H3O+ in (aq)

solutions

41


(aq) solutions

OH– is the strongest base entity that exists in (aq) solutions

stronger base dissolves in water instantly and quantitatively to form hydroxide ions

Example: dissolved ionic oxide compound, like Na2O(s), write oxide ion (O2–) as OH– in (aq) solutions

42


(aq) solutions

an entity can’t act as a base in (aq) system if it’s a weaker base than water, therefore the CB of the six SA not considered a base in (aq) solution

Read Learning Tip – p. 730

43

Predicting Acid-Base Rxn Equilibria Assumptions for Nelson:

Equation represents a single proton transfer, neither of which is water, and in stoich ratio

Strongest acid and base present in significant chemical amounts and approximately equal concentrations

rxn must satisfy these restrictions for Brønsted-Lowry to assume products or reactants favored based on % rxn(Learning Tip – p. 729 ) AND Kc value ( >1 – P; <1 – R)

rxn of H3O+ and OH– is always quantitative (single arrow)

44

Communication Example – p. 730Ammonium nitrate fertilizer is produced by the quantitative reaction of aqueous ammonia with nitric acid. Write a balance acid-base equilibrium equation.

NH3(aq), H2O(l), H3O+

(aq), NO3–(aq)

NH3(aq) + H3O+

(aq) ⇌ NH4+

(aq) + H2O(l)

45

A

B

S

S

A

B

Predict Predominant Acid-Base Rxn1. List all rxn entities as they exist in (aq) solution

Cations, anions, molecules, atoms, water, etc.

2. Using Brønsted-Lowry definitions identify and label all possible (aq) acid and bases

3. Using the relative strengths table identify the strongest acid and strongest base present

4. Write an equation showing a transfer of one proton from the strongest acid to the strongest base and predict the CB and CA to be the products.

5. Predict the approximate equilibrium position

47

Homework Practice Qs – p. 731 #9-11

Lab Exercise 16.C – p. 732 DUE: Thursday, October 8

Extra Practice – try INV 16.2 prediction – p. 732

Read Case Study – p. 733

Section 16.2 Review – p. 738 #1-8

Section 16.2 Extra Exercises handout

48

INVESTIGATION 16.1 – p. 727

Discuss Post Lab

Formal Report

Refer to textbook Appendix B – p. 790

49

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