ACIDS AND BASES

Acids and bases reactions

• The acids react with bases (alkalis), forming salts in a process called neutralization/ neutralisation.

ACID + BASE → SALT + WATER

HCl + NaOH → NaCl + H2O

H-OH

A salt is made.

Neutralization Reaction

Acid + Base Water + Salt (double replacement)

• HCl (aq) + NaOH (aq) HOH (l) + NaCl (aq)

• H2SO4 (aq) + 2 KOH (aq) 2 HOH (l) + K2SO4 (aq)

Example:

• HNO3 (aq) + Ca(OH)2 (aq)

• H3PO4 (aq) + Mg(OH)2 (aq)

Practice:

• HBr (aq) + LiOH (aq)

• H2CrO4 (aq) + NaOH (aq)

(c) 2006, Mark Rosengarten

Salts

• The salt made depends on the acid and alkali used.

• The salt contains the cation from the alkali, and anion of the acid molecule.

The salts of sulphuric acid are known as sulphates.

The salts of hydrochloric acid are known as chlorides.

The salts of nitric acid are known as nitrates.

Are salts acidic? Basic? Or neutral?

Acid-Base Properties of Salt

• Assume that when salts dissolve in water, are completely ionized.

• Nearly all salts are strong electrolyte.

• Acid-base property: due to the behavior of their cations & anions

• Hydrolysis

– Rxn of ions with H2O to form H+(aq) or OH-

(aq)

Acid-Base Properties of Salt

• ANIONS: – From WEAK ACID react w/ water to form

OH- ions & are thus BASIC

– From STRONG ACID do not influence pH

– Have IONIZABLE PROTONS are AMPHOTERIC; behavior are determined by relative magnitude of Ka & Kb

Acid-Base Properties of Salt

• CATIONS:

–From ALKALI METALS & ALKALINE EARTH react w/ water to form H+ ions & are thus ACIDIC

–ALL OTHERS do not influence pH

Qualitative Prediction of pH of a Soln of Salt

1) Salts derived from SB & SA – cation(B+) & anion(A-) do not hydrolyzes; pH=7

2) Salts derived from SB & WA – A-: SCB, B+: do not hydrolyze; pH > 7 (BASIC)

3) Salts derived from WB & SA – B+: SCA, A- : do not hydrolyze; pH < 7 (ACIDIC)

4) Salts derived from WB & WA – B+ & A- :Both hydrolyzes; depends on Ka & Kb

– Ka > Kb = acidic; Ka < Kb = basic

Exercise: Identify whether Acid, Basic or Neutral

• Example

1) NaCl

2) NaClO

3) NH4Cl

4) NH4CN

5) NH4C2H3O2 – Ka of HCN = 6.2 x 10-10

– Kb of NH4OH= 1.8 x 10-5

– Ka of HAc = 1.8 x 10-5

• Practice 1)Ca(NO3)2

2)Ba(C2H3O2)2

3)Al(NO3)3

4)(NH4)2CO3

5)KNO3 Ka of H2CO3 = 4.5 x 10-7

Titration

• Experimental process of reacting a solution of unknown concentration with one of known concentration (standard solution)

• Example: – Suppose we have an HCl soln of an unknown conc & NaOH

soln that we know to be 0.100 M. To determine the conc of HCl, we take specific volume of that soln, say 20.00mL. We then slowly add the standard NaOH soln to it until the neutralization reaction between the HCl & NaOH is complete.

• @ neutralization: mole ACID = mole BASE

Acid-Base Titration

Measuring pH during titration

Typical Set-up

pH Titration Curve

• pH profile of a SA-SB titration

pH Titration Curve

pH Profile of SA-WB titration pH Profile of WA-SB titration

Acid-Base Titration

• Equivalence point/endpoint – The point at which stoichiometrically equivalent quantities

are brought together.

– @ neutralization: mole ACID = mole BASE

– @ equivalence point: mole ACID = mole BASE

• The shape of pH titration curve makes it possible to determine the equivalence point of titration.

– Can also be used to determine Ka of weak acid & Kb of weak base being titrated

Acid-Base Indicators

Methyl red indicator Phenolphthalein

Indicators

They change colour in acid or alkaline solutions.

Different indicators change to different colours.

Used to determine the equivalence point of titration.

Indicators help

you find out

whether a

solution is

acidic or not.

An indicator is a large organic molecule that works

somewhat like a "color dye."

Indicators

(c) 2006, Mark Rosengarten

At a pH of 2:

Methyl Orange = red

Bromthymol Blue = yellow

Phenolphthalein = colorless

Litmus = red

Bromcresol Green = yellow

Thymol Blue = yellow Methyl orange is red at a pH of 3.2 and

below and yellow at a pH of 4.4 and

higher. In between the two numbers, it is

an intermediate color that is not listed on

this table.

Litmus Test

• Litmus is an indicator. It changes colour in acid and alkaline solutions.

• Litmus is red in an acid.

• Litmus is blue in an alkali.

Universal Indicator

• Universal indicator changes colour in acids and alkalis.

Its colour shows the strength of an acid or alkali.

ACIDS ALKALIS N

eutra

l

Natural indicators

• There are natural indicators for acids and bases, and we may find them in our kitchen or garden!

– Red rose flowers

– Bougainvillea flowers

– Red cabbage

– Blue berries

http://tides.sfasu.edu:2006/cdm4/item_viewer.php?CISOROOT=/Digital&CISOPTR=1019&CISOBOX=1&REC=4

Natural Indicators

Red cabbage treated with acids & bases

Components of an Indicator

• To be an effective indicator, weak monoprotic acid, HIn & its conjugate base, In-, must have distinctly different colors.

• In solution, the acid ionizes to a small extent:

HIn(aq) H+(aq) + In-(aq)

[Hin] / [In-] 10 color of acid (Hin) predominates

[Hin] / [In-] 0.1 color of conj base (In-) predominates

[Hin] = [In-] indicator color is a combination of HIn & In-

APPLICATIONS

Applications of Neutralisation

• Indigestion: Our stomach carries around hydrochloric acid. Too much of this leads to indigestion. To cure indigestion, you can neutralise the excess

acid with baking soda or specialised indigestion tablets.

• Insect Stings

Bee stings are acidic

and can be neutralised with

baking soda (bicarbonate of soda).

Wasp stings are alkaline and can be

neutralised with vinegar.

More Applications of Neutralisation

Factory Waste: Liquid waste from factories is often acidic. If it reaches a river it will destroy and kill sea life of many forms. Neutralising the waste with slaked lime can prevent this.

Soil Treatment: When soils are too acidic (often as a result of acid rain) they can be

treated with slaked lime, chalk or quicklime, all alkalis. Plants and crops

grow best in neutral soils.

Caves formation

• Cave formation is based on a chemical

reaction between an acid and a base. This

acid is carbonic acid (H2CO3), and the base is

calcium carbonate (CaCO3), although it is not a

direct reaction.

Caves formation

• Carbonic acid is formed by the reaction of rain water and carbon dioxide from soil. When the water is absorbed by the soil into the ground it reacts with the carbon dioxide present there:

H2O + CO2 → H2CO3 Water + carbon dioxide → carbonic acid

• Carbonic acid is responsible for acid rain.

Caves formation

• The acid water reacts chemically with rocks made of the base calcium carbonate, called limestone, and dissolves them:

H2CO3 + CaCO3 → Ca + H2CO3 Carbonic acid + calcium carbonate → calcium + carbonic acid

Caves formation

• The calcium reacts with the hydrogen carbonate:

Ca + HCO3- → CO2 + CaCO3 + H2O

• The carbon dioxide is given off into cave air to

react again with rain water. Calcium carbonate is deposited, and water is formed. That’s the reason you will always see water inside the limestone caverns.

Caves formation

• These series of chemical reactions are very slow and take thousands of years to produce the characteristic stalagmites and stalactites of these caverns.

http://tides.sfasu.edu:2006/cdm4/item_viewer.php?CISOROOT=/PPY&CISOPTR=81&CISOBOX=1&REC=1

CHEMICAL IMPACT

Resources

Animation of cave formation on Exploring Earth: http://www.classzone.com/books/earth_science/terc/content/visualizations/es

1405/es1405page01.cfm?chapter_no=visualization

Acids & Alkalis Presentation. Retrieved from www.worldofteaching.com

Mark Rosengarten’s Amazing Chemistry Powerpoint Presentation! (2006). Retrieved

from www.markrosengarten.com