Properties of acids and basesProperties of acids and bases• Get 8 test tubes. Rinse all tubes well with water. Add

acid to four tubes, base to the other four.• Touch a drop of base to your finger. Record the feel

in the chart (on the next slide). Wash your hands with water. Repeat for acid.

• Use a stirring rod, add base to the litmus and pH papers (for pH paper use a colour key to find a number). Record results. Repeat for acid.

• Into the four base tubes add: a) two drops of phenolphthalein, b) 2 drops of bromothymol, c) a piece of Mg, d) a small scoop of baking soda. Record results. Repeat for acid.

• Clean up (wash tubes, pH/litmus paper in trash).

Baking soda

Magnesium

Bromothymol

Phenolphthalein

Litmus (blue or red)

pH (# from the key)

Feel (choose slippery or not slippery)

SourBitterTaste

HCl(aq)NaOH(aq)ObservationsObservations

BubblesNRBaking soda

BubblesNRMagnesium

*Yellow*BlueBromothymol

*Cloudy/ white

*PinkPhenolphthalein

RedBlueLitmus (blue or red)

114pH (# from the key)

Not slipperySlipperyFeel (choose slippery

or not slippery)

SourBitterTaste

HCl(aq)NaOH(aq)ObservationsObservations

*Usually, but not always

Historical views on acidsHistorical views on acids

• The more recent Bronsted-Lowry concept is that acids are H+ (proton) donors and bases are proton acceptors

Ionization

+Cl HH

HO

+H

HH O Cl+

• O (e.g. H2SO4) was originally thought to cause acidic properties. Later, H was implicated, but it was still not clear why CH4 was neutral.

• Arrhenius made the revolutionary suggestion that some solutions contain ions & that acids produce H3O+ (hydronium) ions in solution.

Bronsted-Lowry theory

• An acid is a substance from which a proton (H+ ion) can be removed

• A base is a substance that can remove a proton (H+ ion) from an acid

• An acid-base reaction involves the transfer of a proton

Types of Acids

• Acids may have more than one proton to donate.– Monoprotic acids contain 1 hydrogen ion i.e.

HCl(aq)

– Diprotic acids contain 2 hydrogen ions, i.e. H2SO4(aq)

– Triprotic acids contain 3 hydrogen ions, i.e. H3PO4(aq)

Conjugate acid-base pair

Two molecules or ions that are related by the transfer of a proton (H+) are called a conjugate acid-base pair.

(conjugate means “linked”)

The Bronsted-Lowry conceptThe Bronsted-Lowry concept• In this idea, the ionization of an acid by water

is just one example of an acid-base reaction.

• Acids and bases are identified based on whether they donate or accept H+.

• “Conjugate” acids and bases are found on the products side of the equation. A conjugate base is the same as the starting acid minus H+.

+Cl HH

HO

+H

HH O Cl+

acid base conjugate acid conjugate base

conjugate acid-base pairs

Example:HBr(aq) + H20(l) -> H30+(aq) + Br-

(aq)

Conjugate base of an acid is the particle that remains when a proton is removed from the acid (in this example: Br-(aq)

Conjugate acid of a base is the particle that results when the base receives the proton from the acid (in this example: H30+(aq)

Practice problemsPractice problemsIdentify the acid, base, conjugate acid, conjugate base, and conjugate acid-base pairs:

acid base conjugate acidconjugate baseHC2H3O2(aq) + H2O(l) C2H3O2

–(aq) + H3O+(aq)

conjugate acid-base pairs

acidbase conjugate acidconjugate baseOH

–(aq) + HCO3–(aq) CO3

2–(aq) + H2O(l)

conjugate acid-base pairs

Acids

An acid is a substance that produces Hydrogen ions, H+(aq) in water (Arrhenius definition).

The hydrogen ion (H+) bonds with a water molecule to make a hydronium ion, H30+ (aq) (H+ -> H20 = H3O+)

Strong acids

A strong acid is an acid, which ionizes extremely well. Essentially all of the acid molecules are ionized. (lots of H3O+)

Example: HCl, HBr, HNO3, H2SO4, HClO4

Example:HCl (aq) + H2O(l) -> H30+(aq) + Cl-(aq)

H2SO4(aq) +2H2O(l) -> 2H3O+ + SO42-(aq)

(Ionization equations show how they react with water to make H3O+)

Weak acid

A weak acid is an acid which ionizes poorly. Only a small percentage of the acid molecules actually ionize. (a few H3O+)

Example: HC2H3O2

HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H302-

Avoid confusing terms!

• Concentrated/Dilute vs Strong/Weak

• A strong acid such as HCl can be concentrated (12mol/L) or dilute (0.5mol/L)

• Concentrated or dilute refers to the amount of solute dissolved in the solvent

• Strong or weak- refers to the degree of dissociation of ions in solutions.

Bases

A base is a substance that produces hydroxide ions, OH-, in water (Arrhenius defintion)

A strong base is a base that dissociates 100% in aqueous solution.

Example: NaOH(s) Na+(aq) + OH-(aq)KOH(s) -> K+(aq) + OH-(aq)(Ionization equations: show how they make ions)

Weak baseA weak base is a base which ionizespoorly. Only a small percentage of

the base molecules actually ionize.(a few

OH-

Example: NH3(aq) + H2O(l) -> NH4

+(aq) + OH-(aq)