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Experiment #6. Acid Base Indicators. What are acids and bases?. There are many different definitions for classifying a substance as an acid or a base. Definitions are based on different theories. Arrhenius theory of acids and bases. Lowry-Bronsted theory of acids and bases.

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Presentation Transcript
slide1

Experiment #6

Acid Base Indicators

slide2

What are acids and bases?

There are many different definitions for classifying

a substance as an acid or a base.

Definitions are based on different theories

Arrhenius theory of acids and bases

Lowry-Bronsted theory of acids and bases

Lewis’s theory of acids and bases

slide3

Arrhenius theory of acids and bases

Proposed in 1884 by Svante Arrhenius

An acid is a substance that contains hydrogen and produces

H+ in aqueous Solution. A base is a solution that contains

the OH group and produces Hydroxide ions OH-, in

aqueous solution

Neutralization is defined as the combination of H+ ions

with OH- ions to form H2O molecules.

H+ (aq) + OH-(aq) H2O(l)

Very Limited in Scope!!

slide4

Lowry-Bronsted theory

Proposed in 1923 by N. Bronsted and T.M.Lowry

An acid is defined as a proton donor, H+, and a base is

Defined as a proton acceptor

An acid-base reaction is the transfer of a proton from

an acid to a base

The complete ionization of hydrogen chloride, HCl in water

is an acid-base reaction

Step 1: HCl(aq) H+(aq) + Cl-(aq) (Arrehnius)

Step 2: H+ (aq) + H2O(l) H3O+

Overall: HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) (LB)

slide5

Overall: HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) (LB)

conjugate

acid

conjugate

base

acid

base

Overall: NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) (LB)

conjugate

acid

conjugate

base

base

acid

Whether water can act as an acid or base depends

on the other species present

slide6

Autoionization of water

H2O (l) + H2O(l) H3O+ (aq) + OH-(aq)

Simplified notation

H2O(l) H+ (aq) + OH-(aq)

Water is said to be amphiprotic

slide7

Strength of acids

Acids and bases are classified as strong or weak

depending on the extent to which they can ionize.

Higher ionization ≡ Strong

Strong acids and strong bases are completely ionized

Weak acids and weak bases are partially ionized

Strong acid does not necessarily mean that it is the

most corrosive acid. HF is a weak acid but it is one

of the most corrosive acids.

Strength of some acids:

HClO4 > HI > HBr > HCl > HNO3

H2SO4 > HNO3

slide8

Lewis theory

Proposed in 1923 by G. N. Lewis

An acid is any species that can accept a share in an

electron pair. A base is any species that can make

available or donate a share in an electron pair.

A more broader definition. There is no need for

the molecule to have hydrogens.

slide9

What happens when salts are

dissolved in water?

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

Acid

Base

Salt

Water

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

Salt

Water

Base

Acid

Depending on the strength of the acid and base

that are formed in solution, the medium will be

either acidic or basic

slide10

How do I know if the resulting

solution is acidic or basic?

Measure pH of the solution

What is pH?

It is the measure of hydrogen ions in solution

slide11

What is pH?

pH=14.0

Basic

Neutral

pH=7.0

Acidic

pH=1.0

slide12

What are some of the ways to

measure pH?

1. Using an acid-base indicator

2. Using pH paper

3. Using pH meter

4. ……………..

slide13

What are acid-base indicators?

How do they work?

An organic dye, whose color depends on the

concentration of H3O+ or pH

HIn(aq) + H2O(l) H3O+(aq) + In-(aq)

conjugate

acid

conjugate

base

acid

base

When the indicator is added to the solution of interest,

depending on the pH of the solution, the indicator stays

in either its acid form or its conjugate form.

slide14

Indicator"Acidic" Color"Basic" color

(MO)methyl orange Red below pH 4.0 Yellow above pH 4.4

(BG)bromcresol green Yellow below pH 4.5 Blue above pH 4.8

(BB)bromthymol blue Yellow below pH 6.9 Blue above pH 7.3

(TB)thymol blue Yellow below pH 8.0 Blue above pH 8.4

(P)Phenolphthalein Colorless below pH 9.3 Pink above pH 9.7

(AY)alizarin yellow Yellow below pH 10.8 Red above pH 11.2

Indicators cannot give a particular numerical value for pH.

They can only give a range for the pH of the solution.

If we are to pick a suitable indicator for

following the pH change in the reaction,

We have to pick one that is suitable over the pH range of the reaction.

slide15

Ionization of acids

All acids undergo ionization. Strong acids ionize

more than the weak ones. Strong acids are almost

100% ionized. Since the weak acids are partially

ionized, the measure of the extent of ionization is

given by an expression called acid ionization

constant, Ka.

In our experiment today, we would like to measure

the Ka for a weak monoprotic acid.

slide16

What is a monoprotic acid?

Usually acids have one or more hydrogen atoms (H)

which they end up donating as protons (H+).

# of H atoms Name Example

1 monoprotic acid (HA) HCl – hydrochloric acid

CH3COOH - acetic acid

2 diprotic acid (H2A) H2SO4 – sulfuric acid

3 triprotic acid (H3A) H3PO4 – Phosphoric acid

slide17

Ionization constant of an acid

For a monoprotic weak acid (HA) dissolved in water,

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

conjugate

acid

conjugate

base

acid

base

slide18

Ionization constant of an acid

Ionization constant of the acid

slide19

Ionization constant of an acid

Taking log of the equation on both sides,

slide20

Ionization constant of an acid

Multiplying both sides of the equation by -1

slide21

Ionization constant of an acid

Henderson-Hasselbach equation

In the above equation, if we can make the [A-] = [HA]

slide23

Vinitial

Vfinal- Vinitial =Vused

Add Vused amount of

water to the solution in

the volumetric flask

0.2 M

NaOH

Vfinal

Mix the solution in

volumetric flask to the

Solution in the erlenmeyer

flask

100mL

1.0 g of HA

In 100mL of

water

Measure the pH of the

final solution with

indicators and pH paper

250mL

250mL

50 mL of HA

+ 2 drops phenolphthalein