Acids and Bases An Introduction Chapter 4 Nova Scotia Science 10

What do you already know about acids and bases?

What r some common acids? Deoxyribonucleaic acid (DNA) Ribonucleaic acid (RNA) Amino acids (building blocks of protein) Lactic acid (build-up →sore muscles; by- product of cell metabolism and insufficient oxygen) Boric acid (antiseptic) Acetic acid ( in vinegar) Citric acid (in fruits)

What is an acid? “an acid is a substance that produces hydrogen ions in solution” – Arrhenius 1884 HCl (aq) → H + (aq) + Cl - (aq)

What are some common bases? Most soaps/detergents Most drain cleaners Most window cleaners

What is a base? “ a substance that produces hydroxide ions in solution” – Arrhenius NaOH (aq) →Na + (aq) + OH - (aq)

Distinguishing between acids and bases Most solutions of both acids and bases are clear and colourless. We need an indicator to tell them apart. An indicator is a chemical which changes colour as the concentration of H + (aq) and OH - (aq) changes. Two common indicators are litmus and phenolpthalein

Which compounds are acids and which are bases? Acids: Formulas begin with one or more hydrogen atoms (ex. HCl (aq) or H 2 SO 4 (aq)) Names have the word “acid” in them (ex. Hydrochloric acid or sulfuric acid) Bases: Formulas end with one or more “OH” (ex. NaOH (aq) or Ca(OH) 2 (aq)) Names end in hydroxide (ex. Sodium hydroxide or calcium hydroxide)

Characteristics of acids Sour taste (never taste lab chemicals) Stinging feeling on mucous membranes Strong acids will burn your skin Aggressive reactions with metals Conduct electricity Turns blue litmus red Phenolpthalein remains colourless Forms carbon dioxide when reacting with carbonates and non metal oxides when reacting with water

Characteristics of bases taste bitter feel slippery (soap and many drugs are bases) caustic on organic matter conduct electricity Strong bases will burn your skin Turn red litmus blue Turns phenolpthalein pink Does not react with carbonates but forms metallic oxides when reacting with water

Oxides of Elements Acids and bases are formed when oxides of elements react with water. An oxide is a binary compound formed with an element and oxygen e.g. CO, CO 2, Al 2 O 3, N 2 O 5 H 2 O (l) + CO 2 (g) → H 2 CO 3 (aq)

pH : a “powerful” scale ( courtesy of Sørensen – a Danish biochemist) Really means the power or concentration of hydrogen ions in solution The lower the pH the greater the concentration of H + (aq) and the more acidic the solution The higher the pH, the greater the concentration of OH - (aq) and the more basic the solution The scale goes from 0-14 with a midpoint of 7 Solutions with a pH of 7 are neutral

Ways to measure pH pH paper pH meters pH probes

How is pH calculated? You need not know this but for future reference: pH = -log [H + ] E.g. suppose that your concentration was 0.010, enter this in your calculator and press the log button, change the sign, your answer should be 2.0

Properties of Acids and Bases Two factors determine how many H + ions are contained in a solution: concentration and percent ionization Concentration refers to the amount of pure acid dissolved per litre of water Percent ionization refers to the number of molecules that will ionize per 100 that dissolve. Strong acids ionize completely whereas weak acids do not. E.g. sulfuric is strong and acetic is weak Strong bases also ionize completely whereas weak bases do not e.g. NaOH is a strong base but NH 3 is not

Strong Acids and Bases NameFormula Sulfuric acidH 2 SO 4 Hydrochloric acidHCl Nitric acidHNO 3 Carbonic acidH 2 CO 3 Sodium hydroxideNaOH Potassium hydroxideKOH Calcium hydroxideCa(OH) 2 Magnesium hydroxideMg(OH) 2 ammoniaNH 3

Neutralization reactions When an acid and a base react together to form a new compound, the word equation is Acid + Base → Salt + Water A salt is an ionic compound produced when an acid and base react The reaction is also known as a neutralization reaction HNO 3 (aq) + KOH (aq) → KNO 3 (aq)+ H 2 0 (l)

Chemistry of Swimming Pools Acids, bases, and neutralization reactions help maintain water in swimming pools. Water is chlorinated to kill bacteria and algae but chlorine gas is toxic HOCl (aq) +NaOH (aq) → NaOCl (aq) + H 2 O (l) and