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E X P E R I M E N T 1 1

Acidity and Basicity

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Contents are not mine. It's only a compilation of researches for our reports so yeah. I hope it'll be helpful to others.

Text of Acidity and Basicity

E X P E R I M E N T 11

IntroductionAcidity and Basicity

Acids

Any

solution that releases hydrogen ions when added to water and has a pH of less than 7.0

pH it measures the acidity of a liquid by measuring the concentration of hydrogen ions.

PROPERTIES OF ACIDS : Sour

taste paper blue turns red

Litmus

Reactions Neutralize

with metal oxide and hydroxidebas forming water

Bases

Substances

which combines with acid and also known as Alkaline substance. Compound that furnishes the hydroxide ions. Compound that gives or donate hydroxyl ions in water or other substances.

Hydroxyl unit that composed of one or more atom of hydrogen and one of oxygen.

PROPERTIES OF BASES : Slippery Soapy Red

feeling and a biting , bitter taste

litmus blue , turn methyl organic from red to yellow phenolphthalein from colorless to

Turn

red

ProceduresAcidity and Basicity

Using the pH Paper

pH paper in test solution Dip For 10 seconds

Color Chart Match the color obtained in the test soln

Record

Repeat With other test solutions

Using the pH Meter

Buffer Solution Immerse the electrode After that, rinse the electrode with distilled water Wipe with tissue

First Solution Dip the electrode Get the pH reading Record

Distilled Water Rinse the electrode Wipe it with tissue paper

Repeat With other test solutions

Keep the electrode immersed in distilled water when not inuse.

Samples

Chemical Formula Acetic Acid CH3COOH

Structural Formula

Functional Group

Carboxyclic AcidCarboxyclic Acid, Alkyl Halides

Monochloroacetic acid

ClCH2COOH

Acetone

(CH3)2CO

Ketone

Acetamide

CH3CONH2

Amides

Chemical Formula Glysine NH2CH2COOH

Structural Formula

Functional Group

Amines, Carboxyxlic AcidAmines, Carboxyxlic Acid

Lysine

C6H14N2O2

Isopropyl Alcohol

(CH3)2CHOH

Alcohol

Phenol

C6H5OH

Alcohol, Aromatic Compound

Chemical Formula

Structural Formula

Commercial Vinegar

C2H4O2

Calamansi Juice

C6H8O7

Spoiled Milk

C3H6O3

Arrhenius ConceptAcidity and Basicity

Svante

Arrhenius, a Swedish chemist who received a Nobel prize in 1903 for his work on electrolytes, focused on what ions were formed when acids and bases dissolved in water.

One

of the properties that acids and bases have in common is that they are electrolytes--they form ions when they dissolve in water. He came up with the concept or idea that acids dissociated in water to give hydrogen ions (H+) and that bases dissociated in water to give hydroxide ions. (OH-)

Examples:HCl H+ + ClAn acid, like HCl, is something that dissociates in water to give hydrogen ion. NaOH H+ + Cl-

A base, like NaOH, is something that dissociates in water to give hydroxide ion.

Arrhenius

focused on the idea that acids and bases split into ions when they dissolved in water. In a sense, the Arrhenius concept focuses on what the chemical contains or what is there in solution.

Brnsted-Lowry ConceptAcidity and Basicity

With

the Brnsted-Lowry concept we usually refer to a hydrogen ion as a proton. That is because a proton is all that is left when a hydrogen atom loses an electron to become an ion.

Brnsted Acids Proton

(H+) Donor. When an acid reacts, the proton is transferred from one chemical to another. The chemical which accepts the proton is a base.

H+ Cl- +I DONATE!!

+

Cl

Note

that in order for an acid to act like an acid, there needs to be something for it to react with. There needs to be something to take the proton. There needs to be a base.

Brnsted Bases Proton

Acceptor. Opposite of acids. Bases are basic because they take or accept protons. Hydroxide (OH-) ion, for example can accept a proton to form water.

Brnsted

and Lowry realized that not all bases had to have a hydroxide ion. As long as something can accept a proton it is a base. So anything, hydroxide or not, that can accept a proton is a base under the Brnsted-Lowry definition.

The

water molecules that accept protons when HCl dissolves in water are acting as bases.

+ ClHI DONATE!

+

I ACCEPT!

+ Cl-

Some

additional examples of BrnstedLowry bases are shown accepting protons in these equations. These examples do not show the acids which are providing the protons. Ammonia can accept or react with hydrogen ion to give ammonium ion (NH4+)

NH3 + H+ NH4+

Carbonate

ion (CO32- )can accept a hydrogen ion, or accept a proton, to become bicarbonate ion (HCO3-).

CO32- + H+ HCO3 Also,

water molecules, as mentioned before, can act as a base by accepting protons.

H2O + H+ H3O+

Hydroxide, ammonia, carbonate and water are all Brnsted-Lowry bases.

When

a Brnsted-Lowry acid donates a proton, it forms the conjugate base of that acid. When a base accepts a proton, it forms the conjugate acid of that base. Conjugate base and acid are produced as products. The formulas of a conjugate acid-base pair differ by one proton (H+)

Consider

what happens when HCl(g) is bubbled through water, as shown by this equation: The conjugate base of HCl is Cl The conjugate acid of Cl- is HCl Conjugate acid-base pair

HCl(g) + H2O(l) Cl-(aq) + H3O+(aq)Conjugate acid-base pair The conjugate base of H2O is H3O+ The conjugate acid of H3O+ is H2O

To

write the conjugate base of an acid,remove one proton from the acid formula: H+ H2O OH(Conjugate base) H+ HNO3 NO3- (Conjugate base) that, by removing H+, the conjugate base becomes more negative than the acid by one minus charge.

Note

To

write the conjugate acid of a base, add one proton to the formula of the base:

HSO4(Conjugate acid) +H+ C2H3O2HC2H3O2 (Conjugate acid) SO4 In

+H+ 2-

each case the conjugate acid becomes more positive than the base by a +1 charge due to the addition of H+.

Lewis ConceptAcidity and Basicity

The Lewis Concept as an Extension of the Brnsted Concept An

acid is an electron-pair acceptor. A base is an electron-pair donor. An acid-base reaction is the sharing of an electron pair with an acid by a base.

These

three simple definitions constitute the heart of what is now known as the Lewis concept of acids and bases. Experimentally and conceptually, they are an extension of the Brnsted definitions.

The Fundamental Lewis AcidBase Reaction

the formation of a coordinate covalent bond between an acid and a base. The base is the electron-pair donor, the acid the acceptor. The process is called neutralization, or simply coordination. The product is a coordinated compound, coordinated complex, or adduct, made up of an acid portion and a base portion.

A

typical, and oft-cited example is the reaction of the acid boron trifluoride with the base dimethyl ether to form the complex or adduct BF3CH3OCH3:

The

coordinate molecule may be thought of as being made up of the acid portion BF3 and the base portion CH3OCH3

Classification of Lewis AcidsI. Simple Cations. Theoretically all simple cations are potential Lewis Acids, although their strength as acids varies within wide limits. In general, we can expect the acid strength or coordinating ability of cations to increase with:

an increase in positive charge on the ion, an increase in nuclear charge for atoms in any horizontal period, a decrease in ionic radius, and a decrease in the number of shielding electron shells.

This

means that Lewis acidity of simple cations tends to increase for the elements from left to right and from bottom to top in the periodic table. (Periodic Trends)

II. Compounds Whose Central Atoms Has an Incomplete Octet. Among the most important Lewis Acids are compounds whose central atom has less than a full octet of electrons.

III. Compounds in Which the Octet of the Central Atom Can be Expanded.

Although

carbon and silicon belong to the same family of elements, silicon tetrafluoride and silicon tetrachloride are tremendously more reactive than their carbon analogues, carbon tetrafluoride and carbon tetrachloride.

The

explanation is straightforward- the silicon, with its vacant d orbitals, can act as a Lewis acid by expanding its octet. This is illustrated by the reaction of silicon tetrafluoride with fluoride ion to form fluorosilicate ion:

With

no available d orbitals, carbon cannot do this, in keeping with the fact that the elements in the first period of eight in the periodic table can accommodate no more than eight electrons in their valence shell. Actually the silicon halides typify a large group of halides which, with vacant d orbitals, can expand their octets. Some examples are:

These

halides tend to form adducts with halide ions and with organic bases such as ethers (R-O-R). Halides of this type are vigourously hydrolyzed to form an oxy-acid (or oxide) of the central atom and the appropriate hydrogen halide. This reaction depends upon the ability of the halides to act as Lewis acids.

The

first step in the removal of each halogen atom is undoubtedly the acid-base coordination of the acid halide with the base water. This is followed by elimination of the hydrogen halide from the adduct. For the removal of the first chlorine in the hydrolysis of phosphorus trichloride, we believe the pathway or mechanism is

IV. Compounds Having Multiple-bonded Acid Centres. There

are many compounds, particularly organic, in which a multiple-bonded atom can accept a share in an electron pair with a synchronous shift in a pair of electrons of the multiple bond.

By

a slight extension of the Lewis concept, we can classify such compounds as Lewis acids. Although the atom involved does not, in a strict sense, have an unfilled orbital nevertheless an orbital is made available as the incoming base forces the intramolecular electron pair shift.

A

familiar example is carbon dioxide. Consider its neutralization by hydroxide ion to hydrogen carbonate in:

V. Elements with an Electron Sextet. To the extent that oxygen and sulfur atoms participate directly in chemical reactions, they may be regarded as Lewis acids. On this basis, the oxidation with sulfur of sulfite to thiosulfate and of sulfide to polysulfide ion can be classified as acid-base reactions:

Classification of Lewis Base

They are species that contain atoms with lone pairs. Such atoms are called donor atoms. Thus H2O (donor atom, O), NH3 (donor atom, N), Cl-, and CH3CH2OH (donor atoms, O) are immediately recognized as Lewis bases. Because the number of Lewis bases is almost unlimited, it is useful to categorize them based on the number of donatable nonbonding electron pairs that they contain.

a Lewis base which can form only one bond to a Lewis acid

monodentate

name means "one tooth"

chelating / polydentate

has 2 or more donor atoms spaced so that they can attach to the same Lewis acid. Normally the donor atoms must be "spaced" by 2 or 3 intervening atoms.

non-linear, often with 2 or 3 atoms separating the donor atoms

Discussing the ResultsAcidity and Basicity

pH Paper Reading Acetic Acid Monochloroacetic acid 3 1

pH Meter Reading 3.34 1.96

Acetic Acid has a higher pH reading than Monochloroacetic acid. But Monochloroacetic acid is more acidic than Acetic acid.

Why is that so??

Acid Strength is influenced by Inductive EffectsCarboxyclic

acids are the strongest acids among compounds that contains only C, H and O.

Acid Strength is influenced by Inductive Effects

Acetic acid provides the reference mark for a typical carboxyclic acid and it has pKa value of 4.75. The pKa value of monochloroacetic acid is less than that for acetic acid.

Acid Strength is influenced by Inductive Effects The

variation in acidity among structurally similar compounds like these can be explained in the electronegativity values of the substituents. These electronegativity difference manifest themselves by donating or withdrawing electrons through the bonds between atoms, an influence known as inductive effect.

Acid Strength is influenced by Inductive Effects In

the comparison of acetic and monochloroacetic acids, the argument goes like this:

Chlorine, being electronegative, renders the carbon atom adjacent to the carbonyl group partially positive.

Acid Strength is influenced by Inductive Effects In

turn, this withdraws electron density from the carbonyl carbon atom and the oxygen atom, bearing the proton. The effect is to weaken the O-H bond, making the proton more acidic.

Acid Strength is influenced by Inductive Effects Thus

this inductive effect of the chlorine atom makes monochloroacetic acid more acidic than acetic acid. In general, an electron-withdrawing substituent near the COOH group increases the acidity of acetic acid.

Acid Strength is influenced by Inductive Effects The

more electronegative a substituent, the stronger the acid. Conversely, an electron-donating substituent makes the acid less acidic than acetic acid. Alkyl groups are the most common substituents that donate electrons.

pH Paper Reading Acetamide Acetone 9 5

pH Meter Reading 8.19 6.46

Acetamide has a higher pH reading than Acetone. Applying our basic information, pH less than 7 is acidic while pH more than 7 is basic. To conlude, Acetamide is a base while Acetone is an acid. But what makes them a base and an acid?

Amphoteric character (Amides) The

presence of a lone pair of electrons on Nitrogen atom should make acid amide basic in character. But actually, they are very feeble bases. This is because the lone pairs of electrons on the nitrogen atom is involved in Resonance with carbonyl group and is therefore not available for protonation.

Amphoteric character (Amides)

However,

under suitable conditions, acid amide can show basic or acidic character.

Amphoteric character (Amides)

The

presence of lone pair of electrons on the nitrogen atom is resonating structure (I) makes it feebly basic. Acid amide, therefor, act as a base.

Amphoteric character (Amides) Thus

acetamide (a base) reacts with hydrochloric acid (an acid) to form a salt.

Amphoteric character (Amides) Likewise,

the positive charge on the nitrogen atom shown in resonating structure (II)

Implies easy release of proton.

Amphoteric character (Amides) Acid

amide, therefore, acts as an acid. For example, acetamide behaving as an acid reacts with bases Na or HgO to form corresponding salt.

Acidity of Ketones Ketones

are far more acidic (pKa 20) than a regular alkane (pKa 50). This difference reflects resonance stabilization of the enolate ion that is formed through dissociation.

The

relative acidity of the -hydrogen is important in the enolization reactions of ketones and other carbonyl compounds. The acidity of the -hydrogen also allows ketones and other carbonyl compounds to undergo nucleophilic reactions at that position, with either stoichiometric and catalytic base.

pH Paper Reading Glycine Lysine 5 6

pH Meter Reading 6.15 6.17

Glycine has a lower pH reading than Lysine.

But Glycine is more acidic than Lysine.Why is that so??

Chemical Nature of the Amino Acids The

amino acids found in proteins have the following generalized structure:

Chemical Nature of the Amino Acids All

peptides and polypeptides are polymers of -amino acids. Several other amino acids are found in the body free or in combined states (i.e. not associated with peptides or proteins).

Acid-Base Properties of the Amino Acids The

-COOH and -NH2 groups in amino acids are capable of ionizing (as are the acidic and basic R-groups of the amino acids). As a result of their ionizability the following ionic equilibrium reactions may be written: R-COOH R-COO + H+ R-NH3+ R-NH2 + H+

equilibrium reactions, as written, demonstrate that amino acids contain at least two weakly acidic groups.

The

However,

the carboxyl group is a far stronger acid than the amino group. At physiological pH (around 7.4) the carboxyl group will be unprotonated and the amino group will be protonated. An amino acid with no ionizable R-group would be electrically neutral at this pH. This species is termed a zwitterion.

pH Paper Reading Isopropyl Alcohol Phenol 4 3

pH Meter Reading 3.40 3.33

Isopropyl Alcohol has a higher pH reading than Phenol. But Phenol is more acidic than Isopropyl Alcohol.

Why is that so??

Relative Acidities of Alcohols and Phenols The

polar O-H bond of alcohols makes them weak acids. By the Bronsted-Lowry definition, acids are hydrogen ion donors and bases are hydrogen ion acceptors in chemical reactions.

Strong

acids are 100% ionized in water and weak acids are only partially ionized. Weak acids establish an equilibrium in water between their ionized and unionized forms.

Phenols

are one million to one billion times more acidic than alcohols and this is the characteristic property that distinguishes them. Phenols will react with the base sodium hydroxide but alcohols will not.

The

acidity of phenols is explained by resonance stabilization of the phenoxide ion; the negative charge is dispersed throughout the benzene ring as opposed to being concentrated on the oxygen as it is in the alkoxide ion. Electron-withdrawing groups on the benzene ring increase the acidity of phenols.

Chemical components that makes it Acidic.Spoiled Milk Lactic Acid C3H6O3

Yes,

spoiled milk is an acid. The lactic acid makes the milk acidic, milk is said to be sour when it is at a pH level of 4.3-4.5 (acidic)

Chemical components that makes it Acidic.Calamansi JuiceCitric Acid Ascorbic Acid C6H8O7 C6H8O6

Calamansi

juice is an acid. It has mild sour taste. Sour taste is a characteristic of an acidic property.

Chemical components that makes it Acidic.Commercial Vinegar Acetic Acid (Ethanoic acid) CH3COOH

Acetic

acid is the source of the acidity in vinegar. Acetic acid (ethanoic acid) is an organic acid (carboxylic acid) and is classified as a weak acid.

Intramolecular H-Bond on acidity The

effects of intramolecular hydrogen bonding on acidity can be see not just on O-H and N-H, where acidity is greatly reduced, but also on certain C-H groups, which in some cases become the primary source of acidity.