Chapter 14

Acids and Bases in the Environment

Acids and Bases

• Acids and Bases (video)• H+ ions - ?• OH- ions - ?• Soapy slippery - ?• Taste sour - ?• Acids give up H+ when added to water

forming - ?• More Hydronium – pH ___________

Safety

• When diluting a concentrated acid, always add acid to water

• Weak and strong acids and bases can all be dangerous. Handle with care

• Rinse with running water immediately if spilled on skin

COMMON ACIDS

Binary Acids A binary compound consists of two elements. Binary acids have the prefix hydro in front of the full name of the nonmetallic element.

They have the ending -ic. Examples include hydrochloric and hydrofluoric acid.

Hydrofluoric Acid - HFHydrochloric Acid - HClHydrobromic Acid - HBrHydroiodic Acid - HIHydrosulfuric Acid - H2S

COMMON ACIDS Ternary Acids

Ternary acids commonly contain hydrogen, a nonmetal, and oxygen.

The name of the most common form of the acid consists of the nonmetal root name with the -ic ending.

The acid containing one less oxygen atom than the most common form is designated by the -ous ending.

An acid containing one less oxygen atom than the -ous acid has the prefix hypo- and the -ous ending.

The acid containing one more oxygen than the most common acid has the per- prefix and the -ic ending.

COMMON ACIDS

Nitric Acid - HNO3

Nitrous Acid - HNO2

Hypochlorous Acid - HClOChlorous Acid - HClO2

Chloric Acid - HClO3

Perchloric Acid - HClO4

Sulfuric Acid - H2SO4

Sulfurous Acid - H2SO3

Phosphoric Acid - H3PO4

Phosphorous Acid - H3PO3

Carbonic Acid - H2CO3

Acetic Acid - HC2H3O2

Oxalic Acid - H2C2O4

Boric Acid - H3BO3

Silicic Acid - H2SiO3

PROPERTIES OF ACIDS

Corrosive ('burns' your skin) Sour taste (e.g. lemons, vinegar) Contains hydrogen ions (H+) when dissolved in

water Has a pH less than 7 Turns blue litmus paper to a red colour Reacts with bases to form salt and water Reacts with metals to form hydrogen gas Reacts with carbonates to form carbon dioxide,

water and a salt

General Equations and Review

• Note the general equations page 319

• Complete the revision questions 1, 2 page 319

Common BasesBases Sodium Hydroxide - NaOH

Potassium Hydroxide - KOHAmmonium Hydroxide - NH4OHCalcium Hydroxide - Ca(OH)2

Magnesium Hydroxide - Mg(OH)2

Barium Hydroxide - Ba(OH)2

Aluminum Hydroxide - Al(OH)3

Ferrous Hydroxide or Iron (II) Hydroxide - Fe(OH)2

Ferric Hydroxide or Iron (III) Hydroxide - Fe(OH)3

Zinc Hydroxide - Zn(OH)2

Lithium Hydroxide - LiOH

PROPERTIES OF BASES AND ALKALISCorrosive ('burns' your skin) Soapy feel Has a pH more than 7 Turns red litmus paper to a blue colour Many alkalis (soluble bases) contain hydroxyl

ions (OH-) – A base that is soluble in water is called an alkali

Reacts with acids to form salt and water

Johannes Brønsted Thomas Lowry (1879-1947) (1874-1936) Denmark England

The Lowry-Bronsted theory of acids and bases

• An acid is a substance that donates a proton (H+ ion) to another substance

• A base is a substance that accepts a proton (H+ ion) from another substance

• Since a hydrogen atom is simply a proton and an electron, removing the electron leaves a proton, H+ .

The Lowry-Bronsted theory of acids and bases

• Acid – proton donor • Base – proton acceptor • The general reaction for an acid dissolving in

water is

A Brønsted-Lowry acid is a proton donorA Brønsted-Lowry base is a proton acceptor

acid conjugate base

base conjugate acid

The Lowry-Bronsted theory of acids and bases

• HNO3 (l) + H2O (l) H3O+ (aq) + NO3- (aq)

• According to Bronsted-Lowry the water in this equation is acting as a base since it accepts a proton

H+

Acid loses a proton Base gains a proton

Review

• Complete question 3 page 321

IONISATION AND THE PRODUCTION OF THE HYDRONIUM ION

Ionisation is a reaction in which a molecule reacts with water to produce two or more ions

• Water acts as a base accepting a proton from the acid.

• Forms hydronium ion (H3O+)

Hydrolysis

• When an ionic substance dissolves in water, the resulting solution is often not neutral

• This is because the ions can act as acids or bases when reacting with water and produce solutions that are either acidic or basic – known as hydrolysis

• Hydrolysis reaction – An anion reacts with water to produce OH-

– A cation reacts with water to produce H3O+

• Not all cations and anions hydrolyse eg NaCl

Hydrolysis• Ammonium chloride is dissolved in

water. Ammonium and chloride ions are produced. The ammonium ions react as follows

• NH4+ (aq) + H2O (l) H3O+ (aq) + NH3

(aq)

• Is the resulting solution acidic or basic?

Hydrolysis

• Sodium carbonate is dissolved in water, sodium ions and carbonate ions are produced.

• The carbonate ion reacts as follows

• CO32- (aq) + H2O (l) HCO3

- (aq) + OH- (aq)

• Is the resulting solution acidic or basic?

Dissociation of Bases

• When ionic bases dissolve in water, they dissociate or separate into their constituent ions.

• They do not ionise since they do not actually react with the water to produce ions as acids do.

• NaOH (s) ---> Na+ (aq) + OH- (aq)H2O

Dissociation of Bases

• Molecular bases such as NH3 cannot dissociate as they do not contain ions

• Ammonia ionises

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

Neutralisation

• Neutralization Reaction - a reaction in which an acid and a base react in an aqueous solution to produce a salt and water:

• Write the neutralisation equation below as ionic and net ionic equation

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

• Note – can the H+ ion exist by itself in water?• Therefore the ionic equation of neutralisation is• H3O+ (aq) + OH- (aq) 2H2O (l)

Neutralisation

• Neutralisation reactions are one way of producing pure samples of salts.

• Hydrochloric acid and calcium hydroxide mixed – an aqueous solution of calcium chloride results

• 2HCl (aq) + Ca(OH)2(aq) CaCl2 (aq) + 2H2O (l)

• The water can be removed by evaporation• How do antacid tablets work in our stomach

which contains HCl?

Review

• Complete the revision question 4 page 323

Acid – Base terms

• Conjugate acid-base pairs• In an acid-base reaction, the substance acting

as the acid gives away a proton and forms a conjugate base.

• The substance acting as a base, after accepting a proton, forms its conjugate acid.

• An acid-base reaction will therefore form two conjugate pairs, with the formulas of each pair member differing by the H+ ion that was transferred.

Conjugate Pairs

Learning Check!

Label the acid, base, conjugate acid, and conjugate base in each reaction:

HCl + OH-   Cl- + H2O HCl + OH-   Cl- + H2O

H2O + H2SO4   HSO4- + H3O

+ H2O + H2SO4   HSO4- + H3O

+

Polyprotic Acids

• A polyprotic acid can donate more than one proton

• Acids can be classified as monoprotic, diprotic or polyprotic depending on the actual number of protons that can be donated

• Eg HCl – monoprotic (only one to donate)• Eg (Phosphoric acid )H3PO4 – triprotic (3 to

donate)

Amphiprotic substances and ampholytes

• Amphiprotic substances and ampholytes are substances that can act as either acids or bases.

• The way they react depends on the relative strengths of the acids and bases they are being reacted with.

• Water is a common example of an amphiprotic substance

• Ampholytes are electrolytes of ionic substances such as HSO4

- or HPO3-

Review

• Complete the revision questions 5 – 9 page 324

Strengths of Acids and Bases

• The strength of an acid or base is related to the ease with which it donates or accepts a proton– A strong acid donates protons readily– A strong base accepts protons readily

• Weak acids or bases do not donate or accept protons readily

Strengths of Acids and Bases• Conductivity is also related to strength.• A strong acid will react nearly to completion in

water and produce many ions, therefore conducting electricity (a good electrolyte)

• Write the equation for HCl reacting in water• A weak acid will not react to completion and its

solution will be a poor conductor of electricity because not many ions are produced.

• Write the equation for acetic acid CH3COOH reacting in water

Strengths of Acids and Bases

• The strength of a base also affects its conductivity• A base is strong if it produces many hydroxide

ions in solution because the hydroxide ions readily accept protons

• Write the equation for NaOH dissociating • Ammonia is a weak base and does not readily

dissociate, therefore it is a poor conductor of electricity

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

HNO3, HCl, H2SO4 and HClO4 are among the only known strong acids.

Strong and Weak Acids/Bases

The strength of an acid (or base) is determined by the amount of IONIZATION.

The strength of an acid (or base) is determined by the amount of IONIZATION.

Strong and Weak Acids/Bases

• Generally divide acids and bases into STRONG or WEAK ones.

STRONG ACID: HNO3 (aq) + H2O (l) --->

H3O+ (aq) + NO3-

(aq)

HNO3 is about 100% dissociated in water.

• Weak acids are much less than 100% ionized in

water.

One of the best known is acetic acid = CH3CO2H

Strong and Weak Acids/Bases

Strong and Weak Acids/Bases

• Strong Base: 100% dissociated in water.

NaOH (aq) ---> Na+ (aq) + OH- (aq)

Strong and Weak Acids/Bases

Other common strong bases include KOH and Ca(OH)2.

CaO (lime) + H2O -->

Ca(OH)2 (slaked lime)CaO

• Weak base: less than 100% ionized in water

One of the best known weak bases is ammonia

NH3 (aq) + H2O (l) NH4+ (aq) +

OH- (aq)

Strong and Weak Acids/Bases

Strong and Weak Acids/Bases

Weak Bases

Review

• Complete the revision questions 10 – 12 page 327

Strength vs Concentration

• The strength of a solution is determined by the number of ions present

• Strong acid – many ions in solution• Weak acid – few ions in solution• Concentration refers to the amount of an

acid or base that is dissolved in a given volume of water

• It is possible to have a weak, concentrated acid or a dilute solution of a strong acid.

The pH Scale

• The pH scale is usually applied over a range from 1 to 14

• 7 = _____________ ?• 1 – 6 = ___________ ?• 8 – 14 = ____________ ?

pH testing• There are several ways to test pH

– Blue litmus paper (red = acid)– Red litmus paper (blue = basic)– pH paper (multi-colored)– pH meter (7 is neutral, <7 acid, >7 base)– Universal indicator (multi-colored)– Indicators like phenolphthalein– Natural indicators like red cabbage,

radishes

Paper testing• Paper tests like litmus paper

and pH paper– Put a stirring rod into the solution and

stir.– Take the stirring rod out, and place a

drop of the solution from the end of the stirring rod onto a piece of the paper

– Read and record the color change. Note what the color indicates.

– You should only use a small portion of the paper. You can use one piece of paper for several tests.

pH paperpH paper

pH meter

• Tests the voltage of the electrolyte

• Converts the voltage to pH• Very cheap, accurate• Must be calibrated with a

buffer solution

pH indicators• Indicators are dyes that can be

added that will change color in the presence of an acid or base.

• Some indicators only work in a specific range of pH

• Once the drops are added, the sample is ruined

• Some dyes are natural, like radish skin or red cabbage

Review• Read the chapter summary and note the

terminology and concepts

• Complete the multiple choice questions 1 – 11 page 331, 332

• Complete the review questions 4, 5 (a), (b), (d), 8, 12 (a), (b),(c), 17, 19 (a), 21

Extras

• Acid Base review on line

• Review Acids, Bases and pH (video)