Introduction to Acids and Bases

Science 10

Introduction to Acids

Do your muscles ever feel sore after a heavy workout?

If so, you can blame a buildup of lactic acid in your muscles.

Lactic acid is a by-product of cell metabolism when too little oxygen is available.

Sore muscles and sour milk are caused by the same acid.

Introduction to Acids (con’t)

Lactic acid is also produced by bacteria in milk – this is why milk turns sour.

Acids occur naturally in many fruits. Insects such as millipedes, scorpions, and ants use

acids to deter predators. A physician may use a solution of boric acid to rinse

out your eyes, but a drop or two of many other acids would blind you.

Some acids add a tangy, sour flavour to foods and drinks, while others are deadly.

Characteristics of Acids

Corrode metals Taste sour React with bases to form salts and water Produce H+ in solution

Example: HCl H+ + Cl-

HCl + NaOH NaCl + H2O

Introduction to Bases

Bases are bitter-tasting compounds with a slippery feel.

Most soaps and drain and window cleaners are bases.

Introduction to Bases (con’t)

Quinine is a base which is responsible for the bitter flavour of tonic water and it occurs naturally in the bark of the cinchona tree.

Quinine is also used as a drug to treat malaria. Many other drugs are bases, too. For example, lidocaine, a local anaestheic used by

dentists, is a base. Like acids, some bases are harmless while others

are very dangerous.

Word Connect

The word “acid” comes from the Latin acidus, meaning “sour”. Think of the taste of fruits such as lemons or limes. These fruits contain citric acid. Vinegar, a common household product, contains acetic acid.

Chemists sometimes use the term “alkali” for a base that is soluble in water. This word shows the Arabic origin of chemistry. The earliest source of bases was the ash obtained by burning plants. Thus the Arabic word alkali means “ashes of a plant.” What clue does the word “alkali” give you about the kinds of elements that you might find in a basic compound?

Defining Acids

Most acids and bases dissolve in water. In 1884 the Swedish chemist Svante Arrhenius

defined an acid as a substance that produces hydrogen ions in solution, H+ (aq).

For example, HCl (aq), hydrochloric acid is an ionic compound that dissolves in water, splitting into H+ (aq) and Cl – (aq) ions.

The resulting solution is an electrolyte.

Defining Bases

Arrhenius also defined a base as a substance that produces hydroxide ions in solution, OH– (aq).

An example of a base is sodium hydroxide, NaOH (aq). Sodium hydroxide forms OH– ions in solution.

Svante Arrhenius (1859-1927)

Defining Indicators

Most solutions of acids or bases are clear and colourless. Therefore they cannot be distinguished from ordinary water by appearance alone.

The simplest way to distinguish them from water is to use an indicator.

An indicator is a chemical that changes colour as the concentration of H+ (aq) or OH– (aq) changes.

Defining Indicators (con’t)

Two of the most common indicators are phenolphthalein and litmus.

Litmus is a compound that is extracted from lichens, a plant-like member of the fungi kingdom. Litmus paper is made by dipping paper in litmus solution.

pH Scale

Used to define acids and bases

Range: 0-14– 0-6: Acidic– 8-14: Basic– 7: Neutral

pH meter used to identify pH of various substances

pH of Common Substances

Naming Review

HCl H2SO4

H2CO3

HF HNO2

The Oxides of Elements

Oxides of many elements react with water to form an acid or a base.

An oxide is a compound that consists of an element combined with only oxygen. For example, CO (carbon monoxide), CO2 (carbon dioxide), and Al2O3 (aluminum oxide) are oxides. H2SO4 and HNO3 are not.

Amphoteric

pH = 7

Neutral

O

H H

OH

HO+

OH

HH

H+

Water

+-

Check Your Understanding

1. In your own words, summarize Arrhenius’ theory about acids and bases.

2. Balance each of the following skeleton ion equations. Identify the substance on the left of the equation as an acid or a base.

a) H2CO3 (aq) → H+ (aq) + CO3 2- (aq)

b) Ca(OH)2 (aq) → Ca 2+ (aq) + OH- (aq)

c) KOH (aq) → K + (aq) + SO4 2- (aq)

Check Your Understanding (con’t)

3. Chris observes that a drop of an unknown solution on red litmus paper causes no colour change. Chris correctly concludes the solution is either neutral or acidic, but cannot be sure which. Explain. What further test(s) could Chris perform to identify whether the solution is, in fact, neutral or acidic?

pH Calculations

pH = -log[H+]– [concentration]– The pH is directly proportional to the amount of hydrogen

in the solution

pOH = -log[OH-]– The pOH is directly proportional to the amount of

hydrogen in the solution

pH + pOH = 14.00 Kw = [H+][OH-]

– Kw = 1.0 x 10-14

pH Calculations

neutral solution: [H3O+] = [OH-] = 10 –7 M – pH = 7.0

acidic solution: [H3O+] > 10-7 M – pH < 7.0

basic solution: [H3O+] < 10-7 M – pH > 7.0

Practice

Calculate the pH of a solution in which [H +] = 1.0 x 10-9 acidic or basic?

Calculate the pH of a solution in which [OH -] = 1.0 x 10-6 acidic or basic?

Calculate the pH of a solution in which [H +] = 1.0 x 10-3 acidic or basic?

Practice

The pH of a human blood sample was measured to be 7.41. What is the [H+] in solution?

The pOH of water in a fish tank is found to be 6.59. What is the [OH-] for this water?

Conjugate Acid/Base Pairs

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

acid base conjugate conjugate acid base

Conjugate base is everything that remains of the acid molecule after a proton is lost.

Conjugate acid is formed when the proton is transferred to the base.

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

acid 1 base 2 acid 2 base 1

Conjugate base: everything that remains of the acid molecule after a proton is lost.Conjugate acid: formed when the proton is transferred to the base.A conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single proton.

Buffered Solutions

Resists a change in pH when strong acid or base is added to solution

Made by using a weak acid and its conjugate base

Example: acetic acid and sodium acetate– HC2H3O2, Na+, C2H3O2

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