9.3 - Section 4

8/3/2019 9.3 - Section 4

1/36

9.3 - The Acidic Environment

Section #4Theories of Acid Structure

Because of the prevalence and importance of acids, they have beenused and studied for hundreds of years. Over time, the definitions

of acid and base have been refined

8/3/2019 9.3 - Section 4

2/36

Antoine Lavoisier, the French Chemist who pioneered verycareful, accurate quantitative methods.

In the 1780s, Lavoisier noted that non-metal oxides

produced acidic solutions.

From this he concluded

LavoisierTheories of Acid Structure

Acids contain the element Oxygen

The discovery of the acidic compounds of HCl and HCN contradicted this theory.

8/3/2019 9.3 - Section 4

3/36

Both the theories of Lavoisier and Davy were observational definitions. That is,the definitions were based upon experimental observation, and didnt attempt anyreal in-depth explanation of the structure of acids and bases.

DavyTheories of Acid Structure

In 1815, Humphrey Davy noted

Acids contained Hydrogen, and the Hydrogen isreplaced when reacting with a Metal

Davy also noted that compounds of metals and oxygenwere often basic.

8/3/2019 9.3 - Section 4

4/36

Theories of Acid Structure

In 1884 Savante Arrhenius proposed

Arrhenius

Acids produce H+ ions when in solution (aq)andBases produce OH - ions when in solution (aq)

These definitions are the basis of junior high school acids / base study.

This definition is restricted to acids and bases where the solvent is water.

This theory cannot explain the fact that some compounds create acidic & basicsolutions even though they do not contain H+ or OH- ions (eg NH4Cl forms an

acidic solution & K2CO3 forms a basic solution more on this later).

8/3/2019 9.3 - Section 4

5/36

In 1923 Johannes Bronsted & ThomasLowry independently proposed

Acids are proton donorsand

Bases are proton acceptors

Bronsted & Lowry

When HCl molecules dissolve in water, a proton moves from the HCl moleculeto a water molecule (creating an hydronium ion)


Example

HCl(g) + H2O(l) H3O+

(aq) + Cl-(aq)

8/3/2019 9.3 - Section 4

6/36

Hence, an acid/base reaction can be considered to be a Proton TransferReaction.

When Ammonia (NH3) gas dissolves in water, a proton moves from the H2O

molecule to the ammonia molecule (creating an ammonium ion)

Notice from these two examples, water can act as either and acid or a base,depending on what it is reacting with.

This means that water can be either a proton donor or a proton acceptor!

Hence water is an example of an Amphiprotic substance (another example is theHCO3- ion, more later)


Example

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

8/3/2019 9.3 - Section 4

7/36

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

Conjugate Acids & Bases

Remember these definitions

Amphoteric Substance = able to react chemically as either an acid or a baseAmphiprotic Substance = producing and reacting with protons thereforehaving properties of both an acid and an alkali

We have already investigated the following reaction

In the above example NH3 & NH4+ are Conjugate Pairs.


8/3/2019 9.3 - Section 4

8/36

H2O(l) + NH3(g) OH-(aq) + NH4

+(aq)

Conjugate Pairs = differ by one proton & are linked Conjugate pairs are used to describe substances that have such similar molecular

structures thatbecomes the other through the gain or loss of a proton.

So using re-examining the reaction, we find two conjugate pairs


Pair #1: H2O & OH- Pair #2: NH3 & NH4

+

8/3/2019 9.3 - Section 4

9/36

HCl + NH3 Cl- + NH4

+

In summary, an acid-base reaction involves two different conjugate pairs.

conjugate pair 1

conjugate pair 2

It would seem relatively obvious that a strong acid would have a very weakconjugate base (and vice versa).


acid1 + base2 base1 + acid2

Example

8/3/2019 9.3 - Section 4

10/36

Relative Strengths of Bronsted & LowryAcids & Bases Strong Bronsted & Lowry acids form weak conjugate Bronsted & Lowry bases,

and vis versa.


ExampleHCl(g) + H2O(l) H3O

+(aq) + Cl

-(aq)

conjugate pairStrong

Bronsted & Lowry Acid

Weak

Bronsted & Lowry Base

In other words, the HCl donates protons very readily, and the Cl- ion is a verypoor acceptor of protons.

Hence the forward reaction is favoured, and HCl dissociates readily!

8/3/2019 9.3 - Section 4

11/36

Strong Bronsted & Lowry acids form weak conjugate Bronsted & Lowry bases,

and vis versa.


8/3/2019 9.3 - Section 4

12/36

Different acids will produce differing amounts of H+ ions when they dissociate inwater.

Hydrochloric Acid (HCl) is an example of a MonoproticAcid, because itproduces one proton (H

+

) for each HCl molecule.

Sulfuric Acid (H2

SO4

) is an example of a Diprotic Acid , because it producestwo protons (H+) for each H2SO4 molecule.

HCl H+ + Cl-

H2SO4 H+ + HSO4

-

HSO4- H+ + SO4

-

Monoprotic, Diprotic & Triprotic AcidsTheories of Acid Structure

8/3/2019 9.3 - Section 4

13/36

Remember that one of the problems with the Arrhenius theory of acids and baseswas its inability to explain why various salt solutions were acidic or basic.

The BronstedLowry theory provides an explanation for these observations.

pH of Salt Solutions

Phosphoric Acid (H3PO4) is an example of a TriproticAcid, because itproduces three protons (H+) for each H3PO4 molecule.

H3PO4 H+ + H2PO4

-

H2PO4- H+ + HPO4

2-

HPO42- H+ + PO43-


8/3/2019 9.3 - Section 4

14/36

We classify salts that form basic solutions as basic salts, and those that form

acidic solutions are called acidic salts.

The reaction of a salt with water to produce a change in pH is called Hydrolysis. One of the simplest method of determining the nature of a salt (acidic, basic, or

neutral) is to examine how the salt can be produced


Strong Acid + Strong Base Neutral Salt + Water

Weak Acid + Strong Base Basic Salt + Water

Strong Acid + Weak Base Acidic Salt + Water

8/3/2019 9.3 - Section 4

15/36


Neutral salts do not react with water to any appreciable extent.

Consequently, the pH of the water remains unchanged.

Salts such as NaCl, K2SO4 and NaNO3 are neutral salts.

The salts of weak bases and weak acids may also be neutral if the relativestrengths of the acid and base are similar. This is difficult to achieve in practice,and is not on the HSC course.

Neutral Salts

8/3/2019 9.3 - Section 4

16/36

The ammonium ion acts as a weak BronstedLowry acid when dissolved in water.This equilibrium lies to the right and the hydronium ion is produced.


Remember, acidic salts are the salts of strong acids and weak bases.

Acidic Salts

Example Ammonium Chloride NH4Cl

H2O(l) + NH4+

(aq) H3O+

(aq) + NH3(aq)

HCl + NH3 NH4Cl + H2O

strongacid

weakbase

acidicsalt

possible formation

8/3/2019 9.3 - Section 4

17/36

Sodium carbonate Na2CO3 H2CO3 + 2NaOH Na2CO3 + 2H2O

weakacid

strongbase

basicsalt

possible formation

Remember, basic salts are the salts of weak acids and strong bases.

Basic Salts

The carbonate ion acts as a strong BronstedLowry base when dissolved inwater. This equilibrium lies to the right as the and the hydroxide ion is produced.


Example

H2O(l) + CO32-

(aq) OH-(aq) + HCO3

-(aq)

8/3/2019 9.3 - Section 4

18/36

The fluids within the bodies of living things must be maintained in a narrow pHrange in order for our biochemical processes to occur at an optimal rate.

BuffersTheories of Acid Structure

The pH of saliva must be maintained in the range 6.4 7.0 or the amylaseenzyme (that catalyses the breakdown of carbohydrates) will not function.

Gastric Juices (in the stomach) must have a pH around 1.6 in order for enzymessuch as pepsin to catalyse the breakdown of proteins into amino acids.

Cells in the pancreas secrete pancreatic juice whose pH must be maintainedaround 8.5 in order for lipase enzymes to digest fats into fatty acids. Thealkalinity of this fluid also neutralises gastric juices from the stomach as theyenter the small intestine.

Example

8/3/2019 9.3 - Section 4

19/36

Theories of Acid Structure Solutions called Buffersresist changes in pH when small quantities of an acid or

base are added to them.

Natural buffers restrict the pH range of body fluids, ensuring that biochemicalreactions proceed at their required rate.

Buffers are also used in a chemical laboratory to calibrate pH meters, as well asproviding a constant pH environment for chemical processes.

Buffer solutions usually contain a weak BronstedLowry acid and its conjugatebase (or a weak BronstedLowry base and its conjugate acid).

By choosing the correct amounts of the weak acid / base (and their conjugates) in thesolution, the pH of the solution can be fixed to within narrow limits.

How Buffers Work

8/3/2019 9.3 - Section 4

20/36

HClaOH

Example


Consider Acetic Acid in solution

CH3COOH(aq) + H2O(l) H3O+

(aq) + CH3COO-(aq)

weak acid1% dissociation hence very small amounts

of these ions present in solution

So

CH3COOH

The added OH- ions can be easilyneutralised until the CH3COOH is used

up thus the pH of the solution slowlybecomes basic!

The added H3O+ ions cannot be easily

neutralised since there is little CH3COO-

ions to react with them thus the pH ofthe solution becomes more acidic!

8/3/2019 9.3 - Section 4

21/36

Now consider a solution of Acetic Acid & Sodium Acetate

CH3COOH(aq) + H2O(l) H3O+

(aq) + CH3COO-(aq)

present due to originalsolution of acetic acid

large amount present due toadded sodium acetate

HClaOH

Example


So The added OH- ions can be easilyneutralised by the CH3COOH

The added H3

O+ ions can be easily

neutralised by the CH3COO- ions

CH3COO-CH3COOH Hence the pH of the solution will resist

changing because of added acid or base

8/3/2019 9.3 - Section 4

22/36

A Titration is a Volumetric Analysis technique where accurate measurement ofvolumes are used to deduce the concentrations of unknown solutions.

Several basic chemistry skills are required to complete a successful titration,

these include

1. Accurate and precise laboratory skills.

2. Balancing Chemical Equations.

3. Mathematical analysis of the results and deduction of the unknown

concentration.

Theories of Acid StructureTitrations

Types of Titrations There are several different type of titrations, however the most common variety

of titration used in chemistry is the Acid Base Titration.

8/3/2019 9.3 - Section 4

23/36

As the name suggests, this involves the analysis of the reaction of an Acid witha Base (neutralisation reaction).

The reaction is allowed to precede to the Equivalence Point. The equivalence point is the theoretical point where the acid and base have

completely reacted together in molar ratio quantities (given by the balanced chemical

equation for the reaction).

Example

The equivalence point of the above reaction occurs when HCl and NaOH havereacted in a 1:1 mole ratio.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

The equivalence point of the above reaction occurs when H2SO4 and NaOH have

reacted in a 1:2 mole ratio.

H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + H2O(l)


8/3/2019 9.3 - Section 4

24/36

When the equivalence point is reached, the solutions pH will depend upon the

nature of the reactants.

Strong Acid + Strong Base Neutral Salt Solution pH about 7 Strong Acid + Weak Base Acidic Salt Solution pH below 7 Weak Acid + Strong Base Basic Salt Solution pH above 7

If an Indicator is used during a titration, its colour change will signal that thereaction has reached the equivalence point.

The point where the indicator changes colour is called the End Point. An indicator is chosen so that its end point wil l coincide with the

expected equivalence point of the neutralisation reaction.


Example

8/3/2019 9.3 - Section 4

25/36

The most common equipment used to complete a titration are

1. Burettes Rinse the internal surface with 10-15mL of the solution that you

intent to use (holding the burette almost horizontal).

Run this washing liquid through the tap to remove it.

Repeat the process if necessary, & discard the washings.

Place some of the solution to be used in the burette and run

some liquid through the tap for a few seconds to remove any airbubbles.

Fill the burette to the top line, using a filter funnel, and removethe funnel when full.

Make sure the burette is positioned vertically.

Theories of Acid StructurePreparation & Use of Glassware

8/3/2019 9.3 - Section 4

26/36

2. P ipettes NEVER pipette by mouth. Rinse the internal surface with 5-10mL of the solution that you

intent to use & discard the washings.

When filling, do not allow the solution to enter the pipette filleretc

When full, hold vertically and check

Drain the liquid into a prepared conical flask, holding the tip ofthe pipette against the inner surface of the conical flask.

Bottom of meniscus sits on the line No air bubbles are present

3. Conical Flask Wash with water.

Following a titration, empty & rinse, then it can be used inanother titration.


8/3/2019 9.3 - Section 4

27/36

4. Volumetric F lask Used if making standard solutions, or completing dilutions. Rinse the internal surface with water.

All solid (for standard solutions) should be dissolved before fillingthe flask to the line with water.

Add the final amounts of water drop by drop.

Stopper, invert and mix well.

In summary

Wash with water, dont dry rinse with solution


8/3/2019 9.3 - Section 4

28/36

In general, chemists call a solution of known concentration a StandardSolution. In general, the standard substance that is dissolved should

1. Be a water soluble solid obviously it is easier to measure an accurateamounts of a solid than liquid or gas.

2. Have an accurately known chemical formula mixture are therefore notused.

3. Be stable in air we do not want our chosen chemical to be reactingand changing before or during the weighing process.

4. Have a very high purity analytical reagent (AR) grade or laboratoryreagent (LR) grade .

Theories of Acid StructureStandard Solutions

8/3/2019 9.3 - Section 4

29/36

Making a Standard Solutions

The water that is used to dissolve the standard substance should be of the

highest purity possible distilled, deionised, demineralised, triply ionised water,etc

The following steps are used to create a standard solution

1. Determine the required Concentration and Volume for the standardsolution (volume limited to size of volumetric flasks, 100mL, 250mL ).

2. Calculate the number of mole of the substance that is required.3. Calculate the mass of the substance that is required.4. Accurately weigh the required mass.5. DissolveALL of the measured mass using pure water, and transfer this

to a clean & prepared volumetric flask.


8/3/2019 9.3 - Section 4

30/36

6. Fill the volumetric flask to the indicated line.

7. Stopper the flask and mix (invert, rotate, shake ).

8. Calculate the approximate concentration using actual measured mass

9. Label flask with solution details (chemical, date, etc).


ExampleA chemist wished to create 250mL of 0.125M NaOH solution

#mol NaOH = 0.125 0.25

= 3.125 10-2 1.2499 gram of NaOHshould be accuratelymeasured out anddissolved in pure water

mass NaOH = 3.125 10-2 39.998

= 1.2499g

8/3/2019 9.3 - Section 4

31/36

Diluting Solutions Sometimes we may have a solution of known concentration, that we will use to

create an new solution of a desired concentration.

We often use the cv method, that is we use the following formula

C1V1 = C2V2


ExampleA chemist wished to dilute some of a 5M solution of sulfuric acid, to make 250mL of1.5M solution of sulfuric acid.

solution 1(original solution)C1 = 5M V1 = ?

solution 2(new solution)C2 = 1.5M V2 = 0.25L

C1V1 = C2V2

5 V1 = 1.5 0.25

V1 = (1.5 0.25) 5

V1 = 0.075L

V1 = 75mL

75mL of theoriginal solution canbe diluted with purewater in a 250mLvolumetric flask tomake a 1.5Msolution!!

8/3/2019 9.3 - Section 4

32/36

Theories of Acid StructureTitration Methodology The strategy that is employed during a titration is as follows

1. An accurately known volumeof a solution whose concentration isunknown is placed in a conical flask using a Pipette.

unknown concentration accurately known volume

8/3/2019 9.3 - Section 4

33/36

Theories of Acid Structure2. A solution of accurately known concentration is placed in a Burette.

accurately knownconcentration

3. Equipment is setup and anappropriate Indicator isadded to the conical flask.

8/3/2019 9.3 - Section 4

34/36

Theories of Acid Structure4. The solution from the burette is run into the conical flask (while it is

swirled to mix completely).

5. When the indicator reaches its End Point (colour change), the volume ofsolution added from the burette is noted.

Remember at the end point, the acid and base havereacted in mole ratios.

Hence we can deduce the unknown concentration ofthe solution in the conical flask.

8/3/2019 9.3 - Section 4

35/36

The general equation occurring in a titration is as follows

Acid + Base Salt + Water

Since the acid and base used in a titration are both aqueous solutions, the mostcommon properties that we will measure are Concentration and Volume . Also during most titrations, the products (salt & water) are often ignored, it is

therefore the Acidic and Basic concentrations and volumes that are the mostimportant.

# mol Acid

[Acid]Volume

(acid)

# mol Base

[Base]Volume

(base)

mol ratio

Theories of Acid StructureTitration Mathematics

8/3/2019 9.3 - Section 4

36/36

A student prepares a solution of hydrochloric acid that is approximately 0.1M andwishes to determine its precise concentration. A 25.00mL portion of the HClsolution is transferred to a conical flask, and after a few drops of indicator is added,the HCl solution is titrated with 0.0775M NaOH solution. The titration requiresexactly 37.46mL of the standard NaOH solution. What is the molarity of the HCl

solution?

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

# mol NaOH = 0.0775 0.03746 = 2.90 10 -3 molfrom the equation # mol HCl = 2.90 10-3

[HCl] = 2.90 10-3 0.025

= 0.116 M (mol L-1)

NaOH : HCl 1 : 1

Example


9.3 - Section 4

Documents

Transcript of 9.3 - Section 4